Auxinic herbicides are widely used for control of broadleaf weeds in cereal crops and turfgrass. These herbicides are structurally similar to the natural plant hormone auxin, and induce several of the same physiological and biochemical responses at low concentrations. After several decades of research to understand the auxin signal transduction pathway, the receptors for auxin binding and resultant biochemical and physiological responses have recently been discovered in plants. However, the precise mode of action for the auxinic herbicides is not completely understood despite their extensive use in agriculture for over six decades. Auxinic herbicide-resistant weed biotypes offer excellent model species for uncovering the mode of action as well as resistance to these compounds. Compared with other herbicide families, the incidence of resistance to auxinic herbicides is relatively low, with only 29 auxinic herbicide-resistant weed species discovered to date. The relatively low incidence of resistance to auxinic herbicides has been attributed to the presence of rare alleles imparting resistance in natural weed populations, the potential for fitness penalties due to mutations conferring resistance in weeds, and the complex mode of action of auxinic herbicides in sensitive dicot plants. This review discusses recent advances in the auxin signal transduction pathway and its relation to auxinic herbicide mode of action. Furthermore, comprehensive information about the genetics and inheritance of auxinic herbicide resistance and case studies examining mechanisms of resistance in auxinic herbicide-resistant broadleaf weed biotypes are provided. Within the context of recent findings pertaining to auxin biology and mechanisms of resistance to auxinic herbicides, agronomic implications of the evolution of resistance to these herbicides are discussed in light of new auxinic herbicide-resistant crops that will be commercialized in the near future.

Nomenclature: Auxinic herbicides; dominant trait; evolution of resistance; fitness cost; herbicide-resistant crops; mode of action; mechanism of resistance; plant growth regulator; recessive trait.

Mesotrione, topramezone, and tembotrione inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme integral to carotenoid biosynthesis. Research was conducted to evaluate the response of hybrid bermudagrass following mesotrione (280, 350, and 420 g ai ha⁻¹), topramezone (18, 25, and 38 g ai ha⁻¹), and tembotrione (92, 184, and 276 g ai ha⁻¹) applications. Measurements of visual bleaching (VB) and chlorophyll fluorescence yield (F_v/F_m) were evaluated 3, 5, 7, 14, 21, 28, and 35 d after application (DAA). Leaf tissues were sampled on the same dates and assayed for chlorophyll and carotenoid pigments using high-performance liquid chromatography (HPLC). Responses of plants treated with topramezone and tembotrione were similar; these herbicides caused more VB and greater reductions in F_v/F_m, total chlorophyll, lutein, and xanthophyll cycle pigment concentrations than mesotrione 5 to 21 DAA. Increasing mesotrione application rate did not increase VB or lead to greater reductions in total chlorophyll, lutein, or xanthophyll pigment concentrations. Alternatively, increasing topramezone and tembotrione application rates above 18 and 92 g ha⁻¹, respectively, extended VB and pigment reductions. Of the three HPPD-inhibitors tested, topramezone was the most active, because the low (18 g ha⁻¹) rate of topramezone reduced lutein and total xanthophyll pigment concentrations more than the low rate of tembotrione (92 g ha⁻¹) during periods of maximum activity (14 to 21 DAA). No necrosis was observed with any of the treatments, suggesting tank mixtures of topramezone with other herbicides might be required to provide long-term control of hybrid bermudagrass.

Nomenclature: Mesotrione; topramezone; tembotrione; hybrid bermudagrass, Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy ‘Tifway’.

Natural herbicides approved in organic agriculture are primarily nonselective burn-down essential oils applied POST. Multiple applications are often required due to their low efficacy. To address this problem, the in vivo herbicidal activity of manuka oil, the essential oil distilled from manuka tree (Leptospermum scoparium J.R. and G. Forst), was tested on selected broadleaf and grass weeds. While manuka oil exhibited good POST activity when applied in combination with a commercial lemongrass oil–based herbicide, it ultimately demonstrated interesting PRE activity, providing control of large crabgrass seedlings at a rate of 3 L ha⁻¹. Manuka oil and its main active ingredient, leptospermone, were stable in soil for up to 7 d and had half-lives of 18 and 15 d, respectively. The systemic activity of manuka oil addresses many of the current limitations associated with natural herbicides. Additionally, its soil persistence opens up a multitude of new possibilities for the use of manuka oil as a tool for weed management and may be a potential bridge between traditional and organic agriculture.

