Glyphosate was compared with other commonly used corn herbicides at three locations in Kentucky in 1998 and 1999. Sequential glyphosate treatments provided greater than 87% control of common cocklebur, ivyleaf morningglory, common lambsquarters, and giant ragweed. Control of these species with glyphosate treatments was similar to the control with atrazine plus other postemergence (POST) herbicides. Generally, treatments containing s-triazines and chloracetamides applied to the soil surface were not as effective as sequential glyphosate or atrazine plus POST herbicides. Corn yield was not significantly different at any location or for any year, whereas differences in net return occurred at one location in 1998.

Greenhouse studies were initiated to evaluate glyphosate alone and tank-mixed with acifluorfen, CGA 277476, chlorimuron, cloransulam-methyl, fomesafen, imazaquin, or pyrithiobac on seedling johnsongrass, broadleaf signalgrass, pitted morningglory, and hemp sesbania. Johnsongrass and broadleaf signalgrass control by glyphosate was not affected by the selective herbicides applied in mixtures. Pitted morningglory control by glyphosate ranged from 0% with 280 g ai/ha to 67% with 840 g/ha. There was an additive effect when selective herbicides were added to 280 g/ha glyphosate 2 wk after treatment (WAT). When acifluorfen was added to 560 g/ha glyphosate, pitted morningglory control 2 WAT increased to 100% compared with 55% with glyphosate alone. Similarly, the addition of fomesafen or acifluorfen to 840 g/ha glyphosate controlled pitted morningglory 2 WAT by 90 and 98%, respectively, compared with 67% with glyphosate alone. Only tank mixtures of acifluorfen, CGA 277476, or fomesafen, and 840 g/ha glyphosate reduced fresh weight compared with glyphosate alone 4 WAT. Acifluorfen, CGA 277476, and fomesafen controlled pitted morningglory by 85 to 100% when added to 1,120 g/ha glyphosate. Both acifluorfen and fomesafen effectively controlled hemp sesbania without the addition of glyphosate 2 WAT. Chlorimuron and pyrithiobac added to 1,120 g/ha glyphosate increased hemp sesbania control to 88 and 99%, respectively, compared with 45% with glyphosate alone 2 WAT. CGA 277476, cloransulam-methyl, imazaquin, and pyrithiobac were antagonistic to hemp sesbania fresh weight reduction when compared with the expected response, but fresh weights were still reduced more than with the same rate of glyphosate alone.

Studies were conducted from 1992 to 1995 to determine the effect of rimsulfuron on corn spurry, wild radish, narrowleaf goldenrod, and quackgrass and potato cultivars. Rimsulfuron alone gave 100% control of wild radish, 68 to 70% control of corn spurry, and 75 to 90% control of narrowleaf goldenrod. Rimsulfuron plus metribuzin gave 99 to 100% control of wild radish and 85 to 100% control of corn spurry. Quackgrass control varied from 51 to 90% between experiments, indicating that environmental and plant factors affected the activity of the herbicide. Tank mixes with metribuzin slightly reduced the control of quackgrass but resulted in greater foliar injury than that of rimsulfuron to potato. Slight injury to potato plants disappeared within 2 to 3 wk, and tuber yields were not reduced. Russet Burbank, Shepody, and Kennebec potato cultivars had slight (2 to 8%) injury from rimsulfuron. Injury to Shepody (14 to 20%) and Russet Burbank (11%) increased when rimsulfuron was applied with metribuzin. Rimsulfuron had no effect on the number or the fresh weight of sprouts on daughter tubers tested the next spring after application.

A study was conducted to determine the dose–response of purple nutsedge tuber sprouting and plant growth to glyphosate. Tuber sprouting on day 6 satisfactorily fitted the probit model, with a predicted effective dosage causing 50% inhibition of response (ED50) of 30 mM, which is similar to that with 25 mM of ED50 on day 3. In addition, together with the dose–response analysis of bud growth on day 6, it was shown that bud growth of purple nutsedge was more sensitive to root- or tuber-absorbed glyphosate or both than was tuber sprouting; the former had 1.25 mM of ED50. During the V1–2 stage, purple nutsedge injury, including shoot dehydration, wilting, and yellowing, appeared sequentially when glyphosate was absorbed. When compared with the dose–response of shoot greenness of purple nutsedge at the V5–7 stage under the root-absorbed glyphosate treatment, purple nutsedge at the V1–2 stage was less sensitive to root-applied glyphosate. Further studies for determining the efficacy of glyphosate applied on root and foliage of purple nutsedge at the V5–7 stage showed that injury from glyphosate occurred within 7 d after foliar treatment, and the ED50 values of glyphosate for shoot survival and leaf chlorophyll were 13.1 and 14.5 mM, respectively. However, the response of plants was less sensitive to root-absorbed glyphosate, which had an ED50 of 101 mM. This finding might be the result of direct injury of roots caused by glyphosate, resulting in the delayed and diluted effect of this herbicide from root to shoot.

