Experiments were conducted to examine the utility of a spectrophometric leaf disc assay for detecting shikimate accumulation after glyphosate application in sunflower, proso millet, and wheat. The assay was conducted on both greenhouse- and field-grown plants. Glyphosate was applied at five rates ranging from 840 to 53 g ae ha−1. Shikimate accumulation data were generated at 1, 4, 7, and 14 d after application (DAA). Sunflower accumulated shikimate more rapidly and at lower glyphosate rates than the other two species. At 14 DAA, glyphosate at the two highest rates remained detectable in all three species. Plants receiving lower glyphosate doses (210, 105, and 53 g ae ha−1) had begun to grow out of the injury, or at least the shikimate levels in the plants were no longer significantly different than that present in the untreated controls. This spectrophotometric assay is both rapid and simple, with respect to other means of detecting shikimate, and it can be used to detect glyphosate drift. For it to be used by crop managers, samples from potentially drift-affected crops should be taken as soon as possible after the suspected drift event or immediately after the appearance of glyphosate injury.

A 2-yr field study was conducted to measure the growth and development of Russian thistle (Salsola iberica Sennen and Pau # SASKR) in the growing crops of winter and spring wheat (Triticum aestivum L.) and after harvest of these crops. In herbicide-free conditions, few Russian thistle seedlings emerged in winter wheat. Only 50% of these plants survived compared to 92 and 95% survival in spring wheat and crop-free treatments, respectively. Compared to growth in the crop-free treatment, both wheat types suppressed oven-dry weight, height, and width of Russian thistle plants during the crop-growing season and after crop harvest. During the crop-growing season, winter wheat suppressed Russian thistle height and width more than spring wheat. After crop harvest, oven-dry weight of Russian thistle plants grown in winter wheat stubble was suppressed 75% compared to plants grown in spring wheat stubble. Russian thistle plants grown in crop-free, spring wheat, and winter wheat treatments produced 152 100, 17 400, and 4 600 seeds/plant, respectively.

Broadleaf weeds apparently have patchy distributions within a field while POST control decisions are made assuming a regular spatial distribution. As a result, yield loss from weed competition may be overestimated, possibly leading to mistakes in choosing the optimal control treatment. Data on distribution of broadleaf weeds in 14 soybean fields were used in simulation experiments to investigate the potential for improving decision making with information about weed patchiness. The feasibility of modeling weed distribution in individual fields was also examined. Overall, the cost of assuming a regular distribution when making POST decisions was found to be low. Errors that occurred most often involved recommending more intensive control than was actually required, although in a few cases less intensive control was recommended. Error in the yield loss estimated for the uncontrolled population did not indicate the potential for a mistake in decision making for a field. Accurately modeling distribution of weeds within fields may be difficult as a result of correlations between distributions of individual species within a field and variation in distributions between fields.

Corn poppy is the most abundant broad-leaved weed in winter cereals of Mediterranean climate areas and causes important yield losses in wheat. Knowledge of the temporal pattern of emergence will contribute to optimize the timing of control measures, thus maximizing efficacy. The objectives of this research were to develop an emergence model on the basis of soil thermal time and validate it in several localities across Spain. To develop the model, monitoring of seedling emergence was performed weekly during the growing season in a cereal field located in northeastern Spain, during 3 yr. Cumulative thermal time from sowing date was used as the independent variable for predicting cumulative emergence. The Gompertz model was fitted to the data set of emergences. A base temperature of 1.0 C was estimated through iteration for maximum fit. The model accounted for 91% of the variation observed. Model validation in several localities and years showed general good performance in predicting corn poppy seedling emergence ( values ranging from 0.64 to 0.99 and root-mean-square error from 4.4 to 24.3). Ninety percent emergence was accurately predicted in most localities. Results showed that the model performs with greater reliability when significant rainfall (10 mm) occurs within 10 d after crop sowing. Complemented with in-field scouting, it may be a useful tool to better timing control measures in areas that are homogeneous enough regarding climate and crop management.

