Tank-mixed combinations of pyridate and 2,4-DB were synergistic with respect to sicklepod control and independent with respect to Florida beggarweed control and peanut tolerance in greenhouse studies. Leaf absorption of 14C-pyridate was generally enhanced by the addition of 2,4-DB. Translocation of 14C-pyridate out of the treated leaf did exceed 21% of the amount absorbed across all species, and 14C-pyridate was not affected by the addition of 2,4-DB. Sensitivity of peanut, Florida beggarweed, and sicklepod to pyridate was independent of both adsorption and translocation. Metabolic detoxification of pyridate into a benign metabolite was highest in the tolerant peanut, lowest in highly susceptible Florida beggarweed, and intermediate in slightly susceptible sicklepod. Absorption of 14C-2,4-DB by peanut was increased by approximately 10% with the addition of pyridate. No such increase was observed in either Florida beggarweed or sicklepod. Translocation of 14C-2,4-DB out of the treated leaf was not affected by the addition of pyridate in any of the three species evaluated.

Black nightshade was subjected to two degrees of water stress by adding polyethylene glycol 20,000 (PEG) to the nutrient solution 5 d before treatment with glyphosate. The ED50 values for glyphosate, determined from dose-response curves, demonstrated that both degrees of water stress strongly increased the ED50, with and without the surfactant Ethomeen T/25 in the spray solution. The surfactant reduced the ED50 5-, 4.6-, and 6.9-fold at 0, 15, and 20% PEG, respectively. A 14C study demonstrated that unstressed plants absorbed 22% of applied glyphosate. Without surfactant, water stress reduced foliar absorption 2.2-fold at 15% PEG and 4.5-fold at 20% PEG. With surfactant, the foliar absorption was 35% of the applied amount in unstressed and water-stressed plants. The surfactant and PEG reduced the translocation efficiency of glyphosate. The surfactant had the most pronounced influence and reduced the translocation efficiency 1.5-fold at 0% PEG, 2.2-fold at 15% PEG, and 1.8-fold at 20% PEG. Induction or removal of water stress 24 h after glyphosate treatment indicated that plant growth rate is positively correlated with glyphosate efficacy. It was concluded that the surfactant can overcome the adverse influence of water stress on foliar absorption of glyphosate but not the adverse, postapplication influence of water stress on glyphosate efficacy.

The noxious weed yellow starthistle is commonly controlled by the auxinic herbicide picloram. Induction of ethylene synthesis, epinasty, and reduction in shoot growth are typical symptoms of picloram treatment. Picloram did not induce ethylene evolution in the resistant accession RDW-1, though it caused a 250% increase in ethylene evolution in the susceptible wildtype SCI-1. The ethylene synthesis inhibitor aminoethoxyvinylglycine reduced the amount of ethylene induced by picloram in SCI-1 to control levels, but only reduced epinasty by 20% after 6 d. Aminoethoxyvinylglycine did not affect the reduction in shoot weight caused by picloram. The ethylene-releasing compound ethephon induced only a small amount of epinasty and had little effect on shoot weight in either accession. These results suggest that ethylene induced by picloram in wildrype plants plays only a minor role in the herbicidal effects of picloram. Furthermore, the resistance of the RDW-1 accession is not due to the lack of ethylene biosynthesis following picloram application to this accession.

Weed removal experiments in dry edible beans were conducted during 1992 and 1993 at Crookston and Staples, MN. Ten manual weed removal treatments were studied to determine when a natural infestation of weeds first reduced dry bean yield, and when weed removal could be discontinued without further loss of seed yield. Major weeds in order of average biomass production on weedy check plots at dry bean harvest over locations and years were wild mustard, foxtail spp., redroot pigweed, common ragweed, wild buckwheat, hairy nightshade, and common lambsquarters. Hairy nightshade also emerged late in the growing season and could negatively affect harvest efficiency and stain navy beans. Weed removal treatments had little effect on dry bean stands or 100-seed weights of harvested dry bean seed. The critical period for weed control in dry beans was 3 to 5 or 6 weeks after planting (WAP). Thus, weed control practices should begin no later than 3 WAP and continue until at least 5 or 6 WAP for maximum dry bean yields.

