A cooperative study to evaluate the accuracy and precision of bioassays was carried out among investigators at several locations for 3 years. Oats (Avena sativa L., var. Cimarron) seedlings were used as indicator plants for soil-applied 2,4-bis-(isopropylamino)-6-methylthio-s-triazine [prometryne]. Assay results varied a great deal initially, and ranged from 147% below to 234% above the prometryne actually applied. The adoption of uniform conditions and procedures greatly increased the uniformity of the determinations between locations to a range of from 37% low to 0% high in prometryne applied. Chemical analyses of the soils ranged from 13% low to 20% high. Measurements of herbicidal activity included dry and fresh plant weights, plant height, visual injury ratings, and plant water use. The average concentration of prometryne in the soil as estimated with several types of measurements gave a much better measure of prometryne content than any one measurement alone. Prometryne rapidly and greatly affected the utilization of water by oats seedlings and careful measurement of water use showed potential of being a good early indicator of prometryne activity.

Perennial pepperweed (Lepidium latifolium L. # LEPLA) is a serious weed of meadows and riparian habitats in the intermountain area of the western United States. Germination of seeds of this species was tested at 55 constant- and alternating-temperature regimes. No significant differences were found in mean germination at a range of temperatures varying from 0 to 40 C for seedlots collected at the same site in different years or at different locations in the same year. Optimum germination not significantly (P = 0.01) lower than the maximum observed occurred most frequently at 5/40 C (5 C for 16 h and 40 C for 8 h), and 10/40 and 15/40 C. Germination was enhanced by alternating regimes with low night temperatures (0, 2, or 5 C) and high day temperatures (35 and 40 C), in comparison with more moderate constant-temperature regimes.

Common groundsel is an important weed of strawberry and other horticultural crops. Few herbicides are registered for common groundsel control in such crops, and understanding and predicting the timing and extent of common groundsel emergence might facilitate its management. We developed simple emergence models on the basis of soil thermal time and soil hydrothermal time and validate them with the use of field-derived data from Minnesota and Ohio. Soil thermal time did not predict the timing and extent of seedling emergence as well as hydrothermal time. Soil hydrothermal time, adjusted for shading effects caused by straw mulch in strawberry, greatly improved the accuracy of seedling emergence predictions. Although common groundsel generally emerges from sites at or near the soil surface, the hydrothermal model better predicts emergence when using hydrothermal time at 5 cm rather than 0.005 cm, probably because of the volatility of soil temperature and water potential near the soil surface.

Imazamox is an imidazolinone herbicide used to control many grasses and broadleaf weeds in leguminous crops such as soybean, alfalfa, and dry bean; however, imazamox cannot be used on red lentil due to unacceptable injury. Studies were conducted to compare imazamox absorption, translocation, and metabolism in red lentil and dry bean to determine if any or all of these factors contributed to differential crop sensitivity. Radiolabeled imazamox was applied to three young red lentil leaves and the youngest, fully expanded dry bean trifoliolate leaf. Absorption, translocation, and metabolism were followed over a 96-h time course. Red lentil had more rapid absorption compared to dry beans with 64 and 54% of the applied dose absorbed 12 h after treatment (HAT), respectively. Maximum absorption was also greater in red lentil than dry bean, 79 and 61%, respectively. Translocation out of the treated leaf was significantly higher in red lentil compared with dry bean, 16 and 0.5%, respectively, at 96 HAT. Translocation was greater to red lentil roots compared to shoots, 9 and 7%, respectively, at 96 HAT. In dry bean only 14% of applied 14C-imazamox remained intact 24 HAT, while 79% of the radioactivity was imazamox in red lentil 24 HAT. Both species metabolized the herbicide to more polar metabolites. The inherent sensitivity of aceolactate synthase (ALS) from dry bean and red lentil was also evaluated. ALS from both species had similar I50 values for imazamox, 7.2 and 8.2 µM, respectively. The combined effects of increased imazamox absorption and reduced imazamox metabolism are the major contributors to differential selectivity between dry bean and red lentil. Rapid imazamox metabolism in dry bean significantly limited herbicide translocation out of the treated leaf.

