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.

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.

Landoltia was collected and cultured from a canal in Lake County, Florida, where diquat was used repeatedly during the past 20–30 yr for duckweed control. Recent applications of diquat failed to provide adequate control of duckweed, and a new commercial formulation of diquat was suspected. The new formulation was not the cause of reduced efficacy. Static exposures (48 h) to various concentrations of diquat were used to compare the susceptibility of the Lake County landoltia accession to one never exposed to diquat. These static tests indicated that landoltia, from a population with no prior history of herbicide treatment, was extremely susceptible to diquat. The accession from Lake County, FL had developed resistance to diquat, and was also cross resistant to paraquat. The resistance factor was 50 for diquat and 29 for paraquat. The Lake County accession also exhibited reduced ion leakage after diquat exposure under light and dark conditions. This suggests the resistance mechanism to the bipyridylium herbicides in landoltia is independent of photosynthetic electron transport. This research documents the first aquatic plant species that has developed resistance to the bipyridylium herbicides.

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.

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.

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 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.

Palmer amaranth (Amaranthus palmeri S. Watson) is not native to Africa. Based on the presence and persistence of A. palmeri populations, its invasive status in southern Africa is classified as “naturalized.” Globally, A. palmeri is one of the most troublesome weed species in several crops, including soybean [Glycine max (L.) Merr.], maize (Zea mays L.), and cotton (Gossypium hirsutum L.). Certain populations of A. palmeri in various countries were reported to be resistant to herbicides with different sites of action (SOAs). Two biotypes of A. palmeri in the United States reportedly each have resistance to herbicides representing five different SOAs, and between them a total of eight different SOAs are involved. Resistance mechanisms in these biotypes involve target-site and/or non–target site resistance. Here we characterize a specific A. palmeri population that was found in the Douglas district in South Africa and showed resistance to various herbicide SOAs. Initially, this A. palmeri population was discovered in a glyphosate-tolerant cotton field, where it survived glyphosate treatment. Subsequently, greenhouse experiments were conducted to characterize this A. palmeri population for potential resistance to herbicides of additional SOAs, and molecular analyses were conducted to reveal the mechanisms of herbicide resistance. Results indicated resistance to chlorimuron-ethyl and glyphosate in this population, while <90% control (decreased sensitivity) was observed at the label rate for mesotrione, atrazine, saflufenacil, and S-metolachlor. However, glufosinate, tembotrione, acifluorfen, dicamba, 2,4-D, metribuzin, acetochlor, isoxaflutole, diflufenican, and pyroxasulfone were effective at controlling this population. This profiling of herbicide sensitivity has allowed development of programs to control and potentially minimize the spread of this weed. In addition, molecular analysis of EPSPS revealed the role of higher copy number as a mechanism for glyphosate resistance in this population and a Ser-653-Asn target-site mutation likely conferring resistance to the acetolactate synthase–inhibitor chlorimuron-ethyl. No known target-site mutations were identified for the protoporphyrinogen oxidase–inhibitor group.

Seasonal changes in germination behaviour of meadow salsify seeds buried at different depths, and survivorship of seed and plant populations were investigated. Freshly matured seeds were mixed with sterilized soil, placed into nylon mesh bags, and buried at three soil depths in a meadow salsify-infested rangeland. Two germination peaks were observed: one in the fall, 2 to 3 mo after burial, and a second the following spring. Burial depth affected seed dormancy. The number of seeds with enforced dormancy, and innate or induced dormancy, increased with burial depth in the 1989 cohort, and the number with enforced dormancy increased in the 1990 cohort. Burial depth also affected seed survivorship. The survivorship curves for 1989 and 1990 seed cohorts were not different as indicated by the Logrank test. The seed population was nearly depleted after 9 to 10 mo of burial; less than 3 % of the buried seeds remained viable after 13 mo. About 50% of the plants monitored in a natural meadow salsify population in 1990 died before winter, and an additional 35% died during winter. The mortality of the remaining plants during the second growing season was very low. Only 12% of the 1990 cohort remained in October 1991. Mortality of meadow salsify plants was density-independent.

Citrus and avocado trees have occasionally been injured following use of 3–amino–1,2,4–triazole (amitrole) at rates of 12 or more lb/A for control of perennial weeds. The distribution of tree injury relative to sprayed areas as well as the known low volatility of amitrole has indicated that tree injury was not due to volatility. The initial occurrence of symptoms on sucker growth in the interior and on the top foliage rather than lower exterior foliage of the trees would seem to eliminate spray drift as a possible cause. This injury has appeared to be due to spray residues being leached into the soil and taken up by the roots of the trees.

Individual droplets of 14C-2,4-D dimethylamine (DMA) were applied to oriental mustard seedlings to determine the effect of droplet size and herbicide concentration on absorption and translocation 1 or 3 d after application. Absorption of 14C-2,4-D was generally not affected by droplet size which ranged from 198 to 2760 μm. However, the percentage of absorbed 14C-2,4-D that was translocated away from the treated leaf increased as droplet size decreased. Absorption of 14C-2,4-D either increased slightly or was not affected by increased herbicide concentration, but translocation of absorbed 14C-2,4-D was reduced as herbicide concentration increased. Absorption and translocation of 14C-2,4-D DMA were reduced when applied after a drop of 21.3 mM commercially formulated 2,4-D had been applied at the same location on the leaf. These results indicate that large amounts of 2,4-D, either in the form of large droplets or a more concentrated herbicide solution, inhibit translocation of 2,4-D from oriental mustard leaves.

