Field studies were conducted from 1992 through 1994 to evaluate application timing of seven postemergence (POST) broadleaf herbicides alone and in mixtures for control of eclipta and pitted morningglory. Imazethapyr and 2,4-DB did not control eclipta while AC 263,222 applied early postemergence (EPOST) at 0.07 kg/ha provided greater than 90% control in 2 of 3 yr. EPOST applications of bentazon, acifluorfen + bentazon, and pyridate controlled eclipta at least 92% all 3 yr. Lactofen applied EPOST at 0.28 kg/ha provided similar levels of eclipta control in 2 of 3 yr. Imazethapyr controlled pitted morningglory > 70% when applied EPOST. AC 263,222 controlled pitted morningglory a minimum of 83% when applied EPOST at 0.04 or 0.07 kg/ha. Pitted morningglory control was at least 85% with 2,4-DB applied alone or in a mixture with AC 263,222, acifluorfen, imazethapyr, lactofen, or pyridate. Effective weed control increased peanut yields up to 98% over the untreated check.

Spurred anoda is an increasing weed problem in Colorado pinto bean production. This research was designed to evaluate reduced rates of bentazon and imazethapyr applied at various times for spurred anoda control and profitability. In 1993, spurred anoda control with early applications of either bentazon or imazethapyr was superior to late applications. Bentazon provided better spurred anoda control than imazethapyr in June. Spurred anoda control in 1993 was better than in 1994 due to late-emerging weeds and herbicide treatments not providing residual control. In 1994, bentazon controlled spurred anoda better than imazethapyr 1 and 3 wk after late POST, but no treatment provided commercially acceptable full-season control. The number of treatments with gross margins greater than the untreated check increased as pinto bean prices increased, but ranking of gross margin did not change due to pinto bean prices.

A study was conducted in 1994 and 1995 at two Mississippi locations to evaluate preplant incorporated (PPI) and preemergence (PRE) applications of alachlor, clomazone, SAN 582, metolachlor, pendimethalin, and trifluralin, and postemergence (POST) applications of AC 263,222 and imazethapyr alone or followed by clethodim late postemergence (LPOST) for red rice control in soybean. Applications of 110 g ai/ha clethodim increased red rice control when following any earlier herbicide application at one location that harbored a high natural infestation. In 1 yr at one location, red rice seedhead suppression from PPI and PRE herbicide applications alone was greater than 95% due to high activity from herbicides and drought conditions during red rice seedhead development. Early postemergence (EPOST) applications of 30 g ae/ha AC 263,222 suppressed at least 95% of red rice seedheads, regardless of year, location, or clethodim LPOST application. At one location, any treatment where 110 g/ha clethodim followed an earlier herbicide application suppressed red rice seedheads at least 95%. Compared to the nontreated control, only AC 263,222 injured soybean (30%) and reduced soybean yield (200 kg/ha).

Field experiments were conducted in Louisiana to evaluate the effectiveness of tillage, tillage plus herbicide, and herbicide-only fallow programs in controlling bermudagrass. Tillage of fallow fields three times at 3- to 4-wk intervals between row formation in late May and planting in late September or early October reduced bermudagrass infestations in the newly planted crop in December 44 to 51%. When the final tillage operation was replaced by glyphosate postemergence at either 2.24 or 3.36 kg ai/ha in late August, bermudagrass infestation level in December on the surface of the newly planted rows was reduced to below 20%. Two applications of glyphosate at 3.36 kg/ha were more effective than tillage only and as effective as tillage twice followed by glyphosate in reducing bermudagrass infestation levels in the newly planted crop. Application of atrazine at 3.36 kg ai/ha, imazapyr at 0.68 kg ai/ha, metribuzin at 1.68 kg ai/ha, or sulfometuron at 0.14 and 0.28 kg ai/ha to newly formed rows in late May of the fallow period did not reduce the need for tillage or an application of glyphosate during the fallow period to control bermudagrass, but imazapyr at 1.12 kg/ha did. Sugarcane shoot populations in December of the fallow year were similar for the treatment programs despite differences in bermudagrass infestation levels, which ranged from 11 to 94%. However, sugarcane stalk number and height and cane and sugar yield in the plant-cane crop the following year were highest when glyphosate was applied to bermudagrass as two applications either alone or following the application of atrazine or metribuzin, when tillage was followed with one application of glyphosate, and in 1 yr when imazapyr alone was applied to newly formed rows.