Nomenclature: Leptospermone; 2,2,4,4-tetramethyl-6-(3-methyl-1-oxobutyl)-1,3,5-cyclohexanetrione, CAS 567-75-9; large crabgrass; Digitaria sanguinalis (L.) Scop. DIGSA.

Solanum viarum, commonly known as tropical soda apple (TSA), is native to Brazil and Argentina but has become a harmful weed in many countries with tropical climates. This study was conducted to reassess the seed biology of TSA found in South Africa. Cold stratification (14 d), acid scarification (20% H₂SO₄ for 5 min), and sandpaper scarification (30 s) significantly improved percentage germination when compared to the control. The highest germination (99.5%) was achieved when seeds were germinated in 50% Hoagland's nutrient solution (HS). The lowest germination (66%) was recorded in the absence of phosphorus (P) under alternating light conditions. HS without nitrogen (N) completely inhibited seed germination of TSA under constant light conditions. These findings are useful in controlling TSA by amending the levels of N and P in soils. Seed germination of TSA was significantly enhanced by different concentrations of smoke-water and butenolide solution. Smoke-water dilution of 1∶500 v/v and butenolide concentration of 10⁻⁸M showed the highest seedling vigor indices (6,688 and 6,666, respectively) in comparison to the control (1,251) and gibberellic acid (GA₃) concentrations (< 5,327). These findings suggest that germination of seeds or seedbanks of TSA might be successfully stimulated using smoke solutions. Subsequently, patches of seedlings emerging after treatment can be mechanically uprooted to reduce the infestation of TSA. However, justifying this with field trials is essential.

Nomenclature: Tropical Soda Apple; Solanum viarum Dunal SOLVI.

Purple nutsedge is a troublesome C₄ weed, characterized by high photosynthetic efficiency, compared to C₃ weeds. As its dispersal is based on vegetative growth, accurate prediction of its growth could help in arriving at favorable management decisions. This article details the development and validation of predictive models of purple nutsedge spatial growth, based on temperature (thermal model), and temperature and radiation (photothermal model) measurements. Plants were grown in six experiments in the summers of 2008, 2009, and 2010, under different temperature and radiation conditions. Results indicate that under optimal temperatures, radiation becomes the main growth-limiting factor, and is highly related to the final leaf-cover area (R²  =  0.89). Comparison of the thermal and photothermal models showed that under all conditions, including varied temperature and radiation, the photothermal model performs significantly better, with differences in root-mean-square error values reaching up to 0.073, compared to 0.195 with the thermal model. Validation experiments confirmed the ability of the photothermal model to predict purple nutsedge spatial growth accurately.

Nomenclature: Purple nutsedge, Cyperus rotundus L. CYPRO.

Information on the germination and viability of turnipweed seeds could be helpful in developing appropriate management strategies for this weed. Therefore, experiments were conducted to investigate the effects of light, storage conditions, duration of storage or burial, and seed type (naked, i.e., fruit wall was removed and encapsulated in siliques) on germination and viability of turnipweed. The naked and encapsulated seeds (fruit) were kept under five different storage conditions, including dry storage at 25 ± 2 C and 3 ± 1 C and outdoor environments (soil depths of 10, 20, and 40 cm). All seeds were retrieved every 2 mo and tested for germination in light and darkness. At each exhumation date, nongerminated seeds were treated with triphenyltetrazolium chloride to test their viability. The germination of seeds liberated from siliques (85%) was markedly greater than that of seeds in intact siliques (20%). The germination response of naked and encapsulated seeds to light varied between storage conditions and through time. Under indoor conditions (room and cold), both seed types had greater germination percentages in dark on most occasions than those in light. On the contrary, the germination of siliques buried at soil depths of 20 or 40 cm was considerably stimulated by light. Under indoor conditions, the percent viability of both seed types only declined marginally, whereas seeds buried in soil showed high rates of mortality. Seeds in intact siliques persisted longer under either indoor or outdoor conditions than naked seeds.

Nomenclature: Turnipweed, Rapistrum rugosum (L.) All.

Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype.

Nomenclature: Horseweed, Conyza canadensis L. Cronquist ERICA.; grape, Vitis vinifera L.

