(i) .

Intraspecific relationships using morphological characteristics

The morphological variability within Panicum miliaceum has led to many attempts to subdivide this species. Much of this work was carried out in the former USSR in the early 20th century. Most divisions were based on panicle morphology, including the presence or absence of pulvini. Seed/fertile floret colour and shattering of the panicle were other characteristics often used. This literature is summarized in Table 1.

Table 1.

Proposed subdivisions of Panicum miliaceum based primarily on morphological variation, 1885 until the present.

In addition to these subspecies and varieties, many named variants of the species are grown as commercial crops in North America, China, India, Russia and parts of Eastern Europe. In Udaipur, India, Baghel and Maloo (2002) measured 12 characters for each of 34 crop varieties and strains of proso millet grown in two different locations, one with a much higher annual rainfall than the other. Statistical analysis showed highly significant differences among the genotypes for all characters studied, including seed yield. Reddy et al. (2007) tabulated the variability in several morphological and phenological characteristics within 842 accessions of the species from 27 countries grown in a common garden at the gene bank at ICRISAT, Patancheru, Andhra Pradesh, India. All five of Lysov’s (1975) races (see Table 1) were represented.

All of the extensive research presented points to great morphological variability within the species (singular or plural depending on the placement of the Black biotype). Genetic analyses incorporating more recent molecular approaches are providing further insights into these intraspecific relationships.

(ii) .

Classification based on genetic studies of seed traits

As noted in Table 1, Lysov (1975) considered caryopsis colour to be variable and inconsistent. Ayyangar and Rao (1938), Yashovskii (1974), Il’in et al. (1978) and others have attempted to elucidate the genetic control of caryopsis colour in crop varieties. They found the number of genes involved to be small, about four. Krasavin and Ul’yanova (1989) suggested that the weedy subsp. ruderale (Kitag.) Tsvelev could most easily be distinguished from the cultivated subsp. miliaceum by seed colour; the weedy plants have “dirty yellow” seeds, unlike those of the cultivated plants, which have light to bright yellow seeds. They stated that the two subspecies easily hybridize and that the F₁ hybrids have dirty yellow seeds. In the F₂, seed colour segregates in a monohybrid ratio. Later, Krasavin (1991) suggested that cultivated and weedy forms may be distinguished by testing the exposed caryopsis (husk removed) with a solution of 5% iodine in alcohol. In his work, weedy forms turned dark brown after 5–7 min, whereas cultivated forms remained either unchanged or turned light pink. More recently, Graybosch and Baltensperger (2009) studied the waxy (amylose-free) endosperm trait [see section 3(a)] in some USDA accessions. Only six of the 650 accessions had this trait, of which five derived from mainland China. This trait was found to be under the control of duplicate recessive alleles at two loci.

(iii) .

Intraspecific relationships using molecular techniques

Mansfeld (1952), Zhukovsky (1964) and Lysov (1975) regarded Panicum plants with olive–black grains and open panicles as a separate species, P. spontaneum Lyssov ex Zhuk. (see also Table 1). Based on electrophoretic studies, Oestry and de Wet (1981) considered plants with this description to be only a subspecies of P. miliaceum (i.e., subsp. ruderale).

In another electrophoretic study, using seedlings from 110 world-wide collections of proso millet in which 11 enzyme systems were examined, Warwick (1987) detected only very low levels of isozyme variation. This is in contrast to the striking phenotypic and life-history differences among populations of the species. All black-seeded weedy strains of the species, from both Europe and North America, contained the most common multi-locus genotype out of the eight possibilities in this study. In a later paper, Warwick (1990) examined five northwardly colonising North American weed species, including proso millet. In all cases, there were low levels of allozyme variation. She suggested that the limited allozyme variability present in new introductions, particularly in predominantly selfing species, may be a reflection of founder effects.

Although the isozyme and protein electrophoresis patterns may indicate low genetic differentiation among proso millet biotypes, Colosi and Schaal (1997) wondered if the allozyme systems normally selected for study accurately reflected the genetic diversity of colonizing plants. In their paper, Colosi and Schaal employed DNA molecular techniques on 398 individuals representing a range of proso millet biotypes (weeds, crop-like weeds, crops and hybrids) to examine relationships among them. They used Random Amplified Polymorphic DNA (RAPD) markers to assess genetic relationships within the species. The premise of the technique is that several RAPD primers taken together provide a genetic signature of an individual and genetic relatedness of two individuals may be inferred from the degree of similarity of these signatures. Their work revealed that five RAPD markers consistently differentiated wild-proso millet from crops and crop-like weeds. They reported more variability among wild-proso millet samples than expected for a plant that had only been described as present in North America from about 1970 (Harvey 1979), which might suggest multiple introductions rather than a single entry. Their work also indicated possible hybridization between wild and crop biotypes (see section 9). Colosi and Schaal (1997) illustrated the genetic distance of the 97 genotypes identified in their study of 398 individuals, in the form of a phenogram. In this, the wild proso millets separated into two groups. Crop and most crop-like weeds formed another group as did cv. Crown and Crown-like weeds. An additional group was also distinguished. It was comprised of two genotypes that were morphologically intermediate between crop and wild-proso millet. The authors suggested that these two genotypes may have been hybrids. Three Elgin [for description of this biotype see section 2(c), paragraph 2] plants were grouped together. Other than the group of possible hybrids, these Elgin individuals appeared to be genetically closest to wild-proso millet, even though morphologically and phenologically they were the most dissimilar.

M’Ribu and Hilu (1994), in a study of four Panicum species, were also of the opinion that the use of RAPD markers was more effective than previous methods for studying genetic diversity and defining gene pools. In their work, they used 13 accessions of proso millet cultivars from several parts of the world. These grouped according to geographical region of origin.

Hu et al. (2008) employed polymerase chain reaction (PCR) analysis, using six primers, to investigate genetic diversity among 32 Chinese accessions of P. miliaceum and six Indian landraces. The results revealed extensive polymorphism among the accessions. A cluster dendrogram produced by similarity coefficient analysis revealed geographical groupings consistent with those of the M’Ribu and Hilu (1994) study, but not with the results of Karam et al. (2004) described below. Hu et al. (2008) mentioned “hard” (which millet breeders call non-glutinous) and “soft” (glutinous) grain characteristics among the samples. Glutinous varieties have a waxy endosperm (Kaume 2006). This is not a distinction used widely in the literature, but is useful for interpreting how the various millet forms would be used and eaten [see section 3(b)].

Karam et al. (2004) used the Amplified Fragment Length Polymorphism (AFLP) technique on three domestic and nine wild biotypes of proso millet. Subsequent cluster analysis resulted in two distinct groups without any geographical association. Six weedy types with some cultivated characteristics clustered with the domesticated plants and three plants with typical wild-type traits formed another group.

From all of these studies, we conclude firstly that weedy types of proso millet in North America are highly variable and no doubt have several origins, and secondly that intraspecific relationships have not been determined completely.

(i) .

Primarily for crop populations

Proso millet is a warm-season annual adapted to temperate and subtropical areas. Komarov (1934) described it as a typical xerophyte. The ability of the species to complete its life-cycle within 60–90 days enables it to survive (or avoid) heat and drought in arid areas (Shanahan et al. 2000; Zohary and Hopf 2001). According to Hanna et al. (2004), P. miliaceum can grow at higher latitudes (up to 54°N) than other millets and is well adapted to plateau conditions and higher elevations (up to 1200 m in the former USSR and 3500 m in India). Akhalkatsi et al. (2012) noted that it is cultivated in high mountain regions (1000–1800 m) in parts of Georgia in the former USSR. During development the plants need temperatures of 18 °C–23 °C at various stages of growth (Gashkova 2003–2009; Kalinová and Moudrý 2005). Although the plant has a high water-use efficiency, it is the short growing season, rather than drought tolerance, that confers success in hot dry areas, because the species does not grow well under either water stress or, in fact, excessive moisture (Baltensperger 1996; Upadhyaya et al. 2008). In an experiment conducted in Iran to establish the stages of growth at which P. miliaceum is most susceptible to drought stress, Seghatoleslami et al. (2008) found the greatest reduction in seed yield and water-use efficiency at the ear-emergence stage.

In Germany, Öztürk et al. (1981) compared the growth of a C₃ grass, Avena sativa, with that of Panicum miliaceum, a C₄ grass, under a range of soil-water levels. They found that the length and dry weight of the above-ground parts of A. sativa increased, whereas the comparable values for P. miliaceum decreased, as soil water supply increased. The roots of P. miliaceum were longer than those of A. sativa under all water conditions. Also, the root length in P. miliaceum increased in the drier soils whereas root length in A. sativa did not change with changes in soil moisture.

In Wisconsin, Anderegg and Lichtenstein (1981) compared five C₃ species to four C₄ species for the amount of water transpired. All of the C₄ species transpired less than any of the C₃ species, and Panicum miliaceum (red proso variety) transpired the least of all the species.