Seven postemergence (POST) herbicides were evaluated alone and in various mixtures for Palmer amaranth control in peanut from 1992 through 1994. AC 263,222 at rates as low as 0.04 kg/ha controlled ≥ 95% Palmer amaranth. Imazethapyr at rates of 0.05 to 0.07 kg/ha controlled ≥ 90% Palmer amaranth in 2 of 3 yr. Acifluorfen alone or in combination with bentazon or 2,4-DB and lactofen alone controlled > 90% Palmer amaranth in 2 of 3 yr. Bentazon or pyridate failed to provide adequate control (≤ 60%), while 2,4-DB provided good control (80%) in only 1 of 3 yr.

Field experiments were conducted at Dekalb, IL, in 1996 and 1997 to determine the optimum application timing and rate of cloransulam for giant ragweed (Ambrosia trifida) control in soybean (Glycine max). Cloransulam treatments included preplant incorporated (PPI) and preemergence (PRE) applications of 35 g ai/ha and early postemergence (EPOST), postemergence (POST), and late postemergence (LPOST) applications of 18 or 27 g ai/ha. Cloransulam applied at 18 g/ha was also combined with lactofen at 70 g ai/ha at each POST application timing. At 60 d after LPOST, cloransulam applied PPI provided 68% giant ragweed control in 1996 and 1997 compared to PRE applications, which provided 95 and 25% giant ragweed control, respectively. The reduction in giant ragweed control with PRE applications of cloransulam in 1997 was likely due to insufficient rainfall for activation. Cloransulam applied at 18 g/ha EPOST provided 87 and 88% giant ragweed control, respectively, in 1996 and 1997. Cloransulam applied POST provided 97 and 82% giant ragweed control, respectively, in successive years. Delaying cloransulam application until LPOST reduced giant ragweed control to 53 and 47%, respectively, in 1996 and 1997 compared to EPOST and POST. At EPOST and POST application timings, increasing the rate of cloransulam to 27 g/ha or adding lactofen did not improve giant ragweed control. However, giant ragweed control was improved by at least 20% by increasing the rate of cloransulam to 27 g/ha at LPOST. Similarly, applying cloransulam in combination with lactofen improved giant ragweed control by at least 15% at LPOST.

Research was conducted in 1995 and 1996 to determine the potential for commonly used volunteer corn herbicides to control volunteer sethoxydim-resistant (SR) corn in soybean. Greenhouse studies showed that the SR corn hybrid tolerated 181 times more sethoxydim than the susceptible sister hybrid. SR corn also tolerated other acetyl CoA carboxylase (ACCase) inhibitors including fluazifop-P, quizalofop-P, and clethodim with 30 X, 27 X, and 7 × magnitudes of tolerance, respectively, compared with the susceptible hybrid. SR corn exhibited the least tolerance to clethodim, with a control rating of 50% (CR50) predicted at 28 g ai/ha. Field studies at Dekalb and Urbana, IL, showed that quizalofop-P, fluazifop-P, and fluazifop-P plus fenoxaprop at 62, 140, and 140 + 47 g ai/ha, respectively, controlled 22% or less of volunteer F2 SR corn at 30 days after treatment (DAT). Clethodim at 105, 140, and 210 g/ha consistently suppressed 23 to 70% of the volunteer SR corn. Dry weight reductions at 60 DAT showed the same general trend as the visual estimates of control. The field results confirmed the greenhouse data, which suggested SR corn had the least amount of cross-resistance to clethodim compared to other ACCase-inhibiting herbicides. In 1996, only AC 299,263 and imazethapyr plus imazaquin suppressed volunteer SR corn and prevented soybean yield loss at both locations. However, no system completely controlled volunteer SR corn.

