A greenhouse experiment used a replacement series design to compare the vegetative growth 6 wk after emergence in pure cultures and mixtures of winter wheat and Italian ryegrass, with phosphorus (P) levels recommended by soil testing. The planting proportions of wheat and Italian ryegrass were 100 and 0%, 75 and 25%, 50 and 50%, 25 and 75%, and 0 and 100%, respectively. There was no alleopathic interaction between the species. Both species in all pure and mixed cultures had substantially less growth in the low-P than in the recommended P treatment. However, the relative performance of the two species differed between P treatments. In the recommended P treatment in pure culture, Italian ryegrass had more tillers and greater root weight and length than wheat. Pure culture wheat in the low-P treatment exceeded pure culture Italian ryegrass in leaf area, weights of leaves, stems, and roots, and root length. Thus, the growth of wheat was inhibited less by P deficiency than the growth of Italian ryegrass in pure culture. In the 50:50 mixture of the recommended P treatment, wheat had greater leaf, stem, and root weights than Italian ryegrass. In the 50:50 mixture of the low-P treatment, the two species were very similar in growth, except that Italian ryegrass had about three times more tillers than did wheat. Whereas P deficiency limited the growth of wheat less than Italian ryegrass in pure culture, P deficiency did not affect the relative competitiveness of Italian ryegrass as much as wheat in mixed cultures. The ability of Italian ryegrass to compete with wheat when P was limiting may result from a difference in root growth. Italian ryegrass had a greater fresh root length to fresh root weight ratio than did wheat in the low-P treatment in pure culture and in the 50:50 mixture. The greater surface area of Italian ryegrass roots likely enhanced the competitiveness of Italian ryegrass relative to wheat under P-deficit conditions. Thus, the use of the recommended P nutrition from soil testing may be a key component to diminish Italian ryegrass competition in wheat fields.

The effects of surfactants on retention, absorption, and efficacy were determined for bromoxynil, 2,4-D amine, and glyphosate on kochia and Russian thistle. Bromoxynil, 2,4-D amine, and glyphosate retention were similar for both species. Surfactants improved spray retention on kochia and Russian thistle compared with spray mixtures without surfactant. Herbicides mixed with allinol 810-60 surfactant were generally retained better than with MON 0818, oxysorbic 20, or R-11 surfactant. Bromoxynil phytotoxicity was not affected by surfactants, and all surfactants equally enhanced 2,4-D amine phytotoxicity. Glyphosate phytotoxicity to kochia was enhanced only by MON 0818 and oxysorbic 20, and phytotoxicity to Russian thistle was enhanced only by MON 0818. Bromoxynil, with or without surfactants, was absorbed similarly by kochia and Russian thistle. 2,4-D amine and glyphosate absorption were greater with surfactants than without. Kochia and Russian thistle leaves had visibly similar crystalline epicuticular wax structure when plants were grown at ≤ 40% or ≥ 80% relative humidity, which did influence absorption of these herbicides.

In both reduced-tillage and tilled fields, dandelion can be a problem, but the effect of dandelion on canola yield is unknown. This study was conducted to investigate the effect of dandelion infestation on spring canola yield and the influence of tillage on interference and in-field distribution of dandelion. Dandelion distribution was not associated with tillage regimen. Dandelion distribution showed some relationship to past cropping history, for example, the presence of alfalfa in the rotation. The strength of correlation between measures of dandelion infestation level and canola yield loss was associated with tillage regimen. For tilled fields, there was no correlation between reduction in canola yield and any measurements of dandelion infestation level. For reduced-tillage fields, the most reliable measures of dandelion interference level were dandelion ground cover prespray, total dandelion rosette diameter prespray, relative dandelion ground cover prespray, and total dandelion root diameter at crop harvest.

The effect of droplet size on retention, absorption, and translocation of 14C-glyphosate was studied in glyphosate-resistant corn. Fine, medium, and coarse spray droplets were studied using a track-sprayer equipped with commercially available nozzles. Glyphosate-resistant corn was used to obtain measurements at field use rates in the absence of phytotoxicity. Spray retention on corn leaves was calculated based on recovered glyphosate per leaf area, and retention was higher with application of fine droplets (47%) than with application of coarse (38%) and medium (37%) droplets. Absorption in corn leaves was directly correlated with droplet size and reached a plateau 1 d after treatment (DAT) for all droplet sizes. Based on glyphosate recovered 3 DAT, coarse droplets showed the highest absorption (49%), followed by medium (35%) and fine (30%) droplets. Percentage of translocation also increased with droplet size, and translocation was primarily toward strong sink tissues such as roots and young leaves. Our results show that large droplets have slightly reduced retention in corn but have increased absorption resulting in increased translocation of glyphosate to growing sink tissues.

