The impact of POST applications of fluazifop-P on sugarcane grown in Louisiana was determined for 3, 6,11, 22, and 44% of the lowest rate recommended for the control of johnsongrass in soybean (100 g ai ha−1). In field studies, sugarcane injury 4 WAT increased as the rate of fluazifop-P increased. Injury in excess of 80% was observed when fluazifop-P at 44 g ha−1 was applied in May and June and 54% when applied in August Sugarcane stalks treated with fluazifop-P were fewer in number, shorter and lighter, and immature at harvest This resulted in lower Brix and sucrose levels, higher fiber content, and ultimately lower sugar yields. Reductions in sugar yields associated with an application of fluazifop-P closely paralleled visual injury 4 WAT. Injury from application of fluazifop-P was also evident in the subsequent ratoon crop where reductions in spring shoot counts of 14 to 25% were obtained even when fluazifop-P had been applied at 6 g ha−1 the previous year. Greenhouse studies indicated that the response observed in the field with CP 70-321 sugarcane was indicative of the major cultivars grown, with the possible exception of CP 74-383.

Field studies were conducted from 1991 through 1993 to determine the effects of stale seedbed management practices on weed control in peanut. Main plots were four levels of stale seedbed management: deep till (23 cm) and plant the same day (standard system), deep till 6 wk early and shallow till (7.6 cm) at 2 wk intervals prior to planting, deep till 6 wk early and application of glyphosate (1.1 kg ai ha−1) 1 wk prior to planting, and deep till 6 wk early without additional treatment prior to planting. Sub-plots were three levels of weed management following peanut planting; intensive, basic, and cultivation alone. Stale seedbed management practices stimulated weed emergence when followed by other control measures prior to planting. Populations of Florida beggarweed, Texas panicum, and yellow nutsedge were lower when stale seedbeds were shallow tilled at 2 wk intervals prior to planting, resulting in greater peanut yields. Weeds on nontreated stale seedbeds were difficult to control once peanut was planted and reduced yields. Stale seedbed management practices generally had no effect on the quantity of foreign material contaminants originating from weeds, soil, or peanut plant in harvested peanut. These results indicate that shallow tillage on stale seedbeds can reduce weed populations prior to planting and increase peanut yields.

Rust-specific polymerase chain reaction (PCR) primers selectively amplified ribosomal DNA of a rust fungus from infected dyers woad. PCR enabled DNA of the fungus to be detected in symptomatic plants as well as in asymptomatic parts of diseased plants. The use of PCR enabled early detection of rust infections in dyers woad plants during their first season when they are often asymptomatic Dried plant samples stored at room temperature for several months worked as well as lyophilized material for DNA extraction prior to PCR. The PCR detection method should greatly facilitate further studies on the biology and inoculation of this and other systemic rusts that have potential for use in biocontrol of weeds.

Spring small grains were not as competitive with barnyardgrass and witchgrass as winter wheat. Winter wheat grain yields were greater than spring barley or spring wheat in 1986, 1987, and 1988 and oat in 1986 and 1988. Barnyardgrass, stinkgrass, and witchgrass control with glyphosate plus 2,4-D plus atrazine at 0.6 plus 0.8 plus 1.7 kg ha−1 was usually less when the herbicides were applied to stubble of spring small grain versus winter wheat due to the advanced weed growth at treatment Barnyardgrass and witchgrass were more difficult to control than stinkgrass, redroot pigweed, tumble pigweed, kochia, and tumble thistle. No-till corn planted into winter wheat stubble had fewer barnyardgrass and witchgrass than corn planted into spring wheat stubble. The addition of metolachlor plus atrazine at 1.7 plus 0.6 kg ha−1 eliminated differences among small grain cultivars in weed control in corn. Corn grain yields from winter wheat plots were greater than other small grains in 1989 because of better weed control and more crop residue.

