Two rapid, nondestructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants as well as glyphosate-resistant and -susceptible horseweed plants were dipped in solutions of 0, 300, 600, and 1200 mg ae L⁻¹ glyphosate for 3 d and subsequent injury was evaluated. In the second assay, plant sensitivity to glyphosate was evaluated in vivo by incubating excised leaf disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L⁻¹ glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf-dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L⁻¹ rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1,200 mg L⁻¹ rates. The in vivo assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose-dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops and levels were similar to nontreated leaf discs. Shikimate accumulated at high (≥ 21.1 mg ae L⁻¹) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (≤ 10.6 mg L⁻¹) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and might have utility for screening other weed populations for resistance to glyphosate.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L. ‘Dekalb 687RR’, ‘Pioneer 31B13’; cotton, Gossypium hirsutum L. ‘Delta and Pine Land 444RR’, ‘Suregrow 747’; soybean, Glycine max (L.) Merr. ‘Delta and Pine Land 4748’, ‘Asgrow 4702RR’.

The introduction of Clearfield (CL) rice cultivars resistant to imidazolinone herbicides, acetolactate synthase (ALS) inhibitors, has raised concerns of gene flow to weedy rice genotypes collectively called “red rice” that infest rice-growing areas in the southern United States. This experiment was conducted to study hybridization between CL rice and red rice using simple sequence repeats (SSR) markers, identify mutations in the ALS gene of imazethapyr-resistant red rice, and to detect the introgression of the ALS-resistant gene from CL rice into red rice. Natural outcrossing experiments between CL rice and strawhull (SH) red rice were set up in Stuttgart, AR, in 2002 and 2003. Putative red rice hybrids were detected among volunteer plants in the following year. Hybridization was confirmed using SSR markers, and introgression of the resistant ALS gene from CL rice to red rice was detected by ALS gene sequencing. The ALS gene sequences of U.S. rice cultivars ‘Bengal’ and ‘Cypress’, SH red rice, CL rice (CL161), and imazethapyr-resistant red rice/CL rice hybrids were compared. Nucleotide sequences of the ALS gene from the rice cultivars were identical. Three point mutations were present in the SH red rice ALS gene coding region relative to Bengal/Cypress. One of these resulted in the substitution of Asp⁶³⁰ for Glu⁶³⁰. The ALS gene sequences of confirmed hybrids were identical to that of the herbicide-resistant pollen source, CL161. We identified four ALS gene mutations in the herbicide-resistant red rice hybrids relative to the susceptible rice cultivars. One point mutation, resulting in a substitution of Ser⁶⁵³ with Asn, was linked to ALS resistance in callus tissue derived from a Kinmaze rice line from Japan. The other three mutations (Ser¹⁸⁶—Pro, Lys⁴¹⁶—Glu, and Leu⁶⁶²—Pro) are novel. This experiment confirmed that gene flow from imidazolinone-resistant rice resulted in herbicide-resistant red rice plants.

Nomenclature: Imazethapyr; red rice, Oryza sativa L. ORSAT; rice, Oryza sativa L.

False cleavers and catchweed bedstraw are problematic weeds of field crops in high-latitude regions of the northern Great Plains of North America. The abundance of these species has been increasing in areas of greater tillage intensity and frequency. Field experiments were established over 4 site-yr in Manitoba, Canada, and results indicated that the recruitment of false cleavers and catchweed bedstraw was strongly promoted by a single shallow tillage operation with a sweep cultivator in the spring. Percent recruitment levels in 2001 (pooled over sites) were 17 and 46% for the untilled and tilled treatments, respectively. In 2002, the percent recruitment levels for the untilled and tilled treatments, respectively, were 28 and 38% for the Komarno site and 13 and 28% for the Petersfield site. Only a few and minor differences in microsite conditions (soil temperature, soil moisture, and bulk density) resulted from the single spring tillage pass. The single tillage pass caused a significant relocation of simulated seeds (plastic beads) to below the soil surface and deeper into the soil profile (2–4 cm). Mean bulk density and volumetric soil moisture increased significantly with soil depth. The results of this study suggest that the positive effect of tillage on cleavers recruitment was not due to the effect of tillage on microsite conditions per se. Rather, the vertical redistribution of seed by tillage moved these seeds to a place in which the microsite conditions differed from those on the surface and were more favorable for recruitment. False cleavers and catchweed bedstraw recruitment is clearly promoted by tillage (even minor tillage) under field conditions, and farmers might be able to limit recruitment by limiting spring tillage.

