Adjuvants influence weed control and crop tolerance provided by postemergence (POST) herbicides. Adjuvant effects on weed control and corn and soybean tolerance with CGA-248757 and flumiclorac applied alone and soybean tolerance with CGA-248757 and flumiclorac applied in combination with imazethapyr or CGA-277476 were evaluated. Velvetleaf control in the greenhouse and common lambsquarters, redroot pigweed, and common ragweed control in the greenhouse and field with CGA-248757 and flumiclorac were increased by the addition of an adjuvant. However, corn and soybean tolerance to these herbicides was not affected by adding an adjuvant. Laboratory studies suggest enhanced weed control from the addition of an adjuvant resulted from an increase in CGA-248757 and flumiclorac foliar absorption. In greenhouse studies, velvetleaf, common lambsquarters, and redroot pigweed control with CGA-248757 plus imazethapyr; velvetleaf control with flumiclorac plus imazethapyr; redroot pigweed and common ragweed control with CGA-248757 plus CGA-277476; and velvetleaf, redroot pigweed, and common ragweed control with flumiclorac plus CGA-277476 increased by adding an organosilicone adjuvant when compared with adding a nonionic surfactant (NIS). However, in field evaluations of CGA-248757 or flumiclorac plus imazethapyr, the addition of NIS or an organosilicone adjuvant resulted in equivalent soybean injury, common lambsquarters and redroot pigweed control, and soybean yield. Yet in tank mixtures of CGA-248757 or flumiclorac plus CGA-277476, the addition of an organosilicone adjuvant increased redroot pigweed control and soybean yield compared to adding NIS.

Greenhouse and laboratory studies were conducted to examine certain characteristics of RPA 201772 and how they may affect its utility. 14C-RPA 201772 was used to determine the stability of RPA 201772 in various pH spray solutions over time. After 24 h, degradation of RPA 201772 was 20, 36, and 93% in spray solutions adjusted to pH 4.0, 7.0, and 10.0, respectively. The major metabolite was diketonitrile (DKN), which is herbicidally active. In addition, 9 and 15% of the RPA 201772 had degraded into an inactive benzoic acid derivative at pH 7.0 and 10.0, respectively. The differences in herbicidal activity of RPA 201772 and its metabolite DKN were also evaluated. Corn seeds and velvetleaf seeds readily imbibed RPA 201772, but only low levels of DKN were imbibed. Preemergence (applications of RPA 201772 and DKN were equally active on corn and velvetleaf. Further studies determined that the principal sites of uptake of RPA 201772 by corn was roots and seeds for four different corn hybrids. Another study determined that light was required for RPA 201772 activity. Corn shoots from seeds germinated under 14 h of light exhibited rate-dependent injury, while corn germinated and grown in the darkness was not injured.

Five biotypes of common cocklebur that were not controlled with acetolactate synthase (ALS)-inhibiting herbicides were tested in greenhouse and laboratory studies to determine the magnitude of resistance and cross-resistance to four ALS-inhibiting herbicides. In vivo inhibition of ALS was also evaluated. Based on phytotoxicity, all five ALS-resistant biotypes of common cocklebur were > 390 times more resistant than the susceptible biotype to imazethapyr. However, only four of these biotypes were also resistant to another imidazolinone, imazaquin. Two biotypes were cross-resistant to the sulfonylurea, chlorimuron, and the triazolopyrimidine sulfonanilide, NAF-75. One biotype demonstrated intermediate susceptibility to imazaquin, chlorimuron, and NAF-75. In all cases, the resistance exhibited at the whole plant level was associated with an insensitive ALS.

Glufosinate at 1.1 and 2.2 kg/ha injured ‘Koshihikari’ rice lines that were transformed with the BAR gene from 0 to 53%. However, transgenic ‘Gulfmont’ rice was not injured. Rice yields of transgenic ‘Gulfmont’ lines and six of nine ‘Koshihikari’ lines were not affected by 2.2 kg/ha glufosinate. In field studies, flooding reduced the efficacy of glufosinate in controlling red rice, and greenhouse tests determined that glufosinate efficacy was reduced when red rice was submerged between 25 and 50% of its height. Plant heights and dry weights of red rice increased as flood water depth increased at all rates of glufosinate.