Field experiments were conducted in 1995 and 1996 at DeKalb and Urbana, IL, to evaluate weed management systems in glyphosate-resistant soybean planted in rows 76 cm wide. These experiments compared weed control using preemergence (PRE) herbicides followed by glyphosate or postemergence (POST) tank-mix combinations of glyphosate and acetolactate-synthase-inhibiting herbicides with glyphosate applied alone at 0.63 kg ae/ha in single or sequential applications. Overall, the use of a tank-mix partner or a PRE herbicide followed by glyphosate improved weed control compared to a single application of glyphosate. However, weed control with these treatments was not better than with sequential applications of glyphosate. Control of giant foxtail exceeded 90% for single applications of glyphosate except at DeKalb in 1995 when late emergence of giant foxtail occurred after POST applications had been made. A PRE grass herbicide or a late postemergence (LPOST) application of glyphosate was necessary for season-long control of late-emerging giant foxtail. Tank-mixing glyphosate with imazethapyr, cloransulammethyl, and CGA-277476 or applying glyphosate LPOST following these herbicides improved giant foxtail control compared with these herbicides applied alone. A single application of glyphosate controlled common lambsquarters 88% or greater in two of three trials. At Urbana in 1995, a single application of glyphosate controlled common lambsquarters 78% compared to 88 to 96% control with PRE herbicides followed by glyphosate or sequential applications of glyphosate. Velvetleaf control with a single application of glyphosate ranged from 55 to 78%. A PRE application of chlorimuron + metribuzin, cloransulammethyl, or sulfentrazone followed by glyphosate POST, as well as sequential applications of glyphosate, consistently improved velvetleaf control compared to a single application of glyphosate. In some cases, adding glyphosate to a POST application of imazethapyr or CGA-277476 improved control of velvetleaf but decreased velvetleaf control when added to cloransulammethyl.

Field trials were conducted in 1996 and 1997 to determine the influence of glufosinate and glyphosate application rates, application timings, and interrow cultivation on weed control and corn yield. Glufosinate-ammonium rates ranged from 0.18 to 0.41 kg ai/ha, while rates for the isopropylamine salt of glyphosate ranged from 0.21 to 0.84 kg ae/ha. Increasing rates of glufosinate and glyphosate often improved weed control. Control of many of the weed species was improved by delaying herbicide application timing. Weed control was most consistent from late postemergence (LPOST) applications of glufosinate at 0.41 kg ai/ha or glyphosate at 0.84 kg ae/ha. Corn yields were reduced due to incomplete weed control when the lowest rate of glufosinate was applied. Weed control from early postemergence (EPOST) glufosinate and glyphosate applications followed by cultivation was similar to weed control from LPOST glufosinate and glyphosate applications without cultivation. Interrow cultivation following glufosinate or glyphosate application did not affect corn yield.

A field study was conducted over 2 yr to compare efficacy and economics of glyphosate-resistant, sulfonylurea-tolerant, and conventional soybean (Glycine max) weed control programs. Herbicide programs in the three soybean systems provided at least 90% control of browntop millet (Brachiaria ramosa), prickly sida (Sida spinosa), yellow nutsedge (Cyperus esculentus), pitted morningglory (Ipomoea lacunosa), and hemp sesbania (Sesbania exaltata) in most cases and postemergence (POST)-only programs were as effective as preemergence (PRE) followed by POST programs. Control of hyssop spurge (Euphorbia hyssopifolia) ranged from 93 to 100% in glyphosate-resistant soybean and from 88 to 100% in conventional soybean, but control was 60 to 100% in sulfonylurea-tolerant soybean. Sicklepod (Senna obtusifolia) control was at least 91% in glyphosate-resistant and sulfonylurea-tolerant soybean but was 81% for the standard SAN 582 plus imazaquin PRE and acifluorfen plus bentazon early POST treatment in conventional soybean. In glyphosate-resistant soybean, glyphosate applied sequentially resulted in an average yield of 3,020 kg/ha with a net return of $407/ha. In sulfonylurea-tolerant soybean, chlorimuron applied sequentially yielded 2,500 kg/ha with a net return of $271/ha. Conventional soybean yield with the standard herbicide program was 2,770 kg/ha with a net return of $317/ha. Yields for the cultivars were equivalent when the same standard herbicide program was used. When weed control is satisfactory and herbicide costs relatively comparable, yield potential of the cultivar and seed cost, including any technology fee, would be key factors in selecting a weed management system.