Nomenclature: False cleavers, Galium spurium L. GALSP; catchweed bedstraw, Galium aparine L. GALAP.

Invasive weed managers are presented with a complicated and ever-enlarging set of management alternatives. Identifying the optimal weed management strategy for a given set of conditions requires predicting how candidate strategies will affect plant community composition. Although field experiments have advanced our ability to predict postmanagement composition, extrapolation problems limit the prediction accuracy achieved by interpreting treatment means as predictions. Examples of extrapolation problems include nonlinear relationships between competing plants, site-to-site variation in plant population growth rates, and the carrying capacities of desired species and weeds. Our objective was to develop a model that improves predictions of weed management outcomes by overcoming a subset of these problems. To develop the model, we used data from two field experiments in which four Kentucky bluegrass, six western wheatgrass, and six invasive plant (i.e., leafy spurge) densities were combined in field plots. Graphs of our model's predictions vs. observed field experiment data indicate that the model predicted the data accurately. Our model may improve predictions of plant community response to invasive weed management actions.

Nomenclature: Leafy spurge, Euphorbia esula L. EPHES; spotted knapweed, Centaurea maculosa Lam. CENMA; Kentucky bluegrass, Poa pratensis L. POAPR; western wheatgrass, Pascopyrum smithii Rydb.

A better understanding of the persistence of jointed goatgrass seed in soil and its dormancy will lead to the development of more effective weed-management strategies. Three populations of jointed goatgrass were collected from winter wheat fields in Oregon, and grown together with the winter wheat variety ‘Madsen’ in nurseries at Moro and Pendleton, OR. Germination responses of jointed goatgrass and wheat seed were recorded over 14 d at 5/5, 15/10, 15/15, 25/15, 25/25, and 30/20 C day/night temperatures and a 12-h photoperiod. Because jointed goatgrass spikelets often contain two seed, primary and secondary seed germination values were recorded. Secondary seed germination was defined as 3-mm radicle protrusion, and primary seed germination was defined as 5-mm emergence of the second coleoptile. Jointed goatgrass secondary seed germinated when exposed to all temperature regimes. Jointed goatgrass secondary seed germination occurred 3 d earlier in temperature regimes involving 15 C compared to germination at 5/5, 25/25, and 30/20 C. Final germination values for jointed goatgrass secondary seed were greatest when seed were incubated at 25/15 C. Wheat seed germinated at all temperature regimes, although the onset of germination occurred 1 to 1.5 d later at 5/5 C compared to other temperature regimes. Jointed goatgrass primary seed germinated only at 15/10, 15/15, and 25/15 C, and maximum germination occurred at 25/15 C. Dormancy in jointed goatgrass might prevent germination of seed within freshly shattered spikelets until autumn when temperatures are low and moisture is available. Because final germination percentages in jointed goatgrass primary and secondary seed were less than 100%, additional research on factors regulating dormancy is needed.

Nomenclature: Jointed goatgrass, Aegilops cylindrica Host., AEGCY; wheat, Triticum aestivum L., ‘Madsen’.

Croftonweed is an invasive plant in southwest China. We examined the relationships between its invasion patterns and native plant diversity at different spatio-temporal scales. At the 25 m² scale, invasion success was negatively correlated with native plant diversity, indicating that resource availability might be the dominant factor regulating community invasibility. At the 400-m² scale, both negative and positive relationships were detected, possibly identifying a spatial scale threshold where extrinsic environmental factors became more important to community invasibility. At the vegetation province scale, variations in physical environment outweighed the importance of intrinsic biotic factors and positive relationships between diversity and invader success were found. Native plant diversity also inhibited croftonweed over the course of community succession and at the early stages of invasion at local spatial scales. However, the changing relationship might be an artifact of sampling at different spatial scales.