Although it is described as a short-day species [Kumar et al. 1977; Baltensperger 1996; but see section 3(b)], Carpenter and Hopen (1985) found that both wild and cultivated proso millet flowered under all photoperiods (ranging from 10 to 24 h) under experimental conditions. They also found that dry matter production increased with photoperiod. Tayal (1972) reported that maximal values for the dry weights of whole plants were reached earlier under short days (8 h d^-1) than in natural day lengths or long days (18 h d^-1). However, the dry-weight increase, although slower, lasted longer and reached higher maxima in the latter two regimes. In a later paper, Tayal and Nanda (1980) reported that the size of the inflorescence and the number of spikelets were much reduced under short days but the size of floral organs and the embryo did not differ much under the three photoperiods. They concluded that the shorter vegetative period for plants sown later in the growing season (September) is primarily a result of exposure to shorter photoperiods.

In the western Great Plains of the USA where proso millet is grown as a crop, it is susceptible to hail damage. Simulations have demonstrated that seed number, rather than mass, is reduced by hail (Shanahan et al. 2000).

Proso millet plants are sensitive to low air temperatures (+2 °C to -3 °C), but can survive better in comparatively cooler climates than foxtail millet [Setaria italica (L.) P. Beauv.], so that, as a crop, proso millet can be grown in more northerly zones (Marinval 1992). Plants may be damaged by frost, in particular at both the germination and flowering stages (Gashkova 2003–2009). Kalinová and Moudrý (2005) found proso millet plants to be most sensitive to frost from the two-leaf to tillering stages in their experiments. They also noted that the specific temperatures (-1.5 °C to -4.1 °C) for frost sensitivity vary with crop variety.

Plants of P. miliaceum showed decreased biomass accumulation, and decreased grain quality and quantity when they were exposed to 0.08 ppm O₃, 0.5% SO₂ or a combination of 0.04% O₃ + 0.25% SO₂ under experimental conditions. The pollutants had a synergistic effect on the plants (Agrawal et al. 1983).

(ii) .

Weed populations

Patterson et al. (1986) compared the responses of wild-proso millet and Texas panicum (Panicum texanum Buckl.), grown separately or together, to different temperature/photoperiod regimes in a controlled environment. When the species were grown separately, height, tiller number, leaf number and leaf area, and plant dry weight for proso millet were all greater in a 30/24 °C day/night temperature regime than in a 24/18 °C regime over the first 31 days of the experiment. When the two species were grown together, Texas panicum was the superior competitor under the higher temperature regime and wild-proso millet outcompeted Texas panicum under the cooler regime. It was the authors’ opinion that wild-proso millet would be competitively inferior to locally adapted grasses, including Texas panicum, in the southern United States.

Under experimental field conditions where the species was grown in either monoculture or in competition with corn, leaf area index (LAI) and radiation use efficiency (RUE) were greater under the competitive conditions where shading increased the proportion of diffuse light reaching the weed (Gramig et al. 2006). The nature of the crop canopy also affects the growth of the weed (see section 10).

Di Paola and Benvenuti (1993), using chlorophyll luminescence as an indicator of chilling tolerance, found proso millet to be the most sensitive to chilling of the eight weeds they studied.

(i) .

Primarily crop information

To our knowledge, the species has no very specific substrate requirements. Marinval (1992) considered P. miliaceum to be less exacting in its soil requirements than Setaria italica. It thrives on the chernozems of Manitoba and the calcareous, clay and sandy soils found in its areas of distribution in Ontario and Quebec.

The loess plateau of Northern China seems to have been the site of origin of the species as a crop (Zeller 2000). It is still a major crop there (Chen et al. 2007), where it is grown in the semi-arid loess hilly region of the loess plateau. The soils in that region originate from calcareous loess included in the Calcic Cambisol group. It is currently grown as a crop in the Central Black Earth Belt in Russia as well as some non-chernozem regions of Russia and adjacent countries (Gashkova 2003–2009).

In a study of the effects of tillage systems on the growth of proso millet as a crop on the Central Great Plains of the USA, improved growth following autumn weed removal by tillage and/or atrazine was attributed to increased soil NO₃ ^- levels and increased water-use efficiency (Anderson et al. 1986). Investigations into microbial communities and enzyme activities under crop rotations in similar terrain in Akron, CO, were conducted by Acosta-Martínez et al. (2007). The soils there were described as Weld loam; fine smectitic mesic aridic paleustolls. They found that, among other rotation/tillage options, using a crop rotation of corn, wheat and proso millet increased microbial carbon and microbial biomass nitrogen at the 0–5 cm depth, compared with the traditional wheat/fallow rotation [see also Wright and Anderson (2000) in section 7(e)].

Rachie (1975) described the species as being nitropositive, but Senft (1978) reported that, as a crop, proso millet grown on good loam or silt-loam soils has a far lower requirement for supplemental nitrogen (28.02 kg ha^-1) than that needed for wheat (67.24 kg ha^-1). In the loamy argiustolls of eastern Colorado, Rodriguez et al. (1989) found that, for a proso millet crop, the critical level of supplemental nitrogen, below which a growth response would be expected, was 58 kg ha^-1. Phosphorus requirements ranged from 16 kg ha^-1 for soils with very low soil phosphorus, to zero when phosphorus availability was naturally high (Rodriguez et al. 1989). Kalinová and Moudrý (2003) found that plants responded to nitrogen fertilization by an increase in leaf size; this in turn increased the LAI, which had a positive effect on grain/seed yield. In contrast, Anderson et al. (1986) found that adding nitrogen fertilizer decreased seed production in proso millet.

He and Dong (2001) demonstrated experimentally that in soil that is nutritionally heterogenous there is a plastic response by the fine roots (cf. the coarse ‘framework’ roots) of P. miliaceum in length and surface area, rather than a change in biomass to obtain nutrients. Later, He et al. (2004), using P. miliaceum and four soil-nutrient levels, controlled nutrient concentrations in the vicinity of the original plant and the surrounding patch and measured many parameters through the growth cycle of the plants. They demonstrated that plants of proso millet showed a plastic response to a number of fitness components, in addition to a response by fine roots.

Soil salinity is a problem in arid regions of the world and there is interest in salt-tolerant crops. In an experiment to assess variations in salt tolerance among cultivars of P. miliaceum, Sabir and Ashraf (2008) showed that two of 18 accessions from Pakistan had some tolerance to a salt concentration of 180 mM in Hoagland’s solution at the germination and seedling stages, although weights of germinants and seedlings were less than in the control. The experiment did not continue to later growth stages. Salt is often present in irrigation waters and becomes concentrated in the irrigated soil solution. Magistad and Christiansen (1944) tested the salt tolerance of 48 crops grown on irrigated land west of the Mississippi in the United States. Proso millet was one of the eight least salt-tolerant crops tested.

Balan and Bar? (2008) used roots from proso millet seedlings, which had been germinated in Petri dishes, to examine the cytotoxic effects on the root cells of copper acetate and ferrous sulphate solutions of varying strengths, for durations up to a maximum of 48 h. The highest concentration of copper acetate used (0.05%) led to a decrease in cell division. Roots treated for the longest time interval had the fewest cell divisions. Ferrous sulphate treatments resulted in only minor decreases in cell division.

Root length of proso millet seedlings was measured in an experiment by Bona et al. (1993a) in Hungary to examine acid-soil tolerance by fourteen genotypes of this species. Limed (pH 5.1) and unlimed (pH 4.1) versions of Porters soil were used; this is a soil type dominated by exchangeable aluminum ions. Of the genotypes used, four were tolerant and four intolerant. The remaining six were intermediate. In a ranking of acid-soil tolerance of cereal species, Panicum miliaceum was much less tolerant than rye (Secale cereale L.) and oats (Avena sativa), but slightly more tolerant than bread wheat (Triticum aestivum), barley (Hordeum vulgare) and durum wheat (Triticum durum Desf.) (Bona et al. 1993b ). These results differ from a review by Bavec and Bavec (2006), which stated that proso millet does not tolerate acid soil and that it prefers sandy-loam soil.

(i) .

