Industrial hemp is a multipurpose crop cultivated for fiber, seed, human food, and animal feed. Hemp legalization in Texas creates a considerable potential to increase its acreage in semi-arid conditions; however, knowledge is limited on growing hemp optimally in Texas. Best management practices, including weed control, require evaluation for profitable hemp production. As little is known about the herbicide tolerance of hemp, field studies were conducted to test several soil-residual herbicides with different modes of action for phytotoxicity to two hemp cultivars, ‘Yuma’ and ‘Jinma’. The experimental units were randomized three times in a blocked split-plot design with hemp cultivars in the main plots and soil-residual herbicides in the subplots. Ethalfluralin, the mixture of sulfentrazone and S-metolachlor, prometryn, and S-metolachlor, resulted in 60% to 90% and 73% to 100% weed control as compared to the nontreated control in 2021 and 2022, respectively. The highest hemp germination, stand count, and plant height were observed with ethalfluralin and S-metolachlor herbicides; however, no significant differences were observed for hemp germination and plant height compared to the nontreated control. S-metolachlor, ethalfluralin, fomesafen, and prometryn resulted in similar hemp biomass compared to the nontreated control. Overall, the results indicate that hemp is tolerant to ethalfluralin, prometryn, and S-metolachlor, and these soil-residual herbicides were effective for weed control in hemp. The mixture of bicyclopyrone plus S-metolachlor, metribuzin plus S-metolachlor, and mesotrione should be avoided, as they caused significant injury to hemp plants. Future research is needed to test the efficacy of different preemergence and postemergence herbicides that can be used in industrial hemp grown under different environments, making sure the delta-9-tetrahydrocannabinol content of the hemp is below the legal content restrictions.

Atrazine offers growers a reliable option to control a broad spectrum of weeds in grain sorghum production systems when applied PRE or POST. However, because of the extensive use of atrazine in grain sorghum and corn, it has been found in groundwater in the United States. Given this issue, field experiments were conducted in 2017 and 2018 in Fayetteville and Marianna, Arkansas, to explore the tolerance of grain sorghum to applications of assorted photosystem II (PSII)-inhibiting herbicides in combination with S-metolachlor (PRE and POST) or mesotrione (POST only) as atrazine replacements. All experiments were designed as a factorial, randomized complete block; the two factors were (1) PSII herbicide and (2) the herbicide added to create the mixture. The PSII herbicides were prometryn, ametryn, simazine, fluometuron, metribuzin, linuron, diuron, atrazine, and propazine. The second factor consisted of either no additional herbicide, S-metolachlor, or mesotrione; however, mesotrione was excluded in the PRE experiments. Crop injury estimates, height, and yield data were collected or calculated in both studies. In the PRE study, injury was less than 10% for all treatments except those containing simazine, which caused 11% injury 28 d after application (DAA). Averaged over PSII herbicide, S-metolachlor–containing treatments caused 7% injury at 14 and 28 DAA. Grain sorghum in atrazine-containing treatments yielded 97% of the nontreated. Grain sorghum receiving other herbicide treatments had significant yield loss due to crop injury, compared with atrazine-containing treatments. In the POST study, ametryn- and prometryn-containing treatments were more injurious than all other treatments 14 DAA. Grain sorghum yield in all POST treatments was comparable to atrazine, except prometryn plus mesotrione, which was 65% of the nontreated. More herbicides should be evaluated to find a comparable fit to atrazine when applied PRE in grain sorghum. However, when applied POST, diuron, fluometuron, linuron, metribuzin, propazine, and simazine have some potential to replace atrazine in terms of crop tolerance and should be further tested as part of a weed control program across a greater range of environments.

