Field validation studies were conducted in seven Mississippi environments at three application timings to confirm postemergence (POST) recommendations generated by the computer herbicide decision aids HERB and SWC. HERB and SWC agreed on herbicide treatments in only 14% of the location–application timing combinations. Weed scientists involved in the study agreed on treatment recommendations approximately 33% of the time. The HERB model agreed with a faculty member on only one herbicide treatment, while the SWC model was slightly more agreeable in this regard. Subsequent weed flushes, varied production practices, and delayed weed emergence accounted for a majority of the underestimated predictions given by HERB. Only 55% of the predicted values presented for estimated weed control ratings were similar to actual weed control ratings. Over 75% of the predictions that differed from actual weed control values were underpredictions. Recommendations from both computer models were effective in reducing yield loss below that of the untreated check, and recommendations from the HERB model generally improved yield more than those from the SWC model in most instances. HERB and SWC predictions of yield losses with no weed control were not significantly different from the actual yield loss from the untreated check in nine of the 12 instances at Starkville, seven of the 12 instances at Brooksville, six of the 12 instances at Newton, and three of the six instances at Hollandale. The HERB model estimated yield loss similar to that of the actual yield loss 83% of the time, while predictions from the SWC model were accurate 76% of the time. HERB overestimated yield loss in six of 21 application timing–experiment combinations and underestimated yield loss only once. Yield loss was overpredicted as high as 78%. SWC overpredicted yield loss in five of 21 instances and also underestimated in five instances. SWC did not overpredict yield loss to the same magnitude as HERB in many instances.

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).

Soil- and foliar-applied herbicide treatments were evaluated for the control of seedling johnsongrass in the interim between row formation and the planting of fallowed sugarcane fields approximately 90 d later. Soil-surface applications of metribuzin at 1,680 g ai/ha, pendimethalin plus atrazine each at 2,240 g ai/ha, terbacil at 1,680 g ai/ha, and sulfometuron at 35 and 70 g ai/ha and an incorporated application of trifluralin at 2,240 g ai/ha followed by a surface application of atrazine at 2,240 g/ha did not consistently control seedling johnsongrass until the crop was planted. Rhizome johnsongrass populations originating from seedling johnsongrass that escaped the fallow treatments were lowest in the newly planted crop when sulfometuron at 140 to 280 g/ha was applied to the soil surface and when glyphosate was applied POST at 2,240 g ai/ha, particularly as a sequential treatment alone or as a spot treatment at 2% by volume following applications of metribuzin, terbacil, and pendimethalin or trifluralin with atrazine. Sugarcane shoot populations in the fall after planting and sugar yields at the end of the crop's first growing season were highest where fallow treatments minimized johnsongrass development. These treatments also provided broad spectrum control of other seedling weeds, the residues of which influenced crop development.

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.

Johnsongrass control systems that included commercially available corn herbicides applied to crop sequences of continuous corn or corn–cotton–corn rotation were evaluated over a 3-yr period at the Texas Agricultural Experiment Station in Burleson County. Herbicide treatments evaluated were EPTC + R-25788 applied preplant incorporated (PPI); metolachlor applied preemergence (PRE); nicosulfuron applied postemergence (POST); and the combination of metolachlor PRE and glyphosate POST applied after corn harvest. Weed control practices applied to plots planted to cotton in the second growing season provided excellent johnsongrass control (≈ 95%). Visual control ratings indicated that nicosulfuron provided the best johnsongrass control (> 80%) among the herbicides evaluated all three years. Metolachlor and glyphosate combinations provided acceptable johnsongrass control the third year of the experiment in both cropping sequences. Corn grain yield and johnsongrass control were best in cotton-rotated plots. The highest returns observed in continuous corn were from either nicosulfuron or the combination of metolachlor and glyphosate. Net returns for all control systems in corn–cotton–corn rotated plots were greater than the weedy check and were equivalent to the lead systems in continuous corn.

