Field studies assessed weed control and potato injury with rimsulfuron applied postemergence at various rates in combination with various adjuvants. Weed control was influenced by choice of adjuvant and rimsulfuron rate. Rimsulfuron at 0, 9, 18, 26, and 35 g ai/ha was applied with nonionic surfactant (NIS), crop oil concentrate (COC), methylated seed oil (MSO), or silicone-polyether copolymer (SIL). Potato injury was less than 5% for all rimsulfuron rates and adjuvant combinations. Redroot pigweed was controlled greater than or equal to 93% by all treatments except rimsulfuron at 9 g/ha + SIL. Except for redroot pigweed, rimsulfuron treatments with SIL controlled kochia, hairy nightshade, common lambsquarters, and volunteer oats less than with other adjuvants. At lower rimsulfuron rates, weed control with rimsulfuron + MSO tended to be greater than with rimsulfuron + NIS or rimsulfuron + COC. Common lambsquarters control was 75% or less regardless of rimsulfuron rate or adjuvant. Tuber yield generally increased with increasing rimsulfuron rates. Depending on rimsulfuron rate, tuber yield was 10 to 15% lower with rimsulfuron + NIS or rimsulfuron + COC compared to rimsulfuron + MSO, while tuber yield was 18 to 37% lower with rimsulfuron + SIL compared to rimsulfuron + NIS, rimsulfuron + COC, or rimsulfuron + MSO.

Excessive persistence of imazethapyr has been responsible for injury to corn grown after soybean. Factors implicated in corn injury reported from certain parts of the main corn-producing region in South Africa were: leaching of herbicide to deep soil layers during the season of application, followed in the next season by capillary movement to the root zone of corn, and increased bioactivity of herbicide residues following liming of fields. Bioassays were employed to determine to what extent imazethapyr leached in a soil that typically contains less than 10% total clay and 0.1% organic C in the 0- to 300-mm zone and to assess the role of pH and liming in the bioactivity of the herbicide. Undisturbed soil columns were collected in polyvinyl chloride pipe for the leaching experiment. In the greenhouse, the equivalent of 40 g ai/ha imazethapyr was applied on the column surface, followed by leaching with simulated rain of 25 or 50 mm. Leaf area measurements of the test species rapeseed showed that the herbicide was readily leached to at least 30 cm at both water regimes and that it subsequently moved upward in the soil, with evaporation as the driving force. In the other bioassay, the soil was ameliorated with different amounts of Ca(OH)2 or CaCO3 to adjust pH levels to between 5.7 and 7.1 These soil samples were each treated with imazethapyr at rates equivalent to 1.8, 3.75, 7.5, 15, and 30 g ai/ha. The growth response of the test species indicated that where Ca(OH)2 was applied, the bioactivity of imazethapyr in most instances was significantly greater than where CaCO3 was used. At all the imazethapyr rates, the activity of the herbicide increased significantly with an increase in pH from 5.6 to 6.5 where Ca(OH)2 was used, but with CaCO3, activity was significant only at 15 and 30 g ai/ha. Changes in imazethapyr adsorption and in the organic matter in the soil were not monitored, but it is suggested that the increase in herbicide activity caused by Ca(OH)2 may be due to the degradation of organic matter in the soil or to desorption of the herbicide, which would render the herbicide more available for uptake. These effects, if they do occur, are likely to be of significance for herbicide adsorption only in soil with very low organic matter content. Results indicate that imazethapyr could leach easily in coarse-textured soils low in clay and organic matter content and that the type of lime used on those soils may influence the bioactivity of the herbicide.

