Herbicide-resistant (HR) crops are widely grown throughout the United States and Canada. These crop-trait technologies can enhance weed management and therefore can be an important component of integrated weed management (IWM) programs. Concomitantly, evolution of HR weed populations has become ubiquitous in agricultural areas where HR crops are grown. Nevertheless, crop cultivars with new or combined (stacked) HR traits continue to be developed and commercialized. This review, based on a symposium held at the Western Society of Weed Science annual meeting in 2021, examines the impact of HR crops on HR weed management in the U.S. Great Plains, U.S. Pacific Northwest, and the Canadian Prairies over the past 25 yr and their past and future contributions to IWM. We also provide an industry perspective on the future of HR crop development and the role of HR crops in resistance management. Expanded options for HR traits in both major and minor crops are expected. With proper stewardship, HR crops can reduce herbicide-use intensity and help reduce selection pressure on weed populations. However, their proper deployment in cropping systems must be carefully planned by considering a diverse crop rotation sequence with multiple HR and non-HR crops and maximizing crop competition to effectively manage HR weed populations. Based on past experiences in the cultivation of HR crops and associated herbicide use in the western United States and Canada, HR crops have been important determinants of both the selection and management of HR weeds.

Diversity is key for sustainable weed management and can be achieved via both chemical and nonchemical control tactics. Genetically modified crops with two-way or three-way stacked herbicide-tolerant traits allow use of herbicide mixtures that would otherwise be phytotoxic to the crop. Early weed management (EWM) strategies promote the use of PRE herbicides with residual activity to keep the field free of weeds early in the season for successful crop establishment. To evaluate the respective sustainability and practicality of the two chemical-based management tactics (i.e., stacked traits and EWM), we used a population model of waterhemp, Amaranthus tuberculatus (Moq.) Sauer (syn. rudis), to simulate the evolution of resistance in this key weed species in midwestern U.S. soybean [Glycine max (L.) Merr.] agroecosystems. The model tested scenarios with a varying number of herbicide sites of action (SOAs), application timings (PRE and POST), and preexisting levels of resistance. Results showed that both tactics provided opportunity for controlling resistant A. tuberculatus populations. In general, each pass over the field should include at least two effective herbicide SOAs. Nevertheless, the potential evolution of cross-resistance may void the weed control programs embraced by stacked traits and diverse herbicide SOAs. Economic calculations suggested that the diversified programs could double long-term profitability when compared to the conventional system, because of improved yield and grain quality. Ultimately, the essence of a sustainable herbicide resistance management strategy is to be proactive. Although a herbicide-dominated approach to diversifying weed management has been prevalent, the increasing presence of weed populations with multiple resistance means that finding herbicides to which weed populations are still susceptible is becoming increasingly difficult, and thus the importance of reintroducing cultural and mechanical practices to support herbicides must be recognized.

Pyrethroid and carbamate resistance was evaluated in Helicoverpa armigera from 2008 to 2015. Insects were collected as eggs primarily from cultivated hosts in the major cropping areas of New South Wales and Queensland, Australia. Larvae reared from eggs were tested for resistance to fenvalerate, bifenthrin or methomyl in the F0 generation using a topical application of a discriminating dose of insecticide. In 2008–2009, resistance to fenvalerate was 71% and no resistance to bifenthrin was recorded. In the following two seasons, resistance to pyrethroids was relatively stable with fenvalerate resistance ranging from 63% to 67% and bifenthrin resistance ranging from 5.6% and 6.4% in 2009–2010 and 2010–2011, respectively. However, in 2011–2012, pyrethroid resistance had increased to 91% and 36% for fenvalerate and bifenthrin, respectively. Resistance remained above 90% for fenvalerate and above 35% for bifenthrin in the following three seasons from 2012 to 2015. In 2008–2009, methomyl resistance was 33% and declined to 22% and 15% in 2009–2010 and 2010–2011, respectively. Methomyl resistance remained at moderate levels from 2011–12 to 2014–15, ranging from 21% to 40%. Factors that influenced selection pressure of pyrethroid and carbamate insecticides and impacted resistance frequency in H. armigera may have been associated with changes in the composition of the cropping landscape. The rapid expansion of the pulse industry and the commensurate increased use of insecticide may have played a role in reselection of high-level pyrethroid resistance, and highlights the need for an urgent and strategic response to insecticide resistance management in the Australian grains industry.