In the Midwestern United States, winter hilling, consisting of two tillage activities per year, is required in vinifera-grape vineyards for winter protection. However, this practice often leads to severe soil erosion and pesticide offsite movement. The effectiveness of wheat straw mulch as a replacement for soil mounding was investigated as a way of providing winter protection and to mitigate pesticide leaching and runoff. A laboratory experiment was conducted where simazine was applied to wheat straw or bare soil and then followed by simulated rainfalls. When compared with bare soil, straw reduced simazine leaching and runoff by 40 and 68%, respectively. Adsorption or interception, or both, of simazine by straw were responsible for this effect. Additionally, straw reduced soil erosion by 95% and would largely reduce simazine runoff associated with sediment displacement. The first simulated rainfall contributed 70 and 34% of total simazine runoff from bare soil and straw, respectively. In conclusion, mulching with straw during winter months to provide winter protection could be an effective practice for controlling simazine offsite movement and soil erosion in vinifera vineyards.

The relative competitive ability of common lambsquarters and giant foxtail in mixed weed–corn communities was characterized in 1998 and 1999 using empirical models that described late-season weed biomass on the basis of weed density, early-season relative leaf area, or early-season relative shoot volume. Competition coefficients estimated from weed density were inconsistent between years because they indicated that giant foxtail was more competitive than common lambsquarters in 1998 but that common lambsquarters was more competitive than giant foxtail in 1999. In contrast, the competition coefficients based on relative leaf area and relative volume were consistent between years. Competition coefficients estimated from relative leaf area indicated that giant foxtail was more competitive than common lambsquarters in each year. Competition coefficients estimated from weed relative volume indicated that the relative competitive ability of each weed species was similar in each year. Weed relative competitive abilities were characterized further by describing the mechanisms of competition related to shoot height and width growth. Giant foxtail was taller than common lambsquarters shortly after emergence each year, but plasticity of common lambsquarters growth resulted in reduced height differential between the weed species over time. Even so, giant foxtail was taller than common lambsquarters at physiological maturity each year. Coefficients that described the ability of each weed species to crowd neighbors indicated that giant foxtail shoot width was affected more by increased common lambsquarters density and proportion than was common lambsquarters shoot width by giant foxtail. The greater ability of common lambsquarters to crowd neighbors relative to giant foxtail was attributed to the greater leaf area density (LAD) of common lambsquarters compared with that of giant foxtail. Although characterization of shoot height, width, LAD, and biomass elucidated in part the mechanisms of competition between these species, models that accounted for differences in early-season relative plant size were consistent between years, indicating that giant foxtail was equally or more competitive than common lambsquarters in corn.

Mefluidide {N-[2,4-dimethyl-5-[[(trifluoromethyl) sulfonyl]amino] phenyl] acetamide} at concentrations of 2.9 × 10-4 M and above retarded the growth of corn (Zea mays L. ‘Pioneer 3369A’) grown in nutrient culture. Dwarfing was accompanied by some plant distortion and chlorosis of the bases of those leaves formed soon after treatment. The chlorosis was associated with decreased chlorophyll a content. Chlorophyll b content was not affected. The growth-retardant effects of mefluidide could not be reversed by applications of gibberellic acid. Using cucumber (Cucumis sativus L. ‘Ashley’) cotyledon discs, a low level of mefluidide (2.9 × 10-5 M) increased the incorporation of 14C from leucine into protein, while a high concentration (2.9 × 10-4 M) reduced incorporation. Neither concentration affected the absorption of 14C-leucine.

A field study was established in southern Iowa in 1994 to study seasonal and long-term weed population dynamics on land being brought back into production after 8 yr as part of the conservation reserve program (CRP). The study was a split-plot design with four replications; two tillage regimes, two crop rotations, and three herbicide application methods were used. Even though the tillage regime did not influence individual weed population density throughout the study, the no-till (NT) regime had more weeds compared to conventional tillage (CT). However, when weeds were grouped into categories, tillage influenced broadleaf weeds in 1994 and 1996 and total weeds in 1995. Plots under the NT regime had an average of 46 broadleaf weeds m−2 compared to 27 in CT in 1994, with Amaranthus rudis Sauer (common waterhemp) being the most prevalent. NT had a total of 186 weeds m−2 compared to 125 m−2 weeds in CT in 1995; however, in 1996, CT plots had 184 weeds m−2 compared to 121 m−2 in the NT regime. Except for broadleaf weeds in 1994, crop rotation did not influence the number of weeds, and herbicide application methods had the greatest effect on weed populations. Overall, weed populations were greater in 1997, 1996, and 1995 than in 1994 for all herbicide application methods. The no-herbicide treatment had the highest number of weeds throughout the study. The total number of weeds in band and broadcast treatments averaged 41 and 26 m−2 in 1994; 96 and 24 m−2 in 1995; 96 and 12 m−2 in 1996; and 109 and 95 m−2 in 1997. The use of broadcast herbicides in NT should be recommended for land coming out of CRP. Regardless of the herbicide application method or crop rotation, CT plots had better yields for both Zea mays L. (corn) and Glycine max L. (soybean). Glycine max had a better stand compared to Z. mays in the first year, indicating that a rotation starting with G. max might be preferred in the land coming out of CRP.

