Several sequences of POST herbicides applied each year in fall alone, in spring alone, or both in fall and again in spring controlled Canada thistle stands in spring wheat by severely decreasing root biomass and the numbers of adventitious root buds to a depth of 50 cm over four years. These treatments included dicamba applied at 1.7 or 2.2 kg ae ha–1 for the first two successive falls followed in wheat by either chlorsulfuron at 30 g ai ha–1 plus nonionic surfactant, MCPA plus bromoxynil at 280 plus 280 g ha–1, or 2,4-D amine at 560 g ha–1 applied annually for each of four consecutive years from the start. Chlorsulfuron at 30 g ha–1 applied alone in spring for each of four years also reduced and prevented Canada thistle root growth as effectively as a sequence of fall-applied dicamba followed by spring-applied chlorsulfuron in spring wheat.

Variables affecting ultra-low-volume (ULV) application of herbicides for postemergence grass control in soybeans were evaluated in field experiments from 1988 to 1990. Air-assist applications of clethodim at 2.3 and 4.7 L ha–1 were compared with 94 and 187 L ha–1 applications with a conventional hydraulic sprayer. Rate of herbicide, volume of diluent, type of diluent, air pressure, and four other herbicides were evaluated. Clethodim at 28, 56, and 112 g ha–1 applied in paraffinic oil at 2.3 L ha–1 controlled johnsongrass better 10 wk after treatment than equivalent rates applied at 2.3 or 187 L ha–1 in water. At 2.3 L ha–1, barnyardgrass control was improved by applying clethodim in paraffinic oil rather than water. Johnsongrass control 10 wk after treatment was better with clethodim at 28 g ha–1 applied in paraffinic oil than when applied in soybean oil, cottonseed oil, No. 2 diesel fuel, kerosene, or jet A fuel. Low air pressures of 14 or 28 kPa resulted in better control of johnsongrass and barnyardgrass than higher pressures of 56 and 112 kPa. Clethodim, fluazifop-P, haloxyfop, quizalofop, or sethoxydim were more effective on johnsongrass and barnyardgrass when applied in paraffinic oil than in water at 2.3 and 4.7 L ha–1 with an air-assist sprayer.

The influence of various nutrients on parasitism of tomato plants by Egyptian broomrape and on stimulation of broomrape seed germination by strigol analogs was investigated in greenhouse and laboratory experiments. Parasitism occurred most readily in potting media low in fertility. Addition of nitrogenous compounds to potting media resulted in inhibition of broomrape parasitism. Ammonium nitrate with potassium phosphate or ammonium phosphate alone was most effective in reducing parasitism and enhancing growth of tomato plants. In the laboratory, strigol analogs induced 55% germination in broomrape seeds preconditioned in distilled water. Seeds preconditioned in distilled water or various salt solutions, but not treated with strigol analogs, showed 18% germination. GR 24 (5 μM) induced higher germination in seeds preconditioned in 10 mM ammonium nitrate, with or without potassium phosphate, compared with seeds preconditioned in sodium chloride. Preconditioning of seeds in sodium nitrate (10 mM) and treating with GR 24 stimulated, whereas preconditioning of seeds in ammonium salts (1 mM ammonium sulfate or 10 mM urea) and treating with GR 24 inhibited germination significantly.

The effects of the imidazolinone herbicide AC 263,222 on crops that may be rotated with soybean were studied in the greenhouse and field. Corn, grain sorghum, cotton, rice, wheat, soybean, and Italian ryegrass were planted directly into treated soil after AC 263,222 was incorporated. In the field, AC 263,222 at rates as high as 35 g ai ha–1 did not reduce shoot weight or emergence of these species. Some visual injury occurred, but was 20% or less 28 d after planting for all crops. Crops were more sensitive in the greenhouse, where reductions in both shoot and root dry weight were observed. AC 263,222 at 13, 13, and 7 g ha–1 reduced corn, grain sorghum, and Italian ryegrass shoot weights, respectively. Cotton, rice, and wheat tolerated rates from 27 to 55 g ha–1. Biomass reduction occurred with all crops at 55 g ha–1 and above. Because AC 263,222 will be registered for use in soybean at 9 to 18 g ha–1, and potential rotational crops were planted immediately after soil treatment, problems with injury to these crops are not expected.

