The research worker who attempts the application of herbicides to a small area in such a way as to simulate general practice, is faced with a unique problem. The ultimate objective is to apply the chemical to as small an area as is feasible but in such a way that it will correctly represent what the activity on both weeds and crop plants would have been had the application been made on an acreage basis. Small plots are desirable if land or labor are limited, if expensive crops in farmers’ fields are being used, and particularly when the chemicals may be highly toxic, or the activity is unknown.

The effect of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] combinations on various physiological processes was studied in order to establish a basis for the synergistic effect of this mixture on Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea L.]. In contrast to atrazine, alachlor had no effect on the Hill reaction activity of isolated Japanese millet chloroplasts. When used in combination with atrazine, alachlor had no influence on the inhibitory effect of atrazine on the Hill reaction. Atrazine and alachlor combinations reduced chloroplast protein and severely inhibited chloroplast protein synthesis relative to protein synthesis by other particulate fractions. This inhibition appears to be the basis for the synergistic effect of this herbicide mixture on Japanese millet. Although the mechanism of action of the herbicide mixture is not understood, it does not seem to involve inhibition of atrazine detoxification.

Field and laboratory investigations were conducted from 1983 through 1985 to evaluate morphological and environmental influences on the performance of imazaquin on 26 weed species common to the Mississippi Delta. Imazaquin applied preplant incorporated or preemergence at 140 g ai ha–1 controlled 16 species but controlled only five species at 140 g ha–1 foliar applied. Contour graphs accurately predicted imazaquin postemergence rates necessary to control a given weed species at a given time after emergence. Imazaquin applied preemergence with a sequential postemergence application controlled entireleaf morningglory and sicklepod better than a single soil application. Imazaquin at 140 g ha–1 applied preemergence required at least 50 mm of supplemental water following application to control sicklepod and was more phytotoxic to sicklepod grown at 25/10 C day/night temperatures than those grown at 32/22 C.

Dispersal mechanisms of the alien plant species buffalobur during its invasion of cold desert areas in Xinjiang, northwestern China, were investigated. Seeds and fruits were readily moved by water in irrigation canals in the transition zone between natural desert and a farmed oasis. Maximum flotation time in moving canal water was ∼ 4 h for seeds and > 48 h for fruits, and water moved fruits 279 m in 10 min. Also, 100% of the seeds remained viable during 8 wk of flooding in the laboratory. Mean dispersal distance was 3.4 m by wind-driven rolling of detached plants and 0.5 m by ants. Retention time for 50% of fruits on wool of live sheep was ∼ 4 h. Seeds and fruits that fall into the canals (which are without irrigation water from mid-October to April) are cold-stratified during winter, and then during canal cleaning in spring soil and germinable seeds are deposited along the sides of the canals. The disturbed soil is a highly favorable site for plants to grow. The local spread of buffalobur away from the sides of canals is facilitated by sheep, wind, and ants. We conclude that water in the irrigation canals is the primary dispersal agent for seeds of this invasive species and that the best way to control its spread is to prevent plants growing beside the canals from setting seed.

Effective outreach is critical to achieving success in managing herbicide-resistant weeds. Interdisciplinary collaboration is needed to adapt information delivery and to engage communities to address the herbicide-resistance problem. Weed scientists must partner with the production community to adapt herbicide-resistance practices for local needs, to work collaboratively with state and regional stakeholders to create effective resistance-management practices, and to provide an overarching national message as to the causes of, and solutions to, resistance.