Replacement series studies were conducted under controlled conditions to determine the effect of phosphorus (P) rates and population densities on the competitiveness of smooth pigweed and common purslane with lettuce. Densities were 2, 4, and 8 plants per 113 cm2, whereas P rates were 0, 0.4, and 0.8 g PL−1 soil. A P-deficient Histosol (0.3 mg water-extractable P L−1 soil) was used. High P fertility enhanced the competitive ability of lettuce in smooth pigweed-lettuce mixtures. Smooth pigweed was not responsive to P rates. However, luxurious P consumption by smooth pigweed occurred, reducing the amount of the nutrient available for lettuce absorption. In common purslane-lettuce mixtures, the weed was responsive to P rates, increasing its competitive ability, whereas no increase in lettuce competitive ability was observed. Both weed species were more competitive than lettuce. Competition for P appears to be the main mechanism of common purslane interference on lettuce grown in low-P organic soils. Alternative fertilization strategies (i.e., banded applications) may reduce the effect of smooth pigweed on lettuce.

Jointed goatgrass is a major weed in the wheat-producing areas of the western U.S. It shares the D genome with wheat, and interspecific hybrids between the two species occur in the field. The objective of this research was to determine if wheat X jointed goatgrass hybrids could serve to transfer genes from wheat to jointed goatgrass. A backcrossing program was initiated in the greenhouse between wheat X jointed goatgrass hybrids and either jointed goatgrass or wheat to determine the potential for seed set and the restoration of self-fertility. Seed was set by backcrossing with either species as the recurrent parent. Female fertility increased from 2% in the hybrid to 37% in the BC2 plants with jointed goatgrass as the recurrent parent. Partial self-fertility was restored in the second backcross (BC2) generation using jointed goatgrass as the recurrent parent. This indicates that genes could be transferred between wheat and jointed goatgrass after only two backcrosses. The number of bivalents observed in the plants during meiosis appeared to be key to increasing female fertility and self-fertility. Based on the results of this study, it is possible for genes to move from wheat to jointed goatgrass. Any release of a herbicide-resistant wheat should be accompanied by a management plan that would minimize the potential for gene movement between these species.

Cultivated and weedy clones of yellow nutsedge were analyzed using random amplified polymorphic DNA (RAPD) markers to assess the polymorphism within the species and determine if this approach was suitable for identification of cultivar and wild populations. The RAPD markers unambiguously identified all studied clones. Nei-Li similarities were computed and used in an unweighted pair group method using arithmetic average (UPGMA) cluster analyses. Cultivated and weedy clones were clustered in two groups, but two cultivated clones were more closely related to weedy clones than to cultivated clones. The results showed a high level of genetic variability among the clones tested, particularly among the cultivated ones. Identification of yellow nutsedge cultivars and analysis of genetic diversity within and among weedy populations is possible by using only a small number of primers. In this study, seven selected primers discriminated among the 10 tested clones.

The germination behavior of giant foxtail and shattercane seed was investigated to determine whether environment in which maternal plants grow and progeny seed develop affects seed dormancy. Giant foxtail and shattercane seed was collected in 1985, 1991, and 1992. Plants from this seed were grown to maturity in the field and the greenhouse in 1993 and 1994. Experiments were conducted using mature progeny seed collected from these plants to determine the influence of environment of maternal growth on seed germination. Greenhouse-raised giant foxtail seed was more dormant than field-raised seed. Progeny of the 1991 giant foxtail collection had the most dormant seed, whereas progeny of the 1992 collection had the least dormant seed. Field-raised shattercane seed was more dormant than greenhouse-raised seed, but this dormancy was easily overcome by placing them in fluctuating temperatures. Moist stratification of giant foxtail seed increased its overall germination and reduced the differences in germination between individual collections and between field- and greenhouse-raised seed lots. Moist stratification of shattercane seed resulted in increased variability in germination within and among seed lots. Subjecting shattercane seed to freezing and thawing was most detrimental and caused an 89% reduction in germination due to seed death. These data indicate that before overwintering, giant foxtail seed differed in germination percentage, but overwintering reduced these differences. Shattercane seed produced in different environments had similar germination percentage. This suggests that successful overwintering depends on its ability to survive freezing and thawing conditions.

Understanding patterns of shattercane and giant foxtail field emergence during the spring and summer in the Midwest is important to determine the best weed management programs for crop production systems. The primary objective of this study was to determine percentage emergence of shattercane and giant foxtail under field conditions as influenced by soil temperature fluctuations, in 1992 and 1993. A secondary objective was to determine the difference in emergence percentage based on shattercane and giant