There is an ever-larger need for designing an integrated weed management (IWM) program largely because of the increase in glyphosate-resistant weeds, not only in the United States but also worldwide. An IWM program involves a combination of various methods (cultural, mechanical, biological, genetic, and chemical) for effective and economical weed control (Swanton and Weise 1991). One of the first steps in designing an IWM program is to identify the critical period for weed control (CPWC), defined as a period in the crop growth cycle during which weeds must be controlled to prevent crop yield losses (Zimdahl 1988).

Imazamox-resistant wheat varieties carry the Imi1 allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F1) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of the Imi1 allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of the Imi1 allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for the Imi1 alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that the Imi1 allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.

Spotted spurge, a C4 species, is a summer annual weed, introduced to the Golestan province of Iran in 2006. A series of laboratory experiments were conducted at the Faculty of Agriculture, Ferdowsi University of Mashhad, Iran, to evaluate the influence of environmental factors on seed germination of spotted spurge. Seeds germinated over a constant temperature range of 25 to 45 C with a 14-h photoperiod and at 30 to 50 C in continuous darkness. Seeds germinated at alternating day/night temperature regimes of 25/15 to 45/35 C, with maximum germination (> 80%) at alternating day/night temperatures of 35/25, 40/30, and 45/35C. Germination increased from 23.5 to 98% as water potential increased from −0.6 to 0 MPa (control). Germination was > 85% at sodium chloride (NaCl) concentrations up to 80 mM, with no germination at 160 mM NaCl. Germination was not affected by pH, and it was > more than 95% at pH ranging from 4 to 9. The germination of seeds submerged in water for a period less than 3 wk was > 60%, whereas no germination was observed after 9 wk of submergence in water. The results of our study could help to develop effective management strategies for this species. The results also suggest that spotted spurge could invade most tropical regions of Iran.

The influence of environmental factors on germination and emergence of aryloxyphenoxy propanoate herbicide-resistant (AR) and -susceptible (AS) Asia Minor bluegrass were studied in laboratory and greenhouse experiments. Seeds were collected from AR and AS plants cultivated in separate greenhouses under the same environmental conditions. The results revealed that optimum temperatures for the germination of AS biotype were 10 to 25 C or alternating temperature of 15/5 to 30/20 C and light was not necessary. However, maximum germination occurred at 10 C or 15/5 C, and no germination occurred above 15 C or 25/15 C for the AR biotype. The AS Asia Minor bluegrass was consistently more tolerant to environmental stress, as evidenced by their greater germination at same pH value, osmotic potential, and NaCl concentration at 15/5 C compared to the AR biotype. Higher emergence rates were obtained when seeds were sown on the surface of soil for both biotypes. Emergence percentage of the AR biotype was below 14% when buried, whereas the AS biotype had 20% emergence at 2.5 cm burial depth. It is concluded that several environmental factors affect the germination of Asia Minor bluegrass, and the AS biotype showed higher germination percentage and a wider adaptive range under same treatments compared with the AR biotype. Due to the reduced emergence at depth, deep tillage could be an effective management to reduce AR Asia Minor bluegrass infestation in the following crop.

Weeds pose a great problem to farmers worldwide, and controlling weeds demands a high input cost for herbicides and labor. Because of current environmental regulations, a limited number of herbicides are commercially available (with limited modes of action) to control weeds. Smoke water and the biologically active compounds isolated from smoke affect seed germination in a significant way. Smoke water (SW) and karrikinolide (KAR1, the germination stimulant isolated from smoke) have been tested extensively for their ability to promote seed germination in a vast array of plant species. In addition to KAR1, a germination inhibitor, trimethylbutenolide (TMB), was also isolated from plant-derived smoke. The effects of SW, KAR1, and TMB were tested on five major weed species of South Africa: fleabane, hairy wild lettuce, bugweed, spilanthes, and fameflower. Seeds of these weed species were subjected to 16/8 h light/dark conditions or to constant dark conditions at constant temperatures of 20, 25, 30 C and alternating 30/20 C. SW and KAR1 significantly increased germination, whereas TMB significantly inhibited germination of these weed species. Furthermore, TMB treatment reduced the amylase activity of the tested weed seeds compared with the water control. These results indicate the possibility of manipulating germination of certain weed seeds by SW, KAR1, and TMB. Thus, smoke and smoke-isolated compounds could potentially be used in new weed management strategies.