Broadleaf weed control with postemergence applications of CGA-248757 and flumiclorac applied alone and in tank mixtures was evaluated in greenhouse and field studies. In greenhouse studies velvetleaf, common lambsquarters, redroot pigweed, common ragweed, common cocklebur, eastern black nightshade, and wild mustard shoot weight were reduced by 50% from 0.1, 2.9, 0.9, 1.1, 0.8, 0.4, and 1.2 g/ha of CGA-248757 and 0.7, 3.0, 2.4, 3.3, 3.0, 3.4, and 74.1 g/ha of flumiclorac, respectively. Under greenhouse conditions, tank mixtures of CGA-248757 or flumiclorac with atrazine, bentazon, bromoxynil, dicamba, halosulfuron, imazethapyr, lactofen, primisulfuron plus prosulfuron, or 2,4-D increased velvetleaf, common lambsquarters, redroot pigweed, or common ragweed control 14 d after treatment (DAT) when compared with the control provided by atrazine, bentazon, bromoxynil, dicamba, halosulfuron, imazethapyr, lactofen, primisulfuron plus prosulfuron, or 2,4-D applied alone. In field studies, adding CGA-248757 or flumiclorac to atrazine, bentazon, bromoxynil, dicamba, halosulfuron, imazethapyr, lactofen, CGA-277476, primisulfuron plus prosulfuron, or 2,4-D increased crop injury 3 DAT, but there was no reduction in corn or soybean yields as a result. In corn, tank mixtures of CGA-248757 or flumiclorac plus atrazine, dicamba, or 2,4-D increased velvetleaf control compared with treatments without CGA-248757 or flumiclorac. Tank mixtures of CGA-248757 or flumiclorac with primisulfuron plus prosulfuron increased common lambsquarters control compared with primisulfuron plus prosulfuron alone. Similarly, in soybean CGA-248757 or flumiclorac tank mixtures with bentazon or CGA-277476 increased control of redroot pigweed, and tank mixtures with lactofen increased common lambsquarters control. However, season-long broadleaf control in soybean with bentazon, lactofen, and CGA-277476 tank mixtures with CGA-248757 or flumiclorac did not provide acceptable weed control. Compared with imazethapyr alone the addition of CGA-248757 and flumiclorac improved soybean yield and provided season-long eastern black nightshade, common lambsquarters, redroot pigweed, and common ragweed control.

In 1998, field populations of wild radish suspected of being resistant to chlorsulfuron were collected and screened for resistance to this herbicide using the Quick-Test technique. This test successfully identified chlorsulfuron-resistant populations of wild radish. Detailed dose–response experiments with the progeny of these populations confirmed their resistance and validated the use of the Quick-Test for a dicot species. Subsequently in 1999, a random survey was conducted employing this test to establish the current extent of chlorsulfuron resistance in wild radish populations. The survey covered more than 200 fields in the northern, central, and eastern wheatbelt regions of Western Australia. Wild radish plants were collected from wheat crops in 133 of these fields. The Quick-Test method was used to screen these plants with the acetolactate synthase (ALS)-inhibiting herbicide chlorsulfuron. Overall, 21% of randomly collected wild radish populations were found to be resistant to chlorsulfuron.

Field studies were conducted in 1997 and 1998 to evaluate tolerance of nine sweet corn cultivars to CGA 152005 postemergence at 10 and 20 g ai/ha. There was a cultivar by herbicide interaction for visible plant injury, plant height, ear weight, and yield. Sweet corn injury from CGA 152005 increased as rate increased. CGA 152005 at both rates killed ‘DelMonte 2038’. ‘Eliminator’, ‘Reward’, ‘Delectable’, and ‘Reveille’ were injured (10 to 12% plant height reduction), and the remaining cultivars (‘Challenger’, ‘Krispy King’, ‘CNS 710’, and ‘GH 2690’) were tolerant to CGA 152005 at both rates.

Common windgrass is an increasing winter wheat weed problem in Michigan. Postplant incorporated trifluralin, applied at 0.56 kg/ha, reduced common windgrass density by 85% in small plot research and by 70% or greater in production scale sites. In 1994–1995, incorporation implements had no effect on initial wheat density. However in a spring 1995 evaluation, significant wheat injury and stand reduction occurred in response to trifluralin incorporation. Incorporation implement effects on wheat injury were in the order: flex-tine harrow > spike tooth drag > rotary hoe. When trifluralin was shallowly incorporated over wheat planted at least 5 cm deep, no significant wheat injury was observed at any site. Injury from postplant incorporated trifluralin was related to shallow planting depth.