Horse purslane, a C₄ species, is a branched, prostrate, and annual weed of upland field crops throughout the tropics. Experiments were conducted to determine the influence of various environmental factors on seed germination and seedling emergence of two populations of horse purslane. Seeds were collected from rice fields of the International Rice Research Institute (the IR population) and from sorghum fields of the University of the Philippines (the UP population); the two sites were 5 km apart in Los Baños, Philippines. Germination response of both populations was greater at 30/20 C and35/25 C day/night temperatures than they were at 25/15 C alternating day/night temperatures. Germination of both populations was greater in the light/dark regime than in darkness. In dark, depending on the temperature, seed germination of the UP population ranged from 37 to 62%, whereas seed germination of the IR population was < 20%. Exposure to 5 min at 117 and 119 C for the IR and UP populations, respectively, reduced germination to 50% of maximum germination. Osmotic potential of −0.26 MPa inhibited germination to 50% of the maximum for the UP population, whereas the corresponding value for the IR population was −0.37 MPa. Seeds placed on or near the soil surface had maximum emergence, and emergence declined with increase in seed burial depth. Seedling emergence of the UP and IR populations was 74% and 13%, respectively, for seeds placed on the soil surface. For both populations, no seedlings emerged from a soil burial depth of 6 cm or more. Germination and emergence responses to light and seed burial depth differed between the two populations of horse purslane. Residues on the soil surface of up to 6 Mg ha⁻¹ did not influence seedling emergence of either populations. Knowledge gained in this study could contribute to developing components of integrated weed management strategies for horse purslane.

Nomenclature: Horse purslane, Trianthema portulacastrum L.; rice, Oryza sativa L.; sorghum, Sorghum bicolor (L.) Moench ssp. bicolor.

Greenhouse studies were conducted to determine the relative salinity tolerance of foxtail barley and seven desirable pasture grasses. Grass species were reed canarygrass, timothy, altai wildrye, tall fescue, tall wheatgrass, orchardgrass, creeping meadow foxtail, and foxtail barley. Grasses were exposed to increasing electrical conductivity levels of NaCl and CaCl₂ salt solution over time. Grass species were compared using a cumulative value of salt exposure (EC_days), which was calculated to account for the electrical conductivity (EC) and the time a plant was exposed at that level of conductivity. Salinity tolerance varied among grass species. Increasing EC significantly reduced plant biomass of all species. All grass species experienced a 50% biomass reduction (GR₅₀) between 271 and 512 EC_days in 2008 and between 297 and 575 EC_days in 2009. Foxtail barley was among the most salt tolerant (GR₅₀  =  512 and 525 EC_days), requiring the highest salt exposure in 2008 and the second-highest exposure in 2009 to reduce biomass 50%. Grass mortality increased with increasing EC levels. Reed canarygrass and timothy were most susceptible to increasing salinity, with 50% mortality (LD₅₀) of both grass species occurring between 983 and 1,186 EC_days. Moderate salinity tolerance was exhibited by orchardgrass, which required 1,977 and 1,844 EC_days; creeping foxtail, which required 1,998 and 2,431 EC_days; and tall fescue, which required 2,501 and > 2,840 EC_days to LD₅₀ in 2008 and 2009, respectively. Foxtail barley, altai wildrye, and tall wheatgrass were most tolerant of salinity and persisted with little mortality occurring at 3,033 and 2,840 EC_days in 2008 and 2009, respectively. All grass species with higher growth rates than foxtail barley and altai wildrye were more susceptible to salinity, with the exception of tall wheatgrass. Growth rates of foxtail barley and altai wildrye were less than they were for other grasses, suggesting that slower growth rates may aid in salinity tolerance.

Nomenclature: Altai wildrye, Leymus angustus AP/PR (Trin.) Pilg. ‘Mustang’; creeping meadow foxtail, Alopecurus arundinaceus Poir. ‘Garrison’; foxtail barley, Hordeum jubatum L. HORJU; orchardgrass, Dactylis glomerata L. ‘Potomac’; reed canarygrass, Phalaris arundinacea L. ‘Palaton’; tall fescue, Lolium arundinaceum (Schreb.) S.J. Darbyshire ‘Fawn’; tall wheatgrass, Thinopyrum ponticum (Podp.) Z.-W. Liu & R.-C. Wang ‘Alkar’; timothy, Phleum pratense L. ‘Climax’.