Field studies were conducted in North Carolina in 1981 and 1982 to evaluate the efficacy of postemergence over-the-top and postemergence-directed herbicides for control of five morningglory species: entireleaf [Ipomoea hederacea (L.) Jacq. var. integriuscula Gray], tall [Ipomoea purpurea (L.) Roth. ♯3 PHBPU], ivyleaf [Ipomoea hederacea (L.) Jacq. var. hederacea ♯IPOHE], pitted [Ipomoea lacunosa (L.) ♯ IPOLA], and scarlet [Ipomoea coccinea (L.) ♯IPOCC]. The glabrous morningglories (scarlet and pitted) were more easily controlled than the pubescent morningglories (ivyleaf, tall, and entireleaf). Lower soybean injury, higher morningglory control, and greater soybean seed yields were obtained with over-the-top herbicide applications at 4 weeks after planting (WAP) than at 6 WAP. Pitted morningglory was tolerant to low rates of 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid]. This herbicide applied over the top at the R1 stage of soybean growth produced low yields, probably as a result of morningglory interference and herbicide injury to the soybeans. Postemergence-directed applications of linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] and metribuzin [4-amino-6-tert-butyl-3-(methylthio-as-triazin-5(4H)-one] alone or in tank mixtures with 2,4-DB resulted in soybean injury that ranged from 12 to 36%. Highest soybean seed yields (equivalent to weed-free control) from postemergence-directed herbicides were obtained with applications of 2,4-DB, linuron, and a tank mixture of metribuzin and 2,4-DB.

Herbicidal activity of clove oil and its main constituents eugenol, β-caryophyllene, and α-humulene was studied by measuring their effects on cell membrane integrity in broccoli and common lambsquarters plants at the three- and nine-leaf stage, respectively. Roles of essential oil constituents in the overall phytotoxicity of clove oil, dose-response (10 to 160 mM) relationships of their phytotoxicity, and the effect of light intensity on phytotoxicity of clove oil and eugenol were studied. Most of the phytotoxicity of clove oil (2.5% solution) was due to eugenol, its largest constituent. β-caryophyllene and α-humulene played little or no role. Dose-response relationships showed that at equimolar concentration, eugenol was the most phytotoxic essential oil constituent of the clove oil. On a per unit biomass basis, membrane damage in response to clove oil and eugenol sprays decreased with increasing light intensity. This suggests that efficacy of essential oil in causing plant damage could be affected by light intensity experienced by plants prior to the oil spray.

A bioeconomic model is used to estimate how managing glyphosate resistance to horseweed affects short- and long-run profits in corn, soybean, and corn-soybean rotation systems. Model results found that resistance management reduces profits in the first year of implementation, but increases profits in the second and subsequent 18 yr. In all three systems, long run gains begin to outweigh short-run costs (and resistance management “pays for itself”) by the second year. Over a 20-yr horizon, the estimated annual average profit advantage from managing resistance exceeded $158 ha–1 ($64/acre) for corn, $137 ha–1 ($55/acre) for corn-soybean, and $55 ha–1 ($22/acre) for soybean. Seed immigration from a neighbor's field can reduce these gains, but this reduction is negligible if the neighbor also practices resistance management. If the neighbor did not manage resistance, however, the grower's estimated annual profit advantage fell to roughly $123 ha–1 ($50/acre) for corn, $60 ha–1 ($24/acre) for the corn-soybean rotation, and virtually zero for soybean. Methods applied in this study identify corn and corn-soybean rotations as cases where resistance management “pays for itself' quickly and significantly, even if the neighbor does not manage resistance. Continuous soybean presents a more challenging case that may require additional programs and incentives to encourage collective resistance management among growers. Even for continuous soybean, however, joint grower–neighbor resistance management can pay for itself within 2 yr.

Scarification in sulfuric acid consistently increased germination of Opuntia edwardsii sp. nov, O. discata Griffiths, and O. lindheimeri Engelm. ♯ OPULI seeds over that of untreated seeds. Optimum constant temperatures for germination were generally 25 to 35 C and germination was not enhanced by alternating temperatures. There was a trend for increased germination following leaching in water for 12 h which suggested the presence of chemical germination inhibitors. Seeds passed through the digestive tracts of cattle exhibited average germination percentages that were 1.5 times greater than seeds removed from ripe fruits.