Weeds

In Canada, the authors have noted the presence of Abutilon theophrasti Medik in the same fields as P. miliaceum but assumed that the colonizations by the two species were independent of each other, as both of these warmth-tolerant species edged northwards. Setaria viridis (L.) P. Beauv., Panicum capillare and P. dichotomiflorum may be present in the same fields as P. miliaceum (although frequently they occur as separate infestations), but again the authors have assumed this to be partly coincidental and partly a consequence of selective herbicides allowing the build-up of a seed bank of resistant species rather than a formal assemblage of species constituting a community. In surveys of corn fields in Huron County, Ontario infested with Black proso millet in the early 1980s, other common weed species were Convolvulus arvensis L., Equisetum arvense L., Asclepias syriaca L., Elytrigia repens (L.) Desv. Ex B.D. Jacks [= Agropyron repens (L.) P. Beauv.], Setaria viridis, Amaranthus retroflexus L., Abutilon theophrasti, Panicum capillare, Polygonum convolvulus L., Ambrosia artemisiifolia L. and Salsola kali subsp. ruthenica (Iljin) Soó (P.B. Cavers and research associates, unpublished data). A subsequent survey conducted in the same decade in other counties of southern Ontario had the following weeds, starting with the species in the greatest number of fields (28) and progressing to those in the fewest (3): Setaria viridis, Chenopodium album L., Asclepias syriaca, Equisetum arvense, Amaranthus retroflexus, Echinochloa crusgalli (L.) P. Beauv., Elytrigia repens, Cirsium arvense (L.) Scop., Convolvulus arvensis, Panicum capillare, Ambrosia artemisiifolia, Cyperus esculentus L., Polygonum convolvulus, Arctium minus Bernh. subsp. minus, Malva neglecta Wallr. and Phleum pratense L. (P.B. Cavers and research associates, unpublished data). In Prairie Weed Surveys in Canada (Thomas et al. 1998; Leeson et al. 2002, 2005), proso millet was primarily listed as a weed of oat crops in Manitoba.

Cochrane (1984) described P.miliaceum subsp. ruderale in Wisconsin as being locally abundant in cultivated fields of Zea mays and Glycine max, where it is invariably associated with Setaria faberi Herrm., S. viridis, P. dichotomiflorum and Abutilon theophrasti.

In Hungary, Lehoczky et al. (2006) investigated the impact of weeds on soil moisture content in maize (corn) plots. Early in the growing season they found proso millet to be the second most abundant weed (after jimsonweed, Datura stramonium L.). Although proso millet increased its presence to about 10% of the weed flora as the season progressed, by the end of the season it was far behind jimsonweed (50%) and wild hemp, Cannabis sativa L. (25%). Most other weeds of the early floral assemblage had diminished to about 1.7% or less by the last sampling date. Proso millet has been reported as part of a plant community in a village in Eastern Saxony, Germany, together with Ambrosia artemisiifolia, Malva neglecta and Urtica sp. (Gehlken 1998).

In Mongolia, independent of soil conditions, the grasses Panicum miliaceum, Setaria viridis and Avena fatua L. were widely distributed in the Orchon-Selenga-Basin and were found in each of five listed weed associations from that area (Hilbig 1982).

(i) .

10 000–5000 cal BP

(ii) .

5000 to 2000 cal BP

(iii) .

2000 cal BP–present

(i) .

Mostly weeds

Plant height and amount of tillering depend on the crop with which the weed is growing. The Black biotype can reach 2 m or more in corn but only becomes 15–40 cm tall in barley (Bough et al. 1986). Black-seeded plants tend to be about 100 cm tall in white beans (Phaseolus vulgaris) (Dekker et al. 1981). Bough et al. (1986) found that tillering is most extensive in low crops such as soybeans and at low weed-plant densities and, in general, is least in corn crops.

Bough et al. (1986) described height and tillering in the biotypes they studied. A typical stand of the Golden biotype is 80–90 cm tall but one Ontario population was at least 160 cm tall. Tillering is prevalent in this biotype. Height of the Crown biotype is fairly uniform, about 110–140 cm in corn and slightly shorter in soybeans. The Orange-red biotype is comparatively short, rarely more than 40 cm in Ontario but about 70 cm in Manitoba and Quebec. It tillers profusely, particularly towards the end of the growing season. The White biotype grows to 100–110 cm tall and has sparse tillering. The Elgin biotype is the largest and most robust but also the slowest to mature. Its height is 140–160 cm. It has large leaves, large strong culms, and is strong enough to support seed-eating finches (Carpodacus spp.) while they feed.

The Dark red biotype is unique in that flowering stems lying horizontally (after lodging, etc.) can produce vertical shoots from the nodes (Bough et al. 1986).

In young seedlings of P. miliaceum, the first internode lengthens, which enables successful germination and emergence from depths of 5 cm or more (Strand et al. 1973).

Proso millet has a fibrous root system that penetrates the soil to a depth of up to 150 cm and spreads laterally to between 100 and 120 cm. Most of the root system is formed prior to panicle emergence (Gashkova 2003–2009).

(i) .

Mostly crops, weed information near the end of this subsection

In general, darker seeds have heavier husks and suffer less imbibition damage, measured as electrolyte leakage (Khan et al. 1996). According to Lorenz (1983), dark-husked crop varieties have higher tannin content in the grains than those of lighter-husked varieties. Phytate and tannin contents of the grain diminish when the husks are removed. Lorenz (1983) also recorded large differences in crude fibre content among the crop varieties he studied. Lorenz and Hinze (1976) compared the functional characteristics of starches from six proso millet varieties, two foxtail millet (Setaria italica) varieties and one variety each of wheat and rye. All proso millet starches had higher values for water-binding capacity and gelatinization than wheat starch. Four proso millet varieties produced swelling-power values that were similar to that of wheat starch; the other two proso millet varieties had higher values, similar to that from rye starch. Percent solubility values at 90 °C for proso millet varieties were lower than that of wheat starch, with one exception, which had more than twice the solubility of any other proso millet variety.

In a comparison of 20 commercial grain products from eight grain species, whole-grain grits of P. miliaceum had comparatively low values for total phenolic acids but a similar composition of the different phenolic acids (Mattila et al. 2005). The value for caffeic acid was the lowest of all the samples.

Rudramuniyappa and Panchaksharappa (1978) made a detailed histochemical study of seed development in Panicum miliaceum. They found that the zygote is rich in RNA and proteins, as are the shoot and root apices during differentiation, but these substances decline in other tissues as the seeds mature. In the proembryo, RNA and proteins increase and persist in the embryo proper. Lipid bodies are abundant and distributed uniformly in all layers of the embryo from the quadrant to the globular stage, but at later stages they are more confined towards the basal region. The early endosperm is rich in lipids but, as it develops, lipids decline to a low level. The embryo proper is rich in reducing sugars, as are the young shoot-root apices and their axis. They also found that the early endosperm is rich in lipids, but these decline to a low level as it develops.

The reported nutritional composition of proso millet seeds varies slightly. Anderson and Martin (1949) compiled data from three sources, which show approximately 10%–12% protein, 1%–3.4% fat and 71%–75% carbohydrate. Data from Kaume (2006) are similar: 11% protein, 4.2% fat and 72.9% carbohydrate, together with 8.5% dietary fibre. Kaume’s paper also gives details of the mineral and essential amino acid composition of the seeds.

Kalinova and Moudry (2006) evaluated seven Eastern European varieties of proso millet and they agreed that the usual protein content of proso millet seeds is between 11.3% and 12.7%, but the highest value they obtained was 17%. Wheat has similar protein content, around 11%, but proso millet seeds are significantly richer in the essential amino acids leucine and methionine. Leucine content was twice that of wheat and methionine content was three times higher (Kalinova and Moudry 2006). There were significant differences among the seven varieties in protein quantity and quality (Kalinova and Moudry 2006).

Cell wall composition and biogenesis in crop plants of Panicum miliaceum were studied by Carpita et al. (1985). They found that the composition of tissues and cell suspensions of P. miliaceum was similar to those of other graminaceous species. Also, walls of embryonal cells contained much higher proportions of total hemicellulosic sugars than walls of developed or elongated cells, very similar to the pattern in corn coleoptiles. Carpita (1989) made a more detailed study of the chemical composition of the pectin fraction of growing cells of corn and proso millet. He found that the neutral sugar profiles of the two preparations were similar. Both had about 5% rhamnose and a total of about 30% arabinose and xylose. The corn pectin fraction contained about 36% uronic acid, which was mostly converted to galactose during chemical reduction. In contrast, the proso millet pectin fraction was only about 25% uronic acid, which was primarily compensated for by slight enrichment of arabinose and galactose.

The principal components of leaf waxes of Panicum miliaceum are hydrocarbons, esters, aldehydes and alcohols (Tulloch 1982). The esters include esters of triterpene alcohols and the major free alcohol is dotriacontanol. Free triterpene alcohols were also found.

In India, Sinha and Saran (1992) examined the leaves of six C₄ Panicum species to determine the anatomical structure of the photosynthetic pathway. All of these species possess a chlorenchymatous bundle sheath. In P. miliaceum and three other species, there are intervening cells between the metaxylem and the chlorenchymatous bundle sheath cells. This type of anatomical arrangement implies that the species has a NAD-malic-enzyme type of C₄ metabolism. In such NAD-ME type C₄ plants, the bundle sheath mitochondria are functionally differentiated into photosynthetic organelles, containing some enzymes involved in the C₄ dicarboxylate cycle.