Field studies were conducted in 1996 and 1997 at Belleville and Pawnee, IL, to evaluate single and sequential applications of glufosinate on tolerance of glufosinate-resistant corn and annual weed control. Glufosinate caused 0 to 13% corn injury 7 days after treatment (DAT) and 0 to 6% corn injury 28 DAT. Injury was characterized as stunting with glufosinate. Glufosinate at 400 to 1,200 g ai/ha did not reduce final corn height or grain yield. At Pawnee in both years and at Belleville in 1996, a single application of glufosinate at 400 g/ha controlled giant foxtail, velvetleaf, ivyleaf morningglory, and common lambsquarters 85 to 100%. At Belleville in 1997, sequential applications of glufosinate provided greater weed control (87 to 100%) than a single application (0 to 63%) because of weed emergence after application. Weed control with a single application of glufosinate or with nicosulfuron plus bromoxynil was similar at both locations. Height and grain yield of glufosinate-resistant corn were not different from that of glufosinate-susceptible corn (isoline of glufosinate-resistant corn).

Studies were conducted to determine whether glufosinate treatments to glufosinate-resistant cotton caused changes in floral morphology, pollen viability, and seed set. Four glufosinate treatments were included: (1) glufosinate applied postemergence over the top (POST) at the four-leaf stage, (2) glufosinate applied POST at the eight-leaf stage, (3) the first two treatments sequentially, and (4) a POST application at the four-leaf stage followed by (fb) a postemergence-directed stem application (PDS) at the eight-leaf stage. Glufosinate was consistently applied at 0.49 kg ai/ha. A nontreated control was included. Glufosinate treatments did not affect stigma height, length of the staminal column, or pollen viability. However, the distance from the top anther to the tip of the stigma was less in plants treated with an eight-leaf POST treatment than in nontreated plants, although this difference is not likely to influence pollen deposition because in both cases anthers reached above the stigma tip. Plants receiving four-leaf POST fb eight-leaf PDS treatment with glufosinate had eight seeds per boll less than nontreated plants; however, the more rigorous four-leaf POST fb eight-leaf POST treatment did not differ from the nontreated in seeds per boll.

Glyphosate treatments to glyphosate-resistant (GR) cotton have been associated with poor pollination and increased boll abortion. Anatomical studies were conducted to characterize the effect of glyphosate treatments on the development of male and female reproductive organs of cotton flowers at anthesis. In comparison with nontreated plants, glyphosate applied at both the four-leaf stage postemergence (POST) and at the eight-leaf stage POST directed inhibited the elongation of the staminal column and filament, which increased the distance from the anthers to the receptive stigma tip by 4.9 to 5.7 mm during the first week of flowering. The increased distance from the anthers to the stigma resulted in 42% less pollen deposited on stigmas of glyphosate-treated plants than in nontreated plants. Moreover, pollen from glyphosate-treated plants showed numerous morphological abnormalities. Transmission electron microscopy showed the presence of large vacuoles, numerous starch grains, and less organized pockets of the endoplasmic reticulum containing fewer ribosomes in pollen from glyphosate-treated plants than from nontreated plants. Pollen development in glyphosate-treated plants is likely inhibited or aborted at the vacuolate microspore and vacuolate microgamete stages of microgametogenesis, resulting in immature pollen at anthesis. Although stigmas from glyphosate-treated plants were 1.2 to 1.4 mm longer than those from nontreated plants, no other anatomical differences in stigmas were visibly evident. The presence of the GR 5-enolpyruvylshikimate-3-phosphate synthase (CP4-EPSPS) enzyme from Agrobacterium sp. strain CP4 was quantified in reproductive and vegetative tissues using enzyme-linked immunosorbent assay. The content of CP4-EPSPS in the stigma, anther, preanthesis floral bud (square), and flower petals was significantly less than that in the vegetative leaf tissue. Glyphosate effects on the male reproductive development resulting in poor pollen deposition on the stigma, as well as production of aborted pollen with reduced viability, provide a likely explanation for reports of increased boll abortion and pollination problems in glyphosate-treated GR cotton.