Volunteer potato is becoming increasingly detrimental in potato production regions. We assessed methods to manage the weed in field corn with herbicides and arthropod herbivory. In greenhouse trials, new tuber production was reduced at least 95% in ‘Ranger Russet’, ‘Russet Burbank’, ‘Russet Norkotah’, and ‘Shepody’ potato varieties by mesotrione applied at 0.11 kg/ha. In field studies conducted near Paterson, WA, a single application of mesotrione at 0.07 or 0.11 kg/ha applied at the time of tuber initiation (mid-postemergence [MPOST]) controlled potato top growth 96 to 98% in 2002 and 2003. Mesotrione applied at earlier stages of potato growth, preemergence or early postemergence, controlled potatoes less than mesotrione applied MPOST. All herbicide treatments prevented yield loss in field corn compared with nontreated checks. Mesotrione reduced new potato tubers and tuber weight more than any other herbicide. Herbivory of volunteer potato by Colorado potato beetle reduced tuber number 21% and tuber density 23% in the absence of herbicides and appeared to further suppress the weed in combination with herbicides. Whereas treatments containing mesotrione were most effective against volunteer potato, arthropod herbivory supplemented weed suppression and may be an important component in reduced or low-input weed management systems.

A field study was conducted during 1999, 2000, and 2001 at Stoneville, MS, on a Dundee silty clay loam to determine the impact of a rye cover crop with one or two postemergence (POST) herbicide applications on weed control, yield, and net return in narrow-row glyphosate-resistant, glufosinate-resistant, and conventional soybean systems. Cover crop systems included no–cover crop conventional tillage (CT), no–cover crop no-tillage (NT), and rye NT, all with early POST (EPOST), EPOST followed by late POST (LPOST), and no-herbicide weed management. Weed control and net return among glyphosate-resistant, glufosinate-resistant, and conventional soybean systems were similar. One POST ($111/ha) application of herbicides was more profitable than two POST ($79/ha) applications regardless of soybean cultivar and cover crop system. Rye residue reduced total weed density by 9 and 27% and biomass by 19 and 38% compared with no–cover crop CT and NT, respectively. In the rye cover crop, input costs were higher because of the additional cost of seed, planting, and rye desiccation. The additional cost resulted in a lower net return with the rye cover crop ($29/ha) compared with the no–cover crop CT ($84/ha) or NT ($87/ha) system, even though soybean yield in the rye cover crop system was comparable to that from the no–cover crop CT and NT systems. These results showed that because of additional cost, rye cover crop–based soybean production was less profitable compared with existing no–cover crop–based production systems.

A field experiment was conducted over 2 yr to study efficacy of bromoxynil mixtures with pyrithiobac or MSMA applied postemergence (POST) with and without fluometuron or fluometuron plus pendimethalin preemergence (PRE) for control of broadleaf weeds in ultra narrow row bromoxynil-resistant cotton in the Mississippi Delta. Bromoxynil applied POST (single or sequential) provided variable control of common purslane (<9%), sicklepod (<35%), Palmer amaranth (<46%), prickly sida (>75%), hyssop spurge (>79%), hemp sesbania (>96%), and pitted morningglory (100%) at 4 wk after early POST (WAT). Broadleaf weed control increased when PRE herbicides were followed by bromoxynil or bromoxynil plus pyrithiobac or MSMA POST. Weed control generally decreased at harvest compared to 4 WAT, and the decrease was greater in bromoxynil POST-only programs compared to bromoxynil POST following PRE programs. Seed cotton yield with bromoxynil POST-only programs was lower (400 to 2,810 kg/ha) compared to bromoxynil POST programs following PRE herbicides (2,150 to 3,720 kg/ha). Early-season weed interference and variable control of weeds in bromoxynil POST-only programs resulted in greater cotton stand reduction and lower open bolls per plant compared to bromoxynil POST programs following PRE herbicides.

Field experiments were conducted over 3 yr at three locations in Illinois to evaluate the efficacy of glyphosate in glyphosate-resistant soybean planted in rows spaced 19, 38, and 76 cm. Minimal soybean injury (less than 10%) was observed from any glyphosate treatment. Glyphosate treatments controlled 82 to 99% of giant foxtail. Common waterhemp control was increased as soybean row spacing was decreased. Applying sequential glyphosate applications or increasing the glyphosate rate from 420 g ae/ha to 840 g/ha frequently increased common waterhemp control in 76-cm rows. Velvetleaf control with glyphosate was variable, ranging from 48 to 99%. Decreasing soybean row spacing, utilizing sequential glyphosate applications, or increasing the glyphosate rate improved velvetleaf control in at least four of eight site-years. Glyphosate treatments generally resulted in weed control and soybean yield equal to or greater than the standard herbicide treatments. However, glyphosate treatments yielded less than the hand-weeded control in four of eight site-years, suggesting that weed control from glyphosate treatments was sometimes inadequate.