Field experiments were conducted at East Lansing and Clarksville, MI, to evaluate the efficacy of imazamox, imazethapyr, and CGA-277476 plus quizalofop applied postemergence in wide- (76-cm) and narrow- (19-cm) row soybean. Soybean injury from all herbicides was minimal 14 days after treatment (DAT), except for CGA-277476 at 79 g ai/ha plus 69 g ai/ha quizalofop, which caused 30% soybean injury at the Clarksville location. Adding 4 g ai/ha CGA-248757 to 65 g ai/ha CGA-277476 plus quizalofop reduced common ragweed control, but increased redroot pigweed control in wide rows compared to 79 g ai/ha CGA-277476 plus quizalofop. Imazamox at 35 and 45 g ai/ha provided greater common ragweed and common lambsquarters control than imazethapyr at 70 g ai/ha 28 DAT. All herbicide treatments controlled velvetleaf. Common ragweed and common lambsquarters control by all herbicide treatments was enhanced in narrow- compared to wide-row soybean 56 DAT as was redroot pigweed control by CGA-277476 treatments. Total weed biomass and soybean yield in wide-row soybean treated with imazamox at 45 g/ha was not different from the hand-weeded control. In narrow-row soybean, soybean yield was equal to the hand-weeded control for 35 and 45 g/ha imazamox and 70 g/ha imazethapyr. Postemergence herbicide treatments resulted in less weed biomass and greater soybean yield in narrow- compared to wide-row soybean.

Field and greenhouse experiments were conducted to evaluate postemergence (POST) soybean injury and weed control with CGA-277476 and cloransulam-methyl alone and in tank mixtures. In the field, visible soybean injury was 12 to 14% from CGA-277476 and 9 to 13% from cloransulam-methyl 7 d after treatment. Tank mixtures of either herbicide with acifluorfen or acifluorfen plus thifensulfuron were more injurious than CGA-277476 or cloransulam-methyl applied alone. Both CGA-277476 and cloransulam-methyl reduced velvetleaf dry weight 82%, and cloransulam-methyl reduced common ragweed dry weight 92%. Neither herbicide adequately controlled common lambsquarters, redroot pigweed, nor eastern black nightshade. The addition of acifluorfen to the spray solution improved common ragweed, common lambsquarters, redroot pigweed, and eastern black nightshade control with CGA-277476 and improved common lambsquarters, redroot pigweed, and eastern black nightshade control with cloransulam-methyl. Tank mixing thifensulfuron with CGA-277476 or cloransulam-methyl increased common lambsquarters and redroot pigweed control. In the greenhouse, CGA-277476 at 20 g ai/ha reduced velvetleaf dry weight 98%, and 79 g/ha was required to reduce common ragweed dry weight 93%. Cloransulam-methyl at 4.4 g ai/ha reduced velvetleaf dry weight 98% and common ragweed dry weight 94% at 8.8 g/ha. Chlorimuron reduced yellow nutsedge dry weight more than CGA-277476 or cloransulam-methyl. Antagonism of POST graminicide activity by CGA-277476 was grass species and graminicide related. CGA-277476 reduced giant foxtail control by clethodim but not by quizalofop. Cloransulam-methyl tank mixed with clethodim or quizalofop controlled giant foxtail.

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).

Field studies were conducted over 2 yr to evaluate rhizome johnsongrass control with tank mixtures and sequential applications of bromoxynil at 1.7 kg ai/ha and the graminicides clethodim, fluazifop-P, and quizalofop-P. When compared with the graminicide alone, bromoxynil reduced johnsongrass control 28 days after treatment (DAT) 20, 36, and 55% when applied in combination with clethodim, fluazifop-P, and quizalofop-P, respectively. Application of bromoxynil 7, 3, and 1 d before or 1, 3, and 7 d after all graminicides reduced antagonism when compared with the bromoxynil plus graminicide mixtures. Based on johnsongrass control 28 DAT, antagonism was precluded when bromoxynil was applied 3 d before clethodim and 7 d before fluazifop-P and quizalofop-P. Application of bromoxynil 3 and 7 d after quizalofop-P and 7 d after clethodim also precluded antagonism. The antagonistic tank mixtures of bromoxynil and graminicides reduced cotton yield 17% because of reduced johnsongrass control compared with graminicides alone. A 3-d or greater application interval between bromoxynil and the graminicides was necessary to maximize cotton yield.