Soil microbial community structure and activity are linked to plant communities. Weeds may alter their soil environment, selecting for specific rhizosphere microbial communities. Rhizosphere modification occurs for many crop and horticultural plants. However, impacts of weeds in agroecosystems on soil biology and ecology have received less attention because effective weed management practices were developed to minimize their impacts on crop production. The recent development of herbicide resistance (HR) in several economically important weeds leading to widespread infestations in crop fields treated with a single herbicide has prompted a re-evaluation of the effects of weed growth on soil biology and ecology. The objective of this article is to review the potential impacts of herbicide-resistant weeds on soil biological and ecological properties based on reports for crops, weeds, and invasive plants. Persistent weed infestations likely establish extensive root systems and release various plant metabolites through root exudation. Many exudates are selective for specific soil microbial groups mediating biochemical and nutrient acquisition processes. Exudates may stimulate development of microbial groups beneficial to weed but detrimental to crop growth or beneficial to both. Changes in symbiotic and associative microbial interactions occur, especially for arbuscular mycorrhizal fungi (AMF) that are important in plant uptake of nutrients and water, and protecting from phytopathogens. Mechanisms used by weeds to disrupt symbioses in crops are not clearly described. Many herbicide-resistant weeds including Amaranthus and Chenopodium do not support AMF symbioses, potentially reducing AMF propagule density and establishment with crop plants. Herbicides applied to control HR weeds may compound effects of weeds on soil microorganisms. Systemic herbicides released through weed roots may select microbial groups that mediate detrimental processes such as nutrient immobilization or serve as opportunistic pathogens. Understanding complex interactions of weeds with soil microorganisms under extensive infestations is important in developing effective management of herbicide-resistant weeds.

Glyphosate treatments to glyphosate-resistant (GR) cotton can cause increased fruit loss compared with untreated plants, likely due to reductions in pollen viability and alterations in floral morphology that may reduce pollination efficiency. This study was conducted to determine whether both stamen and pistil are affected by glyphosate treatments by measuring seed set from reciprocal reproductive crosses made between glyphosate-treated GR, untreated GR, and conventional nontransgenic cotton. Pollen viability was 51 and 38% lower for the first and second week of flowering, respectively, in GR plants treated with a four-leaf postemergence (POST) treatment and an eight-leaf POST-directed treatment of glyphosate than in GR plants that were not treated. Seed set per boll was significantly reduced when the pollen donor parent was glyphosate treated vs. untreated for the first 2 wk of flowering. There were no significant differences between treatments applied to male parents as measured by seed set at Weeks 3 and 4 of flowering. Seed set was not influenced by glyphosate treatments applied to female parents at any time. Retention of bolls resulting from crosses was reduced by glyphosate treatment of male parents during the first and third week of flowering but was not affected by glyphosate treatment of female parents. The application of gibberellic acid (GA), which has been shown to reverse male sterility in tomato (Lycopersicon esculentum L.) and to enhance boll retention in cotton, was investigated for similar effects in glyphosate-treated GR cotton. The GA treatments to glyphosate-treated plants increased the anther–stigma distance 12-fold, stigma height, and pollen viability in the second week of flowering but decreased the number of seeds in second-position bolls on Fruiting branches 1 through 3, decreased the number of first-position bolls per plant, and increased the number of squares in comparison with glyphosate-treated GR plants not receiving GA. Although GA applications to glyphosate-treated GR cotton have some remedial effect on pollen viability, the GA-induced increase in the anther–stigma difference exacerbates the increase in anther–stigma distance caused by glyphosate, resulting in low pollination.