Experiments were conducted in 1995, 1996, and 1999 to evaluate clomazone preemergence in flue-cured tobacco seedbeds. The seedbeds were fumigated with methyl bromide to kill all the weeds and other pests, and then the subplots designated as grassy were sown with a mixture of equal mass of crowfootgrass, goosegrass, and rhodesgrass. Clomazone rates were 0, 0.14, 0.28, 0.42, and 0.56 kg ai/ha for 1995 and 0, 0.42, 0.47, 0.52, and 0.94 kg ai/ha for 1996 and 1999. The rates of clomazone in 1996 and 1999 were based on the 1995 results where 0.42 kg/ha decreased the dry mass of grasses by 98% compared with the nontreated control, while clomazone at 0.28 kg/ha decreased the dry mass of grasses by 75%, and this was not considered adequate. In 1995, 0.56 kg clomazone/ha decreased the tobacco seedling number but had no effect on the growth of tobacco plants. In 1996 and 1999 clomazone had no effect on the number or dry mass of tobacco seedlings despite causing slight discoloration 4 wk after sowing at all rates in 1996. This discoloration had disappeared by transplanting time. The effect of grass competition on the emergence and growth of tobacco was severe in the nontreated subplots. More grasses emerged in 1999 than in 1996, possibly due to higher temperatures; however, few grasses survived until transplanting time in clomazone-treated areas. All the clomazone rates tested in 1996 and 1999 were satisfactory as there was no crop injury or detrimental effect of the grasses on tobacco in the clomazone-treated grassy subplots. Therefore, the 0.42-kg clomazone/ha rate is considered optimal.

A full-season field study was conducted near College Station, TX to determine the competitive interference of Palmer amaranth on cotton development, yield, and fiber properties and to measure Palmer amaranth development as influenced by intraspecific competition. Palmer amaranth densities ranged from 0 to 10 plants/9.1 m of row. Palmer amaranth decreased cotton canopy volume by 45% at 10 wk after cotton emergence (WAE) and cotton biomass by greater than 50% at 8 WAE at the highest density. Cotton yields decreased linearly from 13 to 54% for 1 to 10 Palmer amaranth plants/9.1 m of row. Cotton lint properties were not influenced by Palmer amaranth density. Palmer amaranth volume and individual biomass were not affected by intraspecific competition at any of the densities.

Management of corn (Zea mays) row spacings and populations has been used for many years to increase corn productivity. In 1998 and 1999, nonirrigated corn was grown in 38-, 56-, and 76-cm row spacings at populations averaging 59,300, 72,900, and 83,900 plants/ha in Michigan. Glufosinate at 0.29 kg/ha was applied to common lambsquarters (Chenopodium album L.) averaging 5 cm in height in each plot. Corn population and row spacing did not influence weed emergence following application of glufosinate. Common lambsquarters biomass and seed production were reduced when grown under canopies of corn planted in populations exceeding 72,900 plants/ha. Common lambsquarters biomass was reduced as corn row spacings were reduced from 76 to 38 cm. Early-season interception of photosynthetic active radiation (PAR) by corn canopies increased as row spacings decreased, but differences were not apparent later in the season. Interception of PAR was similar throughout the season when corn populations exceeded 72,900 plants/ha. Corn yields were not affected by row spacing, but they were increased with corn populations of 72,900 plants/ha or higher.

