Bioassay methods are described for determining the concentration of 4-amino-3,5,6-trichloropicolinic acid in solutions and in soil. These methods are suitable for determining the concentration of the herbicide in the presence of other herbicides. These test methods can detect considerably less than one microgram of herbicide.

The use of pre-emergence herbicides where natural rainfall is low and/or sporadic has not been uniformly successful. In general, the results have been erratic; excellent weed control has been obtained sometimes and a complete lack of weed control at other times. Good weed control is usually obtained when one-half inch or more of rain falls within a few days after application. This result is confirmed in irrigated regions by the fact that overhead irrigation usually gives uniformly good weed control. The standard furrow irrigation usually gives erratic results; however, a furrow irrigation which thoroughly saturates the soil with water to the surface may give good results. Since this type of furrow irrigation is difficult to achieve in practice and the equipment necessary for overhead irrigation is not widely used, the physical incorporation of the herbicides in the top few inches of soil is being widely field tested as a method of application in these areas of low and sporadic rainfall. In general, such incorporation has resulted in a much improved performance under furrow irrigation in the absence of rainfall.

The selective control of weeds in crops by the use of 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron) has had considerable attention the last five years. Some of the problems of applying diuron are how long and at what depths the herbicide remains in the soil after treatment, with different methods and amounts of irrigation.

The announcement of the effectiveness of Isopropyl N phenyl carbamate as a graminicide received considerable interest by weed research workers. However, early attempts to control grasses with this chemical resulted in rather disappointing control. Subsequent work has shown that this can be largely attributed to improper usage of the material. Since this material is supposedly a growth regulating substance, it was reasoned that it should behave in a manner similar to 2,4-D. Study carried on in Oregon proved that this was not the case and that specific conditions are essential to maximum effectiveness of this chemical.

Paper chromatographic separation of hydroxydimethylarsine oxide (cacodylic acid), monosodium methanearsonate (MSMA), sodium arsenate, and sodium arsenite was achieved with the aid of four solvent systems. Aqueous extracts of plant tissues removed essentially all the arsenicals applied, but methanolic fractionation was required before the extracts could be analyzed by paper chromatographic procedures. A standard nitric-sulfuric acid digestion procedure was employed for arsenic analyses, but great care was taken to avoid sulfuric-acid-induced charring by first adding relatively large amounts of nitric acid to drive off chlorides present. Depending upon the amount of chloride present, substantial losses of arsenic as arsine chlorides were observed if the samples charred. Five minutes in fuming sulfuric acid to completely break the carbon-arsenic bonds was another critical requirement for the quantitative determination of arsenic from cacodylic acid and MSMA. The silver diethyldithiocarbamate colorimetric method was useful for detecting as little as 0.6 μg or as much as 20 μg of arsenic per sample.

Laboratory studies were conducted to compare the soil adsorption of aminopyralid and clopyralid with the use of batch-slurry and centrifugation assays. The calculated soil binding constants for both herbicides varied between the two techniques, but the centrifugation assay had a lower coefficient of variation compared to the batch-slurry assay. These results indicate that a centrifugation assay is a more accurate procedure for measuring the interaction of aminopyralid and clopyralid with soils. Aminopyralid adsorbed more tightly than clopyralid to six of the eight soils tested. Adsorption Kd values ranged from 0.083 to 0.364 for clopyralid and 0.106 to 0.697 for aminopyralid. Pearson correlation analysis indicated that binding of both herbicides was highly correlated to soil organic matter and texture but not to soil pH. On average, soil thin-layer chromatography indicated that aminopyralid was less mobile (Rf = 0.82) than clopyralid (Rf = 0.91), although both were mobile. These results suggest that aminopyralid will have a lower leaching potential than clopyralid. Lower potential aminopyralid soil leaching, coupled with low use rates, suggests it may be the herbicide of choice in areas where potential for leaching could be a concern.

Sesbania (Sesbania exaltata (Raf.) Cory) in soybeans was effectively controlled with wax bars containing 2,4-dichlorophenoxyacetic acid (2,4-D). Bars, 22 in long, were attached horizontally 3 in apart to a tractor-mounted boom and the height was adjusted for it to travel 2 to 3 in above crop plants. Six-pound bars, containing 1 lb of 2,4-D gave better control than bars containing less 2,4-D. Bars made of wax with a melting point of 170 F provided more economical control than bars with waxes of lower melting points. Rate application of wax increased with the size of the weeds treated and with reduced tractor speed. Treatments controlled sesbania without visible injury to soybeans.

The herbicide EPTC (ethyl N,N–di–n–propylthiolcarbamate) offers much promise as a soil-borne toxicant for controlling a variety of common weeds. Not only does it control annuals; it is proving very effective and quite specific in the control of nutsedge. In this role it is unique among the newer materials. In California it has held nutsedge in check during the whole growing season in strawberries, peppers, beans, and other row crops.