Short term incubations (4 d) were conducted to assess the effect of a wetting/drying cycle on atrazine sorption, as well as biodegradation, as a function of various atrazine concentrations (ca. 5, 10, and 25 μg g−1 soil) and levels of added crop residues (0, 5, and 10% cornstalks by weight), using a technique that allowed independent analysis of soluble and sorbed atrazine. Soil solution atrazine concentrations decreased, and KdS increased with increasing crop residues. The sorptive capacity of cornstalks for atrazine was estimated to be 860 μg g−1 vs 28 μg g−1 for unamended soil. Drying and rewetting resulted in lower soil solution concentrations and decreased extraction efficiencies (13 to 22%) for sorbed atrazine; the effect was most pronounced with added cornstalks. High recoveries of 14C from soils (combustion data) indicated that atrazine was not lost to volatilization. Rapid rates of biodegradation were observed in cornstalkamended soils shortly after rewetting; degradation was not observed in unamended soil. A longer incubation (6 wk) was conducted with ca. 10 μg g−1 atrazine and 5% cornstalks to assess metabolites and kinetics of biodegradation. Atrazine disappearance was observed after ca. 2 wk with concomitant production of deethyl- and deisopropyl-atrazine at a ratio of ca. 2:1. Dealkylated-atrazine accumulated after ca. 3 wk; there was no evidence for hydroxy-atrazine production. These data suggest that biodegradation may play an important role in atrazine losses in the field despite wetting/drying cycles. In addition, there may be apparent losses of atrazine due to decreased extraction efficiencies as a consequence of wetting/drying cycles, resulting in underestimation of field residues.

Barnyardgrass is a problem in winter wheat fields in the central Great Plains following harvest. Glyphosate plus 2,4-D efficacy on barnyardgrass with or without atrazine was investigated under various watering events at three sites in west central Nebraska. Two experiments were initiated at each site. One experiment dealt with rainfall and the other rainfall plus irrigation. Main plots in stubble fields infested with barnyardgrass were sprayed with glyphosate plus 2,4-D at 0.4 plus 0.7, 0.5 plus 0.8, and 0.6 plus 1.0 kg ae ha−1 alone and with 2.2 kg ai ha−1 atrazine. Barnyardgrass was under drought stress in unwatered areas in 1991 at two sites, but the 1993 site was waterlogged. Glyphosate plus 2,4-D at 0.6 plus 1.0 kg ha1 controlled 62, 99, and 100% of the barnyardgrass at the three sites 30 DAT. Nevertheless, control was reduced to 48, 88, and 35% when atrazine was combined with glyphosate plus 2,4-D. Poorest control came when plants were under stress from drought or waterlogging and atrazine was included in the spray solution. In subplots barnyardgrass plants in a 0.8 m2 area were watered 6, 3, or 1 d before spraying and 1, 4, 7, or 11 d after spraying. By watering drought-stressed barnyardgrass 6 d before spraying, control generally was similar or better than with later watering dates. Watering 6 d before spraying with glyphosate plus 2,4-D plus atrazine at 0.6 plus 1.0 plus 2.2 kg ha−1 reduced but did not eliminate antagonism from atrazine.

Laboratory studies were conducted to determine the effect of rainfall timing and antecedent moisture on atrazine leaching through intact soil cores taken from no-till and conventional-till corn fields. Simulated rainfall was applied to no-till and conventional-till cores 1 to 14 d after atrazine application and, in a second study, one d after atrazine was applied to no-till and conventional-till cores at 1 to 800 kPa soil moisture. Increasing the lag time between atrazine application and rainfall from one to 14 d reduced the amount of atrazine leached by about 50% for both no-till and conventional-till soil cores. During the same time period, the amount of atrazine adsorbed to soil increased by about 30% for both tillages. Soil dryness (antecedent moisture) at the time of atrazine application had no effect on the amount of atrazine leached through conventional-till cores. However, leaching decreased in no-till cores as antecedent moisture decreased from 1 to 33 kPa; drying to 800 kPa had no further effect. The leaching rate of atrazine was much higher for the initial 0.5 pore volume than for the next 1.5 pore volume at all rainfall timing and antecedent moisture levels. This behavior is indicative of preferential transport.