Taniguchi et al. (1995) studied three isozymes of aspartate aminotransferase (AspAT) in proso millet. The cytosolic and mitochondrial isozymes (cAspAT and mAspAT) are major components whose activity and protein levels increased selectively during recovery from a nitrogen deficit. The third isozyme (pAspAT), localized in plastids, is considered a minor isozyme and its level remains unchanged during recovery from a nitrogen deficit. A group of C₄ species, including Panicum miliaceum, utilize three compounds (a NAD-malic enzyme, the mitochondrial isoenzyme mAspAT and a NAD-malate dehydrogenase) to decarboxylate C₄ acids (Kagawa and Hatch 1975). These three compounds are localized in the mitochondria of bundle sheath cells. A more detailed study of regulation of the NAD-malic enzyme in proso millet is found in Chapman and Hatch (1977).

In Japan, Ohnishi and Kanai (1987, 1988) measured the uptake of glycerate and pyruvate into intact chloroplasts isolated from mesophyll and bundle sheath protoplasts of proso millet. Chloroplasts from both sources had similar rates of light-enhanced glycerate uptake even though glycerate kinase, the glycerate metabolizing enzyme, was undetectable in bundle sheath chloroplasts, whereas kinase activity in mesophyll chloroplasts was significant (Ohnishi and Kanai 1988). Chloroplasts from both sources had similar slow rates of pyruvate uptake when they were incubated in darkness, but when incubated in the light the mesophyll chloroplasts had a very rapid and greatly enhanced pyruvate uptake, whereas the bundle sheath chloroplasts did not show this enhancement (Ohnishi and Kanai 1987).

Samples of fresh and decomposed dissolved organic matter (DOM) from ten different plant (crop) sources were compared by Hunt and Ohno (2007) in Maine. Proso millet had the highest fresh weights and highest DOM apparent molecular weight (MW_AP) of the five crop species that were field-weathered. It also had the highest humification index (HIX).

Aarati et al. (2003) found that finger millet (Eleusine coracana Gaertn.) and Panicum miliaceum are relatively tolerant to increasing levels of salinity [in contrast to findings of Magistad and Christiansen (1944) and Sabir and Ashraf (2008); section 5(b)] and both of them have higher levels of expression of an ABA responsive 21 kDa protein than cucumber (Cucumis sativus L.) and other salinity-sensitive species. In fact, cucumber did not show any detectable level of expression of this protein.

Animal wastes from intensive farming are usually spread on fields for fertilization in many parts of the world and these wastes contain drugs that pollute the soil (Migliore et al. 1995). One such drug, sulphadimethoxine, altered normal post-germination development and growth in three species, the C₄ plants Panicum miliaceum and Zea mays and the C₃ plant Pisum sativum L. The magnitude of the toxic effect depended on the bioaccumulation rate and was higher for the C₄ species.

Linoleic acid was isolated from methanol extracts of proso millet as a histone deacetylase inhibitor that showed noncompetitive inhibitory activity towards histone deacetylase and potent cytotoxicity towards human leukemia and prostate cancer cells (Aburai et al. 2007). These authors concluded that millets containing linoleic acid might have anti-tumor activity.

(ii) .

Crops and weeds

Panicum miliaceum is one of a few species that cause hepatogenous photosensitizations in lambs that graze upon their foliage [Miles et al. 1993; see also section 3(a)]. The resulting condition is similar to alveld, a disease appearing in grazing pastures where lambs ingest leaves of Narthecium ossifragum (L.) Huds. Both P. miliaceum and N. ossifragum contain steroidal saponins. The saponins in proso millet are derived mainly from diosgenin and yamogenin. The diseases appear after the saponins are ingested by the sheep and crystalloid material is deposited in and around the bile ducts of the liver; this leads to photosensitization (Miles et al. 1993). Of more importance to humans are the infrequent but severe anaphylactic reactions that individuals may suffer after consuming millet-containing food (Bohle et al. 2003). In their study, these authors found that all of the affected people kept cage birds. They concluded that proso millet plays an important role as an inhalant allergen for people with a predisposition to this allergy. A sensitization to proso millet may subsequently also cause the food allergy.

Chlorophyll fluorescence was measured as an indicator of chilling tolerance in eight weed species by Di Paola and Benvenuti (1993). Panicum miliaceum had the least chilling tolerance of the eight species (i.e., was the most sensitive). Zauralov et al. (1994) exposed 9-d-old seedlings of two cultivated varieties of proso millet, differing in cold resistance, to chilling at 2 °C for periods of 0.5 to 24 h. Chilling enhanced exosmosis and lipid peroxidation, decreased intracellular pH and reduced plant height and weight as well as leaf surface area. The magnitude of the observed changes depended on the inherent cold resistance of the cultivar and the duration of chilling.

(iii) .

Weeds

In a greenhouse study, the responses of two crop species, corn and soybean, and three weed species, velvetleaf (Abutilon theophrasti), redroot pigweed (Amaranthus retroflexus) and wild-proso millet, to applications of five naturally occurring allelopathic chemicals, salicylic acid, p-hydroxybenzoic acid, caffeine, hydroquinone and umbelliferone, were tested (Shettel and Balke 1983). At an application rate of 11.2 kg ha^-1 the allelochemicals inhibited the weed species more than the crop species. At that rate, wild-proso millet was reduced significantly by p-hydroxybenzoic acid, caffeine and umbelliferone. The authors suggested that in agro-ecosystems these chemicals could inhibit the growth of weed, but not crop, species. Some cucumber accessions have shown allelopathy to proso millet in the field [see section 13(c)].

Peterson et al. (1997) tested 12 different synthetic sucrose esters for their ability to inhibit the germination of seeds of Panicum miliaceum and velvetleaf. Tests in Petri dishes revealed that ten of the synthetic sucrose esters could cause more than 90% inhibition of germination in P. miliaceum when applied at a concentration of 100 ppm and one of these esters could cause 80% inhibition at a concentration of 10 ppm. Seeds of velvetleaf were affected to nearly the same extent at a concentration of 100 ppm but had less effect at lower concentrations. These authors pointed out that field evaluation is necessary to determine whether the synthetic sucrose esters could be used to control P. miliaceum when it occurs as a weed in crops.

(i) .

Weeds

Most seeds of the Black biotype remain dormant on or in the soil during their first winter in Ontario, then germinate and emerge between mid-May and mid-June (O’Toole 1982). Seedling emergence of the Black biotype continues intermittently throughout the growing season (Bough and Cavers 1987). Emergence of Canadian biotypes (Black and crop-like weeds) that set viable seeds occurred from May to August, however emergence in early June resulted in optimal growth (P. Cavers and M. Kane, unpublished data). In New Zealand, late-germinating (autumn) plants were able to set seeds within 5 wk of emergence (James et al. 2011). In Ontario, some seeds of the Golden biotype germinate shortly after dispersal during mild autumn weather (Bough et al. 1986) but they do not produce viable seeds (M. Kane and P. Cavers, personal observations). A few seeds of other crop-weed biotypes can survive one winter in or on the soil and they typically germinate with the crop the following spring. The White biotype exhibits the least overwinter survival in the field but its seeds, like those of other biotypes can survive as impurities in crop seed collections and then be sown and germinate with the crop in the following growing season (Bough et al. 1986).

In Colorado, Anderson (1994a) recorded the emergence pattern of a weed community of 16 species in wheat stubble for each of seven successive years from 1 Apr. to 31 Aug. Wild proso millet was one of six species that emerged each year and comprised over 80% of the total seedlings counted. There were two peaks of emergence, late April-early May and late May-early June. Wild proso millet, Setaria viridis, Amaranthus retroflexus and A. hybridus L. predominated in the second peak of emergence.

There is very little published information on the date of panicle emergence. However, in barley and corn fields in Huron County, Ontario the first panicles of the Black biotype appeared between 7 and 21 July in 1980 (O’Toole and Cavers 1983). In the white bean field of this study, planted later, panicles did not appear until 5 Aug.

(i) .

Crops and weeds

Plenchette et al. (1983), used soils collected from a nursery in Pont-Rouge, Quebec, Canada in which P. miliaceum had been grown the previous year. The root fragments of this species in the soil were described as ‘intensely mycorrhizal’ but no species were named.

The presence of arbuscular mycorrhizal fungi (AMF) can be inferred by the concentration of glomulin, an AMF product, in soils (Wright and Anderson 2000). These authors measured soil glomulin content under different crop rotations in the Central Great Plains regions of the USA and found that soils from a rotation of wheat–corn–proso millet had significantly higher glomulin values than soils from other rotations.

Mycorrhizal fungi need a compatible host to complete their life-cycle and sporulate. They are not generally considered host-specific, but some host-fungus combinations may be better (Silva et al. 2005). In a greenhouse experiment conducted in Brazil to evaluate the effects of nutrient solutions and buffers on AMF sporulation, Silva et al. (2005) used inocula of Gigaspora margarita Becker and Hall, Scutellospora heterogama (Nicolson & Gerdemann) Walker & Sanders and Glomus etunicatum Becker & Gerdemann and two hosts, P. miliaceum and sorghum. These plants were grown in a sterile substrate. P. miliaceum as a host elicited higher sporulation in all treatments than those in which sorghum was the host. Their results show that the three fungal species used are able to colonize P. miliaceum.