Four field studies were conducted in 2004 to evaluate corn tolerance, weed control, grain yield, and net returns in glufosinate-resistant (GUR), glyphosate-resistant (GYR), imidazolinone-tolerant (IT), and nontransgenic (NT) corn with various herbicide systems. No significant differences between hybrid systems were observed for weed control. Limited corn injury (< 5%) was observed for all herbicide treatments. A single early POST (EPOST) system without S-metolachlor and sequential POST over the top (POT) herbicide systems, averaged over corn hybrids and PRE and late POST-directed (LAYBY) herbicide options, provide 93 and 99% control of goosegrass, respectively, and at least 83 and 97% control of Texas panicum, respectively. A single EPOST system without S-metolachlor, averaged over corn hybrids and LAYBY treatment options, provided at least 88% control of large crabgrass. When averaged over corn hybrid and PRE herbicide options, a sequential POT herbicide system alone provided at least 98, 99, 98, and 100 control of large crabgrass, morningglory species, Palmer amaranth, and common lambsquarters, respectively. The addition of ametryn at LAYBY to a single EPOST system without S-metolachlor was beneficial for improving control of morningglory species, common lambsquarters, and Palmer amaranth, depending on location. However, the observed increases (7 percentage points or less) are likely of limited biological significance. Grain yield was variable between hybrids and locations because of environmental differences. Consequently, net returns for each hybrid system within a location were also variable. Any POT system with or without ametryn at LAYBY, averaged over corn hybrid and PRE herbicide options, provided at least 101, 97, 92, and 92% yield protection at Clayton, Kinston, Lewiston, and Rocky Mount, NC, respectively. Net returns were maximized with treatments that provided excellent weed control with minimal inputs.

Two Glycine max near-isolines, GL2415 (glyphosate sensitive) and GL2600RR (glyphosate resistant), were compared for susceptibility to Sclerotinia sclerotiorum, the fungus that causes white mold disease (also known as Sclerotinia stem rot). A formulated isopropylamine salt of glyphosate (RU) at 840, 1,680, and 2,520 g ae ha−1; the RU formulation blank containing only adjuvants (RUFB) at 0.4% (v/v); organosilicone at 0.3% (v/v); a crop oil concentrate at 1.0% (v/v); and a nonionic surfactant at 0.5% (v/v) were evaluated for their influence on V5 G. max susceptibility to S. sclerotiorum in the field. The same treatments plus a formulated isopropylamine salt of glyphosate lacking adjuvants at 2,520 g ae ha−1 and a formulated trimethylsulfonium salt of glyphosate at 2,520 g ae ha−1 were applied to GL2600RR G. max in the greenhouse. Results from these studies indicated no differences in fungal growth, disease development, or G. max yield for any of these treatments. To determine if any interaction occurred between the chemicals, plants, and S. sclerotiorum, leaflets from both cultivars were dipped into and immediately removed from solutions of the RU at 4, 7, and 10% (v/v) and the RUFB and adjuvants at the rates described. The resulting lesion developments were similar across both cultivars but not across chemical treatments. Lesion developments were smaller on leaflets treated with RUFB and RU, but larger on leaflets treated with the other three adjuvants. Sclerotinia sclerotiorum mycelia growing on potato dextrose agar were inhibited by high concentrations of RU and of RUFB at 100 mM ae glyphosate and by the three adjuvants at 1% (v/v). Formulated glyphosate lacking adjuvants did not inhibit mycelial growth on potato dextrose agar. The glyphosate resistance trait did not appear to be associated with the susceptibility of G. max to S. sclerotiorum. Neither the glyphosate resistance trait nor the applications of glyphosate and adjuvants influenced G. max susceptibility to S. sclerotiorum.