Granules of isoxaben plus trifluralin and liquid applications of isoxaben plus oryzalin were applied to miniature nursery plots covered with plastic, woven landscape fabric, or gravel with no cover. Herbicide concentrations were monitored in runoff water over a 30 d period. Runoff losses of isoxaben from the sprayable formulation applied to gravel were 5% greater than from plastic and 4% greater than losses from fabric in 1992. Similarly, oryzalin losses from the sprayable formulation applied to gravel were 7% greater than losses from plastic and 4% greater than losses from the fabric ground cover. In contrast, loss of isoxaben from the granular formulation applied to plastic was 7% greater than loss from gravel in 1993. Isoxaben losses from the granular formulation applied to fabric were intermediate. Trifluralin losses from the granular formulation applied to plastic and fabric were both 3% greater than losses from gravel. In addition, isoxaben losses from the granular formulation were 11 and 10% greater than loss from the sprayable formulation applied to the plastic and fabric-covered plots, respectively. Isoxaben losses from the sprayable formulation applied to gravel were 11% greater than losses from the granular formulation. In an experiment to determine herbicide release patterns and the effect of light on residues from the granular formulation of isoxaben and trifluralin in irrigation effluent, water was monitored for 36 d. Approximately 20% of the applied isoxaben and 7% of the applied trifluralin was detected in irrigation water during the 36 d period. These studies indicate that runoff losses and the ultimate fate of isoxaben, oryzalin, and trifluralin applied in nursery settings depend on factors including ground cover composition, herbicide formulation, and photochemical degradation.

Light regulates many facets of plant growth and development through the effects of quantity of total energy and of photons, spectral quality, duration, and photoperiod. Numerous techniques and types of equipment are available for quantifying light in plant canopies. The effect of total quantity of light on weed and crop productivity has been described for many cropping systems. Recent work has focused on other aspects of light, in particular, spectral distribution of light (quality), transient light (sunflecks), and plant adaptation to changing light environments. The altered spectral quality of light in a plant canopy affects plant growth and morphology, which in turn affect competition for light. Dynamic plant response to transient light is also important to canopy photosynthesis and productivity. Plant physiological and morphological adaptation to fluctuating light is another potential factor regulating weed/crop interactions. Current cropping practices such as using smother crops and narrow row spacing exploit plant light responses to promote crop growth and suppress weed growth. A better understanding of plant responses to light quality, transient light, and fluctuating light environments will lead to a better understanding of how to manipulate the light environment in crop canopies to improve weed management.

Greenhouse and laboratory experiments were conducted to determine the effect of soil organic matter on the synergistic interaction between nicosulfuron, a sulfonylurea herbicide, and the insecticide terbufos in corn. Terbufos was applied infurrow at planting to soils containing 1, 3, and 5% organic matter and nicosulfuron was applied POST to corn at the three-leaf growth stage. The combination of terbufos and nicosulfuron reduced corn fresh weight 75, 45, and 41 % in soils containing 1, 3, and 5% organic matter, respectively. Terbufos increased [14C]nicosulfuron uptake into corn grown in both 1 and 5% organic matter soils. Terbufos decreased epicuticular leaf wax deposition on corn leaves by 35 and 18% in 1 and 5% organic matter soils, respectively. [14C]Nicosulfuron was completely metabolized after 24 h in the absence of terbufos. In the presence of terbufos, 19 and 11% of intact [14C]nicosulfuron remained 24 h after treatment while 11 and 6% remained after 48 h in the 1 and 5% organic matter soils, respectively. In vivo acetolactate synthase activity was reduced to 3 and 20% of control by nicosulfuron at 24 h and to 6 and 38% of control at 48 h, respectively, in the 1 and 5% organic matter soils that contained terbufos. These experiments indicated that soil organic matter is an important factor in determining the level of injury expected from the synergistic interaction between nicosulfuron and terbufos in corn.

Efficacy, movement, and degradation of dithiopyr and pendimethalin were monitored after application to perennial ryegrass turf in 1990 and 1991. Dithiopyr at 0.6 kg ai ha−1 reduced large crabgrass infestation by ≥ 90% up to 87 DAT in both years, while pendimethalin at 1.7 kg ai ha−1 provided similar control up to 59 DAT in 1 of 2 yr. Turf thinning due to disease and insect damage in 1990 allowed more herbicide to reach the thatch, mat, and underlying soil in 1991. In both years, more pendimethalin but less dithiopyr was found in thatch 7 DAT than 1 DAT, suggesting greater pendimethalin retention in verdure and thatch. Precipitation was greater after herbicide application in the second year, and concentration in the mat layer reached a maximum by 7 DAT compared to 14 DAT in 1990. Maximum concentrations of dithiopyr and pendimethalin in soil at 0 to 5 cm deep were 20 and 56 μg kg−1, respectively, measured 7 DAT in 1991. Herbicide concentration 5 to 10 cm deep was ≤ 5 μg kg−1 at all sampling dates in 1990. In 1991, dithiopyr and pendimethalin concentrations 5 to 10 cm deep did not exceed 4 and 20 μg kg−1, respectively, measured within 1 DAT. Neither herbicide was detected 10 to 20 cm deep, nor in samples collected 30 cm outside of the experimental plots in either year. At 126 DAT (final sampling date), little herbicide was detected in verdure, but residues were found in most thatch and all mat samples. The estimated time for a 50% reduction in detectable residues (DT50) of dithiopyr and pendimethalin from the sampling zone was 35 and 23 days, respectively.