Annual bluegrass resistance to inhibitors of acetolactate synthase (ALS) and photosystem II (PSII) in managed turf has been confirmed in the southeastern United States. A biotype of annual bluegrass that had developed resistance (R) to the PSII inhibitor simazine was not controlled by POST applications of foramsulfuron or trifloxysulfuron in 2011 or 2012. In whole plant dose-response experiments, trifloxysulfuron, simazine, and indaziflam controlled a susceptible (S) population of annual bluegrass > 91% when applied POST to nontillering plants. However, trifloxysulfuron applications at 3.5 to 223 g ai ha−1 only controlled R annual bluegrass ≤ 40%. Similarly, simazine at 140 to 9,000 g ai ha−1 only controlled R annual bluegrass ≤ 20%. R annual bluegrass plants were more tolerant to indaziflam applied POST to leaf stage plants prior to tillering, as rates > 100 g ai ha−1 were needed to control R annual bluegrass ≥ 96%. No differences in the activity of ALS in R and S plants exposed to increasing foramsulfuron concentrations from 0 to 100 µM were detected suggesting that nontarget mechanisms could explain reduced efficacy of POST herbicide applications in whole plant dose-response experiments. Applications of indaziflam (35 to 70 g ha−1) and oxadiazon (2,240 to 4,500 g ai ha−1) effectively controlled R annual bluegrass when applied PRE. This biotype of R annual bluegrass is the first reported instance of a weed developing resistance to multiple modes of action in managed turf. Education is needed among turf managers regarding the consequences of exclusive use of the same herbicides for annual bluegrass control leading to the onset of herbicide resistance.

Weeds represent a major cause of agricultural losses worldwide. Most weeds share a common set of life history characteristics that predispose them to weediness, two of which are self-compatibility, which allows for ease of colonization through reproductive assurance, and high trait plasticity, which allows for tolerance of a wide variety of environments and abiotic conditions. However, self-fertilization typically comes at the cost of inbreeding depression. This study investigates the role of inbreeding depression and trait plasticity under abiotic stress in the tall morningglory, a widespread self-compatible agricultural weed in the southeastern United States. Results show very little inbreeding depression in this species, likely due to purging of deleterious alleles through repeated founder events in agricultural landscapes. In contrast, abiotic stress induced substantial plasticity in ecophysiological traits, reproductive traits, and biomass allocation. In terms of performance, drought sharply impacted reproduction but not vegetative growth, and nitrogen limitation sharply impacted both. These findings are applicable to the control of weedy morningglory and underscore the usefulness of evolutionary ecology to weed management.

Waterhemp resistance to foliar applications of protoporphyrinogen oxidase (PPO)–inhibiting herbicides has become increasingly disconcerting given the widespread distribution of glyphosate resistance. Fortunately, soil-residual PPO-inhibiting herbicides remain efficacious in waterhemp populations resistant to PPO-inhibiting herbicides; however, these herbicides should theoretically select for the resistant biotype as herbicide concentrations diminish in the soil. Accordingly, the objectives of this research were twofold: (1) evaluate the efficacy of three PPO-inhibiting herbicides, foliar- and soil-applied, on PPO-resistant (PPO-R) and PPO-susceptible (PPO-S) waterhemp, and (2) investigate the differential effects of PPO-inhibiting herbicides on an R biotype and an S biotype during several discrete developmental events relevant to soil–residual herbicide activity (i.e., radicle protrusion, radicle elongation, and waterhemp emergence). Greenhouse and growth chamber experiments indicated that the R biotype was least sensitive to the diphenylether herbicide fomesafen, followed by sulfentrazone and flumioxazin; however, fomesafen plus s-metolachlor improved soil-residual efficacy over fomesafen alone. Growth stage considerably influenced the R : S ratio, decreasing from 38× to 3.4×, when comparing ratios generated from foliar applications and soil-residual applications measuring radicle protrusion, respectively. Overall, this research supports the use of full soil-residual herbicide rates, reinforcing the importance of best management practices to manage the spread of herbicide resistance.