Field experiments evaluated halosulfuron and glyphosate for yellow nutsedge control in glyphosate-resistant field corn in 1997 and 1998. Treatments included single and sequential glyphosate applications with or without halosulfuron. Single glyphosate applications provided less than 75% yellow nutsedge control. Sequential applications with at least 1.68 kg ae/ha of glyphosate provided 85% or greater yellow nutsedge control 82 or 115 d after treatment (DAT). Halosulfuron was required to consistently obtain 80% or greater yellow nutsedge control. Nearly all treatments resulted in 90% or greater velvetleaf control 82 or 115 DAT. At the same rating times, giant foxtail control was 95% or greater for sequential glyphosate treatments and treatments containing acetochlor. Corn treated with sequential glyphosate applications containing at least 1.26 kg/ha of glyphosate or containing halosulfuron resulted in greater corn yields than with single glyphosate applications. Halosulfuron was required for consistent yellow nutsedge control, but halosulfuron did not control grasses.

Field experiments were conducted in 1997 and 1998 to evaluate the effect of mowing followed by hexazinone for West Indian dropseed/giant smutgrass (Sporobolus indicus var. pyramidalis) (hereafter referred to as dropseed) control. The experimental design was a split plot, with mowing (nonmowed [mature] and 35-cm regrowth) as the whole plot and hexazinone rate (0.0 [control], 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha) as the subplot treatments. The application of 0.84 kg/ha hexazinone provided 94 and 81% dropseed control, 365 d after treatment (DAT) during 1997 and 1998, respectively. Increasing application rate to 1.12 kg/ha hexazinone provided 87 and 88% dropseed control, 365 DAT during 1997 and 1998, respectively. Both the 0.84 and 1.12 kg/ha rates provided the same average control (87.5%); however, the 1.12 kg/ha rate provided consistent control over years. Mowing dropseed, followed by hexazinone application at 35-cm regrowth, provided no additional control when compared with no mowing treatments. Rates of hexazinone at 1.40 and 1.68 kg/ha caused phytotoxicity to bahiagrass and increased bare soil surface area, especially 90 and 120 DAT. Phytotoxic effect on bahiagrass and on bare soil decreased 365 DAT, resulting in 75 to 80% total forage cover. Concentration and yield of total nonstructural carbohydrates were significantly lower for the mowed 35-cm regrowth treatment than for the nonmowed plants; however, even in its weakened condition this reduction had no effect on dropseed control.

A field experiment was conducted in 1998 and 1999 at Keiser, AR, to evaluate glyphosate timing and soybean population in reducing hemp sesbania and pitted morningglory interference with dryland and irrigated glyphosate-resistant soybean under a narrow row, no-till production system. Soybean densities following emergence were 247,000, 475,000, and 729,000 plants/ha. Glyphosate was applied at 0.56 kg ai/ha at the V2; V4; V2 and V4; and V2, V4, and R2 stages of soybean. In dry portions of the growing season, glyphosate increased moisture availability for dryland soybean because of control of hemp sesbania and pitted morningglory. As soybean population increased from 247,000 to 729,000 plants/ha, pitted morningglory and hemp sesbania control increased from 60 to 91%, respectively, for the V2 glyphosate application. Control of both species at 14 wk after emergence was at least 90% following the V4 alone treatment and sequential applications, with no differences in control among soybean populations. Untreated, irrigated hemp sesbania produced up to 32 million seeds/ha in 1999. Irrigation did not influence pitted morningglory seed production either year, and untreated pitted morningglory produced a maximum of 1 million seeds/ha in 1998 at 247,000 soybean plants/ha. Three sequential applications of glyphosate reduced pitted morningglory seed production to 9,000 seeds/ha and eliminated hemp sesbania seed production. Soybean yielded 1,297 kg/ha greater under irrigated than dryland conditions, whereas increasing soybean density from 247,000 to 729,000 plants/ha resulted in 416 kg/ha improvement in seed yield. At the densities of pitted morningglory and hemp sesbania present in this study, seed yield of drill-seeded soybean can be maximized following a V4 alone treatment or sequential glyphosate applications.

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.

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss.