Rhizophagus fasciculatus (Thaxt) C. Walker & Schuessler (=Glomus fasciculatum, MUCL 46100) was found to be the predominant AMF in soils of the Dharwad region of India (Channabasava et al. 2015). It was shown experimentally to form a host–fungus relationship with P. miliaceum, a relationship that could be harmed by some fungicides.

Working with mine-spoil waste in the Uttara Kannada region of India, Channabasava and Lakshman (2013) added inocula of two locally abundant mycorrhizal fungi, Glomus fasciculatum (=Rhizophagus fasciculatus) and Acaulospora laevis Gerdemann & Trappe, to potting mixtures containing different proportions of mine waste in which plants of P. miliaceum and Jatropa curcas L. were grown. In soils with 25% mine-spoil waste, both growth and potassium uptake were increased for both plant species in the presence of the mycorrhizal inocula. We infer from their results that each of the two plant species and each of the two mycorrhizal fungal species form good host-fungus relationships.

In a study conducted in Germany, Maier et al. (1997) grew 61 species of Poaceae, including P. miliaceum in a defined nutrient medium. These were inoculated with the AMF Glomus intraradices Schenk & Smith. Although the percentage mycorrhization of roots of these species was as low as 5% in one of the Poa species used, 80% of P. miliaceum roots became infected.

In Israel and India, Azospirillum brasilense Tarrand & Krieg, a bacterium, fixes nitrogen in association with the roots of P. miliaceum (Kapulnik et al. 1981; Rai 1985). Under controlled environments in a phytotron and a greenhouse, plants of P. miliaceum inoculated with A. brasilense as seedlings had faster development to the heading stage, higher shoot and root weights and higher total N weight than uninoculated controls (Kapulnik et al. 1981). In India, one high-temperature-adapted strain of P. miliaceum had more growth and greater nitrogenase and hydrogenase activity at 30 °C to 42 °C than parental or other temperature-adapted strains (Rai 1988).

(i) .

Crop biotypes

Proso millet is self-fertile (Kane and Cavers 1992) and is ordinarily self-pollinated (Ayyangar and Rao 1938). It is considered a self-pollinated crop (Baltensperger 1996) but natural cross-pollination may exceed 10% (Popov 1947). In Russia, Gashkova (2003–2009) linked self- or cross-pollination to ambient temperatures and anther protrusion. In cool and cloudy weather flowers do not open and only self-pollination occurs. Under ‘optimal’ conditions, the anthers dehisce at the time that they emerge from the opening flower; most of the pollen falls on the stigmas inside the flower but some pollination of other flowers in the same panicle is possible. In hot and dry weather, the anthers open after they have emerged, the pollen is released into the air and cross-pollination occurs. Nelson (1984) described techniques to improve the effectiveness of cross-pollination methods in P. miliaceum.

Treatment of plants of Panicum miliaceum growing under continuous light with 100 mg L^-1 of gibberellic acid caused flowers to form. However, the spikelets remained short and sterile (Kumar et al. 1977).

In Madras Province in India, flowers of P. miliaceum open between 1000 and 1200 with individual flowers opening and closing within five to seven minutes (Ayyangar and Rao 1938). In their plant breeding studies, Ayyangar and Rao (1938) found that finger-thumb pressure on flowers induces opening about one hour before normal and this procedure permits emasculation. In Russia, Gashkova (2003–2009) reported flowers opening between 0800 and 0900, with a peak of flowering between 1000 and 1200, and flowering was over by 1500. She found that an individual flower can stay open from five to ten or from 20–40 min, depending on weather conditions. Under warm dry conditions, for early-maturing and mid-season cultivated varieties, anthesis (flower opening) begins on days two or three after the emergence of the panicle. Late-maturing varieties begin anthesis on days four to six, and very late-maturing varieties on days five to eight. At peak flowering, 50–100 flowers may open simultaneously on a single panicle.

In northern cultivation areas in Russia, varieties with loose, spreading and widely branched panicles are most suitable because they have more light penetration and moisture evaporation, whereas in more arid southern areas, biotypes with compact, dense panicles have reduced moisture evaporation and are more suitable (Gashkova 2003–2009).

(i) .

Mostly weeds but some crop information

Seeds of P. miliaceum ripen in basipetal succession (from the top down) on each inflorescence (Kane and Cavers 1992). In crop populations in the central United States, the seeds in the upper portion are ripe and liable to shatter while lower seeds are still green and ripening (Baltensperger 1996), although in Ontario most crop populations have little or no shattering before harvest [see also sections 2 and 13(a)(i)]. Crop populations tend to produce a single panicle from the apical bud, even when the plants are widely spaced. In contrast, plants of the Black biotype and those of the Orange-red crop-like weed have the greatest tendency to produce inflorescences on tillers arising from lateral bud development (Cavers and Bough 1985).

In a field study conducted in India, on two crop varieties of proso millet, Rao et al. (1977) assessed the importance of the upper leaves of the main shoot in their resource contribution to seed development. They concluded that almost 50% of seed development was contributed by the stem and leaf sheaths, with the ear and flag leaf contributing a further 33%. Each of the second, third and fourth leaves contributed about 6%–7%.

Seed production of the Black biotype averaged 92 seeds m^-2 (1.5 million ha^-1) in a barley crop, 2354 seeds m^-2 (34 million ha^-1) in corn and 38 247 seeds m^-2 (426 million ha^-1) in white beans (O’Toole and Cavers 1983).

Mean seed weights for three Black populations, from Huron County, Waterloo County and Glengarry (now Stormont, Prescott and Glengarry) County in Ontario, were 3.69, 3.75 and 3.72 mg, respectively. Mean seed weights for six populations of crops and crop-like weeds ranged from 4.90 to 6.23 mg (Moore and Cavers 1985). In a more detailed study, Bough et al. (1986) provided mean seed weights for Canadian weed and crop-like weed biotypes. They found that average seed weight for the Black biotype ranged from 4.2 mg for seeds ripening early in the season, to 3.9 mg for seeds ripening in mid-season, to 3.0 mg for late-ripening seeds; ranged from 4.3 to 6.0 mg for different populations of the Crown biotype; ranged from 4.9 to 5.9 mg for different populations of the Golden biotype; averaged 6.2 mg for the Orange-red biotype; averaged “about 6 mg” for the White biotype; averaged 5.2 mg for the Dark red biotype; and averaged 6.5 mg for the Elgin biotype. In the mid 1980s, the Crown biotype was the only one with large weed populations in both Southern Ontario and Southern Manitoba (Cavers and Bough 1985). The crop and weed populations in Manitoba had average seed weights from 6.1 to 6.2 mg, whereas the weedy Crown populations from Southern Ontario averaged 4.7 to 5.5 mg per seed.

Motuzaite-Matuzeviciute et al. (2011) studied variation in the lengths and widths of seeds from individual plants of cultivated populations of P. miliaceum. They found variation of 20% or more in both length and width but there was no clear relationship between seed dimensions and position of the seed on the panicle. Kane and Cavers (1992) found for the Black biotype that there was no relationship between the date of seed ripening and the total seed viability.

Seeds of the Black biotype in Ontario shatter readily as they mature and thus seeds are dispersed from early August (O’Toole and Cavers 1983) until the time of the first frost, usually in October (Bough et al. 1986). Seed dispersal of this biotype occurred first in corn fields, next in white-bean (Phaseolus vulgaris) fields and latest in barley fields in Southern Ontario (O’Toole and Cavers 1983). In contrast, also in Ontario, crop-like weed biotypes have little shattering and most crop populations have almost no shattering (Bough et al. 1986). Consequently, most ripe seeds from crop and crop-like weed populations are retained on the parent plant until crop harvest or post-harvest cultivation (McCanny 1986; M. Kane and P. Cavers, personal observations). One exception is a crop-like weed population of the Crown biotype from the former Carleton County, Ontario (near Ottawa), whose seeds shatter more readily than those of other Crown populations (Bough et al. 1986).

Wind has little effect on the dispersal of proso millet seeds (Cavers and Bough 1985). For example, if Black seeds are not dispersed by human action, the vast majority fall within the maximum lateral spread of the parent plant (O’Toole and Cavers 1983).

Panicum miliaceum was one of 146 species recorded as adventive at terminal grain elevators and adjacent railway yards in a 1975 survey at Thunder Bay, Ontario, but none of these adventive species appeared to pose a threat to local agriculture (Lindsay 1977). Jehlik (1995) implicated railway freight transport in the Czech Republic as a means whereby alien weeds, including P. miliaceum, were introduced to that country.