Potato crops often are grown in rotation with cereal crops such as wheat and barley. Imazamethabenz is a postemergence herbicide that selectively controls wild oat and certain other weeds in wheat and barley. Experiments were conducted at the University of Idaho Research and Extension Center, Aberdeen, ID, over a 3-yr period to evaluate the effect of irrigation management in an imazamethabenz-treated wheat crop on imazamethabenz persistence and injury in a subsequent potato crop. The experimental design was a split plot arrangement of a randomized complete block; main plots were seasonal irrigation amounts in wheat (20, 30, 40, or 50 cm), and subplots were imazamethabenz rates applied to the wheat crop (0, 0.26, 0.52, or 1.05 kg/ha). Soil analyses for imazamethabenz residues showed that herbicide degradation during the wheat growing season (i.e., 0 to 128 d after treatment [DAT]) followed first-order kinetics at all irrigation levels. Herbicide degradation rate increased with irrigation rate. Soil samples taken 338 or 351 DAT showed no detectable imazamethabenz residues (<10 ng/g) in most plots. However, some foliar injury to potato grown the year following imazamethabenz treatment was noted (<10% average injury), suggesting that potato is sensitive to imazamethabenz or imazamethabenz acid residues below the analytical detection limit. U.S. No. 1 and total potato tuber yield were not reduced by any treatment compared to the nontreated control. Therefore, under good growing conditions, potato can recover from mild imazamethabenz carryover injury without effects on tuber yield or quality.

Field studies were conducted near Archer, FL, and Vienna, GA, in 1995 and 1996 to investigate pyridate and SAN 582 for weed management in peanut. At Archer, pyridate plus 2,4-DB applied 3 wk after emergence resulted in 75, 72, 59, and 85% early-season control of yellow nutsedge, Florida beggarweed, hairy indigo, and sicklepod, respectively. In Vienna, pyridate plus 2,4-DB resulted in 87 and 55% early-season control of yellow nutsedge and Florida beggarweed, respectively. At both Archer and Vienna, SAN 582 applied preplant incorporated prior to pyridate postemergence (POST) increased control of yellow nutsedge, Florida beggarweed, and hairy indigo; however, peanut yield was not improved. In greenhouse studies, pyridate plus 2,4-DB controlled prickly sida, common cocklebur, and ivyleaf morningglory. Reduced weed control was observed in greenhouse studies when SAN 582 was added to the pyridate plus 2,4-DB POST tank mix.

Field and greenhouse dose-response experiments were conducted to investigate the potential for resistance in a johnsongrass biotype from New Kent County, VA that survived repeated applications of quizalofop and quizalofop-P. During 1996 and 1997, foliar injury (30 to 60%) was initially observed on the johnsongrass at the New Kent field site, but this biotype eventually recovered and survived applications of fluazifop-P, quizalofop-P, and sethoxydim at twice the recommended field use rates. However, applications of clethodim at twice the recommended field use rates during 1997 provided essentially complete control of the New Kent johnsongrass biotype. In greenhouse dose-response experiments, the amount of quizalofop-P required to inhibit shoot growth by 50% (GR50) was 13.6 g/ha in the New Kent johnsongrass biotype and 0.8 g/ha in the susceptible johnsongrass biotype. In response to sethoxydim, the GR50 for the New Kent biotype was 122.5 g/ha while that of the susceptible biotype was 21.6 g/ha. Additionally, the New Kent biotype was least sensitive to fluazifop-P, which provided a GR50 value of 148.7 g/ha for the New Kent biotype and 5.1 g/ha for the susceptible biotype. As in the field trials, the New Kent biotype was sensitive to clethodim, which provided a GR50 value of 49.8 g/ha and 69.1 g/ha for the susceptible biotype. These values indicate that the New Kent biotype was 17 times more resistant to quizalofop-P, 5.7 times more resistant to sethoxydim, and 29.5 times more resistant to fluazifop-P than the susceptible biotype, and that the New Kent and susceptible johnsongrass biotypes are equally sensitive to clethodim. These GR50 values for the New Kent johnsongrass biotype are inconsistent with the much higher GR50 values most commonly observed in graminicide-resistant weed biotypes, and suggest a mechanism of resistance other than an insensitive ACCase in the New Kent johnsongrass biotype.