Cover crops can provide many benefits to peanut and cotton crops planted in rotation including suppressing weeds, conserving soil moisture after termination, increasing soil organic matter, and reducing soil erosion. However, herbicide carryover can affect cover crop establishment. The objective of this study was to investigate the responses of 6 cover crops (daikon radish, cereal rye, oat, crimson clover, winter wheat, and common vetch) to 12 soil residual herbicides. A multiyear (2016–2018), multilocation study was conducted in Macon and Henry counties, Alabama. Herbicide treatments included S-metolachlor, acetochlor, pyroxasulfone, diclosulam, imazapic, chlorimuron-ethyl, bentazon plus acifluorfen, pyrithiobac-sodium, trifloxysulfuron-sodium, diuron, prometryn, and flumioxazin, each applied at 10% of the full-labeled rate. At 42 to 52 and 145 to 149 d after planting (DAP), cover crop plant heights and stand counts were evaluated, as was biomass at 145 to 149 DAP. Treatments varied from year to year but not locations. In 2016, significant stand reductions (P ≤ 0.10) of 36% to 43% in rye and 44% to 75% in wheat were observed at 48 to 52 DAP for S-metolachlor, acetochlor, pyroxasulfone, imazapic, and bentazon plus acifluorfen compared with nontreated plants. Vetch had stand reductions ranging from 14% to 80% for all treatments 50 DAP except for plants treated with prometryn. S-metolachlor, pyroxasulfone, and acetochlor reduced stands of rye, wheat, and vetch more than any other herbicides. In 2017, at 147 to 149 DAP, clover stands were reduced by 29% with diclosulam and by 38% with trifloxysulfuron-sodium. Similarly, radish stands were reduced by 64% with diclosulam treatment. No significant biomass reductions were observed for any cover crop species either year. Oat showed the most tolerance with no treatments reducing any growth parameters either year. Although initial injury and stunting may occur, biomass at termination of cover crops were not affected by herbicide residues evaluated in this study.

Weed control in corn traditionally has relied on atrazine as a foundational tool to control problematic weeds. However, the recent discovery of atrazine in aquifers and other water sources increases the likelihood of more strict restrictions on its use. Field-based research trials to find atrazine alternatives were conducted in 2017 and 2018 in Fayetteville, AR, by testing the tolerance of corn to PRE and POST applications of different photosystem II (PSII) inhibitors alone or in combination with mesotrione or S-metolachlor. All experiments were designed as a two-factor factorial, randomized complete block, with the two factors being (1) PSII-inhibiting herbicide and (2) the herbicide added to create the mixture. The PSII-inhibiting herbicides were prometryn, ametryn, simazine, fluometuron, metribuzin, linuron, diuron, atrazine, and propazine. The second factor consisted of either no additional herbicide, S-metolachlor, or mesotrione. Treatments were applied immediately after planting in the PRE experiments and to 30-cm–tall corn for the POST experiments. For the PRE study, low levels of injury (<15%) were observed at 14 and 28 d after application and corn height was negatively affected by the PSII-inhibiting herbicide applied. PRE-applied fluometuron- and ametryn-containing treatments consistently caused injury to corn, often exceeding 5%. Because of low injury levels caused by all treatments, crop density and yield did not differ from that of the nontreated plants. For the POST study, crop injury, relative height, and relative yield were affected by PSII-inhibiting herbicide and the herbicide added. Ametryn-, diuron-, linuron-, propazine-, and prometryn-containing treatments caused at least 25% injury to corn in at least 1 site-year. All PSII-inhibiting herbicides, except metribuzin and simazine when applied alone, caused yield loss in corn when compared with atrazine alone. Diuron-, linuron-, metribuzin-, and simazine-containing treatments applied PRE and metribuzin- and simazine-containing treatments applied POST should be investigated further as atrazine replacements.

