Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.

Sequential applications of preemergence (PRE) and postemergence (POST) herbicides were applied over a two-year period for large crabgrass and goosegrass control in common bermudagrass. Large crabgrass control was better when prodiamine at 0.3 kg ai ha−1 or oryzalin at 0.8 kg ai ha−1 were applied in late February, and followed by MSMA at 1.1 kg ai ha−1 in June than when prodiamine (0.8 kg ha−1) or oryzalin (2.4 kg ha−1) were applied at recommended rates. Goosegrass control was better when prodiamine was applied at 0.3 kg ha−1 followed by MSMA plus metribuzin at 2.2 + 0.1 kg ai ha−1, or when oryzalin was applied at 1.7 kg ha−1followed by MSMA plus metribuzin at 1.1 + 0.1 kg ha−1 than when prodiamine was applied at 0.8 kg ha−1 or oryzalin was applied at 3.3 kg ha−1. There was no increase in goosegrass control when diclofop was applied in sequence after prodiamine or oryzalin compared to diclofop alone. POST applications of diclofop, MSMA, and MSMA plus metribuzin did not injure common bermudagrass more severely when applied to plots previously treated with prodiamine and oryzalin than with POST herbicides alone.

Field studies were conducted at nine locations in southern Spain during 2 yr to develop models of nodding broomrape competition with sunflower and to establish economic thresholds. At each location, 30 to 35 small plots, each consisting of three sunflower plants, were chosen at random. The infection severity (BIS, no. of emerged broomrapes per sunflower plant) varied from 0 to 35. Plots were harvested at maturity to assess several sunflower and broomrape population variables. The percent sunflower yield reduction averaged over locations due to broomrape was estimated by the equation: % SYR = 1.7 x BIS (r2 = 0.92). Crop yield loss per BIS unit increased with the expected yield and was estimated to be about 25, 50, and 75 kg ha−1 for yields of 1000, 2000, and 3000 kg ha−1, respectively. A consistent relationship could be established between broomrape-infected sunflower yield, crop and broomrape biomass, and BIS parameters: SSYI = 0.2259 x PoBio/(1 + 0.0687 x BIS) (r2 = 0.7820). The BIS economic threshold was about 1.5 and 3.5 for control treatment cost of $ 40 ha−1 and potential yields of 2000 and 1000 kg ha−1, respectively.

Weed control in tomato production systems is difficult because few are registered. The use of rye for weed control and its influence on transplant tomato yields was investigated during 1986 and 1987 at two locations in IN to determine if cover crops can provide an alternative weed management technique. ‘Wheeler’ rye was sown in the fall of 1985 and 1986, and mowed or desiccated with glyphosate at various times before planting ‘IND 812'tomatoes. At the time of glyphosate application, rye residues reduced the growth of overwintering weeds by 93% or more compared to bare ground (no cover crop) areas. The time of desiccating rye prior to planting tomatoes affected the extent of weed suppression by rye residues. In 1986, rye treated 4 wk before planting (WBP) tomatoes provided up to 89% suppression of weed growth at 2 wk after planting (WAP) tomatoes, but no measurable weed suppression 5 WAP tomatoes. Rye treated 2 WBP tomatoes provided up to 97% weed suppression up to 5 WAP tomatoes. In 1987, weed suppression varied between locations and differed from 1986. At Lafayette, rye treated 2 and 1 WBP tomatoes provided greater than 81% suppression of weed growth up to 8 WAP tomatoes. Rye mowed and the residues placed into a plot at a known density also reduced weed growth (60%) 8 WAP tomatoes. At Vincennes, however, rye treated 2 and 1 WBP in 1987 did not reduce weed growth later than 4 WAP tomatoes compared to the unweeded, bare ground treatment. The mowed rye residues at Vincennes suppressed weed growth (96%) up to 8 WAP tomatoes. Tomato yield was correlated to weed suppression. In 1986, tomato yield in the rye treated 2 WBP tomatoes was comparable to yield in the bare ground, weeded controls. However, tomato yield in rye plots treated 4 WBP tomatoes was similar to yield in the bare ground, unweeded control. In 1987, tomato yields in all rye plots (mowed, treated 2 and 1 WBP tomatoes) were similar to tomato yields in the bare ground, weeded control at Lafayette. At Vincennes, only the mowed rye treatment yielded comparably to the bare ground, weeded control. In general, rye plots that were weeded yielded similar to or up to 28% more than a bare ground, weeded control. Tomato yields were not reduced by rye residues. Tomato yields in rye residues that provided effective suppression of weed growth (greater than 80%) for a minimum of 4 to 5 WAP tomatoes were comparable to bare ground, weeded controls.

