Synergistic interaction between the insecticide terbufos and the herbicide nicosulfuron may result in severe injury to corn. Greenhouse and laboratory experiments were conducted to determine if using the imidazolinone-resistant corn ‘Pioneer-3343 IR’ (P-3343 IR) or coating corn seeds with naphthalic anhydride (NA) would reduce herbicidal injury imposed by the nicosulfuron-terbufos interaction. Greenhouse experiments showed nicosulfuron-terbufos interactions resulting in herbicidal injury in both P-3343 IR and ‘DeKalb 689’ (D-689) corn varieties, but the D-689 was more sensitive than the P-3343 IR corn. The greenhouse experiments also demonstrated protection against the nicosulfuron-terbufos interaction by NA seed treatments. Studies with radiolabeled nicosulfuron showed that terbufos inhibited the metabolism of nicosulfuron, but pretreatment of D-689 and P-3343 IR corn seed with NA decreased the inhibition in excised corn leaves. The differences in sensitivity to nicosulfuron in the two corn varieties resulted in part from the differential metabolism and primarily from the differential sensitivity of the target enzyme, acetolactate synthase, to the herbicide.

The time-dependent uptake, translocation, and metabolism of [14C]MG-191 safener and the influence of EPTC on these processes were studied using 5-d-old corn seedlings grown in nutrient solution. Plants absorbed relatively low levels of root-applied radiolabel at both 10 μM and 50 μM [14C]MG-191 rates during 6 d of exposure. Amounts of absorbed radioactivity increased with time. Addition of 50 μM EPTC, in general, did not alter amounts of [14C]MG-191 taken up. Initial translocation of radiolabel from roots to shoots was more rapid at 10 μM [14C]MG-191 treatment compared to 50 μM. EPTC had no effect on radiolabel movement at either safener rate. MG-191 was metabolized rapidly to water-soluble metabolites. Slower formation of water-soluble products was detected only during the initial 3 h in the presence of EPTC. Also, thin-layer chromatographic analyses of water- and hexane-soluble metabolite fractions confirmed rapid transformation of the parent molecule. Because addition of EPTC retarded parent MG-191 metabolism 3 h after treatment, we assume that initial availability of the MG-191 molecule may be prerequisite to its protective effect.

The DNA sequence of an 83-base pair region of the acetolactate synthase (ALS) gene was compared for 10 chlorsulfuron-resistant (R) and three chlorsulfuron-susceptible (S) kochia biotypes. Point mutation in the codon for the proline residue at position 173 in Domain A of the ALS protein was observed in seven of 10 R biotypes. Among these seven R biotypes, mutation to threonine, serine, arginine, leucine, glutamine, and alanine was identified. The mechanism of resistance was determined for the R biotypes that did not have mutation in Domain A; all were resistant due to modified ALS, which indicated that at least one non-Domain A mutation site for resistance exists in kochia. Sequence results indicate that multiple mutations for resistance have occurred, and that geographically widespread ALS-inhibitor resistance in kochia is not the result of a single resistance allele.

The effect of soil moisture, temperature, and light intensity on the spray deposition of fenoxaprop and imazamethabenz applied to wild oat plants was examined by using fluorescent tracer dye. Based on either biomass or total leaf area, the apparent deposition of the two herbicides diminished in the following order: shading > low temperature ≥ drought ≥ “optimum” > high temperature. The enhanced phytotoxicity of both herbicides under shading could be associated with increased spray deposition; and reduced fenoxaprop phytotoxicity under high temperature stress could be related to reduced deposition. Changes in spray deposition were attributed mainly to differences in herbicide interception due to altered plant morphology. Reduced retention for both herbicides was exhibited only in the plants grown at high temperature. Under “optimum” conditions, fenoxaprop phytotoxicity was directly associated with leaf orientation and thus with the proportion of projected leaf area at the time of herbicide spraying. Given similar application conditions, spray deposition of fenoxaprop and imazamethabenz on wild oat could be estimated by determining the ratio of the projected leaf area, as measured by an image analyzer, to the total leaf area.

