Achenes of Scotch thistle (Onopordum acanthium L.) contain a water-soluble germination inhibitor and are sensitive to differences in light quality. Germination of these achenes can be enhanced by the addition of 0.14 mmole of gibberellic acid and 1.0 mmole of potassium nitrate to the substrate. Dissected embryos exhibit germination responses similar to those observed for achenes. Naturally-distributed achenes recovered from the soil in the field increased in germination under fluorescent light and after washing. Achenes will not germinate on the surface of a constant-moisture-supplying substrate when exposed to an unsaturated atmosphere. The physical environment of the seedbed apparently is paramount in controlling germination and persistence of achenes of Scotch thistle.

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.

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.

Experiments were conducted on a Cedarbluff silt loam and a Cecil sandy loam to determine dissipation of imazaquin and imazethapyr as influenced by burning small-grain straw and tillage prior to soybean planting. Corn bioassay detection limits for imazaquin and imazethapyr in the Cedarbluff silt loam were 2.5 to 30 and 5 to 40 ppbw, respectively. Bioassay detection limits for imazaquin and imazethapyr in the Cecil sandy loam were 2.5 to 20 and 10 to 40 ppbw, respectively. Imazaquin and imazethapyr activity was not detectable in soil by 110 to 152 d after treatment. Imazethapyr dissipation was not affected by burning or tillage in the Cedarbluff silt loam and dissipated more slowly in 1989 than imazaquin. Imazaquin dissipation in the Cedarbluff silt loam in 1988 was slower in burned plots than in nonburned plots but was not affected by burning in 1989. No differences were observed between imazaquin and imazethapyr dissipation in the Cecil sandy loam and neither burning or tillage influenced their rate of dissipation in either year. No-till-planted cotton was injured at both locations by imazaquin and imazethapyr that had been applied the previous year.

Corn (Zea mays L.) can be injured by carry-over of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine] in the soil from one crop year to the next. Factors influencing corn tolerance to carry-over concentrations of trifluralin in the soil were studied in controlled-environment chambers and greenhouse experiments. For several hybrids, greater injury occurred at low than at high temperatures. This injury was especially evident for ‘Pioneer 3320’ and ‘Pioneer 3572’ when the soil moisture was at 100% field capacity. Addition of phosphorus fertilizer appeared to have no influence upon trifluralin injury but did interact with alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide]. Alachlor plus trifluralin injured corn in an apparently additive manner. Significant differences were found in genetic tolerance of corn to trifluralin within a group of hybrids. Tolerance of specific hybrids to trifluralin was altered by environmental conditions.

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.

Glyphosate-resistant (GR) crops have been adopted rapidly since their commercial introduction, and with the increase in commercially available crops resistant to glyphosate, continuous use of the same herbicide mode of action is now possible in some crop rotations. A 6-yr study was initiated to investigate the effects of conventional herbicides compared with continuous use of glyphosate in GR or Roundup Ready corn and GR soybean in a corn–soybean and a corn–soybean–winter wheat rotation. Individual experiments were fully phased and established at three locations under conventional tillage (CT) and at two locations under no-tillage (NT). Results indicated that midseason weed ground cover was lower when weeds were controlled with glyphosate; however, in most cases, this did not result in improved corn or soybean yields. Within locations, species richness, which strongly influenced other diversity indicators, was most affected by the herbicide treatments. Including winter wheat in the crop rotation had little effect on corn and soybean weed ground cover, density, and community structure and only affected soybean yield. Moreover, no effects of herbicide system used in previous corn and soybean were observed in winter wheat, with the exception of species diversity in NT, where species diversity tended to be greater when weeds in previous corn and soybean were treated with conventional herbicides. After 6 yr, the effects of continuous use of GR crops in rotation were similar to those reported in previous studies; however, continued monitoring and longer-term investigations of these systems are necessary to detect the early stages of development of herbicide-resistant biotypes.

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.

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.

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 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.

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.

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.

Giant foxtail putatively resistant to acetolactate synthase (ALS) inhibitors has been reported widely in the upper Midwest, typically in fields with a history of ALS inhibitor use in continuous corn or corn–soybean rotation. However, it is not known whether these giant foxtail populations vary in their response to ALS inhibitors. Therefore, our objectives were to confirm and quantify resistance of giant foxtail accessions from Wisconsin, Minnesota, and Illinois to imidazolinone and sulfonylurea herbicides; to determine the mechanism of resistance; and to determine the mechanism of resistance inheritance. Dose–response experiments using three- to four-leaf stage giant foxtail plants in the greenhouse confirmed cross-resistance of the Wisconsin, Minnesota, and Illinois accessions to imazethapyr and nicosulfuron. Based on ED50 values (the effective dose that reduced shoot dry biomass by 50% compared to the nontreated plants), the Wisconsin, Minnesota, and Illinois accessions were 16-, 17-, and 15-fold resistant to imazethapyr, respectively, and 21-, 19-, and 9-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. In contrast, all accessions were susceptible and responded similarly to fluazifop-P. Based on an in vivo ALS assay, the Wisconsin, Minnesota, and Illinois accessions were > 750-, > 320-, and > 670-fold resistant to imazethapyr, respectively, and 1,900-, > 1,900-, and 80-fold resistant to nicosulfuron, respectively, compared to susceptible accessions. To determine the inheritance of resistance traits, hybrid F1 families were generated from crosses between ALS inhibitor–susceptible and -resistant plants from Minnesota. Three distinct plant phenotypes—resistant (R), intermediate (I), and susceptible (S)—were identified in the F2 generation following exposure to imazethapyr. In repeated experiments, these phenotypes segregated in a 1:2:1 (R:I:S) ratio, indicative of a trait associated with a single, nuclear, semidominant allele.

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.

Field studies were conducted to characterize the predominant nonstructural carbohydrates in roots and crowns of purple loosestrife and to determine the seasonal fluctuation of root and crown carbohydrates in three Minnesota wetland habitats. Starch was the primary nonstructural carbohydrate present, and concentrations were consistently higher in roots than in crowns. Starch content decreased following shoot emergence and declined until flower bud formation, at which time levels reached seasonal lows. Following bud initiation, levels of starch increased during flowering, and this trend continued through to plant senescence in late September or October. Sucrose was the predominant soluble sugar in roots and crowns of purple loosestrife, but fructose and glucose were detected. Levels of sucrose in roots and crowns followed the same seasonal trends as starch.

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.

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.