Enhancement of the canker causing ability of Botryosphaeria ribis on melaleuca was studied with respect to stress from simulated drought, low temperature, and defoliation treatments. Low xylem water potential was related to increased level of canker development and subsequent tree mortality. Canker development was enhanced by low temperature treatments with alternating exposure to 6 C for 3 d followed by 4 d at 30 (±5) C for 8 wk. Partial defoliation did not affect canker development but complete defoliation of B. ribis-inoculated ramets resulted in tree mortality within 4 wk. Callusing of melaleuca wounds was either reduced or prevented in stressed trees. These observations suggest that stress induced on the tree enhances the tree-killing efficacy of this fungus.

Control of common lambsquarters by the use of Ascochyta caldina as a postemergence mycoherbicide was studied in corn and sugar beet, in 1992 or 1993. The weed was planted at determined positions in the crops. Plots were treated with suspensions of A. caulina spores, and wetness duration's were varied to create different levels of disease development. Application of A. caulina resulted in necrosis development on, and mortality of common lambsquarters. Average severities of leaf necrosis 1 wk after treatment ranged from 0.01 to 0.75. Average proportions of dead plants 3 wk after treatment ranged from 0.00 to 0.65. Necrosis development and mortality were affected by wetness duration in two experiments. Sublethally diseased plants showed reduced growth. Maximum dry matter was affected by crop and by necrosis development. Numbers of fruits per plant showed a positive, almost linear relationship with plant dry matter weight. Seed weight was less affected by necrosis than number of fruits per plant. Competitiveness of common lambsquarters was reduced after infection by A. caulina. Crop dry matter weight showed a positive relationship with the level of common lambsquarters control. In corn, yield reduction by competition was prevented by application of A. caulina, but not in sugar beet.

Field studies in 1990 and 1991 at six locations in Georgia and one location in North Carolina evaluated AC 263,222 for weed control, peanut tolerance, and yield. AC 263,222 applied early postemergence at 71 g ai ha−1 controlled bristly starbur, coffee senna, common lambsquarters, Ipomoea species, prickly sida, sicklepod, smallflower morningglory, and yellow nutsedge at least 91%. AC 263,222 controlled common cocklebur 77% and Florida beggarweed from 47 to 100%. Crop injury was 4% for AC 263,222 applied once and 12% or less from two applications. Mixtures of bentazon with AC 263,222 did not improve control compared to AC 263,222 alone. Imazethapyr did not improve control of AC 263,222 systems. In several locations, bentazon reduced control of Florida beggarweed with AC 263,222 when applied in a mixture compared to AC 263,222 alone. Weed control from the standard of paraquat plus bentazon applied early postemergence followed by paraquat, bentazon plus 2,4-DB applied POST did not provide the level or spectrum of weed control as AC 263,222 systems.

Field bindweed infests millions of hectares in the Great Plains greatly reducing productivity and value of land. The standard practice for field bindweed control is sweep tillage at 3 wk intervals combined with one or two annual 2,4-D) applications during the 14 mo fallow period in a winter wheat-fallow crop rotation. This was compared to tillage and 2,4-D in conjunction with dicamba or a mixture of picloram+2,4-D applied once during the first October of the first 14 mo fallow period. Also, three no-tillage systems were included using glyphosate+2,4-D at monthly intervals. Two of the treatments were supplemented with dicamba, or picloram+2,4-D as in the sweep tillage system. All treatments controlled field bindweed in two fallow periods and two winter wheat crops, and increased winter wheat yields to about twice the control. Sweep tillage at 3 wk intervals combined with 2,4-D resulted in $36 ha−1 profit for an owner-operator compared to $15 ha−1 loss with no herbicide or tillage treatment. On average no-tillage lost $35 ha−1. Other treatments, although controlling field bindweed, lost from 35 to $186 ha−1. To determine if long-term benefit after control was achieved, average yields for the area were used to calculate profits using normal farming practices. Profits were 136, 78, and $-50 ha−1, respectively, for sweep tillage and 2,4-D, no-tillage, and the untreated check. In a standard 33:67 owner-tenant rental, profits to the owner for the control period were 90, −33, and $43 ha−1, respectively for tillage and 2,4-D, no-tillage, and untreated check. The tenant lost from $24 to 69 ha−1 for the three systems indicating owners must modify rental agreements during a field bindweed control program.