Indirect effects of insect control strategies on weed populations are important to consider when developing robust integrated pest management strategies. Weed seed predation rates were investigated in corn managed under three contrasting treatments based on control practices for corn rootworm: (1) the transgenic crop Cry3Bb Bt corn, (2) the broad-spectrum insecticide tefluthrin, and (3) no insecticide control. This 2-yr field study conducted near Ithaca, NY, involved quantifying seed loss from velvetleaf, common lambsquarters, and giant foxtail in arenas with and without vertebrate exclosures. Velvetleaf and giant foxtail were unaffected by the insecticide treatment; however, average seed predation of common lambsquarters was lower in both the Bt corn (11.9%) and insecticide-treated plots (11.8%) compared with control plots (17.5%) that did not receive any insecticide. Seed predation of common lambsquarters was not affected by the vertebrate exclosure. Lower seed predation in the transgenic Bt corn and insecticide treatments was likely due to nontarget effects on carabids (Coleoptera: Carabidae). Although the reduction in seed predation was modest and limited to only one of the three weed species tested, our results highlight the need for greater risk assessment that includes the ecosystem service of weed seed predation when considering insect pest management options.

Under certain conditions, application of glyphosate to glyphosate-resistant (GR) cotton can lead to fruit shedding and yield reductions. Field studies were conducted at the Texas Agricultural Experiment Station using GR cotton, cv. ‘DeltaPine 5690RR’, to determine if application method and timing affect cotton fruit retention. Glyphosate at 1.12 kg ai ha−1 was precisely postdirected (PD), postdirected with 25% foliage coverage (PDFC), or applied over the top (OT) at the 8- or 18-leaf stage after an initial topical application of 1.12 kg ha−1 glyphosate at the four-leaf stage. In one of the years of this study, 8 PD, 18 PDFC, and 18 OT reduced yield. In 1999 and 2000, 8 PDFC and 8 OT applications of glyphosate caused yield loss, mainly due to lower mean boll weight. Glyphosate applied topically at the eight-leaf stage also affected the Position 1 boll retention throughout the plant in both years. Glyphosate contact with leaves and stems should be avoided when applying glyphosate after the four-leaf stage to prevent possible yield loss.

Field studies were conducted in 1999, 2000, and 2001 to investigate weed control and glyphosate-resistant corn tolerance to postemergence applications of mesotrione at 70, 105, and 140 g ai/ha applied with and without glyphosate at 560 g ai/ha. Mesotrione alone and mixed with glyphosate controlled smooth pigweed greater than 97% and common lambsquarters 93 to 99%. Control of common ragweed and morningglory species was variable. Common ragweed control was generally best when mesotrione was applied at 105 or 140 g/ha, and control increased only in 2000 with the addition of glyphosate. Giant foxtail control was below 25% with all rates of mesotrione, but mixtures of mesotrione plus glyphosate controlled giant foxtail 65 to 75%. Mesotrione injured glyphosate-resistant corn 4 to 24% when averaged over glyphosate rates, and injury was usually increased by higher mesotrione rates, with rainfall after herbicide applications, and in mixtures with glyphosate. Injury was transient and did not reduce corn yields. Mesotrione injury on glyphosate-resistant corn was confirmed in the greenhouse, where all mesotrione treatments reduced glyphosate-resistant corn biomass 9 to 23% compared with the nontreated check.