Field experiments conducted in 1992 and 1993 evaluated transplanted watermelon tolerance to ethalfluralin applied PPI, PRE (before transplanting), and POST (immediately after transplanting) at 1.2 or 2.4 kg ai/ha. Other treatments for comparison included the registered herbicides ethalfluralin POST-directed spray (PDS), ethalfluralin PDS followed by (fb) naptalam POST, bensulide plus naptalam PPI, and a nontreated check. All treatments controlled common lambsquarters and goosegrass 83 to 100% 2 and 6 weeks after treatment (WAT). Watermelon was injured 30 to 77% in 1992 and 14 to 83% in 1993 by ethalfluralin PPI or PRE at 1.2 or 2.4 kg/ha. Ethalfluralin POST was not injurious to watermelon. In 1992, watermelon treated with ethalfluralin POST at 1.2 and 2.4 kg/ha yielded 52 to 62% more fruit than watermelon from the nontreated check. In 1993, yield of transplanted watermelon treated with ethalfluralin POST was similar to that in the nontreated check.

Cloransulam-methyl applied postemergence (POST) following various preplant-incorporated (PPI) herbicides was evaluated in four experiments for weed control in North Carolina soybean over a 2-yr period at three locations. Acifluorfen plus bentazon or chlorimuron alone applied POST injured soybean more than cloransulam-methyl when following any soil-applied herbicide. When following trifluralin PPI, cloransulam-methyl controlled common ragweed, entireleaf morningglory, and pitted morningglory comparable to acifluorfen plus bentazon or chlorimuron. Common lambsquarters and prickly sida control was higher when acifluorfen plus bentazon was applied POST following trifluralin PPI compared to trifluralin PPI followed by cloransulam-methyl or chlorimuron. Acifluorfen plus bentazon or chlorimuron POST controlled yellow nutsedge and smooth pigweed more than cloransulam-methyl POST when following trifluralin PPI. When trifluralin was applied PPI in mixtures with chlorimuron plus metribuzin, flumetsulam, or imazaquin, control of most species was similar regardless of POST treatment used. Soybean treated with cloransulam-methyl yielded 250 kg/ha more than treatments with chlorimuron when these herbicides followed trifluralin plus flumetsulam or trifluralin plus imazaquin. Net returns with different herbicide systems followed trends similar to soybean yield.

Field studies were conducted at two locations in 1997 and 1998 to evaluate crop injury, weed control, yield, and net economic returns of single and sequential postemergence applications of labeled and reduced rates of glyphosate to no-till, glyphosate-resistant soybean planted in narrow rows. Sequential applications provided at least 91% control of giant foxtail, while single applications provided at least 86% control with labeled rates and 68–93% control with reduced rates. Common waterhemp control was slightly higher with sequential vs. single treatments and with labeled rates vs. reduced rates. Velvetleaf control was greater than 96% with all treatments. Common cocklebur control was 90% or higher with all treatments except a single application of glyphosate at 210 g/ha. Lower control of giant foxtail and common waterhemp with single-application, reduced-rate treatments in two of the four trials resulted in lower yields. Overall, sequential applications, regardless of rate, provided greater weed control, yield, and net income and lower coefficients of variation (C.V.s) of net income than reduced-rate single applications. Single-application treatments showed a trend of decreased weed control, yield, and net income and higher C.V.s of net income with reduced rates of glyphosate.

Field studies were conducted in 1991 to 1993 to determine the effects of PRE and POST herbicides alone and in factorial combinations on sicklepod control, weed biomass, and soybean seed yields. Herbicides evaluated were metribuzin PRE, metribuzin + chlorimuron PRE, imazaquin PRE and POST, chlorimuron POST, and AC 263,222 POST POST herbicides alone provided better sicklepod control than PRE herbicides alone. However, sequential (SEQ) treatments [PRE followed by (fb) POST] provided better sicklepod control than single herbicide treatments. Metribuzin and metribuzin + chlorimuron alone or used in a SEQ program provided similar PRE control of sicklepod, and both controlled sicklepod better than imazaquin in 1991 and 1992. Chlorimuron alone or in a SEQ program provided better POST control than AC 263,222 or imazaquin alone when soil moisture levels at planting favored weed seed germination. When soil moisture levels at planting were unfavorable for weed seed germination, the residual activity of imazaquin provided the best season-long control. SEQ herbicide applications allowed soybean to attain higher seed yields than single (PRE or POST) herbicide treatments all 3 yr. However, only metribuzin fb chlorimuron in 1991 and 1992 and imazaquin fb imazaquin and imazaquin fb chlorimuron in 1993 allowed soybean to achieve seed yields similar to weed-free soybean. Soybean seed yields were greater with POST than with PRE herbicides 2 of 3 yr, but no single herbicide treatment provided soybean seed yields similar to weed-free soybean.