Two field experiments were conducted in Oklahoma to determine whether changing wheat row spacing, seeding rate, and cultivar would reduce interference from rye. Wheat row spacing did not affect rye seed production. Averaged over row spacing, increasing wheat seeding rate from 67 to 134 kg/ha reduced rye seed production 21 and 25% in two experiments. At one site, grain yield of rye-infested wheat was increased 27 and 23% by doubling wheat seeding rate in 10- and 20-cm rows. Doubling the seeding rate of rye-infested wheat in 30-cm rows did not increase wheat yield. At a second site when data were pooled over row spacings, increasing the wheat seeding rate from 67 to 101 kg/ha increased yield of rye-infested wheat 21%, but yield was still 36% less than for rye-free wheat. In two additional experiments, rye seeds in harvested wheat were reduced 36% by increasing the wheat seeding rate from 60 to 162 kg/ha, whereas yield of rye-infested wheat was increased 82%. Nine cultivars were compared for competitive ability against rye in four experiments. Of 32 possible cultivar location situations, the rye-induced yield loss of ‘Jagger’ wheat was less than 16 other cultivar location situations, whereas the yield loss of ‘Triumph 64’ wheat was less than 10 other cultivar location situations. Mature wheat height, wheat yield, and wheat maturity classification were each negatively correlated to rye yield in infested wheat at two of four sites. Wheat plant density, head density, sunlight interception, and fall cover capability classification were each negatively correlated to rye yield at one of four sites.

Goosegrass is considered tolerant to quinclorac. The purpose of this study was to determine whether there was a stage of growth at which it was susceptible. Stage of growth studies with goosegrass were conducted that evaluated quinclorac activity at rates of 0, 1, 2, 4, 8, and 16 kg ai/ha applied with 1% (v/v) Merge spray adjuvant. Quinclorac application stages evaluated included preemergence and one- to two-leaf, four- to five-leaf, and one- to two-tiller stage. The effects of root uptake from postemergence applications were also evaluated by comparing treatments with and without a vermiculite soil barrier. Goosegrass was more susceptible to quinclorac when applied preemergence or early postemergence at the one- to two-leaf stage than at later more mature stages. The importance of root uptake on quinclorac activity was also observed, particularly at the four- to five-leaf stage, where the dose required to reduce growth 50% (GR50) in the absence of the soil barrier was 3.4 kg/ha vs. 16 kg/ha in the presence of a barrier. At the most advanced growth stage of one to two tillers, calculated GR50 values were greater than 16 kg/ha whether a soil barrier was present or not. The lowest calculated GR50 value of 2.7 kg/ha for the one- to two-leaf stage with no soil barrier was still approximately 3.5 times higher than the maximum labeled rate for turfgrass.

The effect of glyphosate on aromatic amino acid metabolism in purple nutsedge sprouted tubers and shoots was investigated. Glyphosate at 33.5 mM caused inhibition of bud elongation, increased total free amino acid concentration, and caused rapid accumulation of shikimic acid in sprouted tubers. However, only one aromatic amino acid, tryptophan, decreased quickly to 22% of control 3 d after treatment (DAT) and remained low afterwards. This suggests that the inhibition of bud elongation is due to the rapid accumulation of shikimic acid and the repression of tryptophan synthesis. Foliar application of glyphosate at 14.5 mM to purple nutsedge shoots resulted in the rapid accumulation of glyphosate which was rapidly converted to its metabolite, aminomethylphosphoric acid. Free amino acids in leaves were also increased by glyphosate 3 DAT. The reduction in soluble protein 5 DAT and increased acid protease activity 3 DAT suggests that the late accumulation of free amino acids partially resulted from protein hydrolysis. Shikimic acid accumulated in glyphosate-treated leaves 5 DAT, but the concentration of the three aromatic amino acids was not reduced. This suggests that glyphosate toxicity in purple nutsedge shoots was associated with the rapid accumulation of glyphosate, followed by large accumulation of shikimic acid. Aromatic amino acids deficiency was apparently not a factor in toxicity.