Flixweed [Descurainia sophia (L.) Webb ex Prantl] is a notorious broadleaf weed that is widely distributed in winter wheat–growing areas of China and has evolved resistance to tribenuron-methyl mainly due to target-site resistance (TSR) mutations in acetolactate synthase (ALS). In the current research, two ALS genes were identified in tribenuron-methyl–susceptible (TS) or tribenuron-methyl–resistant (TR) D. sophia. Resistance mutations of Asp-376-Glu and Pro-197-Ala were identified on ALS1 and ALS2 isozymes in TR D. sophia, respectively. The TR D. sophia evolved 10,836.3-fold resistance to tribenuron-methyl and displayed cross-resistance to multiple ALS-inhibiting herbicides with different chemical structures. Dose response experiments and ALS activity assay indicated that two mutated ALS isozymes contributed differentially in resistance to tribenuron-methyl, flucetosulfuron, and pyribenzoxim. In addition, the relative expression level of the ALS1 gene was 2.2- and 1.6-fold higher than ALS2 genes in TR D. sophia at 1 and 7 d after tribenuron-methyl treatment, respectively. In contrast, the relative expression level of ALS1 and ALS2 in TS D. sophia is similar. This is the first research that explored different roles of ALS isozymes in resistance to ALS-inhibiting herbicides, which might provide a new perspective for the weed resistance management.

Silverleaf nightshade (Solanum elaeagnifolium Cav.) is an invasive species that has successfully spread outside its native range to become a noxious weed in 21 states in the United States and 42 countries worldwide. The successful establishment of S. elaeagnifolium outside its native habitat indicates its innate ability to adapt to a multitude of environments. Phenotypic plasticity and/or genetic adaptation have been identified as key mechanisms underlying the adaptive success of invasive species. Whereas phenotypic plasticity allows a species to buffer changes in the environment by altering its phenotypic attributes within the short term, genetic adaptation is responsible for the longer-term adaptability of plants to heterogeneous environments and is dependent on the amount of genetic variation present in the species. In this study, we screened DNA markers that are specific to tomato (Solanum lycopersicum L.) and Solanum lycopersicoides Dunal for their interspecific transferability to S. elaeagnifolium and determined the applicability of the transferable DNA markers in assessing the extent of genetic variation in populations from Lubbock, Littlefield, and Blackwell, TX. Of the 187 markers screened, 78 successfully amplified targets in S. elaeagnifolium, indicating the evolutionary conservation of marker loci across S. lycopersicum, S. lycopersicoides, and S. elaeagnifolium, despite their genetic divergence millions of years ago. Genotyping of S. elaeagnifolium populations using 50 DNA markers that consistently amplified clear bands in more than 60% of the plants identified nine polymorphic markers with 0.014 to 0.621 polymorphism information content. Genetic diversity analysis by DNA marker profiling established genetic variation among populations and within individuals of different populations. Unweighted paired group method with arithmetic mean analysis grouped the plants into six clusters that are generally defined by selection pressures unique to each collection site. Results of the study indicate the capacity of S. elaeagnifolium for genetic differentiation in response to variable selection pressures within the same geographic region.

Amiben (3–amino–2,5–dichlorobenzoic acid) was applied to the surface of various soil types in cylinders which were then leached with water. The leached soil was tested biologically for herbicide activity. Loss was most rapid from sand, followed by clay, with muck remaining phytotoxic much longer. There was only a slight increase in the toxicity of loam soil given repeated treatments with amiben. Incubation of treated soil for various periods of time indicated that microorganism breakdown is important in soils relatively high in organic matter.

Field studies were conducted at three locations in 1993 and 1994 to evaluate weed control and crop response to metolachlor plus combinations of 0.5 × and 1 × label rates of imazaquin applied preplant and imazethapyr applied early postemergence or postemergence in no-till narrow-row soybean production. Giant foxtail, common ragweed, common cocklebur, and large crabgrass population reductions were greater with sequential preplant metolachlor plus imazaquin followed by early postemergence or postemergence imazethapyr than with preplant metolachlor plus imazaquin or early postemergence/postemergence imazethapyr alone. Ivyleaf morningglory was not effectively controlled by any herbicide program. Pennsylvania smartweed populations were reduced with all herbicide treatments. Soybean yields with treatments utilizing 0.5 × rates were usually equal to 1 × rates if imazethapyr was applied early postemergence or postemergence. Net income with reduced herbicide rates was equal to full-label rates and provided no greater risk to net income.