Integration of preplant-incorporated molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) at 4.5 kg ai/ha and a brief postseeding drainage period in water-seeded rice (Oryza sativa L.) gave the best red rice (Oryza sativa L. # ORYSA) control without a significant reduction in cultivar stand density. This combination of practices provided higher cultivar grain yield and lower red rice grain yield than any water management practice alone. Continuous-flood water management provided a substantial reduction in red rice stand density, but it also caused a significant reduction in the cultivar stand density. Under the reduced stand density conditions, the red rice was very competitive and yielded more than the cultivar. Although brief drainage alone gave a significantly lower level of red rice control, it had no significant effect on the cultivar stand density, and the red rice and cultivar yields were not significantly different from the continuous-flood yields. Prolonged drainage had the highest red rice and cultivar stand densities and the highest red rice grain yield, but the cultivar yield was not significantly different from the other plots that were not treated with herbicide. Although this treatment gave the highest total grain yield, its composition had the highest percentage of red rice. Puddling the seedbed did not alter the stands or yields of red rice or cultivar compared to the nonpuddled plots with the same water management and herbicide rate. The application of molinate as a preplant-incorporated treatment or as a preplant postflood treatment before or after puddling gave results that were not significantly different from each other. The superior performance of preplant application of molinate in water-seeded rice in conjunction with a brief postseeding drainage period provides an excellent tool for red rice suppresion.

Granules of isoxaben plus trifluralin and liquid applications of isoxaben plus oryzalin were applied to miniature nursery plots covered with plastic, woven landscape fabric, or gravel with no cover. Herbicide concentrations were monitored in runoff water over a 30 d period. Runoff losses of isoxaben from the sprayable formulation applied to gravel were 5% greater than from plastic and 4% greater than losses from fabric in 1992. Similarly, oryzalin losses from the sprayable formulation applied to gravel were 7% greater than losses from plastic and 4% greater than losses from the fabric ground cover. In contrast, loss of isoxaben from the granular formulation applied to plastic was 7% greater than loss from gravel in 1993. Isoxaben losses from the granular formulation applied to fabric were intermediate. Trifluralin losses from the granular formulation applied to plastic and fabric were both 3% greater than losses from gravel. In addition, isoxaben losses from the granular formulation were 11 and 10% greater than loss from the sprayable formulation applied to the plastic and fabric-covered plots, respectively. Isoxaben losses from the sprayable formulation applied to gravel were 11% greater than losses from the granular formulation. In an experiment to determine herbicide release patterns and the effect of light on residues from the granular formulation of isoxaben and trifluralin in irrigation effluent, water was monitored for 36 d. Approximately 20% of the applied isoxaben and 7% of the applied trifluralin was detected in irrigation water during the 36 d period. These studies indicate that runoff losses and the ultimate fate of isoxaben, oryzalin, and trifluralin applied in nursery settings depend on factors including ground cover composition, herbicide formulation, and photochemical degradation.

Rose rosette disease, lethal to multiflora rose and indigenous to North America, has been proposed as a biocontrol agent for multiflora rose, a noxious weed in the central and eastern United States. Studies in experimental plots showed that the disease can be intensified by grafting infected shoots onto plants in established stands (i.e., augmentation). The rate of disease spread in augmented plots was significantly faster compared to epidemics in nonaugmented plots at 5 locations. Augmentation provided effective control 3 to 5 yr after implementation. Risk to ornamental rose seems to be low under conditions of this study. Plots to assess risk of the disease to ornamental roses, located at distances greater than 100 m from augmentation sites, showed no infection during the 3 yr of this study.