Eight sweet corn cultivars differed in response to 35 g ai ha–1 nicosulfuron or 42 g ai ha–1 primisulfuron. ‘Merit’ sweet corn was killed 4 wk after nicosulfuron or primisulfuron application. Nicosulfuron and primisulfuron reduced height of Merit, 'Silver Queen’, 'Silverado’ and ‘How Sweet It Is'. Ear number of How Sweet It Is, ‘Pinnacle’ and Silverado was reduced by primisulfuron. ‘Landmark’, Silver Queen, and 'Sweetie 76’ were the most tolerant cultivars to nicosulfuron and primisulfuron with yield similar to that of the control.

Chlorsulfuron or metsulfuron applied POST at 10 g ai ha–1 at the four-leaf stage prevented yellow foxtail from forming seed, whereas seed production per pot was 53 to 77% of untreated checks following POST tribenuron or CGA-131036 treatments in greenhouse experiments. Chlorsulfuron applied POST at the two- and four-leaf stage severely reduced total seed production of yellow foxtail primarily by increasing plant mortality, rather than only reducing seed production per plant. Chlorsulfuron at 10 g ha–1 applied at the six- and eight-leaf stages decreased total seed production less than at the two earlier stages, by reducing both plant mortality and seed production per surviving plant. Yellow foxtail was much more susceptible than green foxtail to either chlorsulfuron or metsulfuron.

Yield reductions due to green foxtail competition with hard red spring wheat varied with environment in field experiments conducted in 1984, 1985, and 1986 at Oakes, Langdon, Prosper, and Fargo, North Dakota. Wheat yield reductions ranged from 0 to 47% from 720 green foxtail plants per m2. Inclusion of early season temperature and precipitation, soil texture, and foxtail density into multiple regression analysis of wheat yield reductions significantly increased the coefficient of determination to 0.62 compared with 0.12 for regression based on green foxtail density alone. Wheat yield reduction decreased as green foxtail seeding was delayed after wheat seeding in 1986. Wheat yield generally decreased as time of diclofop application was delayed from 2 to 6 wk after wheat emergence in 1986.

Controlled-release formulations have the potential to reduce atrazine leaching but may reduce herbicide bioavailability and weed control. This research was conducted to determine if starch-encapsulated atrazine produced by the extrusion process could be modified to provide an optimum rate of herbicide release for adequate weed control and reduced leaching potential in sandy soils. The dry flowable (DF) and six starch-encapsulated atrazine formulations of various mesh size (14 to 20 and 20 to 40), clay content (0 and 5%), and extruder processing speed (200 and 400 rpm) were evaluated. All starch encapsulations reduced the amount of atrazine leaching from the surface 5 cm of soil columns by an average of 68% compared with the dry flowable formulation. Compared with the larger starch granules (14 to 20 mesh) 8% more atrazine in the smaller granules leached from the surface 5 cm. Atrazine efficacy was measured using an oat bioassay conducted in the greenhouse. Smaller granules (20 to 40 mesh) provided equivalent efficacy to DF while large granules (14 to 20 mesh) did not. Extruder speed and clay content did not affect efficacy or leaching potential. This study indicates that finer starch granules should result in faster atrazine release and improved weed control while still reducing the leaching potential of atrazine.

Repeated use of sulfonylurea herbicides in continuous, no-till winter wheat selected for a herbicide resistant biotype of prickly lettuce in Idaho. Greenhouse experiments were conducted to compare the relative competitiveness and growth rate of sulfonylurea herbicide resistant (R) and susceptible (S) prickly lettuce. The S biotype of prickly lettuce produced 31% more aboveground biomass than the R biotype averaged over all densities. Both biotypes were equally competitive when analyzed for both inter- and intrabiotype competition. In relative growth rate studies, regression analysis showed that the S biotype accumulated biomass 52% faster than the R biotype. The results of this study showed that the S biotype was superior to the R biotype in biomass production and growth rate, but competitiveness appeared to be equal for both biotypes. Other fitness parameters must be measured before fitness differences between biotypes can be determined.