Nomenclature: Croftonweed, Eupatorium adenophorum Sprengel EUPAD.

The impact of invasive weed management on plant community composition is highly dependent on location-specific factors. Therefore, treatment means from experiments conducted at a given set of locations will not reliably predict community response to weed management elsewhere. We developed a model that rescales treatment means to better match local conditions. The goal of this paper was to determine if this rescaling improves predictions. We used our model to predict leafy spurge stem length density and grass biomass data from field experiments. The experiments consisted of herbicide-treated plots, untreated controls, and, in some cases, grass seeding treatments. When herbicides suppressed leafy spurge, the model explained 21 to 48% more variation in grass response than did mean grass response to the same or similar herbicide treatments applied at other sites. When herbicides killed grass, the model explained 53% more variation in leafy spurge response than did mean leafy spurge response to the same herbicide treatment applied at other sites. We regressed model predictions against observed data and tested the null hypothesis that resulting slope terms were equal to 1.0. Because the null hypothesis was rejected in two of four tests, the model may systematically over- or underpredict in some situations. However, measurement error in the observed data, unintended herbicide injury, or an inaccurate allometric relationship may account for a major proportion of the systematic deviations, and these factors would not cause prediction error in some management applications. Because the model tends to be better than the means from experiments at predicting plant community composition, we conclude that the model could advance managers' ability to predict plant community responses to invasive weed management.

Nomenclature: Leafy spurge, Euphorbia esula L. EPHES.

Glyphosate resistance was found in a rigid ryegrass population in northern California. A sample of the resistant plants were collected and grown under greenhouse conditions. The objective of this study was to evaluate glyphosate resistance in the progeny of the collected plants by recurrent selection, obtain the homozygous resistant and sensitive lines to establish dose-response curves, and to determine the inheritance of glyphosate resistance in rigid ryegrass. Diverse levels of resistance were observed in the first generation with survival of 89, 59, 45, and 9% from glyphosate at 1x, 2x, 4x, and 8x respectively, where x = 1.12 kg ha⁻¹ isopropylamine salt of glyphosate. Clones of plants that died from 1x were allowed to produce seed and were further subjected to recurrent selection to generate the most sensitive plants (S lines), which died from 0.125x glyphosate. The most resistant plants (R lines) were generated from the survivors receiving 8x glyphosate. The ratio between I₅₀ rates for the glyphosate resistant and the glyphosate sensitive plants was > 100-fold. The R and S lines were crossed reciprocally and F₁ progeny of both (R × S) and (S × R) showed intermediate resistance. These survived up to 2x glyphosate. The F₂ progeny were generated by intercrossing of F₁ plants. The ratio of sensitive, intermediate, and resistant plants in the F₂ population before the treatment of glyphosate at 0.125x followed by 8x was 1 : 16, 14 : 16, and 1 : 16 respectively, which corresponded to the Mendelian segregation ratio of two genes. The results indicated that the inheritance of glyphosate resistance in rigid ryegrass from California appeared to be nuclear, incompletely dominant, multigenic, and pollen-transmitted with no indication of maternal inheritance.

Nomenclature: Glyphosate; rigid ryegrass, Lolium rigidum Gaud. LOLRI.

Knowledge of the influence of environmental factors on weed populations is important in developing sustainable turfgrass management practices. Studies were conducted to evaluate the relationship of green and false-green kyllinga population densities with elevation and edaphic factors in turfgrass systems. Studies were conducted on five different golf courses in North Carolina, three affected by green kyllinga, and two affected by false-green kyllinga. According to Spearman correlation coefficients, both green and false-green kyllinga were correlated with increasing soil volumetric water content, whereas correlation of other edaphic variables varied among sites and species. Stepwise logistic regression confirmed the correlation of volumetric water with green kyllinga presence, but model components varied among sites for false-green kyllinga. Increasing green kyllinga populations correlated with increasing soil sodium; however, sodium did not reach a level believed to be detrimental to turfgrass growth. No other variables correlated with green or false-green kyllinga across all sites. We hypothesized that the lack of significant correlations was due to the overall influence of relative elevation on edaphic variables. According to principal components analysis (PCA), relative elevation had a profound impact on the measured edaphic variables at all sites. However, results of PCA at one site differed sharply from other sites. Results from that site demonstrate the potentially strong effects of management practices to alter edaphic trends normally observed with topography.