RIM, or “Ryegrass Integrated Management,” is a model-based software allowing users to conveniently test and compare the long-term performance and profitability of numerous ryegrass control options used in Australian cropping systems. As a user-friendly decision support system that can be used by farmers, advisers, and industry professionals, RIM can aid the delivery of key recommendations among the agricultural community for broadacre cropping systems threatened by herbicide resistance. This paper provides advanced users and future developers with the keys to modify the latest version of RIM in order to facilitate future updates, modifications, and adaptations to other situations. The various components of RIM are mapped and explained, and the key principles underlying the construction of the model are explained. The implementation of RIM into a Microsoft Excel® software format is also documented, with details on how user inputs are coded and parameterized. An overview of the biological, agronomic, and economic components of the model is provided, with emphasis on the ryegrass biological characteristics most critical for its effective management. The extreme variability of these parameters and the subsequent limits of RIM are discussed. The necessary compromises were achieved by emphasizing the primary end-use of the program as a decision support system for farmers and advisors.

Since the beginning of agriculture, crops have been exposed to recurrent invasion by weeds that can impose severe reductions in crop quality and yield. There have been continuing efforts to reduce the impacts of weeds on production. More than 40 yr ago, overreliance on herbicide technology to reduce weed infestations resulted in the selection of adaptive traits that enabled weed survival and reproduction under herbicide treatments (Délye et al. 2007; Powles and Yu 2010; Vila-Aiub et al. 2008). As a result, herbicide resistance in > 200 weed species has evolved worldwide (Heap 2013; Powles 2008).

Crop rotation promotes productivity, nutrient cycling, and effective pest management. However, in row-crop systems, rotation is frequently limited to two crops. Adding a third crop, especially a perennial crop, might increase crop-rotation benefits, but concerns about disruption of agricultural and ecological processes preclude grower adoption of a three-crop rotation. The objective of the present research was to determine whether weed seed banks differ between a sod-based rotation (bahiagrass–bahiagrass–peanut–cotton) and a conventional peanut–cotton rotation (peanut–cotton–cotton) and the importance of crop phase in weed seed-bank dynamics in a long-term experiment initiated in 1999 in Florida. Extractable (ESB) and germinable (GSB) seed banks were evaluated at the end of each crop phase in 2012 and 2013, and total weed seed or seedling number, Shannon-Weiner's diversity (H′), richness, and evenness were determined. ESB increased in H′ (36%), richness (29%), and total number of weed seeds (40%) for sod-based compared with conventional rotation, whereas GSB increased 32% in H′, 27% in richness, and 177% in total number of weed seedlings. Crop phase was a determinant factor in the differences between crop rotations. The first year of bahiagrass (B1) exhibited increases in weed seed and seedling number, H′, and richness and had the highest values observed in the sod-based rotation. These increases were transient, and in the second year of bahiagrass (B2), weed numbers and H′ decreased and reached levels equivalent to those in the conventional peanut–cotton rotation. The B1 phase increased the germinable fraction of the seed bank, compared with the other crop phases, but not the total number of weed seeds as determined by ESB. The increases in H′ and richness in bahiagrass phases were mainly due to grass weed species. However, these grass weed species were not associated with peanut and cotton phases of the sod-based rotation. The results of the present study demonstrated that including bahiagrass as a third crop in a peanut–cotton rotation could increase weed community diversity, mainly by favoring increases in richness and diversity, but the structure and characteristics of the rotation would prevent continuous increases in the weed seed bank that could affect the peanut and cotton phases.

Knowledge of economic threshold level and growth habits of weeds in chickpea is essential to implement timely, effective, and economical weed control treatments. The effect of weed density on growth and yield performance in chickpea using dragon spurge as a test weed was investigated for 2 consecutive yr. Dragon spurge density levels of 5, 10, 15, 20, 25, 30, 35, and 40 plants m−2 were compared with weed-free plots. Each increment in densities of dragon spurge from 5 to 40 plants m−2 reduced plant height, number of fruits, seeds, and dry weight per plant. Chickpea grain yield losses varied between 1 to 63% with dragon spurge density ranging from 5 to 40 plants m−2. The seed protein content of chickpea was found to be significantly reduced above a weed density of 25 plants m−2. Maximum N, P and K uptake by dragon spurge was 1,520, 1,020, and 7,350 mg m−2, respectively. Thus, dragon spurge should be controlled at densities above 5 plants m−2 to achieve optimum chickpea yield.

