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.

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.

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.

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.

A 3-yr study was conducted to assess cranberry bean susceptibility to mechanical weeding using a rotary hoe at preemergence, hook, cotyledon, unifoliate, and first to fourth trifoliate stages of bean development and at different combinations of stages. The experiment was conducted in a weed-free environment. Cultivation with the rotary hoe reduced bean yield only for the treatment that received four cultivations at four different bean growth stages. Three cultivations improved yield compared with no cultivation. Single cultivation done at any of the eight crop growth stages did not affect yield. Crop density at harvest was decreased 6% in the treatments receiving two cultivations and 9% in the treatments receiving four cultivations compared to no cultivation. The effects of the cultivations on grain moisture were not consistent and differed from year to year. Seed weight did not differ among treatments in either year. Because this study was conducted under weed-free conditions, the beneficial effects of cultivating with the rotary hoe are probably mostly related to breaking the soil crust, improving soil aeration, preserving soil moisture, or promoting mineralization of the nutrients required by the crop.

An experiment was conducted in 1998 and 1999 in southeastern Pennsylvania to examine the effect of tillage and soybean row spacing on burcucumber emergence and growth. A second experiment evaluated postemergence (POST) soybean herbicides on burcucumber control. In the tillage and row spacing study, a glyphosate-resistant soybean variety was planted in no-till and reduced-tillage systems in 38- and 76-cm row spacings. In the POST herbicide experiment, chlorimuron, glyphosate, CGA-277476, thifensulfuron, and several combinations of these herbicides were applied at two different POST application timings in 38-cm row soybean planted in a reduced-tillage system. In the tillage and row spacing study, burcucumber emergence was greatest starting in late May through mid-June and mostly ceased by early July, regardless of tillage system or row spacing. Although there was no difference in germination period in either tillage system, preplant tillage increased the number of emerged plants by 110% in 1997 and 70% in 1998 compared to the no-till system. Row spacing had no effect on burcucumber emergence or biomass production. In general, most POST herbicide programs controlled burcucumber, and there was no difference between early POST and mid-POST application timings. Chlorimuron at 13 g ai/ha, chlorimuron plus thifensulfuron, glyphosate, glyphosate plus chlorimuron, and glyphosate plus CGA-277476 provided 87% or greater control of burcucumber 12 wk after planting. These herbicides reduced burcucumber density and biomass by more than 56% in 1997 and 96% in 1998.

Field experiments were conducted in 1997 and 1998 at the Prattville Experiment Field in Prattville, AL and the Wiregrass Substation in Headland, AL to determine if ammonium thiosulfate (ATS) additions to monosodium methanearsonate (MSMA) affects weed control, reduces MSMA-induced cotton injury, lessens the fruiting and maturity delay on cotton caused by MSMA, and lessens MSMA-induced yield reductions in cotton. Treatments were applied before cotton was at the pinhead square stage and the weeds were 5 cm tall. Weeds evaluated were sicklepod, morningglory species, yellow nutsedge, and Texas panicum. ATS additions to MSMA occasionally enhanced control of all weeds 5 to 20%. However, the addition of ATS did not reduce crop injury caused by MSMA, the effects of MSMA on cotton maturity, or yield reductions caused by MSMA.

A random survey of 264 cropping fields in the Western Australian wheat belt was conducted to determine the extent of rigid ryegrass resistance to commonly used acetolactate synthase- and acetyl-CoA carboxylase-inhibiting herbicides. Rigid ryegrass infestation density was assessed and seed samples collected and subsequently tested for resistance to diclofop-methyl, clethodim, chlorsulfuron, and sulfometuron. Of these randomly collected populations, 46% exhibited resistance to diclofop-methyl and 64% to chlorsulfuron, with 37% exhibiting multiple resistance to both herbicides. Only 28% of tested populations were classified as susceptible to both diclofop-methyl and chlorsulfuron, although all but one population were susceptible to clethodim. Large differences in the proportion of fields containing resistant populations were found between agronomic areas, reflecting different cropping and, therefore, herbicide use history. There was no significant association between resistance status and the density at which rigid ryegrass was present. Herbicide-resistant rigid ryegrass populations are now more common than susceptible populations across much of the Western Australian wheat belt.