Nomenclature: False-green kyllinga, Kyllinga gracillima L.; green kyllinga, Kyllinga brevifolia Rottb.; bermudagrass, Cynodon spp.; bentgrass, Agrostis spp.

Multiple cases of ALS inhibitor-resistant weed biotypes are reported for many species, including wild mustard. The physiological extent and molecular basis of resistance to ALS inhibitors was compared in four biotypes of wild mustard from western Canada: a sulfonylurea (SU)-resistant (R) biotype from Manitoba detected in 1992; an SU (ethametsulfuron)-R biotype from Alberta detected in 1993 (metabolism-based resistance); an SU-R biotype from Manitoba detected in 2002; and a SU- and imidazolinone (IMI)-R biotype from Saskatchewan detected in 2002. Herbicide dose-response experiments confirmed that the two Manitoba biotypes were resistant to the SU herbicides ethametsulfuron and tribenuron:thifensulfuron mixture, whereas the Saskatchewan biotype was resistant to both SU herbicides and to imazethapyr, an IMI herbicide. Sequence analysis of the ALS gene detected target site mutations in three of the four R biotypes, with amino acid substitutions Pro₁₉₇ (CCT) to Ser (TCT) [Domain A of the gene] in the two SU-R Manitoba biotypes and Trp₅₇₄ (TGG) to Leu (TTG) [Domain B] in the Saskatchewan biotype. The Alberta SU-R biotype had the same ALS nucleotide and amino acid sequence as the susceptible population at these two positions. Two heterozygous individuals [Trp₅₇₄ (Tt/gG)] were detected in the Saskatchewan biotype, and genetic segregation for nucleotide bases and resistance phenotype was consistent with single gene control. Nucleotide variation in neutral regions of the ALS gene varied with biotype, with no variation in the two Manitoba biotypes, two variants in the Saskatchewan biotype, and 16 neutral nucleotide polymorphisms (0.9%) in the Alberta biotype. The occurrence of at least three different ALS inhibitor-R biotypes in this important weed species is likely to impact negatively on the use of ALS inhibitors, such as the IMIs, and serves as a warning for strict implementation of herbicide rotations to prevent or delay the evolution and spread of such populations.

Nomenclature: Ethametsulfuron; imazethapyr; thifensulfuron; tribenuron; wild mustard, Brassica kaber (DC.) L. C. Wheeler SINAR.

The objective of this study was to examine the effects of crop rotation (spring barley monoculture vs. spring barley–red clover 2-yr rotation), tillage (moldboard plow, chisel plow, no-till), and weed management (intensive, moderate, minimum) on plant–seedbank relationships for 19 weed species. Plant and seedbank density data were collected over 4 yr and analyzed by analysis of variance and correlation analysis to confirm treatment effects on plant–seedbank relationships. The relative frequency (difference between aboveground and seedbank frequency) of many species was more influenced by rotation, whereas species density appeared regulated more by weed management than by other factors. Frequency data confirmed that very few species were ubiquitous over time or treatment, aboveground or in the seedbank. The perennial species, field horsetail, quackgrass, white clover, and perennial sowthistle were more frequent aboveground than in the seedbank. This was also observed for annuals such as common hempnettle, sun spurge, catchweed bedstraw, and annual grasses. Treatment effects on abundance were inconsistent aboveground and in the seedbank across time for 12 of 19 species. The seven species that showed more consistent treatment response for abundance were frequent species present in 50% of the plots both aboveground and in the seedbank. For most species, plant density was correlated with either the previous or current year seedbank, but correlations were rarely of the same magnitude and significance over the years. Common chickweed was the only species for which treatment effects on the plant–seedbank relationship were confirmed for all 4 yr. Correlations between midseason plant populations and subsequent seedbanks confirmed the role of residual populations in replenishing the seedbanks, including those of perennials like quackgrass and dandelion. Overall, plant–seedbank relationships were tenuous for many weed species and varied over time with cropping practices and environment.