Buckwheat is a broadleaved annual species that is often used as a summer cover crop for its quick growth, weed suppressive ability, and ease of management. Tartary buckwheat is a species related to buckwheat, with many of the same traits valued in buckwheat as a cover crop. However, Tartary buckwheat has been reported to grow more vigorously than buckwheat, especially in cool conditions, which might fill a unique niche for vegetable farmers in Wisconsin and other northcentral states. Our research objectives were to determine the effectiveness of Tartary buckwheat relative to buckwheat for weed suppression, both during the cover-cropping phase and after cover-crop termination during cabbage production, and quantify weed suppression, soil compaction, soil nitrogen availability, and cabbage yield in no-tillage (roller-crimped or sickle-bar mowed) and conventional-tillage (rototilled) systems. Across three site-years, we found that buckwheat emerged earlier and produced 64% more shoot dry biomass than Tartary buckwheat. Pretermination weed shoot biomass (predominantly Amaranthus and Setaria spp.) in Tartary buckwheat treatments was approximately twice that of buckwheat, and did not differ from weed shoot biomass in a control fallow treatment. Cabbage yield did not differ between cover crop species nor did yield differ between conventional-tillage cover cropped and control fallow treatments. However, weed biomass was greater, and cabbage yield was reduced, in no-tillage compared to conventional-tillage treatments. We also found evidence of greater soil compaction and less nitrate–nitrogen (NO3–N) availability in no-tillage than conventional-tillage treatments. These results suggest that Tartary buckwheat is not a suitable summer cover crop alternative to buckwheat for weed suppression prior to cabbage production.

Saflufenacil/dimethenamid-P is a relatively new prepackaged herbicide mixture that has the potential to provide enhanced weed control in soybean when tank-mixed with reduced doses of imazethapyr. Six field experiments were conducted over a 3-yr period (2011, 2012, and 2013) near Ridgetown and Exeter, Ontario, Canada, to determine the dose of imazethapyr, applied PRE, that must be added to saflufenacil/dimethenamid-P (245 g ai ha−1) to provide effective weed control in soybean. The predicted dose of imazethapyr PRE for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf 8 wk after soybean emergence (WAE) was 66, 180, 137, and 48 g ai ha−1, respectively. In contrast, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1), the dose of imazethapyr PRE needed for 80% control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was reduced to 11, 80, 48, and 18 g ha−1, respectively. The control of common lambsquarters, common ragweed, green foxtail, and velvetleaf was improved by 21, 23, 34, and 27%, respectively when saflufenacil/dimethenamid-P (245 g ha−1) was added to imazethapyr PRE. Imazethapyr at 104 g ha−1 resulted in soybean yield that was 95% of the weed-free control; however, when tank-mixed with saflufenacil/dimethenamid-P (245 g ha−1) only 54 g ha−1 of imazethapyr was required for the same yield level. Based on this study, PRE application of saflufenacil/dimethenamid-P with reduced doses of imazethapyr has the potential to improve soybean yield and provide acceptable weed control (≥ 80%); however, the extent that imazethapyr dose can be reduced is dependent upon weed community composition.

Substantial resources are spent each year on weed control, but in many cases eradication projects are incomplete. Here we used the computer program NEWGARDEN to model whether alternate geometric patterns of incomplete removal (99% removed) of the increasingly invasive Callery pear from an isolated fragment differentially affect the rate of population recovery and genetic diversity retention. Geometric patterns of remaining founders within the fragment (1% of the fragment area) included: (A) a long rectangular strip centered on one edge; (B) a square at one corner; (C) a central square; or (D) scattered randomly throughout the entire fragment. Population re-growth and genetic diversity retention measures for each geometric removal pattern were modeled under two contrasting gene dispersal patterns (via both offspring and pollen): short versus long dispersal (both leptokurtic relative to the pistillate plant). After 14 bouts of mating, the greatest difference in census size among comparative recovery populations amounted to 393% (centered founders, long gene dispersal > scattered founders, short gene dispersal). The best pattern of removal for suppressing population regrowth was to leave founders scattered throughout the fragment when gene dispersal was short, or at one corner if gene dispersal was long. The only removal pattern that differed substantially in population genetics characteristics was when remnant individuals were left scattered throughout the fragment and dispersal was short (alleles continued to be lost; observed heterozygosity dropped 13.3% and was still rapidly declining; and inbreeding and/or subdivision were moderate (Fit = 0.12) and still rapidly increasing). Such comparative modeling can be used to suggest removal patterns that might greatly outperform other removal modalities in terms of suppressing the return of weed populations. The effectiveness of such modeling will be improved by acquisition of accurate life history information of targeted species.