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 studies were conducted at three locations in 1993 and 1994 to evaluate weed control and crop response to combinations of glyphosate, metolachlor, 0.5 X and 1 X label rates of chlorimuron plus metribuzin applied prior to planting (PP), and 0.5 X and 1 X label rates of imazethapyr applied early postemergence (EPOST) or postemergence (POST) in no-till narrow-row soybean production. Giant foxtail densities were reduced with sequential PP followed by (fb) EPOST or POST treatments. Large crabgrass was reduced equivalently with all herbicide combinations involving chlorimuron plus metribuzin PP fb imazethapyr. Common cocklebur control was variable but was usually greater with treatments that included imazethapyr. Ivyleaf morningglory densities were not reduced with any herbicide combinations. Sequential PP fb EPOST or POST treatments tended to provide slightly better weed suppression than PP-only treatments, but the difference was rarely significant. Soybean yields with treatments utilizing 0.5 X rates were usually equal to 1 X rates.

In field studies in 1991, 1992, and 1993, rimsulfuron at 26 and 35 g ai/ha, sequentially at 26 g/ha, at 26 g/ha plus metribuzin at 280 g ai/ha, and metribuzin at 280 g/ha were evaluated POST for weed control in transplanted ‘Agriset’ tomato. Common lambsquarters was controlled by rimsulfuron at 35 g/ha. Rimsulfuron plus metribuzin gave consistent control of common ragweed, but jimsonweed control was inconsistent and goosegrass control was generally low. Rimsulfuron treatments caused slight (< 12%) temporary injury to new terminal growth of tomato. Yield of tomato fruit was consistently high in the metribuzin, metribuzin plus rimsulfuron, and rimsulfuron sequential treatments. In greenhouse studies, giant foxtail and large crabgrass control by rimsulfuron was above 95 and 85%, respectively, but goosegrass was not controlled. Height of four tomato cultivars was not reduced, but dry weight of ‘Floradade’ and ‘Sunbeam’ was reduced by rimsulfuron.

Premergence (PRE) herbicides were applied to a bermudagrass golf green overseeded with ‘Laser’ roughstalk bluegrass during 1995 and 1996 to determine their effects on roughstalk bluegrass injury and on the transition from a mixed cool- and warm-season grass back to the permanent bermudagrass. Roughstalk bluegrass tolerated single March applications of trifluralin plus benefin at 2.2 kg/ha, pendimethalin at 3.4 kg/ha, dithiopyr at 0.6 kg/ha, prodiamine at 0.8 kg/ha, bensulide plus oxadiazon at 8.4 kg/ha, and bensulide at 11.2 kg/ha. A single March application of oxadiazon plus benefin at 3.4 kg/ha severely injured (35%) the overseeded turf in 1 yr, but not when 1.7 kg/ha was applied in March and repeated in May. One-half recommended rates of oxadiazon and bensulide plus oxadiazon applied in March did not cause any undesirable injury, but when repeated in May, moderate to severe injury (28 to 33%) occurred in June. Oryzalin at ≥ 1.1 kg/ha, benefin plus oryzalin at ≥ 1.1 kg/ha, and oxyfluorfen plus oryzalin at ≥ 1.7 kg/ha severely injured (≥ 35%) roughstalk bluegrass. The rate of transition of mixed roughstalk bluegrass and bermudagrass back to permanent bermudagrass was not affected by any of the PRE herbicides except when oryzalin was applied alone or in combination with benefin or oxyfluorfen.