Five field experiments were conducted during 1987–89 to determine the effectiveness of using activated carbon in transplant water to reduce injury to tobacco from herbicides incorporated before transplanting. Herbicides used were dicamba, chlorimuron, atrazine, lactofen, flumetralin, pendimethalin, pebulate, and imazaquin. Activated carbon was applied in the transplant water at a rate of 9 kg ha–1 in 1987 and 18 kg ha–1 in 1988 and 1989. Measurements taken were visual stunting ratings, yield, grade index, fresh and dry weights of roots and shoots, and percent sugars and total alkaloids in cured leaves. Activated carbon consistently reduced phytotoxicity of imazaquin and chlorimuron. As much as four- and two-fold yield increases were obtained when imazaquin and chlorimuron, respectively, were applied in conjunction with activated carbon. Less consistent results were obtained in reducing the phytotoxic effects of dicamba, atrazine, and flumetralin. Phytotoxicity without activated carbon was not consistently obtained with lactofen, pendimethalin, or pebulate.

Experimental controlled release starch granules (CRSG) containing 5.3% a.i. (w/w) norflurazon or 6% a.i. (w/w) simazine retarded the leaching of both herbicides in loamy sand soil columns when compared to commercial formulations of norflurazon [80% (w/w) dry flowable] or simazine [90% (w/w) water dispersible granule]. Barley bioassays indicated norflurazon and simazine remained in the surface 0 to 2.5 cm of soil when applied as CRSG formulations and moved to a depth of 15 cm when applied as commercial dry formulations and leached with 6 cm of water. CRSG placed on pre-wetted soil columns began to release norflurazon by 7 d at 25 C or 14 d at 15 C, and subsequent leaching moved norflurazon beyond the top 2.5 cm of the soil column.

Experiments conducted in the greenhouse indicated that 2,4-D antagonism of glyphosate toxicity to wheat was sodium salt = butoxyethyl ester ≥ diethanolamine. Isopropylamine salt of 2,4-D generally was not antagonistic to glyphosate phytotoxicity. Isopropylamine salt of 2,4-D did not influence the antagonism of glyphosate by inorganic salts in the spray carrier. Antagonism of glyphosate toxicity to wheat by 2,4-D increased when sodium bicarbonate, calcium chloride, and ferric sulfate were in the spray carrier water. Isopropylamine alone as an adjuvant enhanced glyphosate toxicity to wheat, and overcame ferric sulfate and sodium bicarbonate antagonism of glyphosate. Diammonium sulfate adjuvant overcame antagonism to glyphosate phytotoxicity from 2,4-D, sodium bicarbonate, and calcium chloride each alone or the salts in combination with 2,4-D. Nonionic surfactants differed in enhancement of glyphosate but none overcame antagonism from salts or 2,4-D.

Field experiments were conducted in 1989 and 1990 to evaluate the tolerance of ‘Penncross' creeping bentgrass and ‘Tifway’ bermudagrass fall-overseeded with perennial ryegrass to irrigation water containing simazine. Creeping bentgrass was more sensitive to simazine than the bermudagrass/ryegrass mixture. The predicted critical simazine concentration necessary to reduce creeping bentgrass quality ≤ 60% was 0.22 mg L–1 which corresponded to a cumulative simazine rate of 98 g ha–1. For bermudagrass/ryegrass, the predicted critical simazine concentration and cumulative simazine rate that reduced quality ≤ 60% was 0.68 mg L–1 and 306 g ha–1, respectively.

The interaction of imazethapyr and selected graminicides on annual grass control was studied in field experiments. Tank-mix combinations of imazethapyr with clethodim, fluazifop-P, quizalofop, or sethoxydim resulted in an antagonistic interaction. Control of large crabgrass, fall panicum, and broadleaf signalgrass by each graminicide decreased when tank-mixed with imazethapyr as compared with each graminicide applied alone. Sequential applications of imazethapyr, relative to each graminicide, successfully overcame the antagonism. Imazethapyr applied 5 d before or 1 d after each of the graminicides did not decrease grass weed control compared with each graminicide alone. Imazethapyr applied 3 or 1 d before, or the same day as the graminicides, generally decreased grass weed control.