Genome sequencing of many model systems, including the human, yeast, Arabidopsis, and rice genomes, has been completed. As a complement to genomic techniques, proteomics has emerged to study the composition, structure, function, and interaction of the expressed proteins or proteome in a given cell, tissue, or organism. During the past several years, tremendous progress has been made with proteomic techniques in human and yeast, but progress has been slower in plants due to several factors. Proteomic techniques have been used in plants, especially in agriculturally important crops and weeds, to understand mechanisms of herbicide tolerance and weed resistance. Proteomic techniques, starting from protein extraction to protein identification, have been developed and advanced for large-scale proteomics studies, but limitations and problems for these techniques still exist for plant proteomics. Further technological improvements are needed to enhance quantitative and comparative large-scale proteomics studies. Applications of these techniques may help weed scientists to understand stress tolerance in crops and investigate weediness traits further. Proteomic techniques have unique strengths but also offer several challenges. Together with transcriptomics and metabolomics, these techniques for analyzing global patterns of gene expression offer new and novel techniques for better understanding biological questions of interest to weed scientists in the future.

The unicellular green alga, Chlorella sorokiniana Shihira and Krauss, was cultured in vitro using an inorganic growth medium with various concentrations of the herbicide glyphosate [N-(phosphonomethyl)glycine]. Growth rates, in terms of ceil doublings per day, were determined for cells inoculated into media containing from 5.91 × 10−6 M to 591 × 10−6M glyphosate. Comparisons with a control, which supported 10.4 doublings/day, showed growth only slightly reduced at 5.91 × 10−6M and 11.8 × 10−6M, with averages of 9.7 and 9.5 doublings/day; reduced by more than half at 17.7 × 10−6M with an average of 4.4 doublings/day; and prevented at all higher concentrations. In addition to completely preventing growth at 23.7 × 10−6M and above, glyphosate seemed to cause a deterioration of the inoculum cells. Filtering the media containing 5.91 × 10−6M, 11.8 × 10−6M, and 17.7 × 10−6M through kaolinite prior to introduction of the alga reduced the inhibition previously noted at 17.7 × 10−6M and resulted in growth rates from 9.1 to 9.5 doublings/day.

Experiments were conducted at two sites for 2 yr in the Pacific Northwest dryland cropping region to determine if seeding rate of small-red lentil could enhance weed control with herbicides and increase lentil seed yield. At Pendleton, OR, and LaCrosse, WA, lentil was planted at 22 or 44 kg ha−1 in one direction in all plots. In one-half of the plots, lentil was cross-seeded at right angles with an additional 22 kg ha−1 to provide seeding rates of 22, 44, 22 + 22, and 44 + 22 kg ha−1. Seeding rate main plots were split into three herbicide treatments and an untreated control. Total weed density was reduced by increasing seeding rate at Pendleton both years when averaged over all herbicide treatments. Seeding rate reduced total weed density to a greater extent when herbicides did not adequately control weeds or when herbicides were not applied at Pendleton in 1992. Increased seeding rate also reduced total weed dry weight at Pendleton in 1992 and 1993 and at LaCrosse in 1993. The suppressive effect of increased seeding rate on weed dry weight was more evident when herbicides were not used or when herbicides gave only partial control. Herbicides generally reduced weed density, but the effectiveness of individual treatments was related to the weed species present and environmental conditions present in each experiment. Lentil aboveground dry weight production increased with seeding rate at both locations; however, only in 1 yr did lentil seed yield increase with seeding rate. The primary benefit from increased seeding rate in this study was to reduce weed density and dry weight.

Twelve-day-old corn seedlings (Zea mays L. ‘Funk's G-83’) were exposed to thermal conditions (482 C for 125 msec) approximating those used for flame weed control. Tissue dehydration was evident within 1 min after flame application. One hour after flame exposure, the water content of shoot tissues was 6% less than in nonflamed shoots. Transpirational water loss 16 hr after flame application was reduced by 68% when compared to that of nonflamed seedlings. Assimilation of 14CO2 by flamed seedlings, while substantially decreased when compared with that of nonflamed seedlings, continued on a limited basis even in the most severely injured leaf tips.