Field studies were conducted at three sites in 1993 and 1994 to evaluate weed control, crop response, and economic returns to 0.5× and 1× postemergence rates of chlorimuron, chlorimuron plus quizalofop, bentazon plus acifluorfen, fluazifop plus fenoxaprop, imazethapyr, and imazethapyr plus clethodim in no-till narrow-row soybean production. Common lambsquarters, ivyleaf morningglory, and common ragweed were controlled equally with 0.5× rates applied early postemergence and 1× rates applied mid-postemergence. Control of giant foxtail, large crabgrass, and common cocklebur was generally greater with 1× rates mid-postemergence than with 0.5× rates early postemergence. Soybean yields were equivalent with 0.5× and 1× rates. Soybean yields and net income were highest with 1× and 0.5× rates of chlorimuron plus quizalofop and imazethapyr plus clethodim, and metolachlor preemergence (PRE) followed by 0.5× bentazon plus acifluorfen.

Field trials were conducted in no-till field corn in northern Missouri to compare weed control and crop response of metolachlor plus atrazine applied under five preplant herbicide management practices. The practices consisted of (1) applying the entire dose of metolachlor plus atrazine 15, 30, or 45 d early preplant (EPP), (2) applying a split application of an EPP (67%) followed by (fb) a PRE (33%), (3) applying the entire dose PRE, (4) applying metolachlor PRE fb dicamba early post (EPOST), or (5) applying atrazine alone EPOST. Weed control at 7 wk after planting was more variable with EPP treatments than with EPP fb PRE or PRE treatments. Giant foxtail and fall panicum control was greatest with treatments that included metolachlor PRE. Velvetleaf and common cocklebur control was greatest with atrazine or dicamba EPOST and was generally unacceptable (< 80%) with any soil-applied herbicide treatment. Common lambsquarters control was greater than 90% with all treatments that included metolachlor, atrazine, or dicamba, regardless of timing. Corn yields were slightly higher with EPP fb PRE and PRE fb EPOST dicamba than with EPP alone. The results suggest that herbicide applications made nearer to the date of planting provide less variable weed control and corn yield than treatments made more than 15 d EPP.

Field studies were conducted at five locations in North Carolina and Virginia during 1996 and 1997 to evaluate weed control, peanut (Arachis hypogaea) response, and peanut yield following diclosulam applied preplant incorporated (PPI) and in systems with commercial herbicide standards. All plots received a PPI treatment of ethalfluralin at 840 g ai/ha. Ethalfluralin plus diclosulam controlled entireleaf morningglory (Ipomoea hederacea var. integriuscula), ivyleaf morningglory (I. hederacea), pitted morningglory (I. lacunosa), common lambsquarters (Chenopodium album), eclipta (Eclipta prostrata), and prickly sida (Sida spinosa) as well as and frequently better than ethalfluralin PPI followed by (fb) acifluorfen plus bentazon postemergence (POST), paraquat plus bentazon early postemergence (EPOST) fb imazapic POST, or imazapic POST. Systems with ethalfluralin plus diclosulam PPI at 26 g ai/ha fb acifluorfen plus bentazon POST controlled a broader spectrum of weeds and yielded greater than systems of ethalfluralin PPI fb imazapic POST or ethalfluralin PPI fb acifluorfen plus bentazon POST. Peanut exhibited excellent tolerance to diclosulam PPI at 17, 26, or 35 g/ha.