Greenhouse and laboratory experiments were conducted to determine the effect of DPX-PE350 on the herbicidal activity of fluazifop-P on large crabgrass. Greenhouse experiments indicated that fluazifop-P activity was antagonized by DPX-PE350. This antagonism was reduced by increasing fluazifop-P rates or by applying DPX-PE350 after fluazifop-P. Applying DPX-PE350 24 h prior to fluazifop-P reduced fluazifop-P activity more than applying it 10 min prior. The effect of DPX-PE350 application to large crabgrass on extractable acetyl-CoA carboxylase activity, and the effect of DPX-PE350 on acetyl-CoA carboxylase activity in vitro also was studied. Applying DPX-PE350 to large crabgrass prior to protein extraction did not reduce fluazifop-P inhibition of acetyl CoA carboxylase activity in vitro. The presence of DPX-PE350 in the enzyme assay mix did not affect fluazifop-P inhibition of acetyl-CoA carboxylase activity. The effect of DPX-PE350 on uptake, translocation, and metabolism of fluazifop-P was considered. DPX-PE350 did not affect uptake of fluazifop-P by large crabgrass. Applying DPX-PE350 24 h prior to fluazifop-P decreased translocation of fluazifop-P out of the treated leaf 44 to 53% whereas mixing DPX-PE350 with fluazifop-P decreased fluazifop-P translocation only 15 to 17%. Metabolism of fluazifop-P was not affected by applications of DPX-PE350. These studies suggest that the antagonism of fluazifop-P activity by DPX-PE350 is due to decreased translocation of fluazifop-P out of the treated leaf.

The inheritance of resistance to dicamba in wild mustard was determined by making reciprocal crosses between a resistant (R) population derived from a field treated repeatedly with auxin-type herbicides, and a known susceptible (S) population. The resulting F1 hybrids were selfed to produce F2 populations and backcrossed to the S parent. At the three- to four-leaf stage, parental, F1, F2, and backcross populations were screened for resistance to dicamba at three dosages (50, 200, and 400 g ai ha−1). F1 progeny survived all dosages and exhibited levels of injury similar to the R parental population. F2 populations segregated in a 3:1 ratio of R to S phenotypes. Progeny of backcrosses segregated in a 1:1 (R:S) ratio. Responses of the F1, F2, and backcross populations to treatment with dicamba indicate that resistance is determined by a single, completely dominant nuclear allele.

Copper-containing herbicides are often applied to control photoautotrophic populations to reduce biosynthesis of the earthy odorant geosmin. The objective of this investigation was to determine the effects of copper-containing herbicides on geosmin and biomass biosynthesis by heterotrophic cultures of the bacterium Streptomyces tendae and the fungus Penicillium expansum. Cultures of S. tendae treated with 12.7 mg Cu L−1 (copper sulfate) accumulated 44.6% more biomass and the mycelium contained five-fold greater concentrations of geosmin than controls. Additionally, P. expansum cultures exposed to 12.7 mg Cu L−1 (copper sulfate) accumulated 9.2% more biomass and 18-fold greater concentrations of geosmin than controls. Cultures of P. expansum and S. tendae, treated with copper chloride, copper-ethylenediamine complex, and copper chelate also accumulated more geosmin and biomass than controls. Inductively coupled plasma atomic emission spectrometry of S. tendae of untreated control cultures indicated that cellular copper concentrations were below 0.5 mg Cu kg−1. However, S. tendae cells grown on a medium containing 5 mg Cu L−1 copper sulfate contained 12.88 ± 0.54 mg Cu kg−1, suggesting that cellular accumulation of copper may be associated with increased geosmin biosynthesis. If field microbial populations exhibit similar responses to copper, copper-algicide application may contribute to geosmin-induced “off-flavors” under certain circumstances.

Studies were conducted of one of the structural factors that influences microroughness on johnsongrass leaves. Bicellular trichomes, 47 ± 5 μm long, represented 4 to 5% of all epidermal cells. They secreted a mucilagenous material that covered 8 ± 4% of the leaf surface. Bicellular trichomes occurred in longitudinal rows, intermixed with intercostal cork-silica cells, between rows of stomata. Numbers of bicellular trichomes present per unit area were inversely related to numbers of intercostal cork-silica cells. The trichomes were the panicoid type that are reported not to secrete salts. Johnsongrass trichomes, however, could be induced to discharge salt in the mucilage-type secretions when plants were grown in a soil mixture that was high in lime. Not all secretory constituents were identified, but carbohydrates and callose were found in addition to possible low concentrations of protein. The apical or cap cell of the trichomes stained positively for lipid, protein, and polysaccharide and negatively for pectin, polyphenols, steroids, and alkaloids. The presence of trichomes increases leaf surface microroughness, but the secretion covers wax crystals, decreasing leaf microroughness and likely providing another barrier to herbicide entry through the cuticle. Bicellular trichomes on grain sorghum were similar to those on johnsongrass and also discharged secretions on the leaf surface.