An isolate of Sclerotinia sclerotiorum (Lib.) de Bary collected from a Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] plant in Montana proved pathogenic on Canada thistle in field trials. In addition to attacking the thistle crown and causing wilting and death of the shoots, S. sclerotiorum also infected the root system. The high percentage of thistle shoot kill (20 to 80%) after treatment, and subsequent reduction in plant thistle density the following year, demonstrated the potential of S. sclerotiorum as a biological control agent for Canada thistle in Montana.

The influence of soil pH (4.3 to 7.5) on the phytotoxicity of herbicides incorporated into high organic soils was studied. Phytotoxicity increased as the soil pH increased and reached a maximum at pH 6.5 for the weak aromatic acids 3,6-dichloro-o-anisic acid (dicamba) and (2,4-dichlorophenoxy)-acetic acid (2,4-D) and the weak bases 2,4-bis(isopropylamino)-6-methoxy-s-triazine (prometone) and 3-amino-s-triazole (amitrole). Conversely, phytotoxicity increased as soil pH decreased and reached a maximum at pH 4.3 for the weak aliphatic acid 2,2-dichloropropionic acid (dalapon), the cationic herbicides 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) and 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat), and a nonionic herbicide S-propyl dipropylthiocarbamate (vernolate). Soil pH levels between 4.3 and 7.5 had no effect on the phytotoxicity of (a) the weak aromatic acids 3-amino-2,5-dichlorobenzoic acid (chloramben) and 4-amino-3,5,6-trichloropicolinic acid (picloram); and (b) the nonionic herbicides 2,6-dichlorobenzonitrile (dichlobenil), 5-bromo-3-isopropyl-6-methyluracil (isocil), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), and 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin). A change of one pH unit decreased the phytotoxicity of 2,4-D, dicamba, dalapon, prometone, amitrole, paraquat, and vernolate by a factor of two to four depending on the particular herbicide and the pH values considered.

The basis for differences in response of eastern black nightshade (Solanum ptycanthum Dun.), a tolerant species, and velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH), a susceptible species, to foliar-applied chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide} was investigated by evaluating differences in spray retention and herbicide absorption, translocation, and metabolism. Based upon a foliar-applied rate causing a 50% reduction in dry weight, velvetleaf was greater than 20000 times more susceptible to chlorsulfuron than was eastern black nightshade. The differences detected in spray retention, absorption, and translocation were inadequate to account for the large response differences between the two species. The primary difference found was in the rate of chlorsulfuron degradation. In eastern black nightshade, 69.9% of the absorbed chlorsulfuron was metabolized within 24 h of application and 81.1% within 72 h of application. Only 7.1% of absorbed chlorsulfuron was metabolized in velvetleaf in a 72-h period.

The allelopathic potential of leafy spurge (Euphorbia esula L.) was investigated in the field and greenhouse. Reductions in frequency and density of quackgrass [Agropyron repens (L.) Beauv.] and common ragweed (Ambrosia artemisiifolia L.) were noted where leafy spurge had high densities in the field. Field soil samples taken from areas of moderate and high leafy spurge densities inhibited tomatoes (Lycopersicon esculentum Mill.) in the greenhouse. Growth of seedlings was inhibited up to 60% when leafy spurge leaves, roots, or litter were incorporated into soil, but surface applied leafy spurge litter had no effect. Growth inhibition was demonstrated for crabgrass [Digitaria sanguinalis (L.) Scop.] and tomato when 0.1 to 1.0% (w/w) of leafy spurge leaves or roots were added to soil. Based on these studies we postulate that leafy spurge exhibits allelopathic characteristics which may result from the incorporation of dead or decaying plant material in soil.