Glyphosate-resistant giant ragweed has become an increasing problem, and the potential spread of these biotypes is a threat to production agriculture and to the long-term utility of glyphosate and glyphosate-resistant crops. The fate of glyphosate resistance in a giant ragweed population is dependent on the fitness of the resistant biotype. Our objective was to determine the fitness of glyphosate-resistant giant ragweed in the absence and presence of glyphosate. We compared the growth and seed production of glyphosate-resistant (GR) and glyphosate-susceptible (GS) giant ragweed under field conditions in the absence of glyphosate. A greenhouse study was also conducted to determine the survival of GR and GS giant ragweed and their open-pollinated progeny from the field study under glyphosate-induced selection pressure. In the absence of glyphosate, GR giant ragweed displayed rapid, early season growth, but 50 d after planting, the biotypes were similar in height, shoot weight, and leaf area. During reproduction, the GR biotype flowered earlier and produced 25% less seed than the GS biotype. In the presence of glyphosate, an outcrossing rate of 31% was detected between GR and GS biotypes because 61% of progeny were resistant to glyphosate at 840 g ae ha⁻¹. A second application 14 d later at 2,520 g ae ha⁻¹ completely removed the GS alleles from the population, whereas several homozygous and heterozygous GR plants survived and produced seed. These results indicate GR will persist in the population when subjected to glyphosate and that glyphosate resistance in giant ragweed has the potential to spread rapidly in our current agricultural ecosystem.

Nomenclature: Glyphosate; giant ragweed, Ambrosia trifida L.

Crowfootgrass, a C₄ species, is one of the principal weeds of dry-seeded rice in Asia. Weed management decisions for this species can be derived from knowledge of its seed germination biology. Experiments were conducted in the laboratory and screenhouse to determine the effects of light, alternating day/night temperatures, water stress, seed burial depth, and rice residue on seed germination and seedling emergence of crowfootgrass and to evaluate the response of this weed to commonly available selective POST herbicides in the Philippines. Light stimulated seed germination, but it was not an absolute requirement for germination. Germination in the light/dark regime was greater at alternating day/night temperatures of 25/15 C (92%) than at 30/20 (70%) or 35/25 C (44%). The osmotic potential required for 50% inhibition of maximum germination was −0.23 MPa, although some seeds germinated at −0.6 MPa. Seedling emergence was greatest for the seeds placed on the soil surface (64%), and emergence declined with increased burial depth in soil. No seedlings emerged from a burial depth of 6 cm or greater. Seedling emergence of crowfootgrass was reduced by the addition of rice residue to the soil surface at rates equivalent to 4 to 6 Mg ha⁻¹. Fenoxaprop-p-ethyl ethoxysulfuron at 45 g ai ha⁻¹ provided excellent control of crowfootgrass when applied at the four- (99%) and six-leaf (86%) stage. The information gained from this study could contribute to developing components of integrated weed management strategies for crowfootgrass. Soil inversion by tillage to bury weed seeds below their maximum depth of emergence, use of crop residue as mulch, and early application of an effective POST herbicide could serve as important tools for managing crowfootgrass.

Nomenclature: Bispyribac-sodium; fenoxaprop-p-ethyl ethoxysulfuron; penoxsulam cyhalofop; crowfootgrass, Dactyloctenium aegyptium (L.) Willd. DTTAE; rice, Oryza sativa L.

Weed control in lentil is difficult because lentil is a poor competitor with weeds and few POST broadleaf herbicides are available. Imadazolinone-tolerant lentils have more herbicide options, but the optimum timing for herbicide application is not known. The critical period of weed control (CPWC) is the period in a crop's life cycle when weeds must be controlled in order to prevent yield loss. The objective of this research was to determine the CPWC for lentil. We made lentil remain weedy or weed-free from 0 to 11 aboveground nodes to investigate the durations of weed interference and weed-free period, respectively. It was found that lentil has a CPWC beginning at the five-node stage and continuing to the 10-node stage. There was an inverse relationship between weed biomass and lentil yield; that is, lentil yield was highest when weed biomass is minimal. We propose that the CPWC begins when weeds start to accumulate significant biomass and ends with crop canopy closure. Therefore, to maximize lentil yields, growers should consider using a POST residual herbicide that can control weeds during the CPWC.

