Extension workers are sensing pressure to use soils information and chemical characteristics data to guide farmers in selecting pesticides least prone to leach into groundwater. Our objective was to estimate differences in herbicide migration to groundwater under conditions typical for the Southeast Coastal Plain, and to consider how a farmer might be advised to use such knowledge in selecting herbicides. We used a simple computer code for microcomputers to predict persistence and migration of 17 herbicides through a hypothetical, coarse-textured soil typical of the Southeast Coastal Plain. Appropriate herbicides were selected for several common crop-weed problems, such as sicklepod in soybean and Palmer amaranth in corn. Groundwater was assumed to be 3.15 m below the soil surface. Herbicides selected covered a broad range of half-lives and organic carbon partition coefficients. Only after the first-order degradation rate constant was reduced by a factor of five did predicted soil water concentrations of several herbicides at the groundwater interface reach normal detection limits. Still, predicted concentrations were below the level established for health effects advisory purposes. Due to the large number of uncertainties and the inability to estimate practical benefits, we conclude that data relating to soil and herbicide characteristics cannot be used at this time to override cost effectiveness, efficacy, and other factors normally considered by farmers and Extension professionals in herbicides for weed control.

Mayweed chamomile is an increasing weed problem in cropping systems of the Pacific Northwest. Modern farming practices that utilize conservation tillage systems and heavy application of nitrogen fertilizers have been associated with increased soil surface water potential and decreased soil pH. Therefore, soil water potential, soil pH, and temperature effects on germination and growth of mayweed chamomile were determined in controlled laboratory tests. Germination of mayweed chamomile in soil was greatest at 30 C and a soil water potential of –25 kPa. Germination and seedling growth were similar in soils with pH 4.7, 5.1, and 6.2. Total plant weight was greatest at 20 C and reduced at 10 and 30 C. Shoot dry weight, as a percent of total dry weight, ranged from a low of 54% at 10 C to 78% at 30 C. A soil moisture potential of –10 000 kPa reduced germination and total plant weight by as much as 95% and 80%, respectively.

Barnyardgrass and many broadleaf weeds were severely stunted by POST applications of isoxaben. Most grasses were only moderately affected by isoxaben at rates up to 1 kg/ha. POST activity was enhanced by soil interception, but evident with foliar interception alone. Cell swelling in stems and in interveinal leaf lamina was observed after foliar application. A radiotracer study in redroot pigweed showed that only a small amount (3.3% of applied) of isoxaben entered the leaf following foliar application, and that little (0.08% of applied) isoxaben migrated beyond the point of application. The potential of isoxaben as a POST herbicide may be limited, in part, by poor absorption and translocation.

Greenhouse and field studies were conducted to examine weed control with, and potato tolerance to, rimsulfuron or rimsulfuron + metribuzin applied POST. In greenhouse trials, hairy nightshade control with rimsulfuron applied POST at 9 to 35 g/ha was ≥ 90%, but cutleaf nightshade control was < 30%. In field studies, rimsulfuron at 18 to 35 g/ha provided ≥ 85% control of hairy nightshade, redroot pigweed, tumble pigweed, and shepherd's-purse; 75 to 82% control of volunteer oats; and 25 to 87% control of common lambsquarters. Common lambsquarters control was increased to 92 to 100% when rimsulfuron at 18 to 35 g/ha was applied in combination with metribuzin at 280 g/ha. Hairy nightshade and redroot pigweed control usually were not improved by adding metribuzin to the rimsulfuron spray mixture and tumble pigweed and shepherd's-purse control were improved only slightly. Volunteer oat control with rimsulfuron sometimes was antagonized by the addition of metribuzin to the spray mixture. In weed-free trials, yield of potatoes grown for the fresh market or to produce seed tubers was not reduced by rimsulfuron at rates as high as 70 g/ha.

Barnyardgrass 7 to 25 cm tall was controlled 48 to 74% with paraquat (420 g/ha), 83 to 87% with glyphosate (1120 g/ha), and 85 to 91% with glufosinate (840 g/ha). In most cases barnyardgrass control was not enhanced with addition of residual herbicides metribuzin plus chlorimuron, metribuzin, or imazaquin. Barnyardgrass and seedling johnsongrass no more than 13 cm tall was controlled at least 90% regardless of herbicide treatment. When rhizome and seedling johnsongrass were present, control with glyphosate was 96% compared with 55% for paraquat and 86% with glufosinate. Tank-mixtures of non-selective and residual herbicides generally enhanced control of entireleaf and pitted morningglory, hemp sesbania (15 to 30 cm), and prickly sida (15 to 18 cm). Soybean yields in most cases were not increased with addition of residual herbicides. Yield following glufosinate applied alone was 25% higher than following paraquat, and for all herbicide treatments yields were at least 45% greater than when a non-selective herbicide was not applied.