There is strong circumstantial evidence that the Black biotype was brought to southern Ontario from Wisconsin on used farm equipment (J. O’Toole, personal communication). Initial infestations in Huron County, Ontario were all on farms where farm equipment was shared and at least one of these machines had been imported from an area in Wisconsin with dense infestations of Black proso millet (J. O’Toole, personal communication). A later intensive study by Stephen McCanny confirmed that seeds of P. miliaceum were being moved between and within fields on combine harvesters (McCanny 1986). Experimental evidence demonstrated that, on average, 3.3% of seeds from plants of the Black biotype were carried more than 50 m by combines. Earlier preliminary studies in Huron County, Ontario had shown that isolated patches could be spread to more or less complete coverage in a field within two years (J.J. O’Toole, unpublished data). McCanny et al. (1988) confirmed that much of this spread could be attributed to combine harvesters. McCanny and Cavers (1988) reported that combines carry up to 13 000 seeds at a time out of a field. McCanny et al. (1988) also showed that combines could distribute seeds of the Golden biotype between fields and that 0.9% could be carried more than 50 m within a field on combines. We have observed that farm equipment in Manitoba can be infested with large numbers of seeds of the Crown biotype and that anyone walking through fields contaminated with Crown or Black proso millet on a warm summer day can pick up hundreds of ripe seeds sticking to skin and sweat-moistened clothing (P. Cavers and M. Kane personal observations). Proso millet seeds are also transported in mud that has stuck to footwear and vehicle wheels (Cavers and Bough 1985).

J.F. Alex and R.D. McLaren of the University of Guelph supplied additional information on the dispersal of proso millet by means of their “Weed Alert Program” and through personal consultations. In 1979, Alex and McLaren isolated nine different seed colours from weed populations and regrowth from crops of proso millet in Ontario (O’Toole 1982). In 1980, they reported proso millet as a weed on 72 Ontario farms (O’Toole 1982). Our research group at the University of Western Ontario contacted more than 100 farm operators in Ontario who had reported weed populations of proso millet before 1985. Many of them believed that their infestation originated on farm equipment brought into their fields from infested farms (P. Klumpers, S. McCanny, M. Kane, and P. Cavers, unpublished data).

James et al. (2010) reported that infestations of Panicum miliaceum in New Zealand in the 1960s and 1970s were probably of crop varieties that originated from imported birdseed and as a contaminant in imported seeds of choumollier (Brassica oleracea L.). It is unknown how the black-seeded weed was brought to New Zealand, but it has spread throughout much of that nation on agricultural equipment (T. James, personal communication).

In west-central Minnesota, Forcella et al. (1996) found that for early-maturing weed species, including wild mustard (Sinapis arvensis L.) and possibly wild proso millet, rapid maturation during warm growing seasons resulted in almost complete dispersal of seeds prior to crop harvest by combine. In cool growing seasons, some seeds of these species were retained on the parent plant until combine harvest. They recommended that this information be considered in devising better systems of weed management.

(i) .

Predominantly weed populations

In 1989–1990, Yenish et al. (1992) examined the distribution of weed seeds in the upper 10 cm of two soils in Wisconsin, a silt loam in Arlington that had been sown to a corn/soybean rotation from 1984 to 1988, and a loamy sand in Hancock that had been sown to continuous corn since 1982. Most of the weed seeds were Chenopodium album, Amaranthus retroflexus, Panicum dichotomiflorum, Panicum miliaceum, Setaria pumila (Poir.) Roem. & Schult. and Cenchrus longispinus (Hack.) Fernald. At both sites, over 60% of the weed seeds in the top 10 cm were found in the top one cm in no-till plots and the concentration of weed seeds declined logarithmically with increasing depth. In chisel-ploughed plots, more than 30% of the weed seeds were in the top centimetre and weed-seed density declined linearly with depth.

The longevity of seeds in the seed bank varies from very brief, less than one year and often only a few weeks, to strongly persistent over several to many years (Bough et al. 1986; Colosi et al. 1988; Cavers et al. 1992; J.J. O’Toole, personal communication). At one extreme, in Ontario, seeds of the White biotype of P. miliaceum almost never survive in the soil for more than a few weeks and more than 99% cannot overwinter successfully in the field (Cavers et al. 1992). Most crop-like weeds have little or no survival in the soil over winter but approximately 20% of the seeds of one Golden population collected from Durham County, Ontario survived overwinter in the soil and were still viable but ungerminated in June of the following year (Cavers et al. 1992). This is also one of the very few crop-like weed populations that has spread naturally into neighbouring fields from the original infestation (Cavers et al. 1992; M. Kane and J.C. Colosi, personal observations). Another crop-like weed with appreciable seed survival over winter is the Dark red biotype from Quebec (Bough et al. 1986; R. Néron, personal communication).

In Canada, the one crop population that also became a serious weed was the Crown biotype in Manitoba [Bough et al. 1986; see section 3(a)]. This population has heavier seeds than other weedy Crown populations from Ontario and Quebec (Cavers and Bough 1985). We examined approximately 30 populations of the Crown biotype from a wide geographical range in southern Manitoba, as well as plants arising from a crop population grown at Plum Coulee, Manitoba, all of which appeared to be of an identical type (Bough et al. 1986). These large populations occurred despite the fact that they had transient seed banks (Cavers et al. 1992). Several factors led to this result; more or less continuous corn crops with regular use of atrazine, a herbicide to which proso millet is resistant; slow breakdown of atrazine in Manitoba soils; large numbers of proso millet seeds persisting on the parent until winter; and long cold winters with few periods when seeds could germinate or rot (Cavers et al. 1992).

The earliest study of seed banks of the black-seeded biotype in North America was initiated in 1977 by McNevin and Harvey in Minnesota (McNevin and Harvey 1980b ). They buried approximately 400 seeds at a time (first sample counted and weighed, remaining samples weighed) in a 15 cm long piece of PVC (polyvinyl chloride) drainage pipe covered at both ends with plastic mesh and placed upright in the soil at a depth of 25 cm. They used two different soils, a silt loam and a sandy loam. Samples were removed at different dates and set to germinate (conditions not specified). After one year, 212 seeds from the silt loam and 170 from the sandy loam germinated. These values declined after two years to 73 and 84 seeds germinating from the silt and sandy loam treatments, respectively.

The Black biotype forms large persistent seed banks in Ontario. Early studies by our team in London, Ontario revealed that Black seeds could remain dormant in the soil for at least four years when stored at a depth of 20 cm (Bough et al. 1986). Similar results were obtained by a farmer in Huron County, Ontario who had a large newly-established population of the Black biotype. He worked effectively to prevent any new seed production by this biotype for five years, but in the sixth year he had a large infestation of proso millet in his corn crop and it had to have come from the seed bank (Cavers et al. 1992). In an experiment in London, Ontario, seeds of the Black biotype were stored for various periods at three depths: surface, three cm deep and 20 cm deep. No seed remained viable on the surface for more than two years but some seeds of this biotype remained alive for 30 months at the three and 20 cm depths (Cavers et al. 1992). The pattern of viable seeds in arable soils is exceedingly variable, even from different soil samples taken from a single arable field (O’Toole and Cavers 1981). To study seed distribution, we established a 100 m × 100 m area in a field in Huron County, Ontario that had had dense infestations of the Black biotype for at least three years. At 50 randomly selected points within this area, a 1000 cc soil sample was collected in June 1981, when the field had an almost uninterrupted lawn of proso millet seedlings. There was a range of viable millet seeds per sample from 0 to 180, with no obvious pattern. Almost half of the seed bank in this field had died over the previous winter (Cavers et al. 1992; see also section 10).

In a very large experiment run at eleven sites across the US Corn Belt from Ohio to Colorado and from Minnesota to Missouri, field emergence of weed species was monitored from 1991 to 1994 (Forcella et al. 1997). Seedling emergence of wild-proso millet ranked eighth of 15 species, with emergence in three or more site years and an average of 6.6% emergence per site.

Williams and Harvey (2002) conducted a field experiment in the United States to determine how many wild-proso millet seeds could be returned to the soil each year without increasing future seed-bank populations. Their original seed bank, in 1988, averaged 15 000 seeds m^-2. Starting that year, they added a seed rain of 0%, 3%, 6%, 12%, 24% and 48% of the original seed bank each autumn for five years. After the fifth year, more than 90% of the original seed bank had been lost in all treatments. They estimated that 77% of the seed rain and 68% of the spring seed bank had been lost each year. This meant that the wild-proso millet had to return approximately three times the number of seeds in the soil each year to maintain a constant seed-bank population.