The premix atrazine + S-metolachlor is commonly used to control a wide range of weeds in corn, but it is weak on velvetleaf and several other broadleaf species. RPA 201772, used at reduced rates in combination with atrazine + S-metolachlor, may improve the weed control spectrum. In field studies at Urbana and Dekalb in 1998 and 1999, RPA 201772 was combined with a premix of atrazine at 1,820 g ai/ha and S-metolachlor at 1,408 g ai/ha to compare RPA 201772 rate effects on corn injury, weed control, and yield. RPA 201772 was applied at 0, 26, 53, 78, 105, and 132 g ai/ha early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE). Compared with atrazine + S-metolachlor alone, RPA 201772 combinations did not improve giant foxtail control. However, RPA 201772 at 26 or 53 g/ha increased control of redroot pigweed, common lambsquarters, ivyleaf morningglory, common ragweed, giant ragweed, and velvetleaf in some environments. At Urbana in both years, RPA 201772 at 26 g/ha in combination with atrazine + S-metolachlor increased velvetleaf control 15% compared with the premix alone. RPA 201722 at 26 to 78 g/ha decreased velvetleaf densities with all application methods. Greater weed control and higher corn yield occurred in treatments applied PRE or PPI compared with EPP applications, except where conditions were dry after PRE applications. This research demonstrates the benefit of RPA 201772 at low rates for broadleaf weed control with less potential for crop injury compared with high RPA 201772 rates.

A diagnostic test (Syngenta Quick-Test, QT) used for testing grass weed survivors to herbicides in the field for resistance was evaluated. Cuttings from grass weeds were transplanted into pots to regenerate new leaves, then treated with herbicide. In greenhouse experiments, resistance of known herbicide-resistant blackgrass biotypes to the aryloxyphenoxypropanoate herbicides CGA 184927 and fenoxaprop-ethyl and to the phenylurea herbicide isoproturon was verified by the QT. The findings were similar to those for seedlings grown from seed. Rigid ryegrass from suspect resistant fields in South Australia was sampled and sent by post to Switzerland for QT analysis. Resistance was confirmed in less than 4 wk, which verified resistance as responsible for the field failures. The added features of the QT over current resistance tests suggest a likely fit for in-season testing of surviving weeds and possible follow-up action.

Two field experiments were conducted to evaluate the effects of selected herbicides on Italian ryegrass, combined Italian ryegrass plus wheat forage production, and returns from hard red winter wheat grain. Herbicides included BAY FOE 5043 + metribuzin (4:1 w/w premix), BAY MKH 6562, chlorsulfuron, chlorsulfuron + metsulfuron (5:1 w/w premix), diclofop, MON 37560, pendimethalin, ICIA 0604, and triasulfuron. Italian ryegrass was controlled 80% or more by 28 of 34 treatments. Grain dockage was reduced by 33 of 34 treatments at both sites, and grain yield was improved at both sites by 29 of 34 treatments. No herbicide treatment decreased forage protein content, but forage yield was frequently decreased by controlling Italian ryegrass. Gross returns from wheat grain were improved over the untreated check by all but six treatments at site 1, and by all but one treatment at site 2. The data did not support the hypothesis that returns from forage plus grain could be maximized by delaying Italian ryegrass control until grazing termination.

The experimental sulfonylurea herbicide ASC-67040 was evaluated preemergence (PRE) and postemergence (POST) at 25 to 100 g ai/ha and PRE at 50 g/ha followed by ASC-67040 POST at 50 g/ha for broadleaf weed control in potato and tomato. In Virginia, ASC-67040 POST at 50 and 100 g/ha gave 70 to 98% control of common lambsquarters, common ragweed, and jimsonweed with slight (0 to 6%) ‘Superior’ potato and moderate (0 to 23%) ‘Agriset’ tomato injury. This crop injury was present primarily as chlorosis in terminal growth and disappeared by 3 wk after treatment (WAT). Potato tuber and tomato fruit yields were high in ASC-67040-treated plots. In North Carolina, ASC-67040 PRE and POST at 50 and 100 g/ha controlled 10 species of annual broadleaf weeds and grasses, and at 25 g/ha ASC-67040 gave greater than 90% control of most species. ASC-67040 injured ‘Fontenot’ potato more than Superior and caused 10 to 13% injury to ‘Mountain Spring’ tomato but did not injure ‘Mountain Pride’ tomato. Mountain Spring tomato recovered rapidly and produced high yields. In the greenhouse, yellow nutsedge control by ASC-67040 POST at 25 to 100 g/ha was 59 to 86% 3 WAT and yellow nutsedge height, dry weight, and regrowth dry weight were reduced by all rates. Lower rates of ASC-67040 might control weeds and cause less injury to potato and tomato.