Nomenclature: Lentil, Lens culinaris L. ‘CDC Impact’.

Natalgrass is an invasive species that has become increasingly problematic in natural areas in Florida and other subtropical and tropical regions around the world. Natalgrass is a prolific seed producer, but little information is available regarding its seed biology and ecology. Research was conducted to determine levels of seed dormancy and to examine the effects of light, temperature, pH, water stress, and depth of burial on natalgrass seed germination. In addition, seed persistence under field conditions was examined both on the soil surface and while buried. Seeds appeared to undergo afterripening. Seed germination was not light dependent and occurred from 15 to 35 C, with optimum germination occurring at 20 to 35 C. Germination occurred at pH levels of 6 and 8 and was affected by water stress; no germination was observed at osmotic potentials less than −0.2 MPa. Seeds emerged from depths of at least 5 cm. Under field conditions, germination was reduced after burial; however, burial lengths of 3 to 15 mo did not result in differences in germination levels. Seedling numbers from seed deposits on the soil surface were greatly reduced after 1 mo, and no seedling emergence was observed after 4 mo.

Nomenclature: Glyphosate; natalgrass, Melinis repens (Willd.) Zizka RHYRE, formerly Rhynchelytrum repens (Willd.) C.E. Hubb.).

Spreading dogbane is an important weed of wild blueberry fields that decreases yields and hinders harvest operations. A range of experiments was conducted to evaluate the impact of abiotic factors on dogbane seed germination. Freshly harvested seeds were largely nondormant with viability ranging between 67 and 84%. Prolonged exposure to light neither promoted nor inhibited germination. Germination rates and total seed germination varied with temperature and osmotic potential. Significantly fewer seeds germinated at 5 C compared with 10, 15, and 20 C. There was a significant quadratic relationship between dogbane germination and osmotic potential, with significant numbers of seeds germinating at levels as low as −0.5 MPa. Emergence rates declined exponentially with depth in the soil and as many as 9% of seeds germinated but were unable to reach the soil surface. Results indicate that substantial seed germination in blueberry fields is possible and primary dispersal without wind occurs over a very short distance.

Nomenclature: Spreading dogbane, Apocynum androsaemifolium L.; wild blueberry, Vaccinium angustifolium Ait.

We used the process-oriented niche model CLIMEX to estimate the potential global distribution of serrated tussock under projected future climates. Serrated tussock is a drought-tolerant, wind- and human-dispersed grass of South American origin that has invaded pastures in Australia, Europe, New Zealand, and South Africa. The likely effect of climate change on its potential global distribution was assessed by applying six climate-change scenarios to a previously developed model. The projections of climatic suitability under the current climate revealed considerable scope for spread, with the most suitable areas occurring adjacent to existing naturalized populations in Australia, New Zealand, and Western Europe. Under future climates, projected to the 2080s, the land area suitable for serrated tussock contracts globally between 20 and 27%. Changes in projected potential area under the six scenarios were very similar in all geographical regions apart from North America and New Zealand, where the projections range from little change or contraction under the National Center for Atmospheric Research (NCAR) and Centre for Climate Research (MIROC) global climate models (GCMs) to expansion under the Commonwealth Scientific and Industrial Research Organisation (CSIRO) GCM. Elsewhere, contractions occur in Australia, Asia, South America, and Africa under all six future climate scenarios. By contrast, for Europe, the area climatically suitable for serrated tussock increases under all six scenarios (average increase 47%) through expansions into eastern European countries that are currently unsuitable and through increases in the suitable area in England, Ireland, and Denmark. Since pastoralism is a dominant land use in these regions of Europe, a prudent biosecurity strategy would be to contain the nascent foci of serrated tussock in southern France, along the west coast of Italy, and in the United Kingdom. This strategy could consist of a set of policies to limit human-assisted dispersal of the species' seeds and to reduce wind-borne spread through cultural control of the plant.

Nomenclature: Serrated tussock, Nassella trichotoma (Nees) Hack. ex Arechav.