Field studies were conducted at two locations in North Carolina in 1999 and 2000 to evaluate weed and bromoxynil-resistant cotton response to bromoxynil, pyrithiobac, and MSMA applied early postemergence (EPOST), alone or mixtures in all combinations (two way and three way), and to prometryn plus MSMA applied late postemergence directed (LAYBY). Trifluralin preplant incorporated followed by fluometuron preemergence controlled common lambsquarters, eclipta, and smooth pigweed at least 90%. These herbicides also provided greater than 90% common ragweed control at two locations but only 65% control at a third location. Pyrithiobac and pyrithiobac plus MSMA EPOST increased sicklepod control more than did bromoxynil or bromoxynil plus MSMA EPOST. Bromoxynil and pyrithiobac were more effective for sicklepod control when applied in mixture with MSMA. Bromoxynil plus pyrithiobac EPOST or with MSMA controlled (≥ 90%) common lambsquarters, common ragweed, entireleaf morningglory, prickly sida, and smooth pigweed early season. But the LAYBY treatment of prometryn plus MSMA frequently improved late-season control of entireleaf morningglory, large crabgrass, prickly sida, and sicklepod. A tank mixture of MSMA plus bromoxynil or pyrithiobac and the three-component tank mixture (bromoyxnil, MSMA, plus pyrithiobac) provided a broader weed control spectrum than did either bromoxynil or pyrithiobac alone. Cotton lint yields were increased with all postemergence systems, and the LAYBY treatment of prometryn plus MSMA increased cotton yields in 13 out of 16 comparisons. High cotton yields were indicative of high levels of weed control.

Broomsedge control studies were conducted on six broomsedge-infested pastures in southeastern Oklahoma from 1995 to 1997. Glyphosate applied in spring at 2.24 kg ai/ha decreased broomsedge plant density by 58% 3 mo after treatment (MAT), on areas where the previous year's forage was grazed, and by 95% 3 MAT, where spring fire had removed the old top-growth before glyphosate application. Broomsedge plant density was not affected where glyphosate was applied in spring to sites with old-standing top-growth. Paraquat applied in spring at 0.56 kg ai/ha and spring burning without a herbicide treatment had no effect on broomsedge plant density. Glyphosate at 0.56 and 1.12 kg ai/ha applied in late summer reduced the number of broomsedge stems 1 yr after treatment (YAT) by an average of 65 and 80%, respectively. Paraquat at 0.56 kg/ha applied in late summer of 1995, followed by burning 1 wk after treatment (WAT), decreased broomsedge stem density by more than 60% 1 YAT at four of six locations when compared with mowing in late summer. Burning in November after an October frost decreased broomsedge stem density by more than 47% 1 YAT at four locations. Two consecutive years of burning after frost and paraquat applied in late summer followed by burning 1 WAT reduced broomsedge dry matter production by 68 and 96%, respectively, when compared with mowing in late summer. These data suggest that good to excellent control of established broomsedge is possible with herbicides alone, with a combination of herbicides and late-summer burning, and with fall burning after an early frost in a dry fall. However, broomsedge control was short-lived with all the treatments because of the establishment of new broomsedge seedlings. Thus, it will be important to integrate the destruction of broomsedge plants with proper fertility and grazing management in order to provide satisfactory broomsedge control.

Weed management strategies are needed to control weeds in corn that emerge after postemergence (POST) application of glyphosate or glufosinate. Field trials were conducted from 1996 to 1999 to determine if residual herbicides could be used with glyphosate or glufosinate to provide season-long weed control in glyphosate-resistant or glufosinate-resistant corn. Preemergence (PRE) applications of several residual herbicides followed by POST applications of glyphosate or glufosinate were compared with POST tank mixtures of glyphosate or glufosinate with residual herbicides. All residual herbicides used in combination with glyphosate, when compared with glyphosate alone, increased control of redroot pigweed and common lambsquarters by an average of 20% and resulted in a 4 to 19% increase in control of giant foxtail. Each residual herbicide tank mixture with glufosinate, when compared with glufosinate alone, increased control of common ragweed and giant foxtail. In most instances, weed control was similar for total POST and for PRE followed by POST systems. Velvetleaf control was reduced by 12% when glyphosate was tank mixed with a half rate of atrazine compared with a full rate of atrazine. Common lambsquarters control was reduced by 13% when glyphosate was tank mixed with a half vs. a full rate of acetochlor. Glufosinate tank mixed with half rates of acetochlor + atrazine, flumetsulam, or pendimethalin reduced control of velvetleaf compared with the full rate of these products. Corn yields were not affected by residual herbicide application timings or rates of residual herbicides.