Field experiments were conducted in 1996, 1997, and 1998 at Lacombe and Lethbridge, AB, to determine the influence of early weed competition on field pea yields. Wild oat and Tartary buckwheat were removed from plots at weekly intervals after pea emergence by hand-weeding and maintained weed-free for the remainder of the growing season. Tartary buckwheat produced slightly more biomass than wild oat at Lacombe, whereas wild oat produced much more biomass than Tartary buckwheat at Lethbridge. Weed-free pea yields at Lacombe were always two- to threefold higher than at Lethbridge. At Lethbridge, early competition with Tartary buckwheat in all years, and with wild oat in 1998, did not reduce pea yields. In 1996 and 1997 at Lethbridge, pea yield reductions due to wild oat began at 2 wk after pea emergence; for the next 2 wk yield loss was linear, decreasing at an average rate of 97 kg/ha per day. Early weed competition led to pea yield losses at Lacombe in all 3 yr. The onset of yield loss at Lacombe over the 3-yr period ranged from 1 to 2 wk after pea emergence; for the next 2 to 3 wk yield loss was linear, decreasing at an average rate of 45 kg/ha per day. Yield losses after full-season weed competition ranged from 40 to 70% at both sites. Usually, the beginning of the critical weed-free period was at 1 or 2 wk after pea emergence. Optimum pea yields usually required weed removal very early in the pea life cycle; weed removal beyond 2 wk after pea emergence often protected only suboptimal yields.

Glyphosate-resistant (Roundup Ready®) soybean is widely grown to provide broad-spectrum weed control following sequential applications of glyphosate. Studies were conducted during 1998 and 1999 at Keiser, AR, and in 1999 at Pine Tree, AR, to examine the effect of glyphosate-resistant soybean populations on the number of glyphosate applications needed to maintain 90% or greater control of all weed species and on the gross profit margin from weed management. Soybean was seeded in 19-cm rows at 12 rates ranging from 185,000 to 1,482,000 seeds/ha. Each seeding rate received a single application of 0.56 and 1.12 kg ai/ha glyphosate once weeds were 5 to 7 cm tall, followed by repeat applications when control of any species fell below 90%. Soybean yield and gross profit margin at Keiser were similar both years, and a quadratic curve best described the relationship between seed yield and seeding rate, with maximum predicted and observed yield at 988,000 seeds/ha. Soybean seeding rate affected light interception by soybean and the number of glyphosate applications needed to maintain 90% control of all species. By the later stages of vegetative development, soybean at Pine Tree achieved ≥95% light interception, maximizing yield at all seeding rates. Late-season light interception at Keiser ranged from 88 to 99%, causing soybean yield to vary by seeding rate. Three applications of 0.56 kg/ha glyphosate were needed at seeding rates of 185,000 and 247,000 seeds/ha, whereas higher seeding rates sometimes required only a single application for season-long weed control. Application and herbicide costs were offset by reduced seeding costs; thus, predicted gross profit margin from weed management was optimized at the lowest seeding rate of 185,000 seeds/ha.

An accession of auxinic herbicide-resistant yellow starthistle found near Dayton, WA, was tested to evaluate cross-resistance to growth regulator herbicides and susceptibility to herbicides with different modes of action. Picloram at 0.43 kg ae/ha removed susceptible (S) yellow starthistle plants from a field plot, and surviving resistant (R) plants were dug and moved to the greenhouse. Known S plants were transplanted from a pasture near the R population. The R biotype was reconfirmed as resistant to picloram in greenhouse tests, with resistance ratios of 5.6 and 3.8 for vegetative and reproductive biomass, respectively, and 10.2 for LD50 (lethal dose for 50% of the treated population) data. The R biotype was also cross-resistant to clopyralid and dicamba in all responses and to 2,4-D and triclopyr in vegetative biomass and LD50 data. In field trials, eight herbicides were applied alone and in various combinations with and without addition of picloram. The yellow starthistle population was apparently comprised of differing percentages of R and S plants in 1997 and 1998, as picloram alone controlled 65 to 76% of the yellow starthistle in 1997 and 96 to 97% in 1998. BAS 662 01H (dicamba + SAN 836) at 0.28 or 0.42 kg ae/ha, respectively, or dicamba at 0.56 kg ae/ha were the best alternative treatments in either trial in either year, but only in 1998 did control exceed 85%. Picloram and other auxinic herbicides should continue to be useful for control of mixed R and S yellow starthistle populations. However, effective herbicides with different mode(s) of action integrated with range improvement practices and biological control must be identified for long-term yellow starthistle management.