A mixed microbial culture able to transform alachlor at a concentration of 50 μg ml-1 was obtained from alachlor-treated soil after an enrichment period of 84 days. The microbial community was composed of seven strains of bacteria. No single isolate was able to utilize alachlor as a sole source of carbon. There was no alachlor left in the enriched culture after a 14-day incubation, but only 12% of the 14C-ring-labeled alachlor was converted to 14CO2 through ring cleavage during 14 days in the basal medium amended with alachlor as a sole carbon source. The presence of sucrose as an alternative carbon source decreased the mineralization potential of the enriched culture, but sucrose increased the mineralizing ability of a three-member mixed culture. Thin-layer chromatographic analysis showed that there were four unidentified metabolites of alachlor produced by the enriched culture. Sucrose decreased the amount of two of the four metabolites. The absence of a noticeable decline in radioactivity beyond the initial 12% suggested that the side chain of alachlor was utilized as carbon source by the enriched culture. Little difference in radioactivity between growth medium and cell-free supernatant of the growth medium suggested that the carbon in the ring was not incorporated into the cells of the degrading microorganisms.

Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)one] provided adequate preemergence weed control on direct-seeded or transplanted tomato (Lycopersicon esculentum Mill.) on Plainfield sand and Drummer silty clay loam soils. Metribuzin demonstrated selective postemergence activity in the field but in glasshouse studies tomato cultivars had a wide range of tolerance to metribuzin applied to the foliage. Tomatoes were more susceptible to injury when they were less than 10-cm tall. Metribuzin was more effective in controlling broadleaf weeds than grass weeds, but grass control was acceptable with higher rates. Ultraviolet light decomposed metribuzin and weed control was improved when metribuzin was incorporated into the soil after application. Metribuzin treatment increased fruit yields. No metribuzin activity was found in the plots by the end of the growing season.

Field experiments were conducted to determine the effectiveness of the herbicide antidote R-25788 (N,N-diallyl-2,2-dichloroacetamide) in protecting corn (Zea mays L.) cultivars from EPTC (S-ethyl dipropylthiocarbamate) injury. Preplant-incorporated applications of EPTC, with and without R-25788, were made to a fine sandy loam in 1976 and a loamy sand soil in 1977. EPTC at 6.7 kg/ha was 20 to 30% more injurious to the susceptible cultivars grown in sandy, lower organic-matter soil than in fine sandy loam. R-25788 did not decrease the weed-control efficacy of EPTC. The cultivar ‘TXS114’ was highly tolerant to EPTC at 6.7 kg/ha with or without antidote protection. Most of the other EPTC-susceptible cultivars showed significant stand improvement when R-25788 was used in combination with EPTC. R-25788 did not alleviate EPTC phytotoxicity toward ‘XL55’ and ‘XL379’ cultivars. EPTC applied in combination with R-25788 showed no, or only, slight injury to all other cultivars.

Field experiments were conducted in 1999 and 2000 to determine if (1) seed predation of redroot pigweed plants occurred in agricultural fields, (2) corn-cropping patterns could be manipulated to influence the quantity of weed seed predation, and (3) alterations in corn canopies affected the microenvironment, possibly influencing predator populations. Corn was planted in standard (75 cm) or narrow (37.5 cm) rows with corn population densities ranging from low to very high (2.5 to 10 plants m−2). The extent of seed predation occurring on terminal weed inflorescences in the treatments was evaluated. Predation levels of redroot pigweed were highly variable spatially and temporally. Coleophora lineapulvella Chambers (Lepidoptera: Coleophoridae) was the dominant predator of redroot pigweed seed. Seed predation was higher in 2000 than in 1999 (P < 0.05). On average, C. lineapulvella larvae attacked 11% of the inflorescences in 2000 and 3% of inflorescences in 1999. The proportion of damaged seeds per attacked inflorescence was as high as 93% in 2000 but only 42% in 1999. Row spacing and corn density did not affect levels of weed seed predation (P > 0.05). But canopies of closely spaced corn increased shading to redroot pigweed plants growing below the canopy, consequently decreasing total weed biomass and seed production.