Several crop and weed species were field grown to maturity and their residues incorporated with soil to determine the effects of the plant residues on the growth of subsequent plantings in a spring-fall planting sequence in 1980 to 83. Growth of Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) was studied in 1984 and 1985, and the effects of soil-incorporated Palmer amaranth on the growth of cabbage (Brassica oleracea var. Capitata L.) and carrot (Daucus carota L. var. sativa) seedlings were studied in 1985. Under warm, moist, saline soil conditions and intense sunlight following the spring plantings of the 1980 to 83 study, soil-incorporated residues of Palmer amaranth inhibited the growth of carrot 49% and onion (Allium cepa L.) 68%. Johnsongrass (Sorghum halepense L. Pers. # SORHA) residues inhibited the growth of onion 67%, and in one of two years inhibited the growth of cabbage 26% and common sunflower (Helianthus annuus L. #HELAN) 10%. In 1984 and 1985, Palmer amaranth grown at population densities of 300 plants/m2 attained heights of nearly 2 m and biomass of 2 to 3 kg/m2 within 10 weeks after planting. Phytotoxicity with these biomasses persisted 11 weeks in cabbage and carrot plantings. In 1985, phytotoxicity was dissipated 16 weeks after soil incorporation of the weed.

The joint activity of mesotrione and atrazine can display synergistic effects on the control of both triazine-sensitive and site-of-action-based triazine-resistant (TR) broadleaf weeds. The first objective of this study was to evaluate a PRE application of atrazine followed by a POST application of mesotrione for potential interactions in both site-of-action-based TR redroot pigweed and metabolism-based atrazine-resistant (AR) velvetleaf. Results from these sequential experiments demonstrated that synergism was detected in reducing biomass of the TR redroot pigweed but not in the AR velvetleaf with metabolism-based resistance. The second objective was to evaluate the joint activity of mesotrione and atrazine in a tank-mix application in the AR velvetleaf biotype. Greenhouse studies with the AR biotype indicated that synergism resulted from a tank mix with a constant mesotrione rate of 3.2 g ai ha−1 in mixture with atrazine ranging from 126 to 13,440 g ai ha−1. Chlorophyll fluorescence imaging also revealed a synergistic interaction on the AR biotype when 3.2 g ha−1 of mesotrione was applied with 126 g ha−1 of atrazine beginning 36 h after treatment and persisting through 72 h.

Additions of montmorillonite clay and soil of high organic matter content to growth media consisting of sand and nutrient solution significantly decreased the phytotoxicity of 2-methylthio-4,6-bis(isopropylamino)-s-triazine (prometryne) to wheat [Triticum aestivum L. em. Thell. (aestivum group) “Wakeland”]. Phytotoxicity was decreased to a greater extent at pH 4.5 than at pH 6.5. The reduction in phytotoxicity was attributed to adsorption of the herbicide by the additives. A larger proportion of the prometryne was present in the protonated form at the lower pH and the increased adsorption probably resulted from the coulombic bonding of these cations to the soil additives. Adsorption mechanisms are postulated and discussed.

Giant ragweed germination is delayed by both a physiological dormancy of the embryo (embryo dormancy) and an inhibitory influence of embryo-covering structures (covering structure-enforced [CSE] dormancy). To clarify the roles of embryo and CSE dormancy in giant ragweed seedling emergence timing, we conducted two experiments to address the following objectives: (1) determine changes in germinability for giant ragweed dispersal units (hereafter “involucres”) and their components under natural burial conditions, and (2) compare embryo and CSE dormancy alleviation and emergence periodicity between successional and agricultural populations. In Experiment 1, involucres were buried in crop fields at Columbus, OH, periodically excavated, and brought to the laboratory for dissection. Involucres, achenes, and embryos were then subjected to germination assays at 20 C. In Experiment 2, temporal patterns of seedling emergence were determined at a common burial site. Reductions in embryo and CSE dormancy were compared with controlled-environment stratification followed by germination assays at 12 and 20 C, temperatures representative of soil conditions in spring and summer. Results indicated that overwinter dormancy loss involved sequential reductions in embryo and CSE dormancy. CSE dormancy, which may limit potential for fatal germination during fall, was caused by the pericarp and/or embryo-covering structures within the pericarp. In Experiment 2, successional populations emerged synchronously in early spring, whereas agricultural populations emerged throughout the growing season. Levels of embryo dormancy were greater in the agricultural populations than the successional populations, but CSE dormancy levels were similar among populations. In 12 C germination assays, embryo dormancy levels were positively correlated with time required to reach 95% cumulative emergence (run 1: r = 0.81, P = 0.03; run 2: r = 0.76, P = 0.05). These results suggest that late-season emergence in giant ragweed involves high levels of embryo dormancy that prevent germination at low temperatures in spring.