Field studies were conducted from 1997 to 1999 to evaluate the influence of broadleaf herbicides on double-crop glyphosate-resistant soybean injury and yield at Belleville, IL. Diphenylether herbicides applied postemergence (POST) caused 10 to 48% necrosis 7 days after treatment (DAT). POST herbicides caused 0 to 8%, 8 to 37%, and 0 to 12% height reduction 7 DAT in 1997, 1998, and 1999, respectively. In most instances, diphenylether herbicides did not delay soybean maturity, whereas imazethapyr applied POST delayed soybean maturity in 1998 and 1999. Acifluorfen plus bentazon reduced soybean height at maturity in 1997 and 1998, whereas lactofen, imazethapyr, and imazamox reduced height only in 1998. Diphenylether herbicides did not reduce grain yield when compared to the no-herbicide hand-weeded treatment, but imazamox reduced yield by 18%. Furthermore, the soil herbicides applied preemergence and diphenylether herbicides applied POST did not reduce grain yield when compared to glyphosate alone.

Field trials were conducted in 1997 and 1998 at two locations in Israel to evaluate the efficacy of imazapic applied postemergence (POST) to sunflower for broomrape control under irrigated and nonirrigated conditions. Two sequential treatments of imazapic at 1.5 followed by (FB) 3.0, 3.0 FB 4.5, or 4.5 FB 6.0 g ai/ha on sunflower plants 12 ± 3 and 55 ± 5 cm tall, respectively, reduced sunflower broomrape throughout the growing season under irrigated and nonirrigated conditions. Sunflower growth was not affected by imazapic treatments. It was confirmed, in accordance with an earlier report, that when sequential treatments of imazapic included an application at the sunflower inflorescence developmental stage, the herbicide decreased seed yield in proportion to the applied rate.

Kyllinga squamulata has become problematic in bermudagrass turf in recent years, probably related to shifts in herbicide use strategies. Preemergence and postemergence greenhouse and field herbicide studies evaluated K. squamulata control in bermudagrass turf. Excellent (≥90%) postemergence control at 6 wk after initial treatment (WAIT) followed single and sequential applications of imazaquin at 0.42 kg ai/ha, MSMA plus imazaquin at 1.12 + 0.42 kg ai/ha, and sulfentrazone at 0.56 kg ai/ha. In one of two experiments, unacceptable (>30%) bermudagrass injury occurred with imazaquin at 0.42 and 0.56 kg ai/ha and MSMA plus imazaquin at 1.12 + 0.42 kg ai/ha 1 wk after application; however, plots recovered fully by 2 wk. In the field at 18 WAIT, preemergence control of greater than 70% was obtained with a single application of oxadiazon at 1.12 or 2.24 kg ai/ha and sequential applications of 1.12 kg ai/ha each 8 wk apart. Sequential applications 8 wk apart of sulfentrazone at 0.28 followed by 0.28 kg/ha also provided greater than 70% preemergence control. In two greenhouse studies, oxadiazon at 2.24 kg/ha and sulfentrazone at 0.56 kg/ha provided greater than 90% preemergence control.

A study was conducted in 1999 to evaluate weed control and imidazolinone-tolerant (IMI-tolerant) rice response to 140 g ai/ha imazethapyr at different application timings and to flood depths of 5, 10, and 20 cm at the Rice Research Station near Crowley, LA, and at the Northeast Research Station near St. Joseph, LA. Barnyardgrass control, rice injury, and rice yield were not influenced by flood depth; therefore, data were averaged over flood depths. Season-long barnyardgrass control was greater than 80% at both locations with imazethapyr applied preplant incorporated (PPI), early postemergence (EPOST), and mid-postemergence (MPOST). IMI-tolerant rice injury at 7 d after late postemergence (LPOST) treatment (DAT) was less than 5% for all imazethapyr treatments at Crowley and 1 to 8% for PPI, preemergence (PRE), delayed preemergence (DPRE), and EPOST timings at St. Joseph. However, IMI-tolerant rice injury was 5% or less at 42 DAT for all treatments at Crowley and St. Joseph. IMI-tolerant rice yield at Crowley was 2,780 to 3,000 kg/ha following applications of imazethapyr at PPI, PRE, DPRE, and EPOST, and these yields were higher than the yields of the MPOST and LPOST treatments. At St. Joseph, IMI-tolerant rice yields were equal for imazethapyr applied PPI, DPRE, and EPOST treatments compared with Crowley.