Weed control in organic onion production is often difficult and expensive, requiring numerous cultivations and extensive hand weeding. Onion safety and weed control with mustard seed meal (MSM) derived from Sinapis alba was evaluated in greenhouse and field trials. MSM applied at 110, 220, and 440 g m⁻² severely injured onions and reduced onion stand by 25% or more when applied from planting to the one-leaf stage of onions in greenhouse trials. MSM derived from mustard cultivars ‘IdaGold’ and ‘AC Pennant’ reduced plant dry weight of redroot pigweed with an effective dose that provided 90% weed control (ED₉₀) of 14.5 and 3.2 g m⁻², respectively, in greenhouse trials, whereas the ED₉₀ of MSM from a low-glucosinolate cultivar ‘00RN29D10’ was 128 g m⁻², suggesting that glucosinolate content and ionic thiocyanate (SCN⁻) production contribute to phytotoxicity of MSM. In field trials, weed emergence, onion injury, and onion yield were recorded following single or three sequential applications of MSM from 1.1 to 4.5 MT ha⁻¹ beginning at the two-leaf stage of onions in 2008, 2009, and 2010. By 8 wk after treatment (WAT), onion injury following MSM sequential applications was 10% or less in all 3 yr. Combined over 2008 and 2009, 48 and 68% fewer weeds emerged 3 WAT with MSM at 2.2 and 4.5 MT ha⁻¹, respectively. In 2010, MSM at 2.2 and 4.5 MT ha⁻¹ reduced the number of weeds emerged 4 WAT by 91 and 76%, respectively. MSM treatment did not significantly affect onion yield or size in 2008 and 2009, but in 2010 onion total yield was reduced by 29% by three sequential applications of MSM at 2.2 MT ha⁻¹. MSM has potential to be used as a weed-suppressive amendment in organic production systems, but the risk of crop injury is substantial.

Nomenclature: Redroot pigweed, Amaranthus retroflexus L.; white mustard, Sinapis alba L.; onion, Allium cepa L.

Agricultural production systems that reduce the use of in-crop herbicides could greatly reduce risks of environmental damage and the development of herbicide-resistant weeds. Few studies have investigated the long-term effects of in-crop herbicide omissions on weed seedbank community size and structure. A crop-rotation study was sampled 10 yr after a strictly annual rotation and an annual/perennial rotation were exposed to different in-crop herbicide omission treatments. In-crop herbicides were applied either in all annual crops (control), omitted from oats only, or omitted from both flax and oats. Seedbank densities were greatest when in-crop herbicides were omitted from flax and oats, and this treatment also reduced crop yield. Shannon-Wiener diversity differed among crops in the annual crop rotation and among herbicide omission treatments in the perennial rotation. Herbicide omissions changed the weed-community structure in flax and in wheat and canola crops in the annual rotation enough to warrant alternate control methods in some treatments. The magnitude of the effects on the seedbank parameters depended largely on the competitive ability of the crop in which herbicides were omitted. No yield response to omitting herbicides in oats indicated that standard weed management practices have reduced weed populations below yield-loss thresholds.

Nomenclature: Canola, Brassica napus L. BRSNN; oats, Avena sativa L. AVESA; flax, Linum usitatissimum L. LIUUS; wheat, Triticum aestivum L. TRZAX.

Goosegrass is a problematic summer annual weed in cotton, soybean, and corn production in the southern United States. Glyphosate is labeled for POST control of goosegrass in glyphosate-resistant (GR) cotton, soybean, and corn production. A population of goosegrass in west Tennessee not controlled by glyphosate was examined in greenhouse and laboratory studies. At 21 days after treatment (DAT), a glyphosate-susceptible (SS) biotype was controlled > 90% with glyphosate at rates greater than 210 g ae ha⁻¹. Comparatively, the GR biotype was only controlled 12% at 210 g ae ha⁻¹. Using goosegrass control data, I₅₀ values for GR and SS biotypes were 868 and 117 g ae ha⁻¹, susceptibility, resulting in a resistance factor (RF) of 7.4. Treatment with glyphosate at 210 g ae ha⁻¹ reduced fresh weight biomass of the SS biotype to 5 g per pot compared to 36 g for the GR biotype. A total of 3,360 g ae ha⁻¹ glyphosate was required to reduce fresh weights of the GR biotype to ∼5 g per pot. Using fresh and dry weight biomass data, I₅₀ values for the GR biotype were 3 to 10 times greater than the SS biotype. On each date from 1 to 6 DAT the SS biotype accumulated higher concentrations of shikimate than the GR biotype. Future research should evaluate strategies for managing GR goosegrass with alternative modes of action. To prevent the spread of resistance, additional research evaluating programs for managing glyphosate-susceptible goosegrass in GR crops is also warranted.