Nomenclature: Catchweed bedstraw, Galium aparine L. GALAP; common chickweed, Stellaria media (L.) Cyrillo STEME; common hempnettle, Galeopsis tetrahit L. GAETE; dandelion, Taraxacum officinale Weber in Wiggers TAROF; field horsetail, Equisetum arvense L. EQUAR; perennial sowthistle, Sonchus arvensis L. SONAR; quackgrass, Elytrigia repens (L.) Nevski AGRRE; sun spurge, Euphorbia helioscopia L. EPHHE; white clover, Trifolium repens L. TRFRE; red clover, Trifolium pratense L. TRFPR; spring barley, Hordeum vulgare L. HORVX.

Effects of environmental factors, especially light, on eastern black nightshade seed germination were studied under growth chamber conditions. Germination occurred only when temperatures were 20 C or higher for 12 to 24 h d⁻¹. Optimum germination occurred within the constant temperature range of 28 to 33 C or with alternating temperatures of 30/25 and 35/30 C. In the dark, eastern black nightshade germination was poor except at high alternating temperatures of 30/25 and 35/30 C. Seed germination was enhanced by light when temperatures were suboptimal. Imbibition was a prerequisite for response of eastern black nightshade seed to light. The light requirement for eastern black nightshade germination was partially overcome by cold stratification, gibberellic acid (GA₃), and high or alternating temperatures. Germination of eastern black nightshade seed was promoted by short exposure to sunlight but was inhibited by prolonged exposure, especially when irradiance was high.

Nomenclature: Eastern black nightshade, Solanum ptycanthum Dun. SOLPT.

Laboratory and greenhouse experiments were conducted to determine the effect of light, temperature, moisture stress, solution pH, and burial depth on coffee senna germination and emergence. Seeds germinated equally with or without light, and pretreatment with red or far-red light did not affect germination. Optimum temperature for germination was 25 C, and a high germination percentage (> 70%) occurred from 12.5 to 30 C. The low temperature threshold for germination was between 10 and 12.5 C, whereas the upper threshold was near 45 C. Coffee senna germination in response to moisture stress and solution pH differed at 15 and 30 C. At −0.4 MPa, no germination occurred at 15 C, whereas 15% germination occurred at 30 C. Optimum germination was at pH 6, but further increases in pH had a more negative effect on germination at 15 C than at 30 C. Coffee senna germination ranged from 9 to 12% at pH 3, but was 0% at pH 10, which indicates that coffee senna germination was more tolerant of acidic than basic solutions. Depth-mediated emergence inhibition was sigmoidal, with greatest emergence on the soil surface. Emergence from 2- to 10-cm depths reached 95% of the total emergence 1 to 3 d earlier in a sandy loam than in a sand soil. Mean emergence depth was 1.7 cm in the sand and 2.4 cm in the sandy loam soil. Knowledge gained from this research will be instrumental in developing a better understanding of the requirements for coffee senna germination and emergence, allowing further development and improvement of integrated weed management strategies specific to this troublesome weed.

Nomenclature: Coffee senna, Cassia occidentalis L. CASOC.

Mexican palo-verde is a serious woody weed in tropical parts of the world. Like many such leguminous species, it has relatively large seeds with hard-seeded (physical) dormancy. It therefore has the potential for long-lived seed banks that are difficult to manage. The physiology of hard-seeded dormancy is still relatively poorly understood but has important implications for weed management. We propose that wet heat is a potentially important dormancy release mechanism for summer rainfall tropical regions. We described the relationships between wet heat and dormancy release (in water; three seed sources) and germination (near saturation; single seed source) by testing seeds at constant temperatures between 10 and 60 C. The logistic transformation of the temperature–dormancy relationship was best described by a quadratic equation below a threshold of ∼ 33.6 C and a linear equation above that threshold. The relationship was the same for all seed sources other than a phase shift of up to 6.6 C, which is likely to be of biological significance. Germination occurred between 15 and 40 C and was limited by cold stress at ≤ 20 C and heat stress > 35 C. The sensitivity of dormancy to naturally encountered temperature ranges suggests that wet heat is an important dormancy release mechanism and one that can be exploited when developing management strategies for invasive populations.