The development of a linuron-free weed management strategy for carrot production is essential as a result of the herbicide reevaluation programs launched by the Pest Management Regulatory Agency in Canada for herbicides registered before 1995 and the discovery of linuron-resistant pigweed species in Ontario. Field trials were conducted in one of Ontario’s main carrot-growing regions on high organic soils in 2016 and 2017. Pigweed species seedlings were effectively controlled with PRE treatments of prometryn, pendimethalin, S-metolachlor, or glufosinate. POST treatments of pyroxasulfone and metribuzin followed by predetermined biologically effective dose (≥90% control of pigweed seedlings) of acifluorfen, oxyfluorfen, fluthiacet-methyl, and fomesafen achieved excellent crop selectivity and commercially acceptable pigweed species seedling control under field conditions. Carfentrazone-ethyl or fomesafen applied PRE severely reduced seedling emergence and yield in the wet growing season of 2017. This study demonstrated clearly that an alternative linuron-free strategy can be developed for carrots. The strategy of exploring the potential to use the biologically effective dose of selected herbicides to achieve crop selectivity and control of pigweed species seedlings was verified.

Linuron herbicide has been a mainstay of carrot weed management for years, but uncertainty around regulatory registration review and an increased prevalence of herbicide-resistant weeds have spurred interest in identifying alternatives that can be readily adopted in production. With this context in mind, herbicide programs were evaluated on a coarse-textured, low organic matter soil in 2015 and 2016. Season-long weed control without compromising yield was possible with weed management programs that included prometryn POST instead of linuron. With that said, a PRE herbicide such as pendimethalin was critical to establish an early-season competitive advantage for carrot plants over weeds, and careful attention should be paid to the prometryn rate, as selectivity is marginal. Carrot is often interseeded with a grain nurse crop to mitigate risk of wind erosion. Nurse crop injury was minimal where S-metolachlor, pendimethalin, or prometryn was applied at rates labeled for PRE use in carrot, with the exception of where prometryn was applied at rates above 1.1 kg ai ha−1.

Greenhouse and laboratory investigations using four soils were conducted to determine the influence of seven herbicides on symbiotic nitrogen (N2) fixation by soybeans [Glycine max (L.) Merr. ‘Davis’]. Soils were treated with recommended rates of herbicides immediately after planting. After 8 to 10 weeks the nodulated root systems were assayed for nitrogenase activity using the acetylene (C2H2) to ethylene (C2H4) reduction technique, and total numbers and dry weights of nodules were determined. Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] (0.3 kg/ha) decreased nodule dry weight and C2H4 production per plant per hour in a Convent sandy loam. Alachlor [2-chloro-2’,6’-diethyl-N-(methoxymethyl) acetanilide] (2.2 kg/ha) and prometryn [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine] (0.6 kg/ha) also decreased C2H4 production per plant per hour in a Convent sandy loam, while metribuzin (0.6 kg/ha) increased C2H4 production per plant per hour and per milligram of nodule dry weight in a Mhoon silt loam. Prometryn (1.7 kg/ha) increased production of C2H4 per plant per hour in a Moreland silt loam. None of the herbicides affected C2H4 production, nodule number, or nodule dry weight in a Commerce silt loam.

A compact incorporator-planter, which incorporates herbicides into moist soil simultaneously with planting of cotton (Gossypium hirsutum L.), was constructed and evaluated over 3 yr using six herbicides. Weed control was consistent with that reported for large, cumbersome units utilizing standard planters and incorporating units from ground-driven rotary tillers and was enhanced in seasons in which rain fell during the first 3 weeks after planting. Among the dinitroaniline herbicides, dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine) controlled barnyardgrass [Echinochloa crus-galli (L.) Beauv.], and redroot pigweed (Amaranthus retroflexus L.) most effectively. Cotton stands and yields were equivalent to those obtained using standard cotton-planting methods.

The effect of two dates of initial manual weeding, i.e., before flowering and after flowering, and of no weeding on yield of a local groundnut variety was studied during 1988 and 1989 at Minkoameyos, Yaounde, Cameroon, in comparison with chemical weed control method or control. The use of herbicides ametryn plus promotryn for weed control gave the best mean yield, followed by handweeding after flowering, and handweeding before flowering, in that order. The no weeding treatment had the lowest yield throughout the trial period.