All environmental factors that influence plant growth potentially can affect the ability of weeds and crops to exploit the environmental resources for which plants compete. Stressful levels of environmental factors such as temperature, light, and water and nutrient availability influence weed/crop interactions directly and also may interfere with (or enhance) weed control. Weed and crop species differing in photosynthetic pathway (C3 vs C4) are likely to respond differently to many of these factors. Long-term changes in the atmospheric concentrations of CO2 and other radiatively-active “greenhouse gases” may exert direct physiological and indirect climatic effects on weed/crop interactions and influence weed management strategies. This review focuses on the effects of temperature, light, soil nutrients, water stress, and CO2 concentration on weed/crop interactions with consideration of the potential impact of climate change.

Laboratory studies were conducted to determine the relative sorption, mobility, and degradation rates of triclopyr and 2,4-D on two surface soils and two subsoils from the rice-producing areas of Arkansas. Triclopyr sorption was slightly greater than 2,4-D sorption. However, mobility of the herbicides on a given soil did not differ. Sorption of both herbicides was greatest and mobility lowest on a subsoil with the lowest pH. Triclopyr degradation rates were lower than 2,4-D degradation rates in a dark incubator. The average half life was 138 d for triclopyr and 21 d for 2,4-D. High soil moisture content (0 versus 100 kPa water tension) increased the rate of 2,4-D degradation. Triclopyr degraded more rapidly at 30 C than at 15 C. The dissipation rates of both herbicides were lowest on the soil on which sorption was greatest.

A water-corn oil (unrefined) emulsion has promise as a carrier for application of the mycoherbicide, Colletotrichum truncatum. We investigated this formulation (normal emulsion; oil in water) for influences on conidia germination, appressoria formation, and mycoherbicidal activity of C. truncatum. Germination of conidia after application to hemp sesbania leaves was increased from 30% in water alone to 92% in the water-corn oil emulsion (water:oil, 1:1, by vol). The emulsion did not affect appressoria formation. of the several other oils tested, safflower seed oil enhanced in vitro germination but did not enhance germination and mycoherbicide activity when the conidia-emulsion formulation was applied to hemp sesbania leaves. Application of C. truncatum conidia in the unrefined corn oil emulsion to hemp sesbania seedlings resulted in 89 to 97% control of the weed after a 24 to 72 h dew delay. The emulsion enhanced mycoherbicide efficacy by stimulating conidia germination by protecting the conidia during a dew-free period, hence, increasing weed infection when a dew occurred.

Field experiments were conducted in 1992 and 1993 to determine the timing and amount of rainfall required to activate UCC-C4243 applied preemergence. UCC-C4243 at 0, 70, and 140 g ai ha−1 was applied 1, 7, 14, and 21 d before 0.5 or 2 cm of simulated rainfall. Temporary rainshelters protected field plots from natural rainfall during the 21 d dry period. Herbicide activity was determined in the field by seeding lentil, wheat, common lambsquarters, and field pennycress and in the greenhouse by a sugarbeet bioassay of soil samples (0 to 3 cm depth) taken from all plots immediately before irrigation. UCC-C4243 did not injure wheat; however, lentil population was reduced when simulated rainfall occurred within 7 d after application. Lentil injury was greater with higher herbicide rate and higher water level. UCC-C4243 at 70 and 140 g ha−1 reduced populations of both weed species by 75 and 90%, respectively, when either 0.5 or 2 cm simulated rainfall was received within 1 d after herbicide application. Weed control was reduced with a 21 d delay between herbicide application and water activation. The sugarbeet bioassay showed a linear decrease of herbicide activity over time and also with accumulated photosynthetically active radiation. After 17.9 d, herbicide activity on a dry soil surface decreased 50%. Laboratory investigations show that [14C]-UCC-C4243 on glass slides was photodegraded by near ultraviolet light (290 to 400 nm). Volatilization of 14C-labeled herbicide from glass slides was less than 5% after exposure to turbulent air for 48 h.