Field experiments were conducted in 1996 and 1997 to evaluate the effects of six johnsongrass densities on picker- vs. stripper-harvest efficiency, fiber properties, loan rate, and lint yield loss of cotton. The weed densities employed were 0 (the check), 3, 4, 5, 8, and 15 plants/15 m of row. With three or fewer weeds in 1996 and four or fewer in 1997, harvest efficiencies were 4.9 to 7.6% higher for stripper- than for picker-harvested cotton. At four and higher weed densities in 1996 and at five and higher in 1997, differences in harvest efficiency between the two machines were not significant. For each weed per 15 m of row, stripper-harvest efficiency in 1996 and 1997 was reduced 0.3 and 0.6%, respectively; picker-harvest efficiency was not affected by the johnsongrass densities included herein. Fiber fineness (i.e., micronaire) was significantly reduced at densities of 8 weeds/15 m of row in 1997 and at 15 weeds in both years. A questionable increase in staple length was detected at the 3-weed density in 1996. Reductions in fiber strength were noted in 1997 at densities of 3, 8, and 15 weeds/15 m of row. No influences on fiber length uniformity were shown. In 1996 the loan rate for picker-harvested lint was 570 points/kg higher than for stripper-harvested lint at 8 weeds/15 m of row. In 1997 it was 741, 801, 1,058, 1,225, 1,074, and 1,329 points/kg higher at weed densities of 0, 3, 4, 5, 8, and 15 plants/15 m of row, respectively. In 1997 picker-harvest loan rate was reduced 49 points/kg of lint, and stripper-harvest loan rate was reduced 85 points. Over both years, picker-harvest lint yield was reduced 32 to 43 kg/ha (3.9 to 5.5%) for each weed per 15 m of row, and stripper-harvest lint yield was reduced 29 to 43 kg/ha (3.5 to 5.2%).

Five weed management systems utilizing combinations of cover crops, herbicides, and cultivation were tested in potato in 1994 and 1995 in central Washington. A standard herbicide treatment of metribuzin applied preemergence (PRE) at 0.4 kg ai/ha (standard herbicide weed management system [STD]) was compared with (1) fall-planted winter rye followed by metribuzin at 0.4 kg/ha applied in a band in the potato hill, followed by reservoir tillage (rye cover crop with herbicide-banded and reservoir-tilled weed management system [RYESTD]); (2) cultivation with tine-tooth harrow followed by hilling with shovels and rolling cultivators (total-cultivation weed management system [CULT]); (3) fall-planted rapeseed followed by reservoir tillage (rapeseed cover crop and reservoir-tilled weed management system [RPSD]); and (4) reservoir tillage alone (reservoir-tilled weed management system [RESTIL]). In both years, early-season weed density and final weed biomass were lower in the STD, RYESTD, and CULT systems than in RPSD and RESTIL. Total tuber yield and yield of U.S. #2 or better were greatest in the RYESTD and STD systems in both years. The CULT system reduced early-season weed densities, but tuber yield was reduced 15% in 1994, and yield of U.S. #2 or better was reduced 25% in 1995 compared with the STD system. The RPSD system reduced early-season in-row weed density from 60 to 70% and final weed biomass from 29 to 40% compared with a nontreated check, i.e., a no–cover crop, no-cultivation, no-herbicide weed management system, but tuber yield was 27 to 30% lower than in the STD system. The RYESTD system was an effective alternative weed management strategy that controlled weeds, decreased PRE-applied herbicide inputs 66%, and maintained tuber yield.

Woolly cupgrass seeds are dormant at physiological maturity and lose dormancy while afterripening in winter. Field experiments were conducted with the seeds collected from the locations across Iowa in the fall of 1991 and 1992. The seeds were buried in mid-November 0, 5, and 15 cm below the soil surface. Seeds were extracted biweekly until mid-February and germination tests conducted. Seeds buried below the soil surface were less dormant when compared with seeds on the soil surface. There were also dormancy differences between locations. Differences observed between locations and variations from 1991 to 1992 indicated that the parent plant environment, and the likely genotype, had a significant effect on woolly cupgrass seed dormancy.

Field and greenhouse studies were conducted to evaluate herbicides in pumpkin. Field experiments at three Illinois locations determined weed control and crop injury from clomazone, dimethenamid, ethalfluralin, sulfentrazone, imazamox, RPA 201772, flumiclorac, and halosulfuron applied preemergence. Clomazone plus sulfentrazone controlled redroot pigweed 78 to 99%, ivyleaf morningglory 80 to 97%, common lambsquarters 97%, common purslane 84 to 99%, and velvetleaf 55 to 99%. Imazamox plus clomazone commonly provided more consistent broadleaf weed control than did ethalfluralin plus clomazone. RPA 201772 when used on sandy soils killed the pumpkin. Sulfentrazone initially caused chlorosis and necrosis on pumpkin seedlings when soil organic matter was low (< 1%) or when soil moisture was high. However, plants recovered rapidly, and yields were not affected. In a greenhouse study pumpkin was more tolerant to clomazone plus sulfentrazone than to sulfentrazone.