Field experiments were established to evaluate coffee senna control in cotton with registered PRE, early POST-directed (EPD), and late POST-directed (LPD) herbicides and with pyrithiobac applied PRE and early POST. Fluometuron and norflurazon applied PRE controlled coffee senna 87 and 65%, respectively, 6 wk after planting (WAP). Methazole applied EPD controlled coffee senna 96% 2 wk after application, whereas fluometuron and MSMA applied EPD controlled 70 and 16%, respectively. LPD applications of cyanazine, diuron, and prometryn averaged only 45% control 8 WAP. MSMA applied LPD controlled coffee senna only 12%. Fluometuron and norflurazon applied PRE followed by methazole applied EPD and cyanazine applied LPD controlled coffee senna 98 and 99% 6 WAP, respectively. PRE, early POST, and PRE followed by early POST applications of pyrithiobac controlled coffee senna 88, 89, and 95% 6 WAP, respectively. Seed cotton yields were similar to weed-free cotton with all treatments except MSMA applied EPD and the LPD treatments, all of which provided unacceptable (12 to 49%) control 8 WAP.

Various combinations of methods were evaluated for the control of ivyleaf morningglory growing in cotton. The combinations included chemicals incorporated to two depths or tanked-mixed as postemergence treatments and cultural methods that included in-row cultivation and hand weeding. The most effective combinations of methods included a herbicide such as cyanazine or prometryn in some treatments resulting in reduced ivyleaf morningglory population and improved cotton yields. Cultural methods without chemical assistance gave little or no control of ivyleaf morningglory based on weed populations and cotton yield. Statistically the best combinations had lint yields equal to the weed-free treatment, but the yields were reduced from 114 to 372 kg/ha. Also, most combination treatments had enough plant debris in the lint to reduce fiber grade.

Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], prometryn [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine], and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) were applied pre-plant in safflower (Carthamus tinctorius L. ‘AC-1′) for 6 yr at two locations. Diuron and prometryn reduced crop stands in 2 of 6 yr on the sandier soil at Mesa. Growth of sorghum [Sorghum bicolor (L.) Moench ‘Amak R-10′] in the greenhouse in soil collected 10 months after each herbicide application was reduced by 1.7 kg/ha of trifluralin in 3 of 6 yr in the Yuma silty clay loam and by both rates of trifluralin in 1 of 6 yr in the sandier soil at Mesa. Growth of barley (Hordeum vulgare L. ‘Arivat’) was not reduced in soil treated with any herbicide at either location. Herbicides did not accumulate in winter-planted safflower.

Devil's-claw and smellmelon are summer annual broadleaf weed problems in the southwest cotton production region. Field studies conducted in 1993 and 1994 evaluated the efficacy of pyrithiobac for postemergence (POST) control of devil's-claw and smellmelon compared with soil-applied fluometuron or prometryn. Pyrithiobac at 0.07 and 0.14 kg ai/ha early POST and 0.14 kg ai/ha mid-POST controlled devil's-claw more than 84% at the late rating. Devil's-claw control with mid-POST applications of pyrithiobac at 0.07 kg ai/ha was reduced when compared with other rates and application timings. Smellmelon control with pyrithiobac ranged from 57 to 84% and, except when applied at 0.07 kg ai/ha early POST, was equivalent to or greater than either fluometuron or prometryn. Crop injury with pyrithiobac treatments 16 d after treatment was 5 to 13%, but 33 d later, was no more than 3%.