The effect of DPX-79406 was evaluated on eight corn hybrids in 1993 and on 18 hybrids in 1994. Experiments were conducted each year at Charlottetown, Prince Edward Island, and Stewiacke, Nova Scotia. Corn tolerance was similiar between sites but varied from year to year with certain hybrids. Generally, tolerance was hybrid dependent. Most hybrids displayed chlorosis and stunting 7 days after treatment (DAT) but recovered quickly. Injury at 7 DAT ranged from 8 to 64%. Grain yield indicated that ‘Pickseed 2444,’ ‘Pioneer 3921,’ ‘Pioneer 3979,’ and ‘DK 302’ were tolerant to 25 g ai/ha of DPX-79406. Pioneer 3979 and Pickseed 2444 grain yields were reduced with DPX-79406 at 50 g/ha in 1 yr. ‘DK 291’ and ‘DK 235’ were severely injured both years and did not recover appreciably from early injury. Responses of ‘Pioneer 3984’ and ‘Pickseed 2477’ varied from year to year, and they were considered sensitive to DPX-79406. ‘Pioneer 3947,’ ‘Pioneer 3962,’ ‘Pickseed 2290,’ ‘Pickseed 2299,’ ‘DK 220,’ ‘G-4011,’ and ‘N0565’ were tested only 1 yr and were tolerant to both rates of DPX-79406.

Agrichemical interactions between propanil; the herbicides HOE 30374, pendimethalin, piperophos, quinclorac, and thiobencarb; and the insecticide carbaryl were evaluated under field conditions to find synergistic or additive interactions useful to control a barnyardgrass biotype resistant to propanil (R-BYG) without injuring rice. Propanil and each compound were evaluated at four rates for a total of 16 rate combinations for each additive. Averaged over all experiments, 2- to 3-leaf R-BYG control with propanil alone at 0.83, 1.65, 3.3, and 6.6 kg ai/ha was 33, 53, 62, and 81% at 7 d after treatment (DAT). Propanil-susceptible barnyardgrass (S-BYG) response to propanil alone at similar rates was 52, 73, 88, and 94% control. HOE 30374, carbaryl, piperophos, or pendimethalin in combination with propanil produced synergistic effects on R-BYG. For each compound tested, at least one rate combination with propanil controlled R-BYG > 80% with minimal rice injury (< 20%) at 7 DAT. Use of these combinations of compounds could provide more effective control of this resistant biotype and help prevent its spread.

Glyphosate and adjuvant combinations were applied to rhizome johnsongrass at vegetative and reproductive growth stages to evaluate control and rainfastness in field studies. Using a rainfall simulator delivering 1.3 cm of water in 15 min, plots received either no rainfall or rainfall 15 or 60 min after glyphosate was applied at 2.1 kg ai/ha in combination with the nonionic surfactants Kinetic® HV at 0.25% (v/v) or Induce® at 1.0% (v/v) or the silicone surfactant Break-Thru® at 0.125% (v/v). Regardless of adjuvant, rainfall 15 or 60 min after application reduced johnsongrass control compared with no rainfall. Johnsongrass control 14 d after treatment at the reproductive stage was at least 89% with no rainfall, but no more than 53 and 65% with rainfall at 15 and 60 min, respectively. Based on initial weed control, adjuvants did not consistently improve rainfastness. Johnsongrass regrowth did not occur when glyphosate was applied with either adjuvant. In contrast, for glyphosate applied to johnsongrass in the vegetative stage, addition of Break-Thru improved control over Induce at both 15- and 60-min rainfall timings in one of two experiments. With no rainfall, addition of Kinetic HV and Break-Thru increased johnsongrass control in only one experiment. For application at the vegetative stage, johnsongrass regrowth averaged across rainfall timings was no more than 10%. In other field experiments, glyphosate at 1.4 kg/ha plus nonionic surfactants, silicone surfactant, crop oil concentrate, methylated seed oil, or a blend of silicone surfactant and methylated seed oil were equally effective in reducing johnsongrass regrowth when applied after seedhead emergence. Improved control of vegetative johnsongrass with some adjuvants was not reflected in decreased regrowth.

Field studies were conducted in 1996 and 1997 at three locations throughout southern Ontario with the objective of developing dose-response curves of RPA 201772 for weed control and crop tolerance in corn. The biologically effective doses required to control redroot pigweed, velvetleaf, and wild mustard were 100, 90, and 80 g/ha, respectively. Yellow foxtail was controlled with 100 to 120 g/ha, while rates for common lambsquarters varied from 60 to 130 g/ha, depending on the year and location. Wild buckwheat control was poor (> 30%) at all of the doses tested. RPA 201772 did not reduce corn grain yield; however, temporary crop injury was evident on coarse sandy soils.