Greenhouse cage studies were conducted to determine the influence of shoot morphology and genetic variation on establishment of Spurgia esulae gall midge on seven leafy spurge genotypes. The genotypes were collected from South Dakota, North Dakota, Nebraska, Montana, Wyoming, Manitoba, and Austria. Genotypes from South Dakota and Nebraska were most susceptible to gall formation and had the highest larvae survival, while the genotypes from Montana and Manitoba were most resistant. Morphological characteristics of the leafy spurge stem tips, such as stem diameter, leaf length, width, and area did not correlate with gall formation or larvae survival. Chloroplast DNA restriction fragment length polymorphism analysis of the genotypes identified six chloroplast types among the seven leafy spurge genotypes. The two genotypes most resistant to galling by S. esulae, Manitoba and Montana, had the same chloroplast genotype, but also were closely related to the two most susceptible genotypes. Because eggs were laid on all genotypes, it appears that adult females were not preferentially selecting appropriate host genotypes, but that egg and larvae survival was strongly influenced by genotype.

The influence of wheat variety on the dose-response of annual ryegrass to diclofop-methyl (POST) was examined in the field in 1992 and 1993 in southern New South Wales, Australia. The aim was to determine if planting a strongly competitive variety of wheat improved control of annual ryegrass at reduced doses of diclofop-methyl. Suppression of ryegrass was dependent on herbicide dose, season, and wheat variety. In the absence of herbicide, dry matter (DM) production of annual ryegrass at 300 plants m−2 at anthesis was 500 g ha−1 with Dollarbird and Katunga compared to 1000 g ha−1with Rosella or Shrike in 1992. In 1993, DM was approximately 150 g ha−1 with Dollarbird or Katunga, and 350 g ha−1with Shrike or Rosella. Ryegrass DM was reduced by diclofop-methyl to a greater extent, relative to the weedy unsprayed controls, with less competitive varieties Rosella and Shrike than with the more competitive Dollarbird or Katunga. Diclofop-methyl at 0.28 kg a.i. ha−1 reduced DM of ryegrass growing with Katunga to less than 100 g m−2 in 1992, compared to more than 200 g m2 with the other varieties. In 1993, diclofop-methyl was more effective on ryegrass, and the same dose reduced ryegrass DM to almost zero in all varieties. Grain yields in unsprayed weedy controls of Dollarbird and Katunga were reduced approximately 20% by annual ryegrass compared with yields achieved with herbicides in both years. Yields of Rosella and Shrike in the unsprayed controls were reduced about 40% in 1992 and 60% in 1993. Only small increases in grain yields of all varieties occurred from diclofop-methyl doses above 0.13 kg a.i. ha−1. Poorly competitive varieties were dependent on herbicides to achieve grain yield potential and had a greater risk of weed survival when herbicide efficacy was reduced. In contrast, strongly competitive varieties, likely to retard build-up of weed seed in the soil, are less dependent on herbicides to achieve grain yield potential, and therefore result in reduced weed control cost.

Greenhouse and growth chamber experiments were conducted to determine the reasons for stunted growth and yield suppression of corn noticed sometimes in nicosulfuron-treated corn fields infested with heavy population of johnsongrass. Results indicated that in the absence of johnsongrass, nicosulfuron applied broadcast POST at 35 g ai ha−1 had no effect on corn. However, growth reduction of corn occurred when nicosulfuron-treated johnsongrass and corn were allowed to share the same rooting medium with their root systems intermingled. The reduction in growth was even greater when corn foliage or the soil surface were also treated with johnsongrass. The extent of growth reduction of corn growing with nicosulfuron-killed johnsongrass depended on weed density and herbicide application rate. Greater growth reductions occurred at four johnsongrass plants per pot compared to two and at a higher application rate of 100 μg nicosulfuron per plant. In general, johnsongrass killed by nicosulfuron appeared to be more phytotoxic to corn than plants killed by paraquat. Nicosulfuron provided excellent control of johnsongrass and improved corn growth by two to three times that of not controlling johnsongrass, but it could not elevate corn growth to the level obtained when johnsongrass was controlled by paraquat or in the absence of interference from johnsongrass.