Redstem is an important weed in California water-seeded rice fields because of its aquatic habit, wide distribution, interference with harvest, and resistance to the herbicide bensulfuron. Our objective was to understand the mechanisms of competition for light between rice and redstem, with the goal of improving redstem control. A replicated greenhouse experiment was done in 1993 and 1994. Rice was water-seeded at a rate of 400 seeds m−2, and redstem was seeded simultaneously at approximate densities of 0, 50, and 100 seeds m−2 in continuously flooded 0.77 m2 basins. Plants were harvested once at final harvest in 1993 and twice in 1994, with an additional nondestructive sampling 34 days after seeding (DAS). Despite slower early growth, redstem height exceeded rice height about 45 DAS. At the midseason harvest in 1994 (56 DAS), no effects of redstem competition on any rice response variables were detected. However, at final harvest (110 and 118 DAS, 1993 and 1994, respectively) redstem competition at both treatment densities reduced rice tiller density, panicle density, shoot drymass, and grain drymass. Redstem competition reduced rice growth only after penetrating the canopy. Shade cast by redstem through rice maturity decreased shoot and grain production and increased tiller mortality. Lodging caused by redstem further affected rice growth. Season-long competition from redstem at mean densities of 67 and 110 plants m−2 reduced rough rice yields 31 and 39%, respectively, making redstem the most competitive broadleaved rice weed yet studied. Improved understanding of rice-redstem interactions indicates that using alternative herbicides to bensulfuron is unlikely to increase yield losses to redstem, and that control may be improved by increasing rice plant densities or slightly delaying early season chemical control. Because these strategies are mechanistic, they may also be useful for controlling other rice weeds with growth patterns similar to redstem.

Granule, and in some cases, tablet and ball formulations of bromacil (5-bromo-3-sec-butyl-6-methyluracil), dicamba (3,6-dichloro-o-anisic acid), picloram (4-amino-3,5,6-trichloropicolinic acid), karbutilate [tert-butylcarbamic acid ester with 3(m-hydroxyphenyl)-1,1-dimethylurea], and tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethyl urea} were applied to three areas within 30 km of College Station in the Claypan Resource area of Texas. Woody species growing in the area included blackjack oak (Quercus marilandica Muenchh.), post oak (Quercus stellata Wangenh.), yaupon (Ilex vomitoria Ait.), winged elm (Ulmus alata Michx.), white ash (Fraxinus americana L.), mockernut hickory (Carya tomentosa Nutt.), and tree huckleberry (Vaccinium arboreum Marsh.). Granules were applied either broadcast or in rows at several intervals. Tebuthiuron was the most effective herbicide on oaks, winged elm, and white ash. Tebuthiuron and picloram were equally and most effective on yaupon, mockernut hickory, and tree huckleberry. Tebuthiuron + picloram at 2.2 + 2.2 kg/ha was the most effective herbicide treatment on huckleberry. However, a mixture of tebuthiuron + picloram (1:1 w/w) was usually no more effective than the same rate of tebuthiuron in the mixture applied alone. Tablet and ball formulations of karbutilate and tablet formulations of tebuthiuron were generally equally as effective as granules. Applications of picloram and tebuthiuron granules in rows 1.8, 3.0, 4.6, or 6.1 m apart gave control equal to broadcast applications except for picloram on yaupon. On yaupon all row spacing treatments of picloram were less effective than the broadcast treatment, whereas with tebuthiuron the 6.1-m spacing was least effective. Picloram and tebuthiuron granules applied in rows, approximately at 1.8-m intervals with a tractor, were as effective as granules applied by hand in straight rows.

Competition studies in field trials were conducted between common purslane (Portulaca oleracea L.) and table beets (Beta vulgaris L., var. Detroit Red) and between common purslane and snap beans (Phaseolus vulgaris L., var. Eastern Butter). Common purslane control was most critical during the first 2 weeks after beet and bean emergence. Common purslane control for longer than 2 weeks did not increase beet or bean yields. The longer common purslane was allowed to compete after beet or bean emergence, the more yields were decreased. Common purslane was a stronger competitor in beets than in beans. The faster and taller-growing bean plants provided more competition than did beets. Cultivation between rows until lay-by increased both beet and bean yields.

Seeds of dyers woad (Isatis tinctoria L.) readily germinated at temperatures from 3 to 25 C. Dyers woad seeds in their fruits, from which they do not normally dehisce, had greatly depressed germination compared to that of threshed seeds. The presence of dyers woad fruits in the same Petri dish markedly depressed germination and/or root elongation of threshed dyers woad seeds and the seeds of some competing broadleaf and grass species. The depression was especially evident on seeds of other species of Cruciferae. Similar reductions in germination or root elongation were obtained when water used to soak dyers woad fruits was used to wet germination substrates.

Planting row crops into a stale seedbed has gained popularity in recent years. This concept uses tillage either after harvest in the fall or early in the spring to smooth the seedbed and eliminate ruts and excess residue on the soil surface. Tillage is not used immediately before planting, thus conserving moisture, eliminating costly and time-consuming tillage trips at planting, and allowing more timely planting. While stale seedbed planting minimizes tillage to establish the crop, conservation is not the primary objective of this concept; rather, timely stand establishment following adverse weather conditions has been the driving force behind the popularity of stale seedbed programs. The stale seedbed approach has been used with success on a variety of soils in the southern U.S., but is best suited on the poorly-drained clay soils in the Mississippi River Delta. The system has been adapted for use in soybean, cotton, corn, and rice. A key component of a well-prepared stale seedbed is the absence of weeds at planting. Producers must keep in mind that weeds present and not controlled at planting have the potential to significantly impair stand establishment and crop yields. Although these weeds are often beginning to senesce by the time soybean planting occurs, they are still using soil moisture and shading newly emerging crop plants early in the season.