Studies of the differential responses of soybean [Glycine max (L.) Merr.] cultivars to bentazon [3-isopropyl-1H-2,1,3,-benzothiadiazin-(4)3H-one 2,2-dioxide] revealed slightly greater translocation and about a twofold greater foliar absorption in the sensitive cultivar ‘PI 229.342 (Nookishirohana),’ than in the tolerant cultivar, ‘Clark 63.’ Clark 63 metabolized bentazon faster than did PI 229.342. Bentazon was metabolized to unidentified polar metabolites I and II in Clark 63, and only to metabolite II in PI 229.342. The differential absorption and metabolism resulted in a tenfold greater concentration of bentazon in the treated leaflet of PI 229.342 than in that of Clark 63 at 8 days after foliar treatment. Leaflets contained similar amounts of 14C in petiole uptake studies, but no bentazon was detected in Clark 63 leaflets 24 hr after pulse treatment, whereas about 20% of the methanol-extractable 14C from PI 229.342 leaflets cochromatographed with bentazon. Metabolite I was labile to selected enzymes with β-glycosidic activity, whereas metabolite II was unaffected. Bentazon concentrations of 30 to 50 μM inhibited O2 evolution about 50% in isolated chloroplasts from the two cultivars. Differential bentazon metabolism appears to be the main cause of the differential responses of these soybean cultivars to bentazon.

The chlorophyll levels and growth of oat (Avena sativa L. ‘Elgin’) seedlings were drastically reduced after foliar application of diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate}. The leaves were chlorotic and necrotic with the newly emerging leaves being most affected. Plants treated with 2,4-D [(2,4-dichlorophenoxy)acetic acid] were similar to the controls. When 2,4-D was applied to intact plants in combination with diclofop-methyl the toxicity symptoms produced by diclofop-methyl were reduced. Diclofop-methyl disrupted membrane integrity and inhibited chlorophyll formation in excised leaf segments. Addition of 2,4-D with diclofop-methyl did not change the effects produced by diclofop-methyl alone in excised leaf segments.

A giant foxtail (PCW1) population putatively resistant to fluazifop-P and sethoxydim was identified in a carrot, onion, and corn cropping system in Wisconsin during 1991. Previous, extensive use of fluazifop and fluazifop-P over several years imposed a high level of selection intensity on grass weeds. In a field experiment, fluazifop-P, sethoxydim, and quizalofop at recommended dosages resulted in 58, 53, and 45% plant survival, respectively, within the PCW1 giant foxtail population; no plants survived treatment with clethodim or nicosulfuron, and few plants survived treatment with imazethapyr at recommended dosages. Based on shoot dry biomass reduction in greenhouse experiments, a PCW1 giant foxtail biotype had 16-, > 9-, 4.9-, and 4.2-fold resistance to fluazifop-P, diclofop, quizalofop, and fenoxaprop, respectively, relative to a giant foxtail (AC1) accession that was susceptible to aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) herbicides. The PCW1 biotype had 134-fold resistance to sethoxydim and slight and inconsistent resistance to clethodim. The PCW1 biotype and AC1 accession were equally susceptible to imazethapyr, linuron, and oxyfluorfen. Based on plant survival, a PCW1 giant foxtail accession had 25- and > 143-fold resistance to fluazifop-P and sethoxydim, respectively, relative to the AC1 accession. The selection intensity associated with repeated use of fluazifop and fluazifop-P over 5 yr contributed to the cross-resistance of PCW1 giant foxtail to APP and CHD herbicides.

A laboratory sprayer, designed to make herbicide applications to potted plants and to study spray distribution patterns with different nozzles, is described. An activated charcoal air filter was installed to minimize herbicide contamination to adjoining greenhouse-laboratory facilities during the spraying operation.