A survey of soybean fields containing waterhemp was conducted just prior to harvest in 2012 to determine the scope and extent of herbicide resistance and multiple herbicide resistances among a sample of Missouri waterhemp populations. Resistance was confirmed to glyphosate and to acetolactate synthase (ALS), protoporphyrinogen oxidase (PPO), photosystem II (PSII), and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, but not to 2,4-D. Of the 187 populations tested, 186 exhibited resistance to chlorimuron. The proportions of populations with atrazine or glyphosate resistance were similar, with 30 and 29% of the populations surviving the 3× rates. Lactofen resistance was observed in 5% of the populations, whereas mesotrione resistance was only found in 1.6% of the populations. All populations tested were susceptible to 2,4-D at the 3× rate. At least 52% of the waterhemp populations tested exhibited resistance to herbicides from two mechanism of action. Resistance to atrazine plus chlorimuron as well as glyphosate plus chlorimuron was present in 29% of the populations. Three-way resistance, primarily comprised of resistance to atrazine plus chlorimuron plus glyphosate, was present in 11% of the populations. Resistance to herbicides from four mechanisms of action was found in 2% of the populations, and one population exhibited resistance to herbicides from five mechanisms of action. DNA analysis of a subsample of plants revealed that previously documented mechanisms of resistance in waterhemp, including the ΔG210 deletion conferring PPO-inhibitor resistance, the Trp574Leu amino acid substitution conferring ALS-inhibitor resistance, and elevated 5-enolypyruvyl-shikimate-3-phosphate synthase copy number and the Pro106Ser amino acid substitution resulting in glyphosate resistance, explained survival in many, but not all, instances. Atrazine resistance was not explained by the Ser264Gly D1 protein substitution. Overall, results from these experiments indicate that Missouri soybean fields contain waterhemp populations with resistance to glyphosate, ALS-, PPO-, PSII-, and HPPD-inhibiting herbicides, which are some of the most common mechanisms of action currently utilized for the control of this species in corn and soybean production systems. Additionally, these results indicate that slightly more than half of the populations tested exhibit resistance to more than one herbicide mechanisms of action. Managing the current resistance levels in existing populations is of utmost importance. The use of multiple, effective herbicide modes of action, both preemergence and postemergence, and the integration of optimum cultural and mechanical control practices will be vital in the management of Missouri waterhemp populations in the future.

The extensive use of foliar-applied protoporphyrinogen oxidase (PPO)-inhibiting herbicides indisputably contributes to the continued selection of waterhemp resistant to PPO-inhibiting herbicides (PPO-R). However, the role of soil-residual applications of PPO-inhibiting herbicides in the selection of PPO-R has been clouded by the efficacy that these herbicides have on PPO-R waterhemp. The aim of the present study was to understand if soil-residual PPO-inhibiting herbicides have the potential to influence the proportion of resistant waterhemp in emerging plants as herbicide concentrations diminish in the soil. Greenhouse and field experiments were conducted in a PRE dose-response experiment testing fomesafen or fomesafen plus s-metolachlor in the presence of mixed seed populations of PPO-R and -susceptible (S) waterhemp. The first 20 (greenhouse) or 10 (field) waterhemp plants that survived the residual herbicide treatment were sampled for genotypic analysis to detect the presence of the allele responsible for PPO-R in waterhemp (ΔG210). Relative to the nontreated control, the highest rate of fomesafen increased the frequency of resistance (FOR) by 70% in the greenhouse experiments and 20% in the field experiments. The addition of s-metolachlor did not reduce the fomesafen-induced increase in the FOR in the surviving plants from field or greenhouse experiments. However, the additional herbicide from an alternate site of action (s-metolachlor) substantially improved soil-residual herbicide efficacy over fomesafen alone, which limited the number of waterhemp plants surviving the herbicide application. Thus, the selection for resistance can be delayed with the addition of a herbicide from an alternative site of action by postponing and/or reducing waterhemp emergence. These data strongly suggest that soil-residual PPO-inhibiting herbicides can influence the FOR in a field population, placing even greater importance on the implementation of best management practices such as full herbicide use rates and using herbicides from multiple sites of action to mitigate the risk of selecting for herbicide-resistant weed biotypes.