Glyphosate-resistant Palmer amaranth escaping residual herbicides is difficult to manage in cotton because of its rapid growth and a limited number of effective herbicide options to control emerged plants. An experiment was conducted at two dryland and two irrigated sites in Georgia during 2011 and 2012 to determine if cotton resistant to glyphosate, 2,4-D, and glufosinate could be used to salvage a crop infested with large Palmer amaranth. Three POST herbicide systems, including sequential applications of 2,4-D, sequential applications of 2,4-D plus glufosinate, or 2,4-D followed by (fb) glufosinate, were applied with intervals of 5, 10, or 15 d between POST applications. All three systems were followed by diuron plus MSMA directed at layby. At the dryland sites with high temperatures and drought conditions, no program provided greater than 90% control. However, the 2,4-D plus glufosinate system was at least twice as effective in controlling 20-cm-tall Palmer amaranth and produced at least three times more cotton than the other two systems, when pooled over POST application intervals. Intervals of 10 or 15 d between POST applications were 23 to 27% more effective than a 5-d interval in controlling Palmer amaranth when pooled over POST herbicide systems; yields were nearly twice as much with the 10-d interval compared to 5 d. At the irrigated site, overall weed control was greater with less treatment differences noted. Palmer amaranth that was 20 cm tall at application was controlled 98 to 99%, 92 to 93%, and 81 to 94% by glufosinate plus 2,4-D, 2,4-D fb glufosinate, and 2,4-D fb 2,4-D systems at harvest, respectively. Intervals between POST applications only influenced control by the POST 2,4-D system, and the 10-d interval was more effective than the 5-d interval. Carpetweed, Florida beggarweed, and smallflower morningglory were controlled 99% at harvest by all systems; however, it was noted that control of carpetweed and Florida beggarweed prior to layby was less effective with 2,4-D than systems including glufosinate. In the event of an at-plant residual herbicide failure in fields infested with glyphosate-resistant Palmer amaranth, our research demonstrates that glufosinate plus 2,4-D sequentially applied 10 to 15 d apart followed by a timely layby application controlled the target weeds in cotton with resistance to 2,4-D, glyphosate, and glufosinate.

The effect of herbicide rates on weed control and crop yield is the subject of countless and ongoing research projects. Weed seed banks receive very little attention in comparison. The seed bank resulting from 3 yr (2006 to 2008) of single herbicide rates in a cropping system where glyphosate/glufosinate and corn/soybean were rotated or not was evaluated in a field located in St-Augustin-de-Desmaures, Québec, Canada. Field plots under conventional tillage were seeded in corn every year, or corn and soybean (1 yr). These plots received the same herbicide every year or various glyphosate/glufosinate 3-yr sequences. Subplots were sprayed with a single POST application of the recommended rate of glyphosate (900 g ae ha−1) or glufosinate (500 g ai ha−1) or lower rates. Subplots received the same full (1.0×, recommended) or reduced (0.5×, 0.75×) rate every year. After crop harvest in 2008, soil cores were extracted and the weed seed bank was evaluated. Including soybean in the cropping system resulted in lower seed banks compared to those under continuous corn cropping. Including glufosinate in a glyphosate herbicide sequence increased weed seed banks due to the lower efficacy of the glufosinate rates tested at reducing the seed bank of annual grasses. Higher herbicide rates translated into lower seed banks, up to a certain rate. After 3 yr, the lowest seed bank (full glyphosate rates every year) still had 4,339 ± 836 seeds m−2 and was higher than the initial seed bank (2,826 ± 724 seeds m−2).

Field experiments were conducted in 1999 and 2000 to evaluate early POST (EPOST) and late POST (LPOST) control of common ragweed and giant foxtail with mesotrione at 70, 105, and 140 g ai/ha alone and in mixtures with glufosinate at 300 g ai/ha in glufosinate-resistant corn. Glufosinate-resistant corn injury was frequently higher with mixtures of mesotrione plus glufosinate than with mesotrione applied alone. Mixtures of mesotrione with glufosinate applied EPOST injured corn 6 to 21% in 1999, but in 2000, injury from mixtures was 23 to 30% from LPOST applications. Common ragweed control was above 77% with all treatments, which included 105 g/ha mesotrione. Giant foxtail control was higher at 76 to 78% by mixtures of mesotrione with glufosinate applied LPOST than by mesotrione alone. Corn yields were highest when glufosinate was included in treatments at either application timing. In the greenhouse, mixtures of mesotrione with glufosinate-injured glufosinate-resistant corn 11% or less, but corn biomass was reduced by 25% for the mixture of mesotrione at 105 g/ha plus glufosinate at 350 g/ha. Mixtures of mesotrione with glufosinate can be more effective than mesotrione alone but control of common ragweed and giant foxtail might not be commercially acceptable.