A reduced-till (RT) experiment determined the effect of rye seeding density and method of kill on rye biomass persistence, weed suppression, and tomato yield. ‘Wheeler’ rye was seeded at 56, 110, and 170 kg ha−1 in the fall of 1988 and 1989 in Champaign, IL; Lexington, KY; and Lafayette, IN. One wk before transplanting tomatoes, rye was desiccated either by applying glyphosate at 1.1 kg ha−1 and mowing (RT-glyphosate) or by mowing alone (RT-mowed). Both methods left rye residues on the soil surface as a mulch. Seeding density did not affect rye biomass, weed control, or tomato yield. Rye biomass differed depending on location and year, ranging from 320 to 1150 gm−2. Rye was completely killed by glyphosate, but regrew following mowing. RT-glyphosate suppressed weeds for 4 to 8 wk, which was similar to the suppression from conventional tillage (fall plowing with spring disking and harrowing) with trifluralin and metribuzin (CT-herbicide). Weed communities varied, but redroot pigweed and giant foxtail were present at all three locations. All treatments required supplemental weed management for commercially acceptable control. Except for IL in 1990, processing tomato yields (kg ha−1) in RT-glyphosate with hand-weeding were equal or better than yields in the CT treatments.

Current and projected increases in the concentrations of CO2 and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests. All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the “greenhouse effect.” However, only weeds are likely to respond directly to the increasing CO2 concentration. Higher CO2 will stimulate photosynthesis and growth in C3 weeds and reduce stomatal aperture and increase water use efficiency in both C3 and C4 weeds. Respiration, and photosynthate composition, concentration, and translocation may be affected. Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs. Changes in leaf surface characteristics and excess starch accumulation in the leaves of C3 weeds may interfere with herbicidal control. Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds. Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2 increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.

Weeds are often more competitive with crops at higher soil nutrient levels. This can result in increased dependency on herbicides and tillage to maintain adequate weed control. A number of studies have shown weeds accumulate higher concentrations of nitrogen, phosphorus, potassium, calcium, and magnesium than crops, thus depleting soil nutrient levels more quickly and reducing crop yield. Understanding basic mechanisms and timing of nutrient uptake in weeds and crops can lead to fertilization strategies which will enhance the competitive ability of crops while reducing interference from weeds. Such strategies can include deep band application of fertilizers to the crop row, as opposed to broadcast applications. Banding techniques also reduce the potential for nitrate contamination of surface or groundwater. The addition of nitrification inhibitors or increasing the proportion of nitrogen as ammonium or urea in fertilizer mixtures can restrict growth of ammonium- or urea-sensitive weeds. The timing of fertilizer applications can take advantage of maximal rates of nutrient uptake into crop roots at specific developmental stages. In addition, nutrient use efficiency can be enhanced by choosing appropriate crop cultivars, maintaining effective weed control practices, or altering row spacing or seeding rate to increase accumulation of nitrogen, phosphorus, and potassium in crops.

Broom snakeweed, a perennial rangeland shrub, is highly variable morphologically and can grow under a broad range of environmental conditions. In this study, isozyme analysis using starch gel electrophoresis was used to quantify genetic variability within and among New Mexico populations of broom snakeweed. Eight separate populations of broom snakeweed and one population of threadleaf snakeweed as a comparison were investigated. of the 10 enzyme systems examined, 16 loci were identified in eight populations and two species. Eleven loci were monomorphic in eight populations and two species and five loci were polymorphic in at least one population or species. Genetic variability was large in broom and threadleaf snakeweed populations as determined by isozyme analysis. Genetic variability among broom snakeweed populations was greater than that within populations for the five polymorphic loci. Cluster analysis of genetic distance and identity for the eight populations and two species characterized two major groups. Within broom snakeweed, cluster analysis characterized five groups. The two species shared most common alleles. The genetic variation identified in this research may account for the morphological differences and broad geographical distribution of broom snakeweed.