A 4-yr study (1985, 1987, 1988, 1989) was conducted on the same field plots at Shafter, CA to evaluate the efficacy of prometryn in controlling black nightshade on planting beds of cotton. Two rates (1.7 and 2.2 kg ai ha–1) were applied at two times (mid March before the preplant irrigation and early April at cotton planting) each year. Incorporation of prometryn into moist planting beds with a powered rotary tiller operated at 10 cm deeper resulted in excellent control of black nightshade under low to moderate weed pressure in 1985 and 1987. Control of nightshade with early and late applications of 1.7 kg ha–1 of prometryn under high weed pressure in 1988 was only 70% at harvest, and yield losses of cotton averaged 25%. Yields of cotton treated with 2.2 kg ha–1 of prometryn in 1988 were not significantly different from weed-free plots. Only the late application of 2.2 kg ha–1 of prometryn prevented significant cotton losses under extreme weed pressure in 1989. Cotton yield losses with the other prometryn treatments ranged from 78 to 100%. Losses of cotton in weedy-check plots that received only cultivation ranged from 22% in the absence of rain or irrigation at cotton planting in 1987 to as much as 100% when rain fell in 1988 or plots were irrigated at planting in 1989. Plots hoed one time 4 wk after cotton planting yielded an average of 84% as much seed cotton as weed-free plots. In an attempt to determine why the efficacy of prometryn declined between 1985 and 1989, several experiments were conducted in 1988 to 1990 to discover reasons for this poor control of nightshade. Because efforts failed to provide evidence for the movement of the herbicide with water, the development of weed resistance to prometryn, or accelerated degradation of this herbicide in soil, increasing weed seed populations in soil were believed to have contributed greatly to the declining nightshade control from prometryn. The fact that prometryn applied and incorporated into flat soil provided excellent control of nightshade in 1990 under sprinkler irrigation indicated that both soil moisture and incorporation techniques limited activity of prometryn in planting beds in 1988 and 1989. Incomplete control of nightshade plus good soil moisture at planting contributed to the high weed populations in 1988 and 1989.

Field studies were conducted in five states at six locations from 2002 through 2003 to evaluate weed control and cotton response to early POST (EPOST), POST/POST-directed spray (PDS), and late POST-directed (LAYBY) systems using glyphosate-trimethylsulfonium salt (TM), s-metolachlor, trifloxysulfuron, prometryn, and MSMA. Early POST applications were made from mid May through mid June; POST/PDS applications were made from early June through mid July; and LAYBY applications were made from early July through mid August. Early season cotton injury and discoloration was minimal (< 1%) with all treatments; mid- and late-season injury was minimal (< 2%) except for trifloxysulfuron POST (11 and 9%, respectively). Annual grasses evaluated included barnyardgrass, broadleaf signalgrass, goosegrass, and large crabgrass. Broadleaf weeds evaluated included entireleaf morningglory, pitted morningglory, sicklepod, and smooth pigweed. For the EPOST, POST/PDS, and LAYBY applications, weeds were at cotyledon to 10 leaf, 1 to 25 leaf, and 2 to 25 leaf stage, respectively. Annual broadleaf and grass control was increased with the addition of s-metolachlor to glyphosate-TM EPOST systems (85 to 98% control) compared with glyphosate-TM EPOST alone (65 to 91% control), except for sicklepod control where equivalent control was observed. Annual grass control was greater with glyphosate-TM plus trifloxysulfuron PDS than with trifloxysulfuron POST or PDS, or trifloxysulfuron plus MSMA PDS (90 to 94% vs. 75 to 83% control). With few exceptions, broadleaf weed control was equivalent for trifloxysulfuron applied POST alone or PDS alone or in combination with glyphosate-TM PDS or MSMA PDS herbicide treatments (81 to 99% control). The addition of a LAYBY herbicide treatment increased broadleaf weed control by 11 to 36 percentage points compared with systems without a LAYBY. Cotton lint yield increased 420 kg/ha with the addition of s-metolachlor to glyphosate-TM EPOST treatments compared with systems without s-metolachlor EPOST. Cotton lint yield was increased 330 to 910 kg/ha with the addition of a POST herbicide treatment compared with systems without a POST/PDS treatment. The addition of a LAYBY herbicide treatment increased cotton lint yield by 440 kg/ha compared with systems without a LAYBY.