Experiments were conducted in 1989 and 1990 to investigate the influence of 220 g ai ha–1 AC 263,222, 840 g ai ha-4 glufosinate, 560 g ai ha–1 glyphosate, and 840 g ai ha–1 paraquat applied at R5, R6, R7, and R8 soybean growth stages as harvest-aid treatments on soybean seed quality. Soybean yield was severely reduced by all herbicides applied at R5. Only paraquat and glufosinate reduced soybean yield when applied at R6, and none of the herbicides reduced yield when applied at R7 or R8. Paraquat and glufosinate reduced seed weight when applied at RS and R6. Glyphosate and AC 263,222 reduced seed germination when applied at RS, R6, and R7 growth stages, and normal seedling percentages were also reduced by glyphosate at these growth stages. Glufosinate and AC 263,222 affected normal seedlings only at R5 and R6. Soybean hypocotyl and primary root lengths were reduced by glyphosate and AC 263,222 applications at R5 and R6, whereas glufosinate and paraquat did not affect these variables. Glyphosate applied at R5 reduced shoot weight in 1-mo-old soybean plants. No effects were observed on soybean yield, seed germination or seedling development when herbicides were applied at R8.

Field experiment were conducted to determine interactions of chlorimuron or imazaquin with fomesafen, lactofen, or acifluorfen on three-leaf and eight-leaf common cocklebur, hemp sesbania, pitted morningglory, and prickly sida. Antagonism was the most common interaction with common cocklebur, and was most severe with chlorimuron combined with fomesafen or acifluorfen, whereas lactofen did not antagonize common cocklebur control. Reductions in control were greater when low rates of chlorimuron were used. On three-leaf prickly sida, control synergistically increased when imazaquin was combined with fomesafen or acifluorfen, but the majority of these combinations were additive on eight-leaf prickly sida. Three-leaf pitted morningglory control synergistically increased when 36 g ai ha–1 imazaquin was combined with 210 g ai ha–1 fomesafen or 110 or 220 g ai ha–1 lactofen. With eight-leaf pitted morningglory, synergism occurred when 2 g ai ha–1 chlorimuron was combined with the high rate of any diphenylether herbicide tested, and when 36 g ha–1 imazaquin was combined with 110 g ha–1 lactofen or 210 g ai ha–1 acifluorfen; however, at higher rates of chlorimuron or imazaquin, several antagonistic interactions occurred. Hemp sesbania was controlled over 90% by all combinations, and no interactions occurred.

Clopyralid at 0.3 or 0.6 kg ae ha–1 applied once or twice during the asparagus harvest season for two consecutive years did not significantly reduce the number of spears or total weight of asparagus harvested. In a second study, clopyralid applied at 0.3 kg ha–1 twice during the harvest season and once at layby for two consecutive years did not reduce asparagus yield in the first year of treatment. In the second year, the number and weight of asparagus spears was reduced. Clopyralid applied during the harvest season caused some epinastic growth of asparagus after layby. Clopyralid applied to emerged asparagus spears slightly increased the number of cull spears harvested, but was less damaging than 2,4-D or dicamba.

Multiple applications of fenoxaprop (0.027, 0.036, and 0.045 kg ai ha–1) were field-applied on either 2-, 3-, or 4-wk intervals to ‘Penncross' creeping bentgrass maintained at putting green height in Easton, MD and Silver Spring, MD during 1989 and 1990. At both locations, yearly averaged creeping bentgrass quality ratings decreased with increasing fenoxaprop rate. Creeping bentgrass discoloration and thinning were minimal at 0.027 kg ha–1 fenoxaprop and did not reduce season-long turf quality below acceptable levels at either site in 1989 or 1990. Substantial discoloration and thinning was observed at 0.045 kg ha–1 fenoxaprop and acceptable season-long quality was limited to Easton in 1990. Significant spray frequency treatment effects were noticed at both sites in 1989 but not 1990. Increasing the spray interval from 2- to 4-wk improved creeping bentgrass quality from unacceptable to acceptable in two of the final four evaluation dates at Easton and five of the final six dates at Silver Spring.

Greenhouse studies were conducted to evaluate the effectiveness of Kinetic, a silicone adjuvant that appeared to increase the efficacy and rainfastness of the isopropylamine salt of glyphosate on velvetleaf, sicklepod, barnyardgrass, guineagrass, yellow foxtail, and yellow nutsedge. Simulated rainfall of 1.3 cm in 5 min at 15 and 60 min after herbicide treatment reduced glyphosate efficacy on all weeds. Kinetic enhanced glyphosate efficacy when yellow nutsedge and guineagrass were subjected to post-spray rainfall at 60 min. On velvetleaf, sicklepod, and yellow foxtail, Kinetic improved glyphosate efficacy when the critical rain-free period was reduced to 15 min. Kinetic failed to provide rainfastness for barnyardgrass, thereby indicating a specificity of Kinetic for certain weed species.