Nomenclature: Mexican palo-verde, Parkinsonia aculeata L. PAKAC.

Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited.

Nomenclature: Velvetleaf, Abutilon theophrasti Medic., ABUTH; corn, Zea mays L.

Field experiments were conducted at the Rice Research and Extension Center at Stuttgart, AR, in 1997 and 1998 to evaluate the growth response of Stuttgart strawhull (Stgstraw) red rice to sowing densities of 0, 50, 100, and 150 kg ha⁻¹ of ‘Kaybonnet,’ ‘Guichao,’ and ‘PI 312777’ rice cultivars. PI 312777 produced a greater leaf area index and tiller density than Kaybonnet when grown with red rice. In 1997, Stgstraw seed yields were lower when grown with PI 312777 and Guichao than with Kaybonnet. The increased weed population in 1998 did not increase seed yield production of red rice when grown with the three rice cultivars. The Stgstraw red rice seed yield was reduced when grown with 50 kg ha⁻¹ rice when compared with its yield in monoculture and was reduced further when grown with 100 and 150 kg ha⁻¹ rice. These results demonstrate that red rice was more competitive when compared with the tropical japonica Kaybonnet than the indica PI 312777. Despite its semidwarf stature, PI 312777 tended to suppress red rice more than did Guichao or Kaybonnet. The mechanisms responsible for this difference could be important keys to the effective use of weed-suppressive cultivars in reduced herbicide input systems.

Nomenclature: Red rice, Oryza sativa L. ORYSA; ‘Stgstraw’; rice, Oryza sativa L.; ‘Guichao’, ‘Kaybonnet’, ‘PI 312777’.

Red rice, which grows taller and produces more tillers than domestic rice and shatters most of its seeds early, is a major weed in many rice-growing areas of the world. Field experiments were conducted at Stuttgart, AR in 1997 and 1998 to evaluate the growth response of the Kaybonnet (KBNT) rice cultivar to various population densities of three red rice ecotypes. The ecotypes tested were Louisiana3 (LA3), Stuttgart strawhull (Stgstraw), and Katy red rice (KatyRR). Compared with KBNT alone, LA3, the tallest of the three red rice ecotypes, reduced tiller density of KBNT 51%, aboveground biomass at 91 d after emergence (DAE) 35%, and yield 80%. Stgstraw, a medium-height red rice, reduced KBNT tiller density 49%, aboveground biomass 26%, and yield 61%. KatyRR, the shortest red rice, reduced KBNT tiller density 30%, aboveground biomass 16%, and yield 21%. Tiller density of rice was reduced by 20 to 48% when red rice density increased from 25 to 51 plants m⁻². Rice biomass at 91 DAE was reduced by 9 and 44% when red rice densities were 16 and 51 plants m⁻². Rice yield was reduced by 60 and 70% at red rice densities of 25 and 51 plants m⁻², respectively. These results demonstrate that low populations of red rice can greatly reduce rice growth and yield and that short-statured red rice types may affect rice growth less than taller ecotypes.

Nomenclature: Red rice, Oryza sativa L. ORYSA, ‘KatyRR’, ‘LA3’, ‘Stgstraw’; rice, Oryza sativa L., ‘Kaybonnet’.

Field experiments on suppression of three species (scentless chamomile, field poppy, and canola) by winter wheat sown in two different spatial patterns (normal 12.8-cm rows and a uniform, grid-like pattern) and three densities (204, 449, and 721 plants m⁻²) in two growing seasons were performed. The effects of crop-sowing density and pattern when weeds were controlled by herbicide were also investigated in one season. Weed and crop biomass were measured when weed biomass was at its maximum (late June/early July), and grain was harvested in August. Weed biomass comprised on average 30% of the total (crop weed) biomass in the first year and only 5% in the second year. Weed biomass decreased and grain yield increased with increasing sowing density. Weed biomass was on average 23% lower and grain yield 14% higher in the uniform pattern than in rows. Weed biomass decreased 27% and 38% in the row pattern and 36% and 50% in the uniform pattern by increasing sowing density from low to medium and from low to high density, respectively. When weeds were controlled with herbicide, increasing sowing density had no influence on grain yield, but grain yield was 7% higher in the uniform pattern. Field poppy was the weed with the largest biomass and the largest impact on yield, whereas canola had the lowest biomass and the least impact on yield.