Asiatic dayflower has recently become a troublesome weed in eastern Iowa. This weed demonstrates an extended emergence period and there is anecdotal evidence of glyphosate tolerance. Thus, Asiatic dayflower is difficult to manage in glyphosate-resistant (GR) corn and soybean. Greenhouse experiments were conducted to evaluate the response of Asiatic dayflower to glyphosate applied at different rates and growth stages. Field research was conducted in 2005 and 2006 to evaluate different herbicides for Asiatic dayflower control in soybean. PRE herbicides were applied at planting and POST herbicides were applied 21 and 42 d after planting (DAP). In addition, shikimate accumulation in response to glyphosate was compared among Asiatic dayflower and GR and non-GR corn and soybean. Under greenhouse conditions, a single application of glyphosate (0.84 kg ae ha−1) did not control Asiatic dayflower. Only the highest rate evaluated, 13.44 kg ae ha−1 (16X), was lethal to Asiatic dayflower. Even when applied at an early growth stage (two leaves) and using high rates (3.36 kg ae ha−1), glyphosate controlled Asiatic dayflower just 28%. In the field, metribuzin and KIH-485 controlled Asiatic dayflower 80 and 73%, respectively. Early POST applications (21 DAP) of cloransulam or lactofen controlled Asiatic dayflower 80 and 67%, respectively. A single glyphosate application of 0.86 kg ae ha−1 controlled Asiatic dayflower approximately 50%. Glyphosate-treated Asiatic dayflower and non-GR corn and soybeans accumulated shikimate after application. GR corn and soybeans did not accumulate shikimate in response to glyphosate. Twenty-one days after treatment, all the non-GR soybean and corn plants died; however, Asiatic dayflower plants survived.

Uptake and translocation of 6,7-dihydrodipyrido[1,2-a:2′, 1′-c]pyrazidiinium salt (diquat) in elodea (Elodea canadensis Michx) was investigated by autoradiography and counting of plants treated with diquat-14C. Uptake initially was very rapid followed by a slower accumulation of diquat. Part of this uptake was reversible in water and rapidly reached a saturation value while the slower uptake was nonreversible. This reversibility of uptake appears to indicate an initial passive adsorption probably followed by accumulation. Slight basipetal and acropetal movement of diquat was demonstrated with accumulation in the nodes and axillary buds. Movement was unaffected by light versus dark treatments. Diquat injured treated and adjacent untreated shoots of elodea to almost the same extent. This transfer occurred whether or not the treated and untreated shoots were in contact.

A segment of the debate surrounding the commercialization of genetically engineered (GE) crops, such as glyphosate-resistant (GR) crops, focuses on the theory that implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was conducted in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR corn, cotton, or soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on emerged weed density and diversity was analyzed. Species richness, evenness, Shannon's H′, proportion of forbs, erect growth habit, and C3 species diversity were all greater in agricultural sites that lacked crop rotation or were in a continuous GR crop system. Rotating between two GR crops (e.g., corn and soybean) or rotating to a non-GR crop resulted in less weed diversity than a continuous GR crop. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. The finding that fields with continuous GR crops demonstrated greater weed diversity is contrary to arguments opposing the use of GE crops. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits, or more broadly, GE crops, to provide a more complete characterization of their culture and local adaptation.

The herbicidal action of sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} on corn (Zea mays L. ‘Goldencrossbantam’) was investigated in preemergence and postemergence experiments and hydroponic culture. Soil-applied sethoxydim did not inhibit corn germination. Leaves failed to emerge through the coleoptile with a high herbicide rate (1.6 kg ai/ha), but chlorotic leaves emerged at a lower rate (0.8 kg ai/ha). Growth of corn seedlings treated with a foliar application of 0.2 kg ai/ha was inhibited within 1 day after treatment, but at a lower application rate (0.05 kg ai/ha) growth continued with chlorotic zones on newly expanding leaves. Over the concentration range 3 × 10-5 to 1 × 10-6 M, sethoxydim inhibited the growth of primary roots of corn in hydroponic culture within 24 h. Cytological investigation showed that sethoxydim inhibited cell division but did not interfere with mitosis.

Greenhouse and field studies showed that α,α,α,-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) gave better control of wild cane [Sorghum bicolor (L.) Moench] when incorporated in the soil above rather than below the wild cane seed. Of four herbicides studied in field experiments, trifluralin gave the best control of wild cane in soybeans (Glycine max L., var. Clark) followed by S-propyl dipropylthiocarbamate (vernolate). Herbicides were more effective than cultivation in controlling wild cane, but neither alone was adequate. A combination of cultivation plus herbicides markedly increased soybean yields on wild cane infested areas. These results show that an economical crop of soybeans can be produced on wild cane infested land.