Field experiments were used to assess how distance mediates the nontarget effect on crepe myrtle by two chrysomelid beetles that were introduced to the United States in 1992 for biological control of purple loosestrife. Previous laboratory tests in Germany and concurrent tests in Oregon showed that although the control organisms can feed on crepe myrtle, they cannot complete development. Therefore, we predicted that negative effects on crepe myrtle would decrease with distance from the purple loosestrife stand. To test this prediction, cohorts of both plant species were transplanted at increasing distances (0, 5, 15, 30, and 50 m) from the colonization source. We found that leaf damage inflicted by the beetles was negatively correlated with increasing distance. Damage was significantly lower at each distance for crepe myrtle plants than for purple loosestrife plants, with a mean difference of 22% and a 95% confidence interval ranging from 12 to 31%. Extensive defoliation of crepe myrtle was limited to within 30 m of the edge of the loosestrife stand. Plant yield was negatively correlated with damage: the closer plants were to the purple loosestrife stand, the greater the suppression of biomass in both plant species. However, loosestrife biomass decreased significantly more quickly than crepe myrtle biomass, with a mean difference in slopes of 0.035 and a 95% confidence interval ranging from 0.022 to 0.048. Our results suggest that release of the Galerucella beetles in North America poses little risk to crepe myrtle. Beetles can feed but cannot complete their life cycle on crepe myrtle, and damage to crepe myrtle approaches zero approximately 50 m from the beetle colonization source.

Wind-erodible soil sediments are classified as aggregates and particles < 1 mm in diameter. Aggregate size has been reported to influence pesticide retention and behavior in the soil. Atrazine sorption and desorption isotherms were determined using batch equilibration methods for six aggregate sizes of Barnes loam (fine-loamy, mixed, superactive, frigid Calcic Haplodol) and Brandt silty clay loam (fine-silty, mixed, superactive, frigid Calcic Haplodol) soils. Aggregate and particle sizes used in this study ranged from < 0.11 to > 1.7 mm, and atrazine concentrations ranged from 0.65 to 39.2 μmol L−1. The Kf values for the isotherms were calculated using the Freundlich equation. In the Barnes loam, Kf values were strongly positively correlated to aggregate size, particle size, and organic carbon (OC) content (P = 0.01 for each parameter), whereas in the Brandt silty clay loam, Kf values were less correlated to size and OC content (P ≥ 0.1) but were better correlated to clay content and estimated specific surface area (P = 0.05 for each parameter). Desorption was hysteretic with about 18% more atrazine desorbed from smallest than from largest size fractions. At a concentration of 13.0 μmol L−1, the amount desorbed ranged from 9 to 13.7% of the initial amount of adsorbed atrazine after 5 d, whereas at 39.2 μmol L−1, the amount desorbed ranged from 10.3 to 16% of the amount adsorbed. These data indicate that wind-erodible size aggregates and particles could be a source of herbicide contamination of surface water.

The presence of herbicides in runoff water after application to container plant nurseries warrants investigation of methods to reduce the amount of runoff. During the summer of 2000, field research was conducted at a commercial nursery to determine the effect of a delay in irrigation after herbicide application on herbicide levels in runoff water. Two studies were conducted in June and August. Isoxaben and oryzalin were sprayed on container plants in production beds at a rate of 1.4 and 2.9 kg ai ha−1, respectively. Treatments were pulse irrigated either immediately or 24 h after herbicide application. Pulse irrigation consisted of three 30-min irrigation cycles, with a 90-min rest between cycles, that supplied a total of 1.8 to 2.0 cm of water. Runoff samples were collected from both treatments after 0, 15, and 30 min of runoff flow from each pulse cycle for 3 consecutive d of pulse irrigation. The maximum isoxaben detected in June for the immediate irrigation treatment was 2.2 μg ml−1. The maximum isoxaben detected in August was 2.0 μg ml−1, also from the immediate irrigation treatment. The total isoxaben detected for the treatments ranged from 5.5 to 9.1% of the amount applied. The maximum oryzalin detected in June was 3.8 μg ml−1 for the immediate irrigation treatment. In August it was 2.8 μg ml−1 (for the immediate irrigation treatment). The total oryzalin detected for the treatments ranged from 4.6 to 8.4% of the amount applied. There were no treatment differences in concentrations and amounts of isoxaben and oryzalin. Efficacy was similar for the treatments in both studies. Delaying irrigation onset after herbicide application did not reduce total levels of isoxaben and oryzalin in runoff water. Both herbicides are stable chemicals with relatively long half-lives, and an irrigation delay of 24 h did not cause degradation that resulted in lower levels of runoff.