Nomenclature: Field poppy, Papaver rhoeas L. PAPRH; scentless chamomile, Matricaria perforata Mérat MATIN; canola, Brassica napus L. ‘Karola’; winter wheat, Triticum aestivum L. ‘Terra’.

The Brassicaceae contain glucosinolates, which hydrolyze to form compounds toxic to plants, fungi, nematodes, and certain insects. Lower weed density and biomass in crops grown following incorporation of brassica cover crops suggest that they may contribute to weed management in agricultural systems. Field experiments were conducted to determine whether incorporated brassica cover crops, including canola, rapeseed, and yellow mustard, reduce subsequent weed and crop establishment; a companion paper describes separate but related field experiments that examined the influence of brassica cover crops on plant growth. Emergence rate and total emergence of sixteen weed and crop bioassay species were measured following brassica cover crops, fallow, or incorporated residues of other short-season cover crops including oat, crimson clover, and buckwheat. The bioassay species, representing a range of seed sizes, were chosen to determine whether larger seed size confers protection from residue-mediated effects on emergence. Averaged over bioassay species, brassica cover crops reduced emergence by 23 to 34% compared with fallow; emergence following brassicas was delayed by approximately 2 d. The effects of the incorporated brassica residues were similar to those of the other short-season cover crops, which reduced emergence of the bioassay species by 19 to 39% and delayed emergence by 2 d. Seed size was a poor predictor of a species' establishment. These results suggest that brassica residues are capable of delaying seedling emergence and reducing establishment, although the magnitude of their effects were comparable to other widely available cover crops.

Nomenclature: Canola, ‘Hyola’, Brassica napus L.; rapeseed, ‘Dwarf Essex’, Brassica napus L.; yellow mustard, ‘Idagold’, Sinapis alba L.; crimson clover, Trifolium incarnatum L.; oat, Avena sativa L.; buckwheat, Fagopyrum esculentum Moench.

Field studies have shown that weed density and biomass were lower in crops following incorporation of brassica cover crops compared with fallow but have not determined whether weed-suppressive effects are solely a consequence of reduced establishment, as evidenced in our companion paper, reduced growth of established plants, or both. In 2002 and 2003, canola and yellow mustard were seeded in early May, mowed in early July, and the residues incorporated. Green bean and redroot pigweed were then planted at fixed densities. Plant height and biomass were measured weekly; leaf area and biomass of component plant parts were measured at three harvests. Based on analysis of variance (ANOVA) at discreet sampling points, growth of redroot pigweed and green bean in monoculture or mixture were similar following fallow and incorporated brassica cover crops. However, based on aboveground biomass fitted to a Richards function, redroot pigweed growth in monoculture was reduced by the yellow mustard cover crop compared with fallow in both years (P = 0.007), but the magnitude of this effect was small; the canola cover crop did not affect growth (P = 0.179). Brassica cover crops did not reduce redroot pigweed growth when it was grown in mixture with green bean (P ≥ 0.382). Redroot pigweed competition reduced green bean yield, but incorporated brassica cover crops did not affect green bean growth and yield, nor did they confer a competitive advantage to the crop. Thus, brassica cover crops may suppress the growth of established weed and crop plants, but the magnitude of suppression was less than previously documented for effects on weed establishment.

Nomenclature: Canola, ‘Hyola’, Brassica napus L.; redroot pigweed, Amaranthus retroflexus L. AMARE; yellow mustard, ‘Idagold’, Sinapis alba L.; green bean, ‘Provider’, Phaseolus vulgaris L.