Using starch gel electrophoresis, we examined the genetic variability of kikuyugrass collected from three golf course sites within its geographical range in California. These experiments were conducted to determine the method of spread of kikuyugrass in areas where it is considered invasive. Samples from roughs and fairways of each of these locations were compared. of the 354 plants examined, 12 different genotypes were identified by isozyme analysis, and only three of the nine putative loci varied among the genotypes. Two genotypes, representing 73% of the plants examined, were found at all three geographic locations. Our results imply that under common golf course practices, kikuyugrass is maintained by clonal reproduction. We conclude that low genetic variation does not preclude a plant species from being an aggressive invader.

The impact of weed density and weed distribution on irrigated corn yield was investigated in Colorado. Weed densities examined were 0,33,50, or 100% of the indigenous weed population. A series of weed distribution treatments were achieved by varying the length of the weed-free and weedy zones within the corn row while maintaining a constant weed population of 33 or 50% of the indigenous weed level. Grain yield was affected by weed density, but not by weed distribution. Each additional weed reduced corn yield 8.5 and 2.3 kg ha−1 in 1991 and 1992, respectively. When corn yields were estimated with a computer weed/corn management model, weed densities 5 to 8 wk after planting provided a better yield reduction estimate than weed densities immediately before harvest.

Experiments were conducted to determine the influences of temperature, light, winter storage, and water stress on seed germination of fringed sage. Seeds collected in 3 yr in central Saskatchewan were placed in sealed vials and buried in the soil after harvest, and germination was tested in spring and early summer. Seeds germinated over a wide range of temperatures with alternating 25/15 C being optimal. The range of optimal temperatures was higher for older seeds than younger seeds. The stimulating effect of light on germination varied among collections and incubating temperatures. Total germination and germination rate was limited by water stress and no seeds germinated at osmotic potentials below −0.9 MPa. Seeds hydrated in the autumn and exposed to low winter temperatures had higher germination the following spring than dry seeds exposed to the same conditions. Results suggest that sufficient soil moisture combined with moderate seedbed temperatures are optimal for fringed sage germination. Periodicity of germination may be influenced by variable germination requirements in different aging seeds.

Field studies were conducted at Rosemount, MN, in 1992 and 1993 to quantify the demographic processes regulating the population dynamics of velvetleaf in soybean as part of a corn-soybean rotation. A consistent 6.8 ± 0.5% of the total velvetleaf seedbank emerged each year. Less than 21% of all velvetleaf seedlings survived each year in mixture with soybean, due in part to Verticillium spp wilt infection. The probability of seedling survival varied across time of emergence. Velvetleaf seed production in the absence of crop competition was 125 and 227 seeds plant−1 in 1992 and 1993, respectively. Velvetleaf plants that emerged early produced greater numbers of seed than later emerging plants. Velvetleaf survival and seed production were reduced up to 82% in the presence of crop competition. Soybean yield varied across soybean densities in both years, but was not reduced across velvetleaf densities.

Resistance to fenoxaprop-P and other aryloxyphenoxypropionate and cyclohexanedione herbicides in the wild oat population, UM1, is controlled by a single, partially dominant, nuclear gene. In arriving at this conclusion, parents, F1 hybrids, and F2 plants derived from reciprocal crosses between UM1 and a susceptible wild oat line, UM5, were treated with fenoxaprop-P over a wide range of dosages. Based on these experiments, a dosage of 400 g ai ha−1 fenoxaprop-P was selected to discriminate between three response types. At this dosage, susceptible plants were killed and resistant plants were unaffected, whereas plants characterized as intermediate in response were injured but recovered. Treated F2 plants segregated in a 1:2:1 (R, I, S) ratio, indicative of single nuclear gene inheritance. This was confirmed by selfing F2 plants and screening several F3 families. Families derived from intermediate F2 plants segregated for the three characteristic response types, whereas those derived from resistant F2 plants were uniformly resistant. Chisquare analysis indicated the F2 segregation ratios fit those expected for a single partially dominant nuclear gene system. In addition, F2 populations from both crosses were screened with a mixture of fenoxaprop-Pand sethoxydim. The dosages of both herbicides (150 g ai ha−1 fenoxaprop-P and 100 g ha−1 sethoxydim) were sufficient to control only susceptible plants. Treated F2 populations segregated in a 3:1 (R:S) pattern, thereby confirming that resistance to the two chemically unrelated herbicides results from the same gene alteration.