Pseudomonas syringae pv. phaseolicola formulated with an organosilicone surfactant was tested as a potential bioherbicide for kudzu. Greenhouse studies were performed to determine the effect of kudzu plant age and leaf age on disease severity and growth of the bacterium in planta. Eight-week-old plants were more diseased 4 wk after spraying and had a higher potential for regrowth 7 wk after spraying than 12-wk-old plants. Young leaves developed water-soaked lesions earlier than older leaves and supported higher populations of bacteria. Data from field experiments at two separate locations indicated that single applications in late spring were as effective as multiple applications in early spring and were enhanced by repeat application. Three months after the beginning of the experiment, extensive regrowth occurred for all bioherbicide treatments and no differences were noted between treatments and the nontreated control.

WEEDSIM is a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB. WEEDSIM recommendations resulted in average annual herbicide applications of 1.1 kg ai ha−1 for PRE+ and 1.0 kg ai ha−1 for POST, compared to 3.5 kg ai ha−1 for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha−1 for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg−1 for corn and $220 Mg-1 for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha−1) and POST ($522 ha−1) than for HERB ($455 ha−1). In general, WEEDSIM appeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.

Metolachlor and metribuzin loss in runoff was determined in three soybean tillage systems with and without a 4 by 2 m tall fescue vegetative filter strip. Soil erosion plots were 4 by 22 m with 3% slope. Regression analysis was used to describe herbicide concentration in runoff, and to determine if vegetative filter strips reduced herbicide concentration. Analysis of covariance indicated no difference in concentration of metolachlor or metribuzin in runoff from the three tillage systems within any vegetative filter strip treatment. Metolachlor loss in 1991 was highest from the no-till monocrop without a vegetative filter strip, and it was 65g ha−1 or approximately 2% of the amount applied. In 1992 and 1993, the no-till doublecrop had a total loss of 120 and 147 g ha−1, respectively, approximately 4% of the amount applied. Similar results were noted with metribuzin, but total loss was as high as 46 g ha−1 or 11 % of the amount applied in 1993 from a no-till doublecrop system without a vegetative filter strip. When a vegetative filter strip was present, losses of metribuzin and metolachlor were reduced over 85% in 1993, and totaled 1.2 and 0.5%, respectively, of the amount applied. The vegetative filter strip reduced herbicide and suspended solids from runoff produced by a conventional-till production system to levels equal to or lower than a no-till doublecrop system.

Hydrolysis and adsorption of the sulfonylurea herbicides rimsulfuron and primisulfuron were investigated under laboratory conditions. Degradation in aqueous solutions was determined between pH 3 and 8 and between 5 and 35 C. Rimsulfuron degraded faster than primisulfuron. Half-lives at different temperatures at pH 4 ranged from 0.16 to 5.8 d for rimsulfuron and from 0.47 to 16.1 d for primisulfuron. Primisulfuron hydrolysis at 30 C decreased with increasing pH, but rimsulfuron decreased up to pH 7, then increased, suggesting rapid degradation in alkaline conditions. Adsorption, measured in six Colorado soils, was complete within 2 h in all soils. Adsorption isotherms were determined by batch equilibration of 75 to 600 μg L−1. Rimsulfuron was adsorbed in amounts 3.7 to 7 times higher than primisulfuron, with adsorption distribution coefficients (Kd) ranging from 0.1 to to 1.18 for primisulfuron and from 0.71 to 5.1 for rimsulfuron. Adsorption of both herbicides was slightly correlated with soil clay content, and primisulfuron adsorption was also correlated with organic matter content.

Bentazon degradation in soil typically proceeds with development of bound residue. Low sorption of bentazon suggests that this residue consists of degradation products; however, there is little data on the sorption behavior of these products. This study was undertaken to determine the sorption of bentazon and the degradation products 2-amino-N-isopropyl benzamide, 2-aminobenzoic acid, and N-methyl bentazon in Dundee silt loam and Sharkey clay, two common agricultural soils of the Mississippi Delta. Greater sorption of bentazon and degradation products in the Sharkey soil was related to finer texture and higher organic C content. Isotherms were nonlinear, with sorption increasing in the order bentazon ≪ 2-amino-N-isopropyl benzamide < N- methyl bentazon. In general, methanol extraction indicated reversible sorption of these compounds. Therefore, it is unlikely that sorption of either 2-amino-N-isopropyl benzamide or N-methyl bentazon contributes to bound residue. In contrast, essentially all of the 14C sorbed from radiolabeled 2-aminobenzoic acid solution remained bound after methanol extraction. However, due to degradation of 2-aminobenzoic acid, it was not possible to conclude the extent to which this compound contributed to bound 14C. Sorption of 14C from 2-aminobenzoic acid exceeded that of N-methyl bentazon, and at low initial concentrations (≤ 20 μM) was nearly 1000-fold greater than bentazon sorption.