Four field experiments were conducted to measure the effects of seven Ipomoea hederacea (L.) Jacq. densities on Gossypium hirsutum L. lint yield, stripper-harvest efficiency, and fiber properties. The seven densities were 0, 2, 4, 6, 8, 10, and 12 weeds 10 m−1 of row. Data were used to develop prediction models to compare with those previously constructed that used fewer experiments and fewer weed densities in this range. Gossypium hirsutum lint yield in kilograms per hectare and as a percentage of the weed-free control best fit a linear regression model. Gossypium hirsutum lint yield reductions for each increase of one weed 10 m−1 of row ranged from 30.7 to 36.2 kg ha−1 at Chickasha and from 35.4 to 36.4 kg ha−1 at Perkins. Lint yield reductions for each weed 10 m−1 of row ranged from 3.8 to 6.9% at Chickasha and from 3.9 to 6.0% at Perkins. All plots could be mechanically stripper harvested, except for the 12-weed density at Chickasha in 1994 and the 10- and 12-weed densities at Perkins in 1996. Harvest efficiencies were not significantly different in any experiment. The only fiber properties to display significant differences were micronaire and strength at Chickasha in 1994. Prediction models calculated herein were highly similar to those previously constructed.

Common lambsquarters is an annual weed of many important crops. Ascochyta caulina is a plant pathogenic fungus that causes necrotic lesions on the leaves and stems of common lambsquarters. The objective of the present study was to estimate the effect of plant N supply on the biocontrol activity of A. caulina isolates against common lambsquarters. In greenhouse experiments replicated groups of common lambsquarters plants raised with different N supplies were sprayed with various isolates and concentrations of A. caulina 3 wk after planting. Height, number of leaves, total leaf area, fresh and dry weight, and tissue N concentration of common lambsquarters 4 wk after emergence increased significantly with increasing N supply. Disease development was positively related to increasing plant tissue N and also to increasing spore concentration. Fungal spore concentration also had a positive effect on the plant tissue N percentage. Ascochyta caulina isolate W90-1 caused a greater dry weight reduction in common lambsquarters than isolates I-001 and NW-6 did.

Several dozen herbicide systems were evaluated over a 4-yr period for weed control in stubble no-till planted corn (Zea mays L.). Crop tolerance and corn yield data also were obtained. Paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) and glyphosate [N-(phosphonomethyl)glycine] afforded excellent desiccation of all emerged vegetation each year. The oil-soluble formulation of dinoseb (2-sec-butyl-4,6-dinitrophenol) also afforded complete contact control of emerged vegetation in 1975, the only year it was evaluated. A herbicide system consisting of an acetanilide plus a triazine afforded the most consistent residual weed control. The specific herbicides affording satisfactory control largely depended on the weed spectrum. Fall panicum (Panicum dichotomiflorum Michx.), large crabgrass [Digitaria sanguinalis (L.) Scop.], and yellow nutsedge (Cyperus esculentus L.) were the weeds most difficult to control with the herbicide systems used in these studies. Corn yields were influenced primarily by the degree of weed control achieved, and in several instances by secondary effects of stand reduction or injury.

A population of junglerice from Sunflower County, MS, exhibited resistance to fenoxaprop-P-ethyl. An 11-fold difference in ED50 (the effective dose needed to reduce growth by 50%) values was observed when comparing the resistant population (249 g ae ha–1) with susceptible plants (20 g ae ha–1) collected from a different field. The resistant population was controlled by clethodim and sethoxydim at the field rate. Sequencing of the acetyl coenzyme A carboxylase, which encodes the enzyme targeted by fenoxaprop-P-ethyl, did not reveal the presence of any known resistance-conferring point mutations. An enzyme assay confirmed that the acetyl coenzyme A carboxylase in the resistant population is herbicide sensitive. Further investigations with two cytochrome P450 inhibitors, malathion and piperonyl butoxide, and a glutathione-S-transferase inhibitor, 4-chloro-7-nitrobenzofurazan, did not indicate involvement of any metabolic enzymes inhibited by these compounds. The absence of a known target-site point mutation and the sensitivity of the ACCase enzyme to herbicide show that fenoxaprop-P-ethyl resistance in this population is due to a non–target-site mechanism or mechanisms.

Invasive species such as Canada thistle pose a significant threat to ecosystems. The risk of introducing invasive species has increased with human activities, and the effects of such events have economic and aesthetic impacts. Native to Europe, Canada thistle is now established throughout temperate North America. Although there is documentation of early occurrences to North America, little is known on how it has become established in diverse habitats or how it continues to spread. We examined genetic diversity within and among nearly 1,700 Canada thistle individuals from 85 North American locations with the use of seven microsatellite markers in order to address these questions. PAUP and STRUCTURE programs were used to assess genetic diversity and relationships within and between populations. Populations exhibited greater within-population diversity (> 60%) than expected for a reported clonally reproducing species. Total diversity of sampled locations in North America (0.183) was less than previously reported for European locations (0.715), but the greater mean difference between North American populations (0.264 relative to 0.246 from England) suggests strong founder effects or restriction of gene flow influencing individual populations. Furthermore, analyses identified numerous instances where individuals from geographically distant regions clustered together, indicating long-distance translocation of propogules. However, isolation by distance analysis showed significant correlation between location and population genetic distances (r = 0.1917, P = 0.006). Within populations, nearly 92% of individuals sampled harbored unique multilocus genotypes, strongly suggesting that sexual reproduction is common. Within populations, analysis of genetic structure indicated significant admixture of genotypes throughout the invasive range in North America. The recurrent distribution of seed throughout North America has led to a highly diverse gene pool and increased the adaptive success Canada thistle to a wide variety of habitats. Future technologies developed for control of Canada thistle should consider this diversity.