Atrazine is an important herbicide for broadleaf weed control in corn. Use rates have declined in many corn production systems due to environmental concerns and the availability of other effective herbicides, especially glyphosate in glyphosate-resistant hybrids. However, using multiple effective herbicide modes of action is ever more important because occurrence of herbicide-resistant weeds is increasing. An experiment to compare application timings of reduced rates of atrazine to benefit resistance management in broadleaf weeds while protecting corn yield was conducted in Wisconsin across four site-years in 2012 and 2013. Herbicide treatments consisted of five atrazine rate and timing combinations and three POST base herbicides: glyphosate, glufosinate, and tembotrione. Metolachlor was applied PRE at 2.1 kg ai ha−1 for grass control in all treatments. A linear regression model estimated that atrazine rates ≥ 1.0 kg ai ha−1 applied PRE would prevent exposure of common lambsquarters plants to POST herbicides, but giant ragweed and velvetleaf exposure was not influenced by timing. Corn yield was also not influenced by atrazine rate and timing combinations at the α = 0.05 level; however, at P = 0.06, corn yield was greater for atrazine applied PRE at 1.1 kg ha−1 than for atrazine applied PRE at 0.5 kg ha−1, POST at 1.1 kg ha−1, or not at all. In summary, higher rates of atrazine applied PRE may improve yield, as reported by others, but this study concludes reduced rates of atrazine (i.e., ≤ 1.1 kg ha−1) applied to corn in a POST tank mixture combination provided more consistent control of giant ragweed, velvetleaf, and common lambsquarters compared with atrazine applied PRE. This information should help direct atrazine application timing applied POST when applied at low rates to improve proactive herbicide resistance management.

The influence of herbicide placement and plant growth stage on the absorption and translocation patterns of 14C-glyphosate in glyphosate-resistant cotton was investigated. Plants at four growth stages were treated with 14C-glyphosate on a 5-cm2 section of the stem, which simulated a postemergence-directed spray (PDS) application, or on the newest mature leaf, which simulated a postemergence (POST) application. Plants were harvested 3 and 7 d after treatment and divided into the treated leaf or treated stem, mature leaves, immature leaves and buds, stems, roots, fruiting branches (including the foliage on the fruiting branch), squares, and bolls. The PDS versus POST application main effect on absorption was significant. Absorption of 14C-glyphosate applied to stem tissue was higher in PDS applications than in POST applications. Plants receiving PDS applications absorbed 35% of applied 14C-glyphosate, whereas those receiving POST applications absorbed 26%, averaged over growth stages at application. Absorption increased from the four-leaf growth stage to the eight-leaf stage in POST applications but reached a plateau at the eight-leaf stage. Plants with PDS applications showed an increase in absorption from the four- to eight- to twelve-leaf stages and reached a plateau at the 12-leaf stage. Translocation of 14C-glyphosate to roots was greater at all growth stages with PDS treatments than with POST treatments. Herbicide placement did not affect translocation of 14C-glyphosate to squares and bolls. Squares and bolls retained 0.2 to 3.7% of applied 14C-glyphosate, depending on growth stage. Separate studies were conducted to investigate the fate of foliar-applied 14C-glyphosate at the four- or eight-leaf growth stages when harvested at 8- or 10-leaf, 12-leaf, midbloom (8 to 10 nodes above white bloom), and cutout (five nodes above white bloom, physiological maturity) stages. Thirty to 37% of applied 14C-glyphosate remained in the plant at cutout in four- and eight-leaf treatment stages, respectively. The concentration of 14C-glyphosate in tissue (Bq g−1 dry weight basis) was greatest in mature leaves and immature leaves and buds in plants treated at the four-leaf stage. Plants treated at the eight-leaf stage and harvested at all growth stages except cutout showed a higher concentration of 14C-glyphosate in squares than in other plant tissue. Accumulation of 14C-glyphosate in squares reached a maximum of 43 Bq g−1 dry weight at harvest at the 12-leaf stage. This concentration corresponds to 5.7 times greater accumulation of 14C-glyphosate in squares than in roots, which may also be metabolic sinks. These data suggest that reproductive tissues such as bolls and squares can accumulate 14C-glyphosate at higher concentrations than other tissues, especially when the herbicide treatment is applied either POST or PDS during reproductive stages (eight-leaf stage and beyond).