Field experiments were conducted to evaluate common cocklebur and entireleaf morningglory interference on soybean under irrigated and nonirrigated conditions. Total leaf area index (LAI), LAI within the soybean canopy, crop growth rate, and seed yield of soybean were decreased more by common cocklebur than by entireleaf morningglory. Interference from entireleaf morningglory, common cocklebur, or both species reduced soybean yields 21, 57, and 64%, respectively, with irrigation and 12, 60, and 76%, respectively, without irrigation. Soybean yield reduction from interference with entireleaf morningglory, common cocklebur, or both species was not influenced by soybean date of planting. Soil water was extracted from greater soil depths by soybean growing with the weeds than by soybean alone. High WUE without irrigation suggests that soybean uses water more efficiently when soil moisture is limiting than when soil moisture is available under irrigated conditions.

A model of competition for light between peanut and three broadleaf weed species has been developed to run with the PNUTGRO model. The model simulates shading of the peanut canopy by reducing the total daily PAR received by the peanuts in a manner that realistically represents timing and quantity of light capture by the weeds. Data were collected in nursery plots of Florida beggarweed, sicklepod, and wild poinsettia in 1989, 1990, and 1991. These data provided the values for the critical parameters: maximum attenuation of PAR by the weed, time when the weed overtops the peanut canopy, time when maximum attenuation is reached, and the distance of influence of the weed. Florida beggarweed overtopped the peanut canopy 52 DAP, and reduced PAR reaching the peanuts 45% by 73 DAP. Sicklepod overtopped the peanut canopy 42 DAP and reached an attenuation of 41% 79 DAP. Wild poinsettia overtopped the peanut canopy 44 DAP, and had an attenuation value of 39% 85 DAP. The distances of influence were 162, 150, and 192 cm for Florida beggarweed, sicklepod, and wild poinsettia, respectively. Observed yield losses in the distance of influence were 26, 27, and 22%, respectively. The model predictions accounted for at least 90% of the yield losses observed in field studies. The model also proved capable of simulating competitive differences between morphologically and phenologically different populations of Florida beggarweed. Simulation models will play an important role in reducing the expenditure of time and resources required to document yield losses due to weeds in peanuts.

The nonuniform spatial distribution of weeds complicates sampling, modeling, and management of weed populations. Principles of a rational approach to analysis of weed spatial distribution, combining classical and spatial statistics, are presented using data for cumulative emergence of common lambsquarters in no-tillage soybean fields in 1990 and 1993. Classical statistics, dispersion indices, mean/variance relationships, and frequency histograms confirmed that raw and loge-transformed data were not normally distributed, that populations were aggregated, and that large-scale trends in population means violated assumptions of spatial statistics. Detrending was accomplished by median polishing loge-transformed data and confirmed by evaluation of standardized residuals and frequency histograms. Detrended residuals were used to construct omni-directional and uni-directional semivariograms to describe the spatial structure of the populations. A spherical model fit to the data was verified by cross validation. Semivariograms showed that common lambsquarters density was spatially autocorrelated at distances to 16 m, with more than 30% of the variance in density due to distance between field locations. Comparisons of kriged estimates and their standard deviations with and without detrending indicated that estimates using detrended data were more appropriate and more precise. Kriged estimates of common lambsquarters density were used to draw contour maps of the populations.