Glyphosate [N-(phosphonomethyl)glycine] was applied in the spring and fall to leafy spurge (Euphorbia esula L. # EPHES) in the field at rates of 0.14 to 4.48 kg ai/ha. Fall applications of glyphosate at rates of 0.56 to 4.48 kg/ha stimulated axillary branching and caused an increase in the number of stems/m2 by the end of the following summer in a dense population. This was a result of shoot growth from buds on the crown region of the root system. The absorption and translocation of 14C-glyphosate applied to leaves of mature leafy spurge plants were evaluated at prebloom, full-bloom, and senescence phenological stages. Approximately 81% of the 14C-glyphosate applied to the leaves of senescing plants was absorbed. There was a decrease in the proportion of 14C translocated out of the treated leaf when applications were made after full bloom. Translocation of 14C to the treated stem, nontreated stems, root crown, and roots did not differ with phenological stage in 1983. Translocation and concentration of 14C in most plant parts in 1985 differed with phenological stage. Translocation to the crown buds as a percentage of the 14C absorbed was highest at the senescence stage. At senescence and before soil freezing, leafy spurge crown buds demonstrate transient but active elongation toward the soil surface. This active development may account for the enhanced translocation of 14C into the crown buds.

Bulb onions are poor competitors and volunteer potato, commonly observed in western USA onion fields, is difficult to manage. To improve the understanding of onion and weed interactions, relationships were quantified among volunteer potato density, onion yield, and volunteer potato tuber production using hyperbolic or linear models. Onion yield losses because of volunteer potato interference occur at densities commonly observed in the field. A volunteer potato density as low as 0.067 plants m−2 resulted in a 10% reduction in crop yield. Asymptotic yield loss (A parameter) was 100% and achieved with 4 volunteer potato plants m−2. Volunteer potato competition limits onion bulb size, resulting in a lower quality and thus a less-valuable crop. Volunteer potato tuber density and biomass increased linearly with initial weed density as high as 8 volunteer potato m−2. Onion yield loss from volunteer potato competition occurs to a greater extent and at a lower weed density than demonstrated in previous research on small-seeded annual weed species.

Herbicide-resistant weeds pose a considerable threat to agriculture, but their resistance mechanisms are poorly understood. Differential gene expression analysis of a weed subjected to herbicide treatment is a key step toward more mechanistic studies. Such an analysis, often involving quantitative real-time PCR (qPCR), requires suitable reference genes as internal controls. In this study, we identified optimal reference genes in the noxious weed, Japanese foxtail. This weed has evolved resistance to acetyl-coenzyme A carboxylase (ACCase) inhibitors. We analyzed the stability of eight commonly used candidate reference genes (glyceraldehyde-3-phosphate dehydrogenase [GAPDH]; ubiquitin [UBQ]; capsine phosphatase [CAP]; beta-tubulin [TUB]; eukaryotic initiation factor 4a [EIF4A]; elongation factor-1 alpha [EF1]; 18S ribosomal RNA [18S]; 25S ribosomal RNA [25S]) from root, stem, and leaf tissue of plants that were either resistant or sensitive to ACCase inhibitors, with or without herbicide stress, using qPCR. The results were further ranked and analyzed using geNorm, NormFinder, and BestKeeper software. These analyses identified EF1 and UBQ in roots, EF1, TUB, CAP, and 18S in stems, and EF1, GAPDH, and 18S in leaves as suitable references for qPCR normalization. We have identified a set of reference genes that can be used to study herbicide resistance mechanisms in Japanese foxtail.