A study was conducted near Garden City, KS with irrigated corn to determine how the integration of a terminated winter wheat cover crop with various atrazine rates would affect Palmer amaranth control and corn water use efficiency (WUE). Without atrazine, the presence of a winter wheat cover crop, killed in the boot stage, resulted in a threefold weed biomass reduction in irrigated corn. The highest rate of atrazine completely masked the weed control effect of the cover crop, producing a greater than 15-fold reduction regardless of the presence or absence of the cover crop. A terminated winter wheat cover crop without atrazine elevated corn yield in only two of nine location-yr, and in one instance, depressed yield. However, a terminated wheat cover crop elevated corn yield in six of nine location-yr combinations when used in conjunction with 1.6 kg ha⁻¹ atrazine. Although increases in WUE associated with reductions in soil water evaporation produced by the cover crop seemed to be responsible for some of the increase in corn grain yield and stored soil water at the end of the summer growing season, end of season Palmer amaranth biomass had a more profound impact.

Nomenclature: Atrazine; Palmer amaranth, Amaranthus palmeri S. Wats., AMAPA; corn, Zea mays L. ‘DK 592SR’; winter wheat, Triticum aestivum L. ‘TAM 107’.

Soil and surface residues from cotton field studies in Stoneville, MS (1994 through 1996) and Florence, SC (1995 through 1996) were sampled to evaluate effects of cover crop and tillage on herbicide dissipation. Mississippi treatments included tillage (conventional [CT]; none [NT]) and cover crop (ryegrass; none [NC]). South Carolina treatments included tillage (CT; reduced tillage [RT]) and cover crop (rye; NC). Fluometuron was applied preemergence (PRE) in both Mississippi and South Carolina, and norflurazon was applied PRE in Mississippi. Soils were sampled various times during the growing season (depths: 0 to 2 cm, 2 to 10 cm). Cover crop residues were sampled from RT or NT cover crop areas. Soil and cover crop sample extracts were analyzed for herbicides. Soil organic carbon tended to increase with tillage reduction and presence of cover crop and was positively correlated with herbicide sorption, especially in the surface. Across locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in surface soil, with higher herbicide concentrations in CT at early samplings, but differences were insignificant later on. The most consistent effects were observed in RT/NT with cover crops, where cover crop residues intercepted applied herbicide, impeding subsequent movement into soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives from 3 to 11 d. Herbicide retention in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., fluometuron K_d = 7.1 [in rye]; K_d = 1.65 [in Mississippi Dundee soil CT, NC]) and herbicide co-metabolism as cover crop residues decomposed. A fluometuron metabolite, desmethyl-fluometuron, was observed in most soil and cover crop samples after 1 wk. Only minimal herbicide or metabolite moved into the subsurface, and little treatment effect could be ascribed to herbicide or metabolite movement below 2 cm.

Nomenclature: Desmethyl fluometuron; fluometuron; glyphosate; norflurazon; paraquat; pendimethalin; cotton, Gossypium hirsutum L. ‘Stoneville 506’, ‘Delta & Pine Land DES 119’; Italian ryegrass, Lolium multiforum Lam.; rye, Secale cereale L.

Herbicides targeting grass plastidic acetyl coenzyme A carboxylase (ACC) are effective selective graminicides. Their intensive use worldwide has selected for resistance genes in a number of grass weed species. Biochemistry and molecular biology have been the means of determining the herbicidal activity and selectivity toward crop plants of ACC-inhibiting herbicides. In recent years, elucidation of the tridimensional structure of ACC and identification of five amino acid residues within the ACC carboxyl transferase domain that are critical determinants for herbicide sensitivity shed light on the basis of ACC-based resistance to herbicides. However, metabolism-based resistance to ACC-inhibiting herbicides is much less well known, although this type of resistance seems to be widespread. A number of genes thus endow resistance to ACC-inhibiting herbicides, with the possibility for various resistance genes that confer dominant resistance at the herbicide field rate to accumulate within a single weed population or plant. This, together with a poor knowledge of the genetic parameters driving resistance, renders the evolution of resistance to ACC-inhibiting herbicides unpredictable. Future research should consider developing tactics to slow the spread of resistance. For this purpose, it is crucial that our understanding of metabolism-based resistance improves rapidly because this mechanism is complex and can confer resistance to herbicides with different target sites.