A field experiment was conducted for 2 yr near Scottsbluff, NE, to evaluate the growth and yield response of nine commercial sugarbeet cultivars to five postemergence herbicide mixtures applied two times. Sugarbeet cultivars varied in their response to herbicides. Three of the five postemergence herbicide mixtures reduced sugarbeet leaf area at 9 wk after planting (WAP), while two treatments had no significant effect. Root yield reductions from herbicide treatments ranged from 3 to 11% averaged over the nine cultivars. Two applications of desmedipham plus phenmedipham at 180 plus 180 g/ha reduced root yield 3% compared to untreated plots. Two applications of clopyralid at 100 g/ha, triflusulfuron at 18 g/ha, or ethofumesate at 160 g/ha plus desmedipham plus phenmedipham resulted in a 17, 29, and 22% reduction in leaf area at 9 WAR respectively, and a 6, 8, and 6% reduction in root yield, respectively, compared to untreated plots. Sethoxydim at 280 g/ha plus desmedipham plus phenmedipham did not cause additional sugarbeet injury.

Observations in a citrus grove indicated that hairy beggarticks numbers were higher in plots receiving thiazopyr alone or thiazopyr with oxyfluorfen than in nontreated plots. At 120 days after treatment (DAT), approximately 90% more hairy beggarticks plants had emerged in plots receiving thiazopyr at 0.56 kg ai/ha than in nontreated plots. A tank mix of thiazopyr plus oxyfluorfen, each at 0.56 kg/ha, resulted in a 55% increase in hairy beggarticks number at 120 DAT. Greenhouse studies using seeds collected from sites with or without herbicide history produced similar results. Oxyfluorfen at 0.28 kg/ha stimulated seed germination but injured seedlings by 47% or more at 36 DAT when applied at higher rates. Thiazopyr applied at 0.56 kg/ha antagonized the preemergence activity of oxyfluorfen at 0.28 or 0.56 kg/ha. Thiazopyr at 1.12 kg/ha stimulated hairy beggarticks germination by 37% compared with nontreated containers.

Field experiments were conducted over 3 yr at two locations in northwest Kansas to evaluate the efficacy and crop tolerance of EXP 31130A alone and with other herbicides in corn. EXP 31130A alone and in combination with acetochlor, atrazine, or metolachlor controlled kochia and redroot pigweed 93% or greater regardless of location, year, or tillage system. Green foxtail control in 1997 and 1998 was > 90% with EXP 31130A in combination with chloroacetamide herbicides but varied among experiments with EXP 31130A alone. Control of puncturevine with EXP 31130A alone or in tank mixtures was 75% or greater at five of six sites. EXP 31130A alone and in combinations caused minor, early-season, corn leaf bleaching and/or stunting under conventional tillage. However, no injury was detected in no-till systems. In 1996, no-till corn receiving EXP 31130A treatments yielded similarly or up to 29% more than hand-weeded corn. Yields did not differ among treated, untreated, and hand-weeded corn in 1996 under conventional tillage, where weed interference was not as intense as under no-till conditions. Conventional-tillage corn receiving EXP 31130A alone or in tank mixtures in 1997 yielded similarly to hand-weeded corn. Averaged over experiments, corn yield in 1998 increased by 10 to 18% with applications of EXP 31130A alone or in combinations compared to untreated corn.

Duration and spectrum of preemergence (PRE) weed control following isoxaben application at 0.56, 0.84, and 1.12 kg ai/ha in spring, fall, or spring plus fall (double) application were evaluated by field experiments. Residual activity of isoxaben was assessed by monitoring weed counts at two locations for 12 mo after treatment (MAT). Buckhorn plantain and dandelion control from spring-applied isoxaben at 1.12 kg/ha was > 90% at 4 MAT in Blacksburg but dropped to < 51% at 12 MAT. Isoxaben at 1.12 kg/ha applied in fall provided ≥ 90% control of buckhorn plantain for 8–9 MAT, with control ranging from 69 to 91% at approximately 12 MAT. Dandelion control with fall-applied isoxaben at 1.12 kg/ha ranged from 60 to 75%, 8–9 MAT. Fall application of 1.12 kg/ ha isoxaben at Virginia Beach controlled henbit and cornspeedwell for 2–3 MAT. Multiple applications of isoxaben improved weed control at Virginia Beach, but results were inconclusive at Blacksburg. In comparison, oxadiazon controlled white sweet clover and dandelion for 1 MAT following spring application in Blacksburg, but < 65% broadleaf control was observed at 4 MAT. Broadleaf weed control following single application of oxadiazon at 3.36 kg/ha was < 60% at 9 MAT Poor weed control resulted from spring applications of isoxaben and oxadiazon when a 25-cm precipitation occurred within 2 wk after application in one study.