In studies conducted from 1991 through 1994, researchers investigated the effects of several crop rotations and herbicide programs on crop yield and populations of common lambsquarters, common ragweed, Amaranthus spp., and jimsonweed at two sites. Crop rotations included continuous corn, continuous soybean, corn–soybean, and corn–tomato–soybean, and herbicide programs were the split-plots and included continuous use of acetolactate synthase (ALS)–inhibitor herbicides, continuous use of non–ALS-inhibitor herbicides, annual rotations between ALS- and non–ALS-inhibitor herbicides, combinations of ALS- and non–ALS-inhibitor herbicides in the same year, and no herbicide. Weed control and weed populations generally were affected by an interaction between crop rotations and herbicide programs. After 4 yr, common lambsquarters control was lowest, and populations were highest where fomesafen was used alone for four consecutive years or in rotation with other herbicides. Although common ragweed populations were low at site 2, control at both sites was generally lowest from treatments that included only ALS-inhibitor herbicides. Common ragweed populations were highest at site 1 in 1992 and 1993 following continuous applications of ALS-inhibitor herbicides. Jimsonweed populations were also low at site 2, but control at site 1 in tomato was low. Jimsonweed control from fomesafen and the combination of butylate plus atrazine in soybean and corn, respectively, was variable. Amaranthus spp. populations decreased as the study progressed, and in 1993, control was over 90% from all treatments, except in the case of the treatment combining butylate plus atrazine. Corn and soybean yields varied with year and site, and yields of these crops and tomato were related to rainfall and weed control.

Enhanced foliar injury was obtained with sprays containing the herbicide linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] and the insecticide carbaryl (napthyl-N-methylcarbamate) in tolerant carrot (Daucus carota L. ‘Chantenay’). Synergism occurred when the compounds were applied as tank mixes or applied at 1 day intervals. The photosynthetic rate of parsnip (Pastinacea sativa L. ‘Hollow Crown’) leaf discs exposed to both chemicals was greatly reduced when compared to tissues receiving linuron or carbaryl alone. Carbaryl enhanced the penetration of 14C-linuron into intact carrot leaves and parsnip leaf discs. The rate of linuron metabolism was also decreased in the presence of carbaryl. Both of these factors may contribute to the enhanced toxicity obtained with this combination of pesticides.

Feasibility of supercritical CO2 fluid extraction of imazaquin from spiked soil (3.21 μmol kg-1) as an alternative to a conventional extraction method was investigated. The supercritical fluid extraction method involved single-step extraction of herbicide from soil with no further sample cleanup procedures. Extraction parameters were optimized for maximum herbicide recovery. Adding water as a modifier to air-dried soil significantly improved herbicide recovery. Extracting a 1-g soil sample with supercritical CO2 at 0.80 g ml-1 density and 3 ml min-1 flow rate, 80 C extraction temperature, 6 min static extraction followed by 25 min dynamic extraction, and analyte trap temperature of 40 C was optimum for maximum herbicide recovery. When optimum supercritical fluid extraction conditions were used, imazaquin recovery from three texturally different soils ranged from 55 to 64%, which was comparable to a conventional extraction method (63%). The supercritical fluid extraction method consumed 4 ml methanol and 75 ml supercritical CO2 and took approximately 1 h for sample extraction.

Sweet corn is planted over a long season to temporally extend the perishable supply of ears for fresh and processing markets. Most growers' fields have weeds persisting to harvest (hereafter called residual weeds), and evidence suggests the crop's ability to endure competitive stress from residual weeds (i.e., crop tolerance) is not constant over the planting season. Field studies were conducted to characterize changes in the residual weed community over the long planting season and determine the extent to which planting date influences crop tolerance to weed interference in growth and yield traits. Total weed density at harvest was similar across five planting dates from mid-April to early-July; however, some changes in composition of species common to the midwestern United States were observed. Production of viable weed seed within the relatively short growth period of individual sweet corn plantings showed weed seedbank additions are influenced by species and planting date. Crop tolerances in growth and yield were variable in the mid-April and both May plantings, and the crop was least affected by weed interference in the mid-June and early-July planting dates. As the planting season progressed from late-May to early-July, sweet corn accounted for a great proportion of the total crop–weed biomass. Based on results from Illinois, a risk management perspective to weeds should recognize the significance of planting date on sweet corn competitive ability. This work suggests risk of yield loss from weed control failure is lower in late-season sweet corn plantings (June and July) than earlier plantings (April and May).