Field studies were undertaken in 1992 and 1993 to investigate the control of wild oat and green foxtail in flax with reduced rates of fluazifop-P and clethodim applied as tank-mixtures. Fluazifop-P plus clethodim at 50 + 18 g ai/ha controlled wild oat and green foxtail and was as effective as full rates of either herbicide applied alone. These rates represent a 20% reduction in total amount of active ingredient required to control wild oat and green foxtail. Application of fluazifop-P, and/or clethodim prior to the 3- to 4-leaf stage failed to control late emerging grass weeds. Application of graminicide mixtures at or after the 3- to 4-leaf stage controlled late emerging grass weeds and did not affect flax yield. When applied late, fluazifop-P at 175 g/ha tended to reduce flax yield, although weed control was acceptable and no foliar injury was observed following treatment. The efficacy of graminicide mixtures was reduced by addition of bromoxynil plus MCPA to the spray mix.

Seven annual weed species were treated under greenhouse conditions with six rates of either nicosulfuron, rimsulfuron, or a 1:1 mixture (DPX-79406). Herbicide rates in g ai/ha producing 90% dry biomass reduction (GR90) were calculated for each weed species. Accordingly, weeds were classified as very susceptible when GR90 ≤ 9, susceptible when 9 < GR90 ≤ 18, moderately susceptible when 18 < GR90 ≤ 27, and tolerant when, GR90 > 27. Giant and green foxtails were very susceptible to the three herbicides. Proso millet was very susceptible to rimsulfuron and DPX-79406, and susceptible to nicosulfuron. Wild oat was susceptible to nicosulfuron and DPX-79406, and moderately susceptible to rimsulfuron. Redroot pigweed was very susceptible to nicosulfuron; whereas, it was susceptible to rimsulfuron and DPX-79406. Common ragweed and smooth crabgrass were tolerant to the three herbicides. In a second experiment, we used regression estimate analysis with an additive reference model to evaluate nicosulfuron and rimsulfuron interaction on corn, soybean, common ragweed, and smooth crabgrass. We found these herbicides synergistic on smooth crabgrass, antagonistic on soybean, less than additive on common ragweed, and additive on corn.

Soil-water profiles of cotton, several weeds, and bare soil were compared in research conducted in 1989 and 1990 near Perkins, OK. Neutron probe access tubes were installed prior to plant establishment to facilitate nondestructive volumetric water content determinations at selected depths throughout the season. Planting was in a double-circle pattern concentric around each neutron probe access tube. Phenological and soil-water data were collected weekly. As plants began to senesce, aboveground biomass yield data were collected. In 1989, rainfall frequently replenished soil-water in the upper soil profile. The plants used the recharge water to varying degrees, particularly late in the season. Common cocklebur and johnsongrass extracted soil-water from greater depths than cotton and velvetleaf throughout the season. In 1990, the largest differences detected between bare soil and weeds early in the season were measured for common cocklebur, devil's-claw, and silverleaf nightshade. Later in the season, however, there was little difference in soil-water content in the upper soil profiles for all species. Johnsongrass emerged later than the other species in 1990 which affected its relative soil-water extraction.

Field-studies were conducted over two years to determine the area of influence and duration of interference of wild poinsettia in soybean. Soybean canopy width averaged across years was reduced approximately 10% beginning at 6 wk of interference for both the 0- to 10-and 10-to 20-cm distances from the weed. Soybean dry weights decreased from 14 to 38% within 20 cm of the weed for 12 through 18 wk of interference. In 1990, soybean yield within 10 cm of wild poinsettia was similar to distances of 10 to 20 and 20 to 40 cm, but was less than that for distances greater than 40 cm from the weed. Weed interference resulted in a 9.5% yield reduction in the 0- to 10-cm distance when compared with the 80-to 100-cm distance. In contrast, yield of soybean in 1991 growing within 10 cm of the weed was less than at greater distances, corresponding to an 18% yield reduction when compared with 80 to 100 cm. Differences in wild poinsettia dry weights when growing alone and when growing within the soybean row occurred after 6 and 8 wk of interference in 1990 and 1991, respectively. In 1991 when rainfall during the growing season was twice that of the previous year, canopy width and dry weight of wild poinsettia growing within the soybean row were reduced an average of 57 and 82%, respectively, compared with weeds growing alone.

Field experiments conducted in 1988 and 1989 evaluated clomazone alone and in a systems approach for weed control in peanut. Clomazone PPI at 0.8 kg ai/ha controlled common ragweed, prickly sida, spurred anoda, and tropic croton better than ethalfluralin and/or metolachlor applied PPI. POST application of acifluorfen plus bentazon was not needed to control these weeds when clomazone was used. Acifluorfen plus bentazon improved control of these weeds when clomazone was not used and generally were necessary to obtain peanut yields regardless of the soil-applied herbicides. Alachlor PRE did not improve clomazone control of any weed species evaluated. Fall panicum and large crabgrass control was similar with clomazone or clomazone plus ethalfluralin.