Field and greenhouse studies were conducted in 1995 and 1996 in Kansas to determine the efficacy of 21 herbicide treatments for control of tumble pigweed, Palmer amaranth, redroot pigweed, and two biotypes of common waterhemp in soybean. In field studies, nine of eleven preemergence treatments controlled all four species 90% or more. However, pendimethalin and trifluralin controlled Palmer amaranth, redroot pigweed, and tumble pigweed less than the other preplant incorporated and preemergence treatments. With the exception of flumiclorac and NAF-75, postemergence treatments controlled 75 to 90% of all four species. A biotype of common waterhemp collected in Iowa was not controlled by acetolactate synthase-inhibiting herbicides. Greenhouse results were similar to field experiments. Results suggest at least 90% control of these Amaranthus species is possible with proper herbicide selection.

Wild carrot is becoming a serious weed problem in Michigan continuous no-tillage crop production. Greenhouse and field research was conducted to identify effective management strategies for wild carrot control in no-tillage cropping systems. In the greenhouse, PRE applications of acetochlor plus dichlormid (5.8:1 w:w), cyanazine, linuron plus chlorimuron (18:1), and metribuzin plus chlorimuron (10:1) and POST applications of bentazon, CGA-152005, clopyralid, cyanazine, and MON 12000 provided the greatest control of wild carrot seedlings. In the field, PRE and POST treatments containing chlorimuron consistently controlled overwintered wild carrot greater than 71% at 30 d after the POST application in no-tillage soybean. Atrazine, MON 12000, nicosulfuron, and primisulfuron applied POST consistently controlled overwintered wild carrot greater than 78% at 30 DAT in no-tillage corn. Glyphosate at 0.84 or 1.68 kg ae/ha applied in October to established wild carrot provided greater than 74% control the following spring. Early preplant (EPP) applications of glyphosate at 0.84 kg/ha in no-tillage soybean gave 95 and 24% control of overwintered wild carrot in St. Clair and Lenawee Counties, MI, respectively, at 70 DAT. PRE applications of glyphosate at 0.84 kg/ha controlled overwintered wild carrot less than 69% at 58 DAT in no-tillage corn and soybean. Fall applications of 2,4-D ester at 1.12 kg ae/ha provided 18 and 88% control of wild carrot the following spring in Clinton and Lenawee Counties, respectively. EPP applications of 2,4-D ester at 1.12 kg/ha provided 7 and 72% control of overwintered wild carrot in St. Clair and Lenawee Counties, respectively.

Field studies were conducted in 1995 and 1996 at three locations in Illinois to determine soybean response to combinations of thifensulfuron and bentazon. Thifensulfuron was applied at 2.2 to 8.8 g ai/ha alone or in combination with 280 to 560 g/ha of bentazon. Soybean injury 30 d after treatment ranged from 0 to 22% when thifensulfuron was applied alone at 2.2 g/ha. Increasing thifensulfuron rate to 8.8 g/ha increased soybean injury to a range of 12 to 44%. Soybean grain yield was significantly reduced compared to the yield of untreated soybean when thifensulfuron was applied at 4.4 and 8.8 g/ha in two of five and four of five experiments, respectively. The addition of bentazon to thifensulfuron consistently reduced soybean injury and stunting. In many cases, increasing the bentazon rate to 420 g/ha decreased soybean injury from thifensulfuron to a greater extent than 280 g/ha. In cases where thifensulfuron decreased soybean yield, the addition of 420 or 560 g/ha of bentazon restored yields to levels that were not lower than untreated soybeans. These studies demonstrate that thifensulfuron at 2.2 to 8.8 g/ha in combination with bentazon at 420 g/ha may be safely applied to soybean for broadleaf weed control.

Little is known about the biology and control of hophornbeam copperleaf, a weed of increasing importance in the Midwest. More than 2 wk of cold stratification and a 0.2% KNO3 solution increased germination of hophornbeam copperleaf. Germination at constant 30 C was 47% and alternating 30/20 C was 65%. Scarification did not increase hophornbeam copperleaf germination. Within the first 600 growing degree days after soybean planting, plant height, leaf area, and dry matter accumulation of hophornbeam copperleaf grown in soybean and alone were similar. Subsequently, leaf area and dry matter accumulation of hophornbeam copperleaf grown alone were greater than of those grown in soybean. In contrast, plant height of hophornbeam copperleaf grown in soybean was greater than when grown alone. Hophornbeam copperleaf grown alone produced up to 12 510 seeds/plant, whereas hophornbeam copperleaf grown with soybean produced 980 seeds/plant. Of 13 postemergent herbicides evaluated on hophornbeam copperleaf in soybean, only lactofen, acifluorfen, and fomesafen controlled 80% or more. Lactofen at 210 g ai/ha consistently controlled more than 95% of the hophornbeam copperleaf.