Field studies were conducted to determine the effect of crop species, tillage, and rye mulch on sicklepod growth. Competition from soybean and sunflower reduced sicklepod biomass by 19 and 77%, respectively. Sicklepod height was reduced by rye mulch. Rye root residue caused 49% reduction in sicklepod biomass, while whole plant rye mulch reduced weed growth an additional 12%. Crop competition reduced sicklepod shoot biomass to a greater extent when rye residue was present. Crop seed yield was 32% lower in tilled plots compared with no-tillage due to more weed competition in the tilled plots. In the presence of rye root residue, soybean yield showed a two-fold increase over no mulch or whole plant rye mulch. Sunflower yield increased by 30% in the presence of rye residue compared with no mulch.

Field research was conducted in Tennessee at Jackson in 1991 and at Milan in 1992 to compare the effect of MSMA and pyrithiobac on cotton development, yield, and quality. In separate treatments to different plots, pyrithiobac at 0.14 kg ai ha–1 did not affect development, yield, and quality of cotton. MSMA at 2.24 kg ai ha–1 decreased cotton plant internode length 10 to 15% and height by 15% 5 wk after late treatment both years. MSMA applied to 50 cm cotton reduced subsequent plant height in 1992, but other plant characteristics were not affected. In 1991, MSMA increased squares and decreased blooms and bolls for monopodia and sympodia position one and two, which suggested a delay in plant development. In 1991, mechanical harvest lint yields were decreased by MSMA at first harvest while increasing second harvest lint yields. However, in 1991 only MSMA applied late decreased total harvest lint yield 20%. Plant mapping data determined that the yield decrease was a result of decreased yields at sympodia positions one and two. Cotton seed arsenic analysis indicated that MSMA-late increased arsenic levels by ≈ 1 ppmw in 1991 for sympodia at position one and two compared to the untreated check, while position two contained the highest level of 1.8 ppmw.

Greenhouse studies were conducted to determine the influence of plant density and spray volume on the retention, spray deposition, efficacy, and translocation of the amine salt of triclopyr in purple loosestrife. More spray solution was retained on leaves at 935 Lha−1 than at 94 Lha−1 at populations of 0, 4, or 8 nontarget neighbors. Spray coverage decreased with decreasing height within the plant canopy when spray cards were placed in the top, middle, and soil surface adjacent to the central target plant. Within a population, spray card coverage generally increased as spray volume increased. Regrowth from the crown was affected by spray volume, and uniform spray coverage of the plant was required for adequate control of vegetative regrowth and was achieved with spray volumes of 374 and 935 L ha−1 spray volume. Regrowth of purple loosestrife was greater at 94 Lha−1 at all three plant populations indicating that less herbicide penetrated the canopy to reach the basal portion of the plant. A laboratory experiment was conducted to investigate the translocation of radiolabelled triclopyr to roots and crowns of purple loosestrife. Only 0.3 to 1.4% of absorbed 14C-labelled material was translocated to roots and crowns. Low spray volumes and dense stands of purple loosestrife would likely result in poor control because inadequate amounts of triclopyr reach the basal portion of the plant and translocate to vegetative propagules.

Field studies were conducted at two locations in Missouri in 1990 and 1991 to evaluate control of velvetleaf, ivyleaf morningglory, and cocklebur in soybean with label rates (1×) or reduced rates (0.5×) of four PPI herbicides followed with or without cultivation and POST application of one fourth (0.25×) the label rate of imazethapyr. Weed control and soybean yield with the 0.5× rate of preplant clomazone, imazaquin, metribuzin, and metribuzin + chlorimuron followed by either cultivation or imazethapyr POST was equivalent to a 1× rate of the PPI herbicides followed by either cultivation or imazethapyr. Weed control and soybean yield with the 0.5× rates of the PPI herbicides without cultivation or POST imazethapyr were lower than with 1× rates of PPI herbicides.

The effects of imazaquin and imazethapyr on cotton planted the year following application to soybean were studied on one silt loam and two clay soils in Arkansas from 1989 to 1991. Imazaquin was applied to soybean at 0.07 kg ai ha−1 PPI and POST, 0.14 kg ha−1 PPI, PRE, and POST, and sequentially PPI or PRE followed by POST at 0.14 kg ha−1 per application. Imazethapyr was applied at 0.035 kg ha−1 PPI and POST, 0.07 kg ha−1 PPI, PRE, and POST, and sequentially PPI or PRE followed by POST at 0.07 kg ha−1 per application. Soil residues of 0.07 kg ha−1 imazaquin and imazethapyr did not injure cotton at any location. Residues of 0.14 kg ha−1 imazaquin applied sequentially reduced cotton shoot dry weight for 6 wk after emergence on clay but not on silt loam soil. The PPI followed by POST sequential imazaquin treatment (0.28 kg ha−1 total imazaquin) delayed initiation of fruiting and reduced yield and lint quality.