Eight POST herbicides were evaluated alone or in various mixtures for spiny amaranth control in peanut in 1991 and 1992. Acifluorfen alone or in combination with bentazon, lactofen alone, or 2,4-DB alone applied late POST controlled spiny amaranth more than 80% at an early season rating. Lactofen applied early POST and 2,4-DB applied late POST controlled more than 90% late season. Imazethapyr and AC 263,222 controlled 72 to 90% of spiny amaranth, whereas bentazon and pyridate failed to provide adequate control.

Field trials were conducted in New Jersey during 1991 and 1992 to evaluate the efficacy of nicosulfuron, primisulfuron, dicamba, and clopyralid on horsenettle control in field corn. Horsenettle biomass was reduced 74%, 68%, 64%, 61%, 48%, and 40% by primisulfuron + dicamba, primisulfuron, nicosulfuron + dicamba, dicamba, clopyralid, and nicosulfuron, respectively. No treatment was statistically more effective than dicamba. Corn yields were not influenced by the herbicide treatments. Horsenettle populations were not reduced by any treatment.

The basidiomycete fungus, Chondrostereum purpureum Fr. Pouzar, has been found to be a good potential bioherbicide candidate for control of hardwood vegetation in forests. However, its interaction with some agrochemicals that are normally employed in forest protection was not known. Employing two concentrations (0.01% v/v and 0.1% v/v) of several agrochemicals, a laboratory study was conducted to assess their impacts on the infective mycelia. Except for Bond, and Suntan gel-2, most adjuvants, sunscreen agents, and pesticides (herbicides, fungicides, and insecticides) were fungitoxic at 0.1%. Fungitoxicity was concentration dependent and the results indicate that the possibility of tank-mixing the basidiomycete with agrochemicals appears limited and the possibility of agrochemical residue effects on survival of the basidiomycete is implied.

Field experiments were established in 1991 and 1992 on silty clay and sandy loam soils to evaluate various split rates of early PPI and PRE (to soybean) selective herbicides with and without paraquat for sicklepod and pitted morningglory control in stale seedbed soybean. Metribuzin at 360 g ai/ha plus 60 g ai/ha chlorimuron tank-mixed with 700 g ai/ha paraquat controlled sicklepod and pitted morningglory 83 and 91%, respectively, 4 wk after planting. Without paraquat, sicklepod and pitted morningglory control was only 65% and 67%, respectively. Imazaquin at 140 g/ha PRE tank-mixed with paraquat controlled sicklepod 78% and pitted morningglory 92%. Without paraquat, control was 38% and 84%, respectively. Early PPI applications of metribuzin plus chlorimuron or imazaquin at the full rate alone or followed by paraquat at planting resulted in poor control. With sequential treatments (PPI followed by PRE) the addition of paraquat at planting did not usually improve control, and either imazaquin or metribuzin plus chlorimuron provided equivalent control when compared with the full rate of either herbicide applied PRE. Season-long weed control was not obtained with any treatment in any experiment, and the crop was not harvestable.

Three corn root bioassays were evaluated for detecting imazaquin in soil. Two techniques, one which utilized a cone-shaped tube as the growth container and another, a petri dish, were compared to a method that utilized a thin layer of soil between two 20 by 20 cm glass plates. Corn root growth responded logarithmically to imazaquin regardless of bioassay method. Corn was most sensitive to low imazaquin concentrations when grown using the glass plate apparatus. At a low concentration (0.5 ng/g) of imazaquin, corn root length was reduced 6% using the cone-tube, 2% using the petri dish, and 24% using the glass plate method. In contrast, the cone-tube method provided a better measure of high imazaquin concentration (200 ng/g) than the other methods.

Field experiments conducted over three years at Stoneville, MS evaluated invert emulsion and oil diluents on the efficiacy of glyphosate and MSMA on purple nutsedge. The herbicides were applied at 0.6 and 1.1 kg ai/ha in an invert emulsion (oil-in-water) and a paraffinic oil at 19 L/ha (low-volume) and in water with a non-ionic surfactant (0.25% v/v) at 187 L/ha (high-volume). Purple nutsedge control with both herbicides was greater with the low-volume, invert emulsion and oil diluent applications than with high-volume, water diluent applications. Glyphosate and MSMA at 1.1 kg/ha in the oil diluent controlled purple nutsedge at least 88% when compared with the invert emulsion (≥ 75 and ≥ 76%, respectively) and water (≥ 42 and ≥ 44%, respectively) diluents. Cotton injury and seed cotton yields were unaffected by any treatment with MSMA but glyphosate with each diluent caused unacceptable cotton injury and seed cotton yield reduction.