Herbicides used in containerized plant production have a potential to move off-site in irrigation runoff water. A granular formulation of isoxaben plus trifluralin was applied to a commercial container plant nursery bed. Herbicides were monitored in irrigation runoff and collection pond water over a period of 60 d. Greatest quantities of both herbicides were lost during the first runoff event following application. Approximately 9.2 and 0.7% of the applied isoxaben and trifluralin, respectively, moved from the application site in runoff water within 5 d after treatment. Herbicide concentrations in runoff water were highest (0.75 μg ml−1 isoxaben and 0.08 (μg ml−1 trifluralin in 1992) during the first 0.25 h following herbicide application. Concentrations in the runoff collection pond climaxed following the first runoff event and decreased to less than 1 ng ml−1 within 60 d after treatment. Photodegradation of isoxaben within the surface 2.5 cm of pond water was greater in light as compared to dark. These studies indicate that isoxaben and trifluralin move from the site of application, but that neither accumulate in the runoff water collection ponds.

Solid phase extraction (SPE) procedures were used to determine the recoveries of herbicides typically used in containerized ornamental plant production from water samples. Recoveries from C18 cartridges and disks were compared for each of 12 herbicides with variations in elution solvent and volume of elution solvent tested. Recoveries for nine of the herbicides from the cartridges and disks using acetone as an elution solvent were not affected by SPE matrix. Fluazifop recovery was greater with the disks, while napropamide and oxadiazon recoveries were greater with cartridges. Both cartridges and disks yielded low recoveries (23 to 47%) of benefin and prodiamine. Changing the elution solvent from acetone to acetonitrile resulted in 10% improvement for the recovery of benefin and a three- to four-fold increase in recovery of prodiamine. Acetonitrile decreased recoveries of napropamide, oryzalin, oxadiazon, oxyfluorfen, and pendimethalin from cartridges. For the disks, oxyfluorfen, prodiamine, and trifluralin had increased recovery, while fluazifop, oxadiazon, and simazine had decreased recovery with acetonitrile as the elution solvent. Increasing the amount of acetone eluting solvent increased the recovery of prodiamine and oxyfluorfen while decreasing the recovery of fluazifop, pendimethalin, simazine, and trifluralin. Binding capacities of oryzalin on cartridges and disks averaged 13.2 and 7.8 mg, respectively. The advantage of the disk lies in the greater volume of water that can be processed, while the higher cost and greater variability are disadvantages. Cartridge extraction yielded good recoveries with lower standard deviations, and used less organic solvent. Selection of an SPE extraction method depends upon the herbicides under evaluation, expected levels, and the water volume being processed. Both SPE techniques offer advantages over traditional liquid-liquid extraction methods such as reduced requirements for organic solvent and sample preparation.

Field studies indicated thiazopyr at 220 g ha−1 injured soybeans planted in no-till, and 340 g ha−1 injured soybean in 1 of 2 yr planted into tilled soil. Thiazopyr half-lives in three field studies ranged from 12 to 14 d, indicating minimal potential for carryover injury to sensitive rotational crops. Thiazopyr dissipation under controlled conditions was slower than under field conditions. Half-lives at 15 and 30 C were 133 and 43 d, respectively. This indicates that microbial degradation is an important mechanism in thiazopyr dissipation, but that other loss mechanisms also play a role. A method using methanol extraction of moist soil samples followed by gas chromatography using a nitrogen-phosphorous detector had > 90% recovery of thiazopyr.

Atrazine, metribuzin, and clomazone half-lives averaged over treatments and seasons were approximately 27, 22, and 55 d, respectively. Clomazone dissipation was not affected by the presence or absence of a soybean crop. Atrazine and metribuzin dissipation was not affected by crops in 1992, but was more rapid in no-crop plots than in cropped plots in 1993. The difference may have been the result of higher soil water content with no-crop (a few weeds present) in 1993 than either corn or soybean. Lower soil moisture may have slowed soil microbial activity, thus suppressing atrazine and metribuzin degradation in the 1993 growing season. Few significant correlations were found between herbicide half-life or herbicide concentration and cocklebur growth, although one would expect these to be an index of activity.