Crop and weed competition studies rarely determine how plant-to-plant interactions alter the structure and physiology of crop roots. Soybean has the ability to detect neighboring weeds and to alter growth patterns including the allocation of resources to root growth. In this study, we hypothesized that low red : far red light ratio (R : FR) reflected from aboveground vegetative tissue of neighboring weeds would alter soybean root morphology and reduce root biomass and nodule number. All experiments were conducted under controlled conditions in which resources of light, water, and nutrients were nonlimiting. Low R : FR reflected from aboveground neighboring weeds reduced soybean seedling root length, surface area, and volume, including the number of nodules per plant. An accumulation of H2O2, an increase in malondialdehyde (MDA) content, a reduction in flavonoid content, and a decrease in 1,1-diphenyl-2-picrylhydrazyl (DPPH)–radicle scavenging activity were observed. The reduction in flavonoid content was accompanied by a decrease in the transcription of GmIFS and GmN93 and an increase in transcript levels of several antioxidant genes. These molecular and physiological changes may have a physiological cost to the soybean plant, which may limit the plant's ability to respond to subsequent abiotic and biotic stresses that will occur under field conditions.

Common lespedeza is a problematic summer annual weed in bermudagrass lawns, with limited PRE herbicides available for control. Dithiopyr is a pyridine herbicide primarily used for PRE grassy weed control but has shown potential efficacy for controlling annual legumes. The objectives of this research were to evaluate efficacy and behavior of dithiopyr in common lespedeza. In a 3-yr field experiment, sequential dithiopyr applications at 0.42 or 0.56 kg ai ha−1 beginning in late winter and single applications of dithiopyr at 0.56 kg ai ha−1 in spring controlled common lespedeza ≥ 88%. Single and sequential applications of indaziflam at 0.035 and 0.053 kg ai ha−1 provided poor control (< 70%) of common lespedeza by late summer. In laboratory experiments, bermudagrass and common lespedeza had similar foliar absorption of 14C-dithiopyr, averaging 10% of the 14C applied, and both species retained > 80% of 14C in the treated leaf at 72 h after treatment (HAT). Common lespedeza translocated 6 times more root-absorbed 14C to shoots than bermudagrass and had 2.8 times greater absorption (Bq mg−1) at 72 HAT. In metabolism experiments, parent herbicide levels measured ≥ 84% of extracted 14C in both species at 1, 3, and 7 d after treatment. Overall, dithiopyr effectively controls common lespedeza in bermudagrass as a PRE treatment in spring. Susceptibility of common lespedeza to dithiopyr is associated with acropetal translocation and greater herbicide concentrations compared with a tolerant species, bermudagrass.

Cover crops are becoming increasingly common in cotton as a result of glyphosate-resistant Palmer amaranth; hence, a field experiment was conducted in 2009 and 2010 in Marianna, AR, with a rye cover crop used to determine its effects on the critical period for weed control in cotton. Throughout most of the growing season, weed biomass in the presence of a rye cover crop was lesser than that in the absence of a rye cover crop. In 2009, in weeks 2 through 7 after planting, weed biomass was reduced at least twofold in the presence of a rye cover compared with the absence of rye. In 2009, in both presence and absence of a rye cover crop, weed removal needed to begin before weed biomass was 150 g m−2, or approximately 4 wk after planting, to prevent yield loss > 5%. Weed density was less in 2010 than in 2009, so weed removal was not required until 7 wk after planting, at which point weed biomass values were 175 and 385 g m−2 in the presence and absence of a cover crop, respectively.