Experiments were conducted in 1991 and 1992 to evaluate the weed control effectiveness from several rates of AC 263,222 applied PPI and PRE (36 and 72 g ai/ha), early POST (EPOST) (18, 36, 54, or 72 g/ha), POST (18, 36, 54, or 72 g/ha), and EPOST followed by (fb) POST (27 fb 27 g/ha or 36 fb 36 g/ha). These treatments were compared to the commercial standard of bentazon at 0.28 kg ai/ha plus paraquat at 0.14 kg ai/ha EPOST fb bentazon at 0.56 kg/ha plus paraquat at 0.14 kg/ha plus 2,4-DB at 0.28 kg ae/ha. Application method had little effect on weed control with AC 263,222. In contrast, application rate affected control. Purple nutsedge, yellow nutsedge, prickly sida, smallflower morningglory, bristly starbur, common cocklebur, and coffee senna were controlled at least 82% with AC 263,222 at 36 g/ha (one-half the maximum registered use rate) regardless of application method. AC 263,222 at 72 g/ha (registered use rate) controlled sicklepod 84 to 93%, Florida beggarweed 65 to 100%, and Ipomoea morningglory species 89 to 99%. A single application of AC 263,222 at 36 g/ha or more controlled all weeds (with the exception of Florida beggarweed) as well or greater than sequential applications of bentazon plus paraquat fb bentazon, paraquat, and 2,4-DB. All rates of AC 263,222 applied POST and all application methods of AC 263,222 at 72 g/ha had better yields than the pendimethalin control.

A field study was conducted in 1994 and 1995 to determine the effect of MON 12037, linuron, and ametryn on purple nutsedge density in field corn in a 2-yr rotation with cotton. Each corn treatment included cultivation and noncultivation. A standard treatment for purple nutsedge control was applied to all cotton plots in the second season of the corn–cotton rotation. Purple nutsedge shoot densities were reduced in cultivated MON 12037, noncultivated MON 12037, and cultivated ametryn in the corn–corn and corn–cotton rotations. Purple nutsedge tuber population was reduced in the corn–corn rotation by MON 12037 with and without cultivation, cultivated linuron, and cultivated ametryn treatments relative to the noncultivated check. In the corn–cotton rotation, purple nutsedge tuber population was reduced only by the cultivated MON 12037 and cultivated ametryn treatments relative to the noncultivated check.

Field studies were conducted at five locations in North Carolina and Virginia in 1996 and 1997 to evaluate weed control and peanut (Arachis hypogaea) response to diclosulam that was applied preemergence (PRE) and in systems with commercial standards. All plots received a preplant incorporated (PPI) treatment of ethalfluralin at 840 g ai/ha. Diclosulam controlled common lambsquarters (Chenopodium album L.), eclipta (Eclipta prostrata L.), entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray), ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.], pitted morningglory (Ipomoea lacunosa L.), and prickly sida (Sida spinosa L.) as well as and frequently better than the commercial standards of acifluorfen plus bentazon applied postemergence (POST), paraquat plus bentazon early POST followed by (fb) imazapic POST, or imazapic POST. Systems with ethalfluralin PPI plus diclosulam PRE at 26 g ai/ha fb acifluorfen plus bentazon POST controlled a broader spectrum of weeds and yielded greater than systems of ethalfluralin PPI fb imazapic POST or ethalfluralin PPI fb acifluorfen plus bentazon POST. Peanut exhibited excellent tolerance to diclosulam PRE at 17, 26, or 35 g/ha.

Studies were conducted at Clayton, Goldsboro, and Rocky Mount, NC, to evaluate weed and cotton response to herbicide programs in glyphosate-resistant cotton. Just prior to cotton harvest, programs containing norflurazon, trifluralin, fluometuron, glyphosate, MSMA, cyanazine, and/or pyrithiobac in various combinations controlled common lambsquarters, common ragweed, goosegrass, ivyleaf morningglory, and smooth pigweed at least 94%. Glyphosate-resistant cotton injury was no more than 5%. Yields for glyphosate programs differed only at Clayton, where glyphosate programs containing residual herbicides yielded more than glyphosate alone. Depending on location, programs utilizing glyphosate as needed required a minimum of two and a maximum of four applications to prevent yield loss when minimal soil-applied herbicides were used. Other as-needed programs required one-three glyphosate applications, depending on location. For comparison based on application, herbicide, and adjuvant costs, the standard program of trifluralin preplant incorporated (PPI), pyrithiobac postemergence (POST), and fluometuron plus MSMA postemergence-directed (PD) was $119/ha compared with trifluralin PPI followed by (fb) two applications of glyphosate ($54/ha) or four applications of glyphosate ($94/ha).