Experiments were conducted to determine peanut tolerance to CGA-362622 applied preemergence (PRE) and postemergence (POST) and to determine the potential for CGA-362622 applied PRE and POST to cotton to injure peanut grown in rotation the following year. CGA-362622 at 3.75 and 7.5 g ai/ha applied PRE visually injured peanut 11 and 16%, respectively, at 5 wk after treatment (WAT) but did not influence peanut yield. POST treatments at 3.75 and 7.5 g/ha injured peanut 63 and 93%, respectively, at 4 WAT and reduced peanut stand by 53 and 89% at 11 WAT, respectively. Peanut pod yield was reduced 73.1 and 97.9% by CGA-362622 POST at 3.75 and 7.5 g/ha, respectively, compared with the untreated weed-free control. CGA-362622 PRE at 3.75 and 7.5 g/ha reduced peanut pod yield 7.5 and 12.6%, respectively. Cotton was injured 9% or less by CGA-362622 PRE or POST at 3.75 or 7.5 g/ha and up to 25% with CGA-362622 POST at 15 g/ha. However, CGA-362622 did not influence weed-free cotton lint yields, regardless of method or rate of application. Peanuts grown in rotation were not injured, and yields were not influenced by CGA-362622 applied PRE or POST the previous year to cotton.

Field trials were conducted at two sites in both 1997 and 1998 to evaluate soybean response and weed control with glufosinate alone or combined with quizalofop, lactofen, imazethapyr, flumiclorac, or bentazon plus acifluorfen in narrow-row, glufosinate-resistant soybean. Soybean injury ranged from 0 to 21% at 2 wk after treatment (WAT) and from 0 to 5% by 4 WAT. Glufosinate alone at 0.29 and 0.4 kg ai/ha controlled velvetleaf, common waterhemp, common ragweed, morningglory species, and giant foxtail greater than 85% in all studies. Mixtures containing glufosinate and other herbicides controlled these species greater than 81% but did not improve control over glufosinate alone. Estimates of weed biomass closely reflected visual control evaluations. However, giant foxtail biomass was higher for mixtures of glufosinate plus lactofen, flumiclorac, or bentazon and acifluorfen, indicating possible antagonism of glufosinate activity. At both locations, soybean yields were similar among most treatments, but that of the glufosinate plus lactofen treatment was lower when compared with other treatments. Additional trials evaluated soybean response and weed control with a preemergence herbicide followed by glufosinate postemergence (POST), glufosinate applied once or twice POST, and mixtures of glufosinate plus imazethapyr or flumiclorac POST in wide-row soybean. Glufosinate applied twice controlled common waterhemp, morningglory species, prickly sida, common cocklebur, and giant foxtail up to 39% greater than did glufosinate applied once. The addition of imazethapyr, but not flumiclorac, to glufosinate improved weed control when compared with glufosinate alone.

Experiments were conducted from 1998 to 2000 at Rocky Mount, NC, in weed-free environments to determine soybean tolerance to preplant (PP) applications of trifloxysulfuron and the potential for trifloxysulfuron applied preemergence (PRE) and postemergence (POST) to cotton to injure soybean grown in rotation the following year. Trifloxysulfuron at 3.75 and 7.5 g ai/ha applied PP 2 wk before seeding injured conventional soybean less than 5%, whereas no injury was observed when seeding was delayed 4 or 6 wk after PP treatment. No injury to sulfonylurea-resistant soybean (SR) was observed for any treatment. Soybean yields were not influenced by trifloxysulfuron treatment. Cotton injury was 7% or less with trifloxysulfuron applied PRE or POST at 3.75 and 7.5 g/ha. Trifloxysulfuron at 15 g/ha PRE or POST injured cotton a maximum of 14 to 18%. Trifloxysulfuron did not reduce cotton lint yields regardless of method or rate of application. Both conventional and SR soybean were not injured nor were yields influenced by trifloxysulfuron applied PRE or POST the previous year to cotton.