Lloyd and Cavers (2002) compared dormancy, viability and seedling vigour from fresh seeds of the Black biotype, several crop-like weed biotypes and a white-seeded crop with seeds from the same populations that had been stored for 15 yr at 5 °C and 40% relative humidity. In a greenhouse study, seedling emergence from aged seeds of the most weedy biotypes (Black, Dark red), planted 1 and 3 cm deep, was faster and more complete than from fresh seeds, demonstrating that seeds of these biotypes lost dormancy, but not viability, during storage. Aged and fresh seeds of the Crown and White crop-like weeds emerged equally quickly and with no loss of viability during storage. In contrast, aged seeds of the white-seeded crop had 70% viability or less (compared with 100% for fresh seeds). Seeds sown on the soil surface had the most variable seedling emergence patterns. Fresh and aged seeds of the Crown biotype had rapid complete emergence. In contrast, the Black biotype had slow and intermittent emergence, reaching 77% for aged seeds and 28% for fresh seeds after three months. In a separate study, emerged seedlings from aged crop seeds had up to 70% mortality before flowering, whereas seedling mortality from aged seeds of five weed and crop-like weed biotypes was negligible.

In a germination test in an incubator, the low threshold temperature for germination of wild proso millet in the USA was 6.9 °C (Wiese and Binning 1987). In Iran, Kamkar et al. (2006) found that the base (lowest) temperature for germination of cultivated P. miliaceum seeds was 9.9 °C, the optimum temperature for fastest germination was 40.2 °C and the ceiling (highest) temperature for germination was 47.8 °C. In the United States, crop seeds of proso millet germinate well between 10 °C and 45 °C but do not germinate at 5 °C or 50 °C (Theisen et al. 1978, cited in Baltensperger 1996).

Van der Kley and Schelwald (1987) tested germination of seven weed biotypes of proso millet in incubators under complete darkness and with 15 h d^-1 of light. There were two diurnal temperature regimes; 25/10 °C and 35/20 °C with a 15/9 h day/night in each case. The seven biotypes were Black, Golden, White, Orange-red, Dark red, Crown and Elgin. For five biotypes, germination under the higher temperatures was rapid and over 80% in both light/dark and completely dark regimes within two days. The Dark red seeds had slightly less germination (70% vs. 80%) and the Black seeds much less (15% in light and 30% in darkness). A few Dark red seeds and a large proportion of the Black seeds remained dormant but viable. Germination under the lower temperature regime proceeded more slowly but after five days had reached close to 100% for the Elgin, Crown, Golden and White biotypes in both light regimes. The Orange-red seeds germinated to over 90% in darkness after five days but only to 60% in light/dark conditions. The Dark red seeds had 80% germination in complete darkness after five days, but only half that much in the light/dark regime. Approximately half of the Black seeds had germinated after five days in both the darkness and light/dark regimes.

Di Paolo (1995) investigated the role of the husk (lemma and palea) in causing dormancy in seeds of proso millet. She tested germination of freshly ripened Black seeds in Petri dishes in an incubator set at diurnal cycles of 25 °C, light for 14 h and 10 °C, dark for 10 h. Intact seeds did not germinate but remained dormant and viable. Black seeds with the husk removed (i.e., the caryopses) and huskless seeds placed in a Petri dish with their removed husks had 98% and 96.6% germination respectively and the few ungerminated seeds were dead. She concluded that the seeds had a husk-induced dormancy. Khan et al. (1996) tested germination from seven weed biotypes and the Crown crop biotype of proso millet and found that the darker the husk, the more dormant were the seeds. They discovered that the darker seeds had heavier husks, imbibed and germinated more slowly and suffered less imbibition damage (measured as electrolyte leakage).

Mándy and Szabó (1973), working in Hungary with crop varieties of proso millet, found that the optimum temperatures for seedling emergence were 18 °C–21 °C at five cm depth. Freshly ripened seeds gave “poor” germination but after a rest period of three to five months, differing for varieties, full germination was achieved. Exposure of fresh seeds to cold stratification had no effect. In contrast, Striegel and Boldt (1981), working with black-seeded proso millet, found that optimum germination occurred at 30 °C and that dormancy could be overcome by the absence of light during germination, by moist pre-chilling at 5 °C or by nine weeks of dry storage at 22 °C.

Beres (1993) reported that germination of weed populations of proso millet in Hungary was unaffected by sodium chloride concentrations of 50, 100 or 150 mM but in a concentration of 200 mM there was a 15%–16% reduction in germination. This author also reported that seeds of proso millet can germinate at pH values from three to eight and that during germination these seeds are less sensitive to drought than those of Chenopodium album.

In Argentina, De Caire et al. (1976) found that aqueous and ether extracts from an algal mass of Nostoc muscorum Ag. significantly accelerated germination of seeds of Panicum miliaceum; a lower concentration of the ether extract was most effective.

(i) .

Predominantly weeds

For all biotypes, input of seeds to the soil depends on the crop with which the weed was growing and the weed-control measures used during the season (Cavers et al. 1992). Cavers et al. (1992) found that in Ontario very few Black seeds were produced in barley or winter-wheat crops but large numbers could be found in corn and white-bean crops. The greatest numbers of Black seeds were dispersed in September in Huron County, Ontario (O’Toole and Cavers 1983). The timing of seed production and dispersal also were governed by the crop and by weed-control measures. For example, Cavers et al. (1992) found that small plants in a barley field produced very few seeds by early September in Huron County, but more seeds ripened after crop harvest and were shed later in September and in October, after seed dispersal in other crops was almost complete.

Although there is often a huge increase in the proso millet seed bank following a dense weed infestation, there is usually a large decrease in the seed bank before the onset of the next growing season. In a field of white beans in Huron County, the seed bank of the Black biotype increased to 28 470 seeds m^-2 in October 1980 but this number had decreased by more than 10 000 m^-2 by April 1981 (Cavers et al. 1992). Many seeds on the soil surface were destroyed by rodents, which left only fragments of lemmas and paleas on the soil surface in the spring, and many other seeds rotted over the winter. Seed banks of the crop and crop-like weed biotypes seldom persist beyond one winter, with the exception of the Dark red biotype in Quebec and one Golden population from Durham County in Ontario (Cavers et al. 1992).

(i) .

As weeds

In Minnesota, Robinson (1985) noted that crops of P. miliaceum gave rise to volunteer plants for several ensuing years. In a field with silt-loam soil, seeds of seven proso millet cultivars, five white-seeded and two red-seeded, were sown in October 1978 following harvest of a crop of ‘Minsum’ proso millet. After these seeds had been sown, crops of sorghum were grown for five successive years. Eight tillage treatments, including a no-till control, were applied to plots within the field for the five years. In all plots, volunteer proso millet plants were removed before seed production from 1979 to 1983 inclusive. In all treatments, a total of more than 700 000 seedlings ha^-1 of volunteer proso millet emerged over the five years. The no-tillage control was the only treatment where emergence was reduced to zero after four years. In contrast, 73 and 254 volunteer seedlings ha^-1 emerged in the fifth year after spring and fall moldboard-plough treatments, respectively. Robinson (1985) provided evidence that volunteers of crop biotypes can emerge in several successive years after a proso millet crop in the USA (although in his study seed colours of the emerged volunteer plants were not determined).

(i) .

Weeds and crops

Moore and Cavers (1985) compared seedling vigour in crops, Black biotype weeds and crop-like weeds of proso millet. Most differences in seedling vigour could be attributed to dissimilar seed weights among the various biotypes. Seedlings of crop-like weeds, with larger seeds, had greater seedling vigour than those of the Black biotype. However, early-season growth in the absence of competition from crops is extremely rapid in all biotypes (Bough and Cavers 1987). In Australia, Murray (1998) focussed on the role of density-dependence during the seed germination period of the life cycle for twelve phylogenetically-diverse plant species. Panicum miliaceum (purchased crop seeds, biotype not specified) was one of four species that had a significant decrease in the proportion of seeds germinating at higher densities of conspecifics. These four species subsequently were each treated at low densities with a leachate solution obtained from high-density conspecifics. For P. miliaceum, this resulted in a significant decrease in the proportion of seeds germinating at the simulated high densities. Murray concluded that the reduced germination percentages caused by the leachate would benefit the species by reducing high-density effects and possibly enabling some seeds to delay germination until a more-favourable time of lower conspecific density. A problem with this study is that Murray identified P. miliaceum as a perennial.

Anderson and Nielsen (1996) studied seedling emergence of weeds in winter wheat stubble in Colorado for three years. Wild-proso millet emergence was greater in tilled than in no-till plots, whereas the reverse was true for kochia [Kochia scoparia (L.) Schrad.], volunteer wheat and green foxtail. In this study, emergence of wild-proso millet began in late May and continued until August.

(i) .