Field experiments were conducted to evaluate weed control and cotton response to glyphosate or glufosinate applied alone or with residual herbicides applied in the last POST-directed application (LAYBY) in glyphosate- and glufosinate-resistant cotton. Glyphosate (0.86 kg ae/ha) or glufosinate (0.47 kg ai/ha) were applied alone over the top of glyphosate- or glufosinate-resistant cotton early POST (EPOST) followed by (fb) late POST (LPOST) fb one of the herbicides applied either alone or with a residual herbicide at LAYBY. Glyphosate- and glufosinate-based treatments were applied only to glyphosate- and glufosinate-resistant cotton, respectively. Residual herbicides evaluated included prometryn (1.12 kg ai/ha), fluometuron (1.12 kg ai/ha), diuron (1.12 kg ai/ha), oxyfluorfen (1.12 kg ai/ha), pendimethalin (1.0 kg ai/ha), prometryn + trifloxysulfuron (1.33 kg ai/ha + 12 g ai/ha), or linuron + diuron (0.56 + 0.56 kg ai/ha). Glyphosate-and glufosinate-based weed management systems with and without residual LAYBY herbicides resulted in little to no injury to cotton. Three applications of glyphosate or glufosinate alone provided better full-season control of most species when compared to two applications of either herbicide. The addition of a residual herbicide to glyphosate or glufosinate at LAYBY did not improve cotton yields, but did improve overall control of barnyardgrass and yellow nutsedge and reduced weed dry biomass present at time of cotton harvest when compared to three applications of glyphosate or glufosinate alone.

Studies were conducted to evaluate absorption and translocation of 14C-glyphosate in glyphosate-resistant (GR) cotton. Both commercial GR cotton events [glyphosate-resistant event 1, marketed as Roundup Ready®, released 1997 (GRE1), and glyphosate-resistant event 2, marketed as Roundup Ready Flex®, released 2006 (GRE2)] were evaluated at the four-leaf and eight-leaf growth stages. Plants were harvested at 1, 3, 5, and 7 d after treatment (DAT). Glyphosate absorption, as a percentage of applied, increased over time with 29 and 36% absorption at 7 DAT in four-leaf GRE1 and GRE2 cotton, respectively. In eight-leaf cotton, glyphosate absorption (33% at 7 DAT) was not different between events. Glyphosate translocation patterns were not different between events or harvest timings and exhibited a source–sink relation. Observed translocation differences between cotton growth stages were probably due to reduced glyphosate export from the treated leaf of eight-leaf cotton. An additional study compared absorption and translocation of 14C-glyphosate and 14C-sucrose in 5- and 10-leaf GRE2 cotton. Averaged over trials, 14C compounds, and growth stages, cotton absorbed 28% of the applied dose at 14 DAT. On the basis of the percentage of 14C exported out of the treated leaf, glyphosate and sucrose translocation patterns were similar, indicating that glyphosate may be used as a photoassimilate model in GRE2 cotton.