Field studies were conducted near Archer, FL, and Vienna, GA, in 1995 and 1996 to investigate the effects of pyridate formulation and adjuvants on broadleaf weed control in peanut (Arachis hypogaea). Pyridate formulations SAN 319H 450EC 361LZ, SAN 319H 450EC 216LZ, and SAN 319H 600EC 418LZ were evaluated at two rates, 1.07 and 2.14 kg/ha. Pyridate at 1.07 kg/ha plus 2,4-DB at 0.23 kg/ha were evaluated alone and with five adjuvants. The adjuvants included a crop oil concentrate, a nonionic surfactant, a nonionic surfactant with organosilicone blend, urea ammonium nitrate plus a nonionic surfactant, and chlorothalonil (a fungicide) plus a nonionic surfactant. No pyridate treatment injured peanut. Pyridate formulation did not affect broadleaf weed control. Increasing pyridate rate increased weed control and yield. Mixing 2,4-DB with pyridate generally enhanced sicklepod (Senna obtusifolia) and common cocklebur (Xanthium strumarium) control. Florida beggarweed (Desmodium tortuosum), smallflower morningglory (Jacquemontia tamnifolia), hairy indigo (Indigofera hirsuta), sicklepod, and common cocklebur control with pyridate was not enhanced by adjuvants. Adding chlorothalonil to pyridate plus 2,4-DB did not affect weed control or peanut injury.

Field experiments conducted in Texas at six locations from 1996 to 1998 evaluated peanut tolerance to imazapic applied postemergence. Imazapic at 71 g ai/ha was applied weekly from ground cracking to 56 d after ground cracking (DAGC). Visible injury 70 DAGC ranged from 0 to 40%. No reduction in canopy height, canopy width, yield, or grade was observed at harvest following any imazapic treatment.

Field experiments were conducted during 1998, 1999, and 2000 in Santa Fe, Argentina, to evaluate chlorimuron, clethodim, flumetsulam, 2,4-DB, glyphosate, and haloxyfop for control of broadleaf and grass weeds in established alfalfa. Herbicides were applied to alfalfa, 6 to 10 cm in height, after first or second cuttings. Clethodim and haloxyfop were highly effective against bermudagrass, barnyardgrass, and knotroot foxtail, and alfalfa yields were increased. 2,4-DB was effective against Scotch thistle, swinecress, turnipweed, and birdsrape mustard, but no effects were observed on alfalfa yields. Chlorimuron, flumetsulam, and glyphosate were effective against burning nettle, common chickweed, common lambsquarters, spiny cocklebur, and Persian speedwell, but they injured alfalfa and decreased yields.

Several commercial and experimental adjuvants were evaluated for efficacy in enhancing activity and selectivity of quinclorac in two cultivars of canola and four turfgrass species. Weed species investigated included cleavers, annual sowthistle, large crabgrass, and goosegrass. Turfgrass species evaluated included Kentucky bluegrass, perennial ryegrass, tall fescue, and creeping bentgrass. A variety of adjuvant types were selected, such as a methylated seed oil (Sunit II), a petroleum-based crop oil concentrate, the silicone-based Sylgard 309, a cationic fatty amine ethoxylate surfactant (Frigate), and modified crop oils (Dash and Merge). Adjuvant efficacy was evaluated by calculating the quinclorac rate required to reduce plant growth 50% (GR50) based on rates applied at 0, 15.6, 31.2, 62.5, and 125 g ai/ha. For goosegrass, quinclorac rates evaluated were increased to 250, 500, 1,000, and 2,000 g ai/ha. All evaluated adjuvants provided similar enhancement of control for cleavers and annual sowthistle. Sylgard 309 was the least effective adjuvant for control of large crabgrass. Goosegrass was tolerant to quinclorac across the evaluated rate range regardless of adjuvant; therefore, GR50 values could not be determined. None of the adjuvants alone caused phytotoxicity to canola or any turfgrass species.

This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction–ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.