Nomenclature: Corn, Zea mays L.; cotton, Gossypium hirsutum L.; goosegrass, Eleusine indica (L). Gaertn.; soybean, Glycine max L.

Field and pot studies were conducted in Central New York to determine the potential weed-management benefits of a buckwheat cover crop grown before winter wheat. Specific objectives were to determine buckwheat residue effects on (1) emergence and growth of winter annual weeds; (2) wheat establishment and yield; and (3) emergence of summer annual weeds in the spring following overwinter seed burial. In a field study, buckwheat was sown at two timings (July or August), mowed, and either incorporated or left on the soil surface. Winter wheat was drilled into buckwheat residue in September and weed and crop growth were monitored. In a complementary pot study, four winter annual weeds were sown in soil removed from buckwheat and bare-soil plots at 0 or 15 d after incorporation and monitored for emergence and early growth. To assess buckwheat residue effects on spring emergence from overwintering seeds, seeds of three weed species were buried in buckwheat residue and bare-soil plots in the fall, exhumed in April, and evaluated for emergence. To investigate the mechanism for possible effects of buckwheat residue on overwintering seeds, two levels each of seed treatment (none or fungicide) and fertilization (none or 170 kg ha⁻¹) were applied before burial. Buckwheat residue had no negative effect on wheat yields but suppressed emergence (22 to 72%) and growth (0 to 95%) of winter annual weeds, although effects were often small and inconsistent. Buckwheat residue had no effect on the emergence of buried weed seeds in spring. However, fungicide treatment enhanced the emergence of Powell amaranth seeds by 12.5 to 25.5% and of barnyardgrass seeds by 0 to 12%. Our results suggest that buckwheat residue can contribute to weed management in wheat cropping systems, but that further studies investigating the mechanistic basis for the inconsistent selective effects of buckwheat residue on weeds are needed before buckwheat use can be optimized.

Nomenclature: Powell amaranth  =  green pigweed, Amaranthus powellii S. Wats. AMAPO; hairy galinsoga, Galinsoga ciliata (Raf.) Blake GASCI; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; common chickweed, Stellaria media (L.) Vill. STEME; yellow rocket, Barbarea vulgaris Ait. f. BARVU; corn chamomile, Anthemis arvensis L. ANTAR; shepherd's-purse, Capsella bursa-pastoris (L.) Medik. CAPBP; winter wheat, Triticum aestivum L.; buckwheat, Fagopyrum esculentum Moench.

Field studies were conducted in Knoxville, TN, over a 2-yr period (2007 and 2008) to determine the field dissipation rate and efficacy of pyroxasulfone, acetochlor, dimethenamid, and s-metolachlor to broadleaf signalgrass. Depending on rainfall patterns, pyroxasulfone at 209 g ai ha⁻¹ provided broadleaf signalgrass control of > 75%, which was equal to or superior to acetochlor at 1,740 g ai ha⁻¹, dimethenamid at 1,500 g ai ha⁻¹ and s-metolachlor at 1,420 g ai ha⁻¹. Pyroxasulfone provided residual control into the growing season and provides a tool for resistance management of later-emerging weeds. Herbicide dissipation was rapid in all soils (half-life usually < 20 d), although it was slower in a dry year. The order of herbicide dissipation and half-life in days in the 2 yr was acetochlor (3.5, 5 d) > dimethenamid (5, 9 d) > s-metolachlor (8.8, 27 d) > pyroxasulfone (8.2, > 71 d). There was poor correlation between observed weed control at 45 d after treatment and chemically determined herbicide concentrations at that same time, with ∼ 40% difference in 2007 and ∼ 50% difference in 2008.

Nomenclature: Acetochlor; dimethenamid; pyroxasulfone, s-metolachlor; broadleaf signalgrass, Urochloa platyphylla (Nash) R. D. Webster.