Field studies were conducted in 2007 and 2008 to evaluate fall applications of herbicides to control glyphosate-resistant (GR) horseweed before planting cotton. Fall treatments were compared with spring treatments for control of GR horseweed and effect on seed cotton yield. Fall and spring treatments with and without residual herbicides were also compared. No differences were observed for control of GR horseweed or seed cotton yield between fall and spring application timings. However, a difference was observed between fall applications with and without a residual herbicide. Fall applications that contained residual herbicides provided 86% control of GR horseweed and yielded 2,360 kg/ha of seed cotton. Fall applications that did not contain a residual herbicide only provided 70% control of GR horseweed and yielded 2,010 kg/ha of seed cotton. No benefit was observed from spring applications that contained a residual herbicide. This research indicates that glyphosate-resistant horseweed can be controlled with fall- or spring-applied burndown herbicides, and fall applications should include a residual herbicide for best results.

Field studies were conducted at four locations in North Carolina in 1998 and 1999 to evaluate the use of the Herbicide Application Decision Support System (HADSS™) for weed management in nontransgenic, bromoxynil-resistant, and glyphosate-resistant cotton. Weed management systems included trifluralin preplant incorporated (PPI) plus fluometuron preemergence (PRE) or no soil-applied herbicides. Postemergence (POST) options included bromoxynil, glyphosate, or pyrithiobac early POST (EPOST) followed by (fb) MSMA plus prometryn late postemergence–directed (LAYBY) or herbicide recommendations given by HADSS. Glyphosate-resistant systems provided control equivalent to or better than control provided by bromoxynil-resistant and nontransgenic systems for smooth pigweed, Palmer amaranth, large crabgrass, goosegrass, ivyleaf morningglory, and fall panicum. Trifluralin PPI fb fluometuron PRE fb HADSS POST provided equivalent or higher levels of weed control and yield than trifluralin PPI fb fluometuron PRE fb bromoxynil, glyphosate, or pyrithiobac EPOST fb MSMA plus prometryn LAYBY. The trifluralin PPI fb fluometuron PRE fb HADSS POST systems controlled large crabgrass at Goldsboro and fall panicum better than HADSS POST-only systems in nontransgenic cotton. Cotton yield and net returns in the glyphosate-resistant systems were always equal to or higher than the nontransgenic and bromoxynil-resistant systems. Net returns were higher for the soil-applied fb HADSS POST treatments in 8 of 12 comparisons with HADSS POST systems without soil-applied herbicides. Early-season weed interference reduced cotton lint yields and net returns in POST-only systems.

Thirteen field trials were conducted in 1999 and 2000 to evaluate postemergence (POST) weed control with single applications of bromoxynil at 420 or 560 g ai/ha, glufosinate at 291 or 409 g ai/ha, glyphosate at 1,120 g ai/ha, pyrithiobac at 36 or 72 g ai/ha, or sulfosate at 1,120 g ai/ha. Additional treatments evaluated included two applications with glufosinate at both rates in all possible combinations, two applications of glyphosate, and two applications of sulfosate. Weeds were 2 to 5 cm or 8 to 10 cm tall for annual grass and broadleaf weeds whereas yellow nutsedge and glyphosate-resistant corn were 8 to 10 cm tall. All herbicide treatments controlled 2- to 5-cm common cocklebur, Florida beggarweed, jimsonweed, ladysthumb smartweed, Pennsylvania smartweed, pitted morningglory, prickly sida, redroot pigweed, smooth pigweed, and velvetleaf at least 90%. All herbicide treatments except pyrithiobac at either rate controlled 2- to 5-cm common lambsquarters, common ragweed, and tall morningglory at least 90%; pyrithiobac at the lower rate was the only treatment that failed to control entireleaf and ivyleaf morningglory at least 90%. Bromoxynil and pyrithiobac at either rate controlled 2- to 5-cm sicklepod 33 to 68% whereas glufosinate, glyphosate, and sulfostate controlled ≥99%. Glyphosate and sulfosate applied once or twice controlled hemp sesbania less than 70% and volunteer peanut less than 80%. Bromoxynil and pyrithiobac were the least effective treatments for control of annual grass species and bromoxynil controlled Palmer amaranth less than 80%. Glufosinate controlled broadleaf signalgrass, fall panicum, giant foxtail, green foxtail, large crabgrass, yellow foxtail, seedling johnsongrass, Texas panicum, and glyphosate-resistant corn at least 90% but controlled goosegrass less than 60%. Glyphosate and sulfosate controlled all grass species except glyphosate-resistant corn at least 90%. In greenhouse research, goosegrass could be controlled with glufosinate POST plus a late POST-directed treatment of prometryn plus monosodium salt of methylarsonic acid.