Field studies were conducted in Mississippi on a Marietta loam during 1988, 1989, and 1990, and in Tennessee on a Statler loam during 1988 and 1989, to determine effect of tillage on chlorimuron persistence. A corn shoot bioassay was conducted to determine bioavailable chlorimuron in the soil. Tillage system affected chlorimuron persistence only in 1989 at Tennessee. Chlorimuron half-life was 10 to 31 d longer in both tillage systems in Mississippi in 1988 compared to 1989 and 1990, probably due to low soil moisture immediately after application in 1988. In Tennessee, chlorimuron half-life was 71 d shorter in the no-till system in 1988 compared to 1989. Differences did not occur between locations in the conventional tillage system in 1989. Differences in chlorimuron persistence between years and locations were most often attributed to soil water and temperature. Soil pH differences between the two locations did not consistently affect chlorimuron persistence.

Sorption and mobility of 14C-bentazon were evaluated using soil solution and soil-thin-layer chromatography, respectively on three Coastal Plain soils. The proportion of bentazon adsorbed, was pH and concentration dependent and ranged between 28 to 47, 31 to 61, and 17 to 68% for the Dothan loamy sand, Greenville clay, and Troup loamy sand, respectively. Bentazon sorption decreased as soil pH increased from 5.4 to 5.8 on the Troup loamy sand, regardless of bentazon concentration. Sorption of 1 and 10 ppm bentazon decreased as soil pH increased from 5.0 to 5.7 on the Greenville clay and 6.1 to 6.4 on the Dothan loamy sand. Bentazon was least mobile on the Greenville clay and mobility increased as pH increased. Mobility of bentazon was greatest on the Troup loamy sand and was not pH dependent. Bentazon mobility decreased as soil pH increased for the Dothan loamy sand.

This research examined the effect of mechanical soil disturbance (none or tilled) and legume crop residues (none or hairy vetch) on fluometuron dissipation for 2 yr from the top 0 to 8 cm of soil in a 10 yr field experiment. Soil pH in the upper 0 to 8 cm was ≤ 5.6, and soil organic matter was highest in plots not-tilled and plots which had a vetch cover crop. Calculated initial half-lives of fluometuron ranged from 19 to 38 d in the 2 yr. Neither tillage nor cover crop influenced early-season fluometuron dissipation. However, there were detectable amounts of fluometuron in all treatments 1 yr after application.

The importance of various factors influencing the evolution of herbicide resistance in weeds is critically examined using population genetic models. The factors include gene mutation, initial frequency of resistance alleles, inheritance, weed fitness in the presence and absence of herbicide, mating system, and gene flow. Where weed infestations are heavy, the probability of selecting for resistance can be high even when the rate of mutation is low. Subsequent to the occurrence of a resistant mutant, repeated treatments with herbicides having the same mode of action can lead to the rapid evolution of a predominantly resistant population. At a given herbicide selection intensity, the initial frequency of resistance alleles determines the number of generations required to reach a specific frequency of resistant plants. The initial frequency of resistance alleles has a greater influence on the evolutionary process when herbicides impose weak selection, as opposed to very strong selection. Under selection, dominant resistance alleles increase in frequency more rapidly than recessive alleles in random mating or highly outcrossing weed populations. In highly self-fertilizing species, dominant and recessive resistance alleles increase in frequency at approximately the same rate. Gene flow through pollen or seed movement from resistant weed populations can provide a source of resistance alleles in previously susceptible populations. Because rates of gene flow are generally higher than rates of mutation, the time required to reach a high level of resistance in such situations is greatly reduced. Contrary to common misconception, gene flow from a susceptible population to a population undergoing resistance evolution is unlikely to slow the evolutionary process significantly. Accurate measurements of many factors that influence resistance evolution are difficult, if not impossible, to obtain experimentally. Thus, the use of models to predict times to resistance in specific situations is markedly limited. However, with appropriate assumptions, they can be invaluable in assessing the relative effectiveness of various management practices to avoid, or delay, the occurrence of herbicide resistance in weed populations.

This paper describes the application of airborne video data with global positioning system and geographic information system technologies for detecting and mapping Chinese tamarisk infestations in the southwestern United States. Study areas were along the Colorado River in southwestern Arizona, the Rio Grande River in extreme west Texas, and the Pecos River in west-central Texas. Chinese tamarisk could be readily distinguished on conventional color video imagery in late November when its foliage turned a yellow-orange to orange-brown color prior to leaf drop. The integration of the global positioning system with the video imagery permitted latitude/longitude coordinates of Chinese tamarisk infestations to be recorded on each image. The global positioning system latitude/longitude coordinates were entered into a geographic information system to map Chinese tamarisk populations along the three river systems.