In the Midwestern United States, winter hilling, consisting of two tillage activities per year, is required in vinifera-grape vineyards for winter protection. However, this practice often leads to severe soil erosion and pesticide offsite movement. The effectiveness of wheat straw mulch as a replacement for soil mounding was investigated as a way of providing winter protection and to mitigate pesticide leaching and runoff. A laboratory experiment was conducted where simazine was applied to wheat straw or bare soil and then followed by simulated rainfalls. When compared with bare soil, straw reduced simazine leaching and runoff by 40 and 68%, respectively. Adsorption or interception, or both, of simazine by straw were responsible for this effect. Additionally, straw reduced soil erosion by 95% and would largely reduce simazine runoff associated with sediment displacement. The first simulated rainfall contributed 70 and 34% of total simazine runoff from bare soil and straw, respectively. In conclusion, mulching with straw during winter months to provide winter protection could be an effective practice for controlling simazine offsite movement and soil erosion in vinifera vineyards.

Nomenclature: Simazine; grape, Vitis vinifera L.; wheat, Triticum spp.

Assessing belowground plant interference in rice has been difficult in the past because intertwined weed and crop roots cannot be readily separated. A ¹³C discrimination method has been developed to assess distribution of intermixed roots of barnyardgrass and rice in field soils, but the suitability of this approach for other rice weeds is not known. ¹³C depletion levels in roots and leaves of rice were compared with those of 10 troublesome weed species grown in monoculture in the greenhouse or field. Included were C₄ tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf signalgrass, fall panicum, and large crabgrass; C₄ sedge, yellow nutsedge; and C₃ species: red rice, gooseweed, and redstem. Rice root δ¹³C levels averaged ∼ −28‰, indicating that these roots are highly ¹³C-depleted. Root δ¹³C levels ranged from −12‰ to −17‰ among the tropical grasses, and were −10‰ in yellow nutsedge, indicating that these species were less ¹³C depleted than rice, and were C₄ plants suitable for ¹³C discrimination studies with rice. Among the C₄ species, bearded sprangletop and yellow nutsedge were most and least ¹³C depleted, respectively. δ¹³C levels in shoot and root tissue of pot-grown plants averaged 6% greater for C₄ plants and 9% greater for rice in the field than in the greenhouse. In pots, shoots of rice typically were slightly more ¹³C depleted than roots. A reverse trend was seen in most C₄ species, particularly for broadleaf signalgrass and plants sampled from field plots. Corrections derived from inputs including the total mass, carbon mass, carbon fraction, and δ¹³C levels of roots and soil increased greatly the accuracy of root mass estimates and increased slightly the accuracy of root δ¹³C estimates (∼ 0.6 to 0.9%) in samples containing soil. Similar corrective equations were derived for mixtures of rice and C₄ weed roots and soil, and are proposed as a labor-saving option in ¹³C discrimination root studies.

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv.; bearded sprangletop, Leptochloa fusca (L.) Kunth var. fascicularis (Lam.) N. Snow; Amazon sprangletop, Leptochloa panicoides (J. Presl) A. S. Hitchc.; broadleaf signalgrass, Urochloa platyphylla (Nash) R. D. Webster; fall panicum, Panicum dichotomiflorum Michx.; large crabgrass, Digitaria sanguinalis (L.) Scop.; yellow nutsedge, Cyperus esculentus L.; gooseweed, Sphenoclea zeylanica Gaertn.; redstem, Ammannia coccinea Rottb.; red rice, Oryza sativa L.; rice, Oryza sativa L.

Although foliar herbicide absorption has been studied intensively, there is currently no standardized method for data analysis when evaluating herbicide absorption over time. Most peer-reviewed journals require the treatment structure of data be incorporated in the analysis; however, many herbicide absorption studies published in the past 5 yr do not account for the time structure of the experiment. Herbicide absorption studies have been presented in a variety of ways, making it difficult to compare results among studies. The objective of this article is to propose possible nonlinear models to analyze herbicide absorption data and to provide a stepwise framework so that researchers may standardize the analysis method in this important research area. Asymptotic regression and rectangular hyperbolic models with similar parameterizations are proposed, so that the maximum herbicide absorption and absorption rate may be adequately modeled and statistically compared among treatments. Adoption of these models for herbicide absorption analysis over time will provide a standardized method making comparison of results within and among studies more practical.