Field studies were conducted from 1995 through 1998 to evaluate citronmelon control with postemergence (POST) herbicides. Imazapic at any application timing and late postemergence (LPOST) applications of 2,4-DB were the only herbicides that provided >80% control of citronmelon late season. Other herbicides such as acifluorfen, imazethapyr, lactofen, and pyridate provided acceptable (>75%) early-season control of citronmelon, but control was inadequate at harvest.

A study was conducted to evaluate weed control with imazethapyr applied at different timings or a combination of timings with or without an early POST application of bensulfuron, bentazon plus aciflurofen, bispyribac, carfentrazone, halosulfuron, propanil plus molinate, or triclopyr in drill-seeded imidazolinone-resistant rice. Control of barnyardgrass, red rice, and alligatorweed was insufficient with imazethapyr at 87 g/ha PRE or at 53 g/ha late POST alone. Imazethapyr at 87 g/ha PRE followed by imazethapyr at 53 g/ha late POST controlled red rice 86% or greater but did not control hemp sesbania. The imazethapyr PRE followed by late POST combination was also weak on barnyardgrass and alligatorweed early in the season. The addition of an early POST application of the other herbicides to the imazethapyr combination improved overall weed control, especially hemp sesbania control. Among the early POST herbicides, bispyribac, carfentrazone, or propanil plus molinate were more effective in helping improve the overall weed control, resulting in better rice grain yield.

Field studies were conducted to evaluate weed control, tuber yield, gross return, economic return on investment (EROI), and net return in glyphosate-resistant ‘Ranger Russet’ potato in 2000 and 2001 at the University of Idaho Aberdeen Research and Extension Center near Aberdeen, ID. Three types of weed control programs were evaluated: a total glyphosate program of single or sequential applications (TGLY), tank mixtures of glyphosate and residual herbicides applied early postemergence (GLY + RES EPOST), and residual preemergence herbicides followed by (fb) a late postemergence glyphosate application (RES PRE fb LPOST GLY). A standard rimsulfuron + metribuzin + nonionic surfactant EPOST treatment was included for comparison. The standard EPOST treatment and all glyphosate-containing treatments controlled hairy nightshade 88 to 99%. RES PRE fb LPOST GLY treatments improved hairy nightshade control compared with the RES PRE components applied alone. All herbicide treatments controlled kochia 87 to 99% and green foxtail 87 to 100%. Redroot pigweed and common lambsquarters were controlled ≥85 and ≥89%, respectively, by all herbicide treatments except a single EPOST application of glyphosate at 420 g ae/ha. Depending on the year, sequential applications of glyphosate, GLY + RES EPOST, or RES PRE fb GLY LPOST treatments controlled weeds better than single EPOST glyphosate applications. Single LPOST glyphosate applications generally controlled kochia, redroot pigweed, common lambsquarters, and green foxtail better than single EPOST applications. However, single EPOST glyphosate applications controlled hairy nightshade better than a single LPOST application of glyphosate at 420 g/ha. RES PRE fb GLY LPOST treatments improved redroot pigweed, common lambsquarters, and green foxtail control, compared with the RES PRE components applied alone, depending on the RES PRE component and the year. Sequential applications of glyphosate at 840 g ae/ha and the standard nonglyphosate EPOST, GLY + RES EPOST, and RES PRE fb GLY LPOST treatments generally provided similar weed control. No crop injury was observed as a result of any herbicide treatment. Sequential applications of glyphosate at 840 g/ha had better tuber yields and economic returns than a single EPOST or LPOST application of glyphosate at 420 g/ha or a single LPOST application of glyphosate at 840 g/ha. A single EPOST application of glyphosate at 420 g/ha had lower tuber yields and economic returns than a single EPOST application of glyphosate at 840 g/ha. The RES PRE alone treatments, except metribuzin + pendimethalin, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha. Glyphosate + rimsulfuron resulted in lower tuber yields than sequential applications of glyphosate at 840 g/ha, whereas EROI and net returns were similar. All other combinations of glyphosate and residual herbicides except glyphosate + pendimethalin EPOST, had similar tuber yields, EROI, and net returns as sequential applications of glyphosate at 840 g/ha.