Seeds of a suspected glyphosate-resistant giant ragweed biotype from Lauderdale County, TN, were collected from a continuous cotton field in fall 2007 after plants were nonresponsive to multiple glyphosate applications. The objectives of this research were to (1) confirm resistance by quantifying the response of the putative resistant biotype to glyphosate compared to a susceptible biotype from a nonagricultural area, (2) quantify shikimate accumulation over time in both biotypes, and (3) determine the effectiveness of POST-applied herbicides labeled for use in cotton in controlling both biotypes at three growth stages. The susceptible biotype had a 50% lethal dose of 407 g ae/ha of glyphosate compared with 2,176 g/ha for the resistant biotype when treated at the four-node stage, a 5.3-fold level of resistance. The resistant biotype accumulated 3.3- to 9.8-fold less shikimate than the susceptible biotype at 1 to 7 d after treatment. The resistant biotype was less responsive to glyphosate as treatment was delayed past the two-node stage, much more than the susceptible biotype. Glufosinate, MSMA, and diuron controlled both biotypes by at least 90%, regardless of size at application. Prometryn, flumioxazin, carfentrazone-ethyl, fomesafen, and trifloxysulfuron controlled both biotypes by at least 89% when applied at the two-node stage, but control generally diminished with later application timings. Pyrithiobac was not effective in controlling either biotype, regardless of size at application. Hence, there are effective herbicide options for controlling glyphosate-resistant giant ragweed in cotton, and the resistant biotype does not appear to exhibit multiple resistances to other herbicides.

Five studies were conducted at Clayton, Rocky Mount, and Lewiston-Woodville, NC, in 2001 and 2002, to evaluate weed management, crop tolerance, and yield in strip- and conventional-tillage glyphosate-resistant cotton. Cotton was treated with two glyphosate formulations; glyphosate-IP (isopropylamine salt) or glyphosate-TM (trimethylsulfonium salt), early postemergence (EPOST) alone or in a mixture with S-metolachlor. Early season cotton injury was minimal (3%) with either glyphosate formulation alone or in mixture with S-metolachlor. Weed control and cotton yields were similar for both glyphosate formulations. The addition of S-metolachlor to either glyphosate formulation increased control of broadleaf signalgrass, goosegrass, large crabgrass, and yellow foxtail 14 to 43 percentage points compared with control by glyphosate alone. S-metolachlor was not beneficial for late-season control of entireleaf morningglory, jimsonweed, pitted morningglory, or yellow nutsedge. The addition of S-metolachlor to either glyphosate formulation increased control of common lambsquarters, common ragweed, Palmer amaranth, smooth pigweed, and velvetleaf 6 to 46 percentage points. The addition of a late postemergence-directed (LAYBY) treatment of prometryn plus MSMA increased control to greater than 95% for all weed species regardless of EPOST treatment, and control was similar with or without S-metolachlor EPOST. Cotton lint yield was increased 220 kg/ha with the addition of S-metolachlor to either glyphosate formulation compared with yield from glyphosate alone. The addition of the LAYBY treatment increased yields 250 and 380 kg/ha for glyphosate plus S-metolachlor and glyphosate systems, respectively. S-metolachlor residual activity allowed for an extended window for more effective LAYBY application to smaller weed seedlings instead of weeds that were possibly larger and harder to control.