Predominantly weeds

Carpenter and Hopen (1985) grew plants of the black-seeded weed and a cultivated white-seeded variety of proso millet in pure stands at different densities in a greenhouse experiment in Illinois. Plant height of both biotypes decreased linearly with increasing planting density. Both biotypes exhibited decreased shoot dry mass per plant at higher densities and the percentage of plants with panicles also was lower at higher densities. Nevertheless, there was still substantial panicle production by both biotypes even at the highest density of 5540 plants m^-2. In Ottawa, Ontario, Warwick and Thompson (1987) ran a similar but much larger experiment using the White, Golden, Orange-red, Crown and Black biotypes of proso millet. Pure stands of each biotype were grown in trays in a greenhouse at densities equivalent to 111, 237, 474 and 978 plants m^-2. The Golden biotype had more than 10% mortality at all four densities, with a maximum of 20% at the two highest densities. The White biotype had increased mortality at the two highest densities but the other three biotypes only had increased mortality (less than 12%) at the highest density. All biotypes had decreased biomass per plant and delayed flowering with increasing density. The White and Golden biotypes also had reduced weight per seed with increasing density. Van der Kley and Schelwald (1987) grew seedlings of four biotypes, Black, Orange-red, White and Crown at different densities in pure stands in a greenhouse in London Ontario. The treatments involved seedlings sown (planted) at spacings of 25, 10, 5, 4, 3 and 2 cm. The experiment was harvested 42 days after seedling emergence, when virtually all plants had grown out of the seedling stage. There was almost no mortality at the two lowest densities (widest spacings) but up to 25% died at the highest densities, with the greatest mortality for the Black biotype. At the end of the experiment, the Crown biotype had the highest mean weight per plant. Black had the lowest values but the differences were only significant at the lowest density. Plants from all biotypes tillered at the lowest density, but only the Black biotype tillered at the second-lowest density and no seedlings tillered at the four highest densities. The Black biotype did not flower at any density; the Crown biotype flowered at the lowest density and only in the last three days of the experiment. The White and Orange-red had 80% and 84% flowering, respectively, in the 10 cm spacing and less at all other densities.

Eberlein et al. (1990) studied growth and development of three “wild-proso millet biotypes” and one cultivated biotype (Crown) under non-competitive conditions in the field in Minnesota. All of the “wild” biotypes had greater leaf area and dry weight at maturity than Crown. They also produced more seeds per plant than Crown, up to twice as many for one wild biotype. Crown headed earlier than all of the wild types. Although all of the “wild” biotypes could be classed as the Black biotype of Bough et al. (1986), they did differ in plant height, seed shattering, reproductive potential, seed size and seed dormancy characteristics. Eberlein et al. (1990) concluded that two of these wild biotypes had more weedy characteristics than the third one.

The “escape hypothesis” (Howe and Smallwood 1982) predicts that a seed’s chance of survival increases with distance from its parent, in situations where the parent is an isolated single plant. McCanny and Cavers (1987) tested this hypothesis in Ontario using six weedy biotypes of proso millet growing in two different crops, corn and soybeans. Although there were no clear differences among the different biotypes or between crops, the overall results supported the escape hypothesis. McCanny and Cavers interpreted these findings to have been the result of density-dependent mortality. In a later paper, McCanny and Cavers (1989) examined the effects of parent plant size and fecundity on the seed shadows and mature offspring patterns around isolated parent plants of six biotypes of proso millet. The most important result, irrespective of biotype, was that the more fecund groups had more distant cohorts of mature offspring. The two prime causes of this pattern were greater density-dependent mortality near the most fecund parent plants and the larger seed shadows of the larger sized parents.

In a greenhouse experiment, Warwick and Thompson (1987) grew five weed biotypes (Black, White, Golden, Orange-red and Crown) together at three densities, 30, 60 and 90 seeds of each biotype per tray (equivalent to just over 1000, 2000 and 3000 seeds m^-2). All biotypes had reduced survivorship with increasing density but, in contrast to the experiment with pure stands described in the same paper, the Black biotype had the greatest mortality at all three densities, whereas the White and Golden biotypes had the highest survivorship at the two higher densities. In terms of total fresh weight per tray, the taller-growing, later-flowering White biotype totally dominated the mixture and the Golden biotype had the second highest fresh weight at each density. The Crown and Black biotypes had very low fresh weights at all densities. The Crown biotype had the highest percent allocation to reproductive weight, whereas Orange-red had the lowest allocation.

In general, early-maturing sweet corn hybrids were less competitive with wild proso millet than later-maturing ones (So et al. 2009a ). Williams II et al. (2012), using four morphologically different sweet-corn hybrids, found that characteristics of the crop (the maternal crop environment) influenced the seed characteristics of wild-proso millet. The weed plants were more productive and their seeds were larger, with greater germinability, in crops that matured faster and had a smaller canopy. Under these latter crops, the seed colour of wild-proso millet was also slightly lighter. Conversely, an early crop-canopy spread at the time of weed emergence, and an almost complete crop canopy later, suppressed weed fitness (So et al. 2009b ). Wilson (1993a) studied the effects of different densities of wild-proso millet on the yield of dry beans in Nebraska. He found that it was most important to remove wild-proso millet plants during the first four weeks of the dry-bean growing season. In contrast, if the dry-bean crop was kept free of proso millet for four weeks after planting, then the yield was comparable to the yield for dry beans kept weed-free all season. In a four-year experiment with corn, also in Nebraska, Wilson (1993b) tested the effects of different pre-plant tillage methods on densities of six major weed species. The most effective of these methods on proso millet was ‘ridge-till’ where 50% of the weed seeds were left in the topmost 0–7 cm, 92% of weed seeds directly over the corn row were removed and 50% of the weed seeds were moved to an area 25 cm away from the corn row. After this treatment, proso millet plant density was much lower than it was after other pre-plant tillage treatments.

(i) .

Crops

There have been numerous studies of population dynamics in crops of proso millet. Agdag et al. (2001) grew proso millet using six different row spacings, three tillage regimes (irrigated, dryland and no-till) and with either no weeds or under weedy conditions in the United States Great Plains. Row spacing had little effect on seed weight, but narrower rows improved seed yield and weed control. In areas of limited moisture, a row spacing of 19 cm appeared optimal. In Southern Germany, Gebauer et al. (1987) grew two C₃ crop grasses, barley and oats, and two C₄ crop grasses, proso millet and barnyard millet (Panicum crus-galli L. [= Echinochloa crusgalli (L.) P. Beauv. var. frumentacea (Link) W. Wight (see Small 2009)] in pure and mixed cultures in field plots. Low and high nitrogen fertility levels were incorporated in the experiment. In mixed cultures, P. miliaceum was least able to compete; its shoot biomass, but not its shoot length, was drastically reduced when compared with its performance in pure stands. In North Dakota, Berglund (2007) also reported that crops of proso millet are known for their poor seedling vigour and are poor competitors with weeds. One exception is the crown variety in Manitoba [see section 3(a)]. Also, Anderson (2000b) hypothesized that, in a semi-arid area with erratic precipitation, sowing a crop of proso millet during a period of minimal weed emergence and using a combination of cultural weed-control methods would enable producers to control weeds in that proso millet crop without herbicides.

(i) .

Mammals

(ii) .

Birds and other vertebrates

(iii) .

Insects and other arthropods

(iv) .

Nematodes and other non-vertebrates

(i) .

Fungi

(ii) .

Bacteria

(iii) .

Viruses

(iv) .

Other diseases

No information has been found.

Crops and weeds

Witchweeds (Striga spp.) are parasites on many cereal and leguminous crops (Weerasuriya et al. 1993) and they can cause devastating losses (Sugimoto et al. 1998). In the United States, Siame et al. (1993) found that seed germination of Striga asiatica (L.) Kuntze is stimulated by active compounds in root exudates of host crop plants. They identified strigol from proso millet and corn root exudates as the major stimulant of germination in seeds of S. asiatica.

In the United States, Putnam and Duke (1974) hypothesized that the predecessors of many species now grown for food or fibre may have possessed allelopathic substances that would have allowed them to compete effectively with other species in their native habitat. They tested this possibility by collecting 526 accessions of cucumber (Cucumis sativus) from 41 nations and grew them under controlled conditions with two indicator species, Panicum miliaceum and Brassica hirta Moench, as representatives of genera with economically important weed species. There was a wide variety of responses from P. miliaceum, with some accessions greatly reducing fresh weight but other accessions having no effect and one actually causing increased fresh weight. Putnam and Duke (1974) then took leachates from two cucumber accessions that had significantly reduced fresh weight in P. miliaceum and used leachate from one accession that had no effect as a control. The leachates from the “toxic” accessions inhibited seedling emergence and subsequent growth in P. miliaceum but the “non-toxic” accession had no effect. They concluded that it would be possible to produce crop genotypes that could compete successfully with weeds by means of allelopathic interactions. In a subsequent field study, Lockerman and Putnam (1979) tested two cucumber accessions against P.miliaceum and found that one accession reduced fresh weight by 58% and population numbers by 84% whereas the other accession was only half as effective against this weed. The allelopathic effects were suppressed during periods of increased rainfall.

In an experiment under simulated no-till conditions in the greenhouse, Barnes and Putnam (1986) reported that weedy proso millet had 35% less emergence when exposed to rye residues than in a control treatment. The effects on radicle elongation were greater than on percent germination. In another study in the United States, Kalanchoe daigremontiana Raym.-Hamet & H. Perrier caused allelopathic reductions in seed germination and reduced seedling development in P. miliaceum (Groner 1975).