Mathematical growth analysis techniques were used to study the effects of simulated field temperatures and chilling events on the growth of the exotic noxious weed itchgrass (Rottboellia exaltata L. f.) and adapted varieties of corn (Zea mays L. ‘DeKalb XL43’ and ‘DeKalb XL395′), and soybean [Glycine max (L.) Merr. ‘Corsoy’, ‘Williams', and ‘Tracy’] in climate-controlled greenhouses and growth chambers. Itchgrass grew vigorously and produced seed in temperature regimes simulating the warmest 4 months of the growing season in the Gulf Coast states (30/25 C, day/night), the central Midwest (27/21 C), and the northern Midwest (23/18 C). At 30/25 C day/night, itchgrass produced 707 g of dry matter and 1907 seed per plant after 119 days. Itchgrass produced 621 g dry matter and 1429 seed at 27/21 C and 499 g dry matter and 1160 seed at 23/18 C. Itchgrass and adapted varieties of corn and/or soybeans were grown in four additional temperature regimes simulating the first 5 weeks of the growing seasons for corn and soybean at Madison, Wisconsin (19/11 C day/night); for soybean at Carbondale, Illinois (24/20 C); for corn at Waycross, Georgia (20/14 C); and for soybean at Baton Rouge, Louisiana (27/23 C). After 24 days of growth, half the plants in each regime were exposed to 3 days of chilling (11/4 C for Madison, 17/10 C for Carbondale, 15/6 C for Waycross, and 24/18 C for Baton Rouge) and returned to the original growth regimes for recovery. The chilling treatments reduced dry matter production, net assimilation rate, and leaf area duration more in itchgrass than in corn or soybean. After recovery, the previously chilled itchgrass plants had greater reductions in height, leaf area, and dry weight, compared to unchilled controls, than did corn or soybean. Weed/crop ratios in height, leaf area, and dry weight for itchgrass/corn were significantly reduced by chilling. In the two cooler regimes, weed/crop ratios in leaf area and dry weight for itchgrass/soybean were significantly reduced by chilling. Because of its sensitivity to cool temperatures, itchgrass, is unlikely to be an important early-season competitor with corn or with soybean outside the South.

Comparative spread of droplets of several different oils and water with different adjuvants on leaf surfaces was investigated. Spread was better on the lower surface of johnsongrass leaves than on upper leaf surfaces with nine of 14 oils studied; two spread best on upper leaf surfaces, and three were equal in spread on both surfaces. Differences in spread coefficients did not appear to be directly related to surface tension, viscosity, or mid-boiling point of the oils. Soybean or cottonseed oils did not spread as well as petroleum oils but methylated soybean and sunflower oils had high spread coefficients on both upper and lower leaf surfaces. All but one petroleum-base oil spread three to four times better on leaf surfaces than on oil-sensitive paper. Water mixtures of an organosilicone surfactant spread much better on water-sensitive paper and on johnsongrass leaves than water with conventional adjuvants, but spread of paraffinic oils exceeded that of any water-adjuvant mixture. Spread coefficients of most petroleum-base oils were better on lower than upper leaf surfaces. Spread usually increased as the age of leaves increased from 14 to 56 d. Water droplets with adjuvant had at least an 86% weight loss after 6 min, but low volatile paraffinic oil droplets had little weight loss 2 d after application.

Sheep sorrel is an invasive, creeping perennial weed of lowbush blueberry fields that decreases yields and hinders harvest. Much of the basic phenology of sheep sorrel in blueberry fields is unknown and not documented in peer-reviewed journals. Three levels of fertilizer (0, 20, and 40 kg N ha−1) and two levels of hexazinone (0 and 1.92 kg ai ha−1) were applied to three vegetative-year blueberry fields to determine their effects on root and shoot growth, biomass allocation, and seed production of sheep sorrel plants. Hexazinone efficacy varied widely between sites, but suppressed shoot biomass, achene number and weight, and reproductive biomass, as well as the reproductive : shoot biomass ratio. Fertilizer tended to increase achene number and increased sheep sorrel shoot biomass in the absence of hexazinone, but had no effect on achene weight, root biomass, or reproductive biomass. When fertilizer was applied, sheep sorrel allocated resources to sexual reproduction at the expense of vegetative growth.