A simulation model was developed to predict the population dynamics and economics of velvetleaf control in a corn-soybean rotation. Data compiled from the literature were used to parameterize the model for two situations, one in which velvetleaf was infected by a Verticillium spp. wilt and one without infection. Verticillium was assumed to have no effect on corn or soybean yield. In the absence of control, simulated seedbank densities of a Verticillium-infected velvetleaf population were 5 to 50 times lower than for an uninfected velvetleaf population. The model was used to evaluate a threshold weed management strategy under the assumption that velvetleaf was the only weed and bentazon the only herbicide available for its control. In the absence of Verticillium, an economic optimum threshold of 2.5 seedlings 100 m−2 afforded the highest economic returns after 20 yr of simulation. Simulations in which velvetleaf was infected in 8 out of 20 randomly assigned years indicated a 6% increase in annualized net return and an 11 % reduction in the number of years that control was necessary. Sensitivity analysis indicated the parameter estimates having the greatest impact on economic optimum threshold were seedling emergence and survival, maximum seed production, and herbicide efficacy. Under an economic optimum threshold of 2.5 seedlings 100 m−2, management practices that manipulate the most sensitive demographic processes increased annualized net return by up to 13% and reduced long-term herbicide use by up to 26%. Results demonstrate that combining an economic optimum threshold with alternative weed management strategies may increase economic return and reduce herbicide use.

Dry bean producers have limited postemergence broadleaf weed control options. The purpose of this research was to determine whether bentazon increased dry edible bean tolerance to postemergence imazethapyr applications. Imazethapyr and bentazon were applied with a petroleum oil adjuvant to ‘Olathe’ pinto bean in the field and greenhouse. Imazethapyr injured pinto bean 7 DAT in the field and greenhouse. Chlorophyll a content, a quantitative measure of bean chlorosis, decreased compared to the untreated control following imazethapyr application. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, bean injury decreased and chlorophyll a increased compared to imazethapyr alone. Fifty three g ha−1 of imazethapyr delayed physiological maturity by 8 and 15 d compared to the untreated control in 1991 and 1992, respectively. Pinto bean seed yields were not reduced compared to the untreated control. When 840 g ha−1 of bentazon was tank-mixed with 53 g ha−1 of imazethapyr, maturity was not delayed. 14C-Imazethapyr absorption decreased by more than 40% and translocation of 14C from the treated leaf decreased by more than 50% when 14C-imazethapyr was tank-mixed with bentazon compared to 14C-imazethapyr alone. The addition of 20 mM Na-acetate inhibited absorption of 14C-imazethapyr, but did not inhibit translocation of 14C. The decreased absorption and translocation of imazethapyr when tank-mixed with bentazon likely accounts for the safening effect observed in greenhouse and field studies.

Field studies were conducted over 3 yr in central Spain to investigate the tolerance of common vetch and oat to some preemergence herbicides and their effects on weed control, forage yields, and botanical composition of the forage mixture. Pendimethalin was the only herbicide that injured common vetch. Pronamide and pronamide plus diuron injured oat by affecting plant emergence. Prevalent weed species were fumitory, henbit, and wild buckwheat. All herbicides provided more than 90% control of fumitory and most herbicides except pronamide provided more than 90% control of henbit relative to the untreated check. Wild buckwheat stands were reduced by isoxaben (68%), linuron (40%), prometryn (69%), pronamide (86%), and pronamide plus diuron (61%). More than 90% control of prostrate knotweed was achieved with isoxaben, pronamide, and terbutryn. Pronamide and pronamide plus diuron reduced forage yields and increased vetch in the forage. The untreated vetch and oat monocrop treatments showed the competitive advantage of oat over vetch and weedy species.

Field experiments were conducted in 1991 and 1992 to evaluate differential soybean cultivar tolerance to SAN 582H and metolachlor applied PRE, as influenced by soil moisture. Eighteen d after planting, a 3× rate of SAN 582H (3.0 kg ai ha−1) reduced plant populations of ‘Wright,’ ‘Bedford,’ and ‘Stonewall’ and metolachlor did not. At leaf drop, neither herbicide reduced crop height with optimum moisture. With excessive moisture, 3× rates of SAN 582H reduced plant height of Wright, Bedford, ‘Walters,’ ‘Young,’ ‘Brim,’ and ‘Bryan,’ and metolachlor reduced height of ‘Sharkey.’ The greatest yield reductions were due to excessive moisture, regardless of herbicide. Neither herbicide reduced yields with optimum moisture nor did a 1× rate with excessive moisture. However, with excessive moisture, a 3× rate of metolachlor (5.0 kg ai ha−1) reduced yield of Bedford by 470 kg ha−1, and the 3× rate of SAN 582H reduced yield of ‘Rhodes,’ Walters, and ‘Centennial’ more than 500 kg ha−1. Overall, soybean tolerance to both herbicides was good, but if used above recommended rates and with excessive moisture, injury can occur.