In greenhouse studies, control of common ragweed (Ambrosia artemisiifolia L. ♯ AMBEL) and common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE) was achieved whether or not soybeans [Glycine max L. (Merr.) ‘Ransom’] partially shielded the weeds from foliar applications of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}. Excellent control (> 90%) of common ragweed was obtained in the greenhouse 2 and 4 weeks after plant emergence. Best control (> 90%) of common cocklebur was obtained 2 weeks after plant emergence. A simulated rainfall of 0.6 cm occurring 1 min after acifluorfen application did not decrease control or fresh weight of common ragweed in greenhouse studies. The weed control efficiency of acifluorfen on common cocklebur was reduced when the herbicide was applied intermittently within 6 h of the 0.6-cm simulated rainfall. The weed control efficiency of acifluorfen on both weed species was also reduced when the herbicide was applied intermittently within 6 to 12 h of a 1.3-cm simulated rainfall in greenhouse studies. In field studies, 2.5 cm of simulated rainfall within 12 to 24 h after acifluorfen application reduced control of common ragweed.

Seeds of honey mesquite (Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell) were germinated and grown in aqueous solutions of the sodium salt of (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) or the potassium salt of 4-amino-3,5,6-trichloropicolinic acid (picloram). Elongation of germinating seedling hypocotyls and roots was progressively inhibited by increasing concn of each herbicide up to about 1 or 2 mg/L the first 2 days. After 5 and 10 days, progressively more inhibition occurred at 5 to 10 mg/L of 2,4,5-T and 1 to 2 mg/L of picloram. Four-month-old honey mesquite seedlings were treated with foliar sprays of the 2-ethylhexyl ester of 2,4,5-T or the potassium salt of picloram. Both herbicides caused curling of the stem tip, death of the growing point, cracking below the leaf, and increased numbers of lateral roots. Both herbicides caused phellem cells to enlarge radially, inner cortext and phloem parenchyma to proliferate, and xylem vessels to lignify without enlarging. Starch granules disappeared from treated plants within 8 days and re-occurred in treated plants only 53 days after spraying. Picloram was slightly more toxic to honey mesquite seedlings than 2,4,5-T.

Red rice (Oryza sativa L. # ORYSA) control in soybeans [Glycine max (L.) Merr.] was 93% or more with shallow-incorporated alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] at 3.9 kg ai/ha, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.8 kg ai/ha, and trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] at 1.7 kg ai/ha. Junglerice [Echinochloa colonum (L.) Link. # ECHCO] control was 80% with alachlor but 90% or more with metolachlor and trifluralin when applied at the same rates. Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 560, fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy]phenoxy]propanoic acid} at 280, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} at 140, and mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl)sulfonyl] amino] phenyl] acetamide} plus bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4 (3H)-one 2,2-dioxide] at 140 plus 840 g ai/ha applied postemergence on two- to four-leaf red rice provided at least 87% control. Junglerice control with these treatments was 85% or more with the exception of fluazifop and mefluidide plus bentazon.

Over a 3-yr period the placement of weeds in relation to cotton (Gossypium hirsutum L.) plants greatly influenced the seed cotton yields. After 3-yr of competition the cotton with no weeds yielded about 2300 kg/ha. With weeds confined to between-the-rows placement, yields were about 850 kg/ha. When weeds were confined to in-the-row placement, the cotton was completely shaded and produced no yield. After 3-yr of intense competition throughout each entire growing season no statistical differences were demonstrated between the competitiveness of large crabgrass [Digitaria sanguinalis (L.) Scop.], spurred anoda [Anoda cristata (L.) Schlecht.], prickly sida (Sida spinosa L.) and velvetleaf (Abutilon theophrasti Medic.) to cotton.