Experiments were conducted at three locations in North Carolina in 1999 and 2000 to evaluate weed management systems in strip-tillage peanut. Diclosulam was evaluated with standard preemergence (PRE), early postemergence, and postemergence (POST) herbicide systems in a factorial treatment arrangement. Preemergence treatments that contained diclosulam controlled common lambsquarters, common ragweed, and eclipta by 100%. Diclosulam PRE controlled entireleaf morningglory by 88%, ivyleaf morningglory by ≥ 90%, pitted morningglory by ≥ 81%, and prickly sida by ≥ 94%. Yellow nutsedge control with diclosulam ranged from 65 to 100% depending on location, whereas POST systems containing imazapic controlled yellow nutsedge by at least 89%, regardless of PRE herbicides. Peanut yields and net returns were reflective of levels of weed management. Systems that included diclosulam PRE plus POST herbicides consistently provided high yields and net returns. Clethodim late POST was required for full-season control of annual grasses, including broadleaf signalgrass, goosegrass, large crabgrass, and Texas panicum.

Field studies were conducted at three sites in Delaware from 1997 to 1999 to evaluate fall glyphosate applications followed by postemergence (POST) field corn herbicides on horsenettle control and shoot densities. The fall treatments were either no fall treatment or 2.2 kg ai/ha glyphosate as a preharvest treatment in soybean at least 2 wk prior to frost. POST treatments were applied the following spring in no-tillage field corn 4 wk after planting (WAP) and included glyphosate, CGA 152005 plus primisulfuron plus dicamba, halosulfuron plus dicamba, atrazine plus dicamba, nicosulfuron plus rimsulfuron plus atrazine plus dicamba, or nicosulfuron plus dicamba. The fall glyphosate application delayed horsenettle shoot emergence in the spring and resulted in > 90% control at the time of POST in-crop applications. At 11 WAP, the highest horsenettle control was observed with a fall glyphosate application followed by POST in-crop treatments of CGA 152005 plus primisulfuron plus dicamba, halosulfuron plus dicamba, or nicosulfuron plus rimsulfuron plus dicamba and no fall treatment followed by CGA 152005 plus primisulfuron plus dicamba.

Field studies were conducted in 1998 and 1999 at two Illinois locations to compare flumioxazin and pendimethalin for preemergence (PRE) weed control and determine the benefit of these herbicides when followed by (fb) postemergence (POST) herbicides, glyphosate, imazethapyr, and imazamox. In early weed control ratings taken before POST applications, flumioxazin alone at 105 g ai/ha or pendimethalin alone at 1,120 g ai/ha resulted in less than 80% control of giant foxtail, but controlled common lambsquarters at least 85% in all experiments. Control of large-seeded broadleaf weeds with flumioxazin or pendimethalin varied greatly between experiments. At Urbana in 1998, where moisture was adequate before and after PRE applications, flumioxazin controlled velvetleaf, common cocklebur, and ivyleaf morningglory at least 90%. With the latter two species, control decreased in the subsequent year, probably because of the reduced precipitation after the PRE applications. Sequential applications including a PRE herbicide provided control up to 25% greater than did POST only treatments. At Dekalb in both years and Urbana in 1998, soybean yields were greater with most treatments containing a PRE fb POST application than with the treatments containing only POST applications.

Field experiments were conducted at three locations in North Carolina in 1998 and 1999 and one location in Virginia in 1998 to evaluate weed management systems in peanut. Treatments consisted of diclosulam alone preemergence (PRE), or diclosulam plus metolachlor PRE alone or followed by (fb) bentazon plus acifluorfen postemergence (POST). These systems were also compared with commercial standards of metolachlor PRE fb bentazon plus acifluorfen POST or imazapic POST. Our data indicate that diclosulam PRE plus metolachlor PRE in conventional tillage peanut production usually controlled common lambsquarters, common ragweed, prickly sida, and entireleaf morningglory. But control of spurred anoda, goosegrass, ivyleaf morningglory, large crabgrass, and pitted morningglory by this system was inconsistent and may require additional POST herbicide treatments. Systems that included diclosulam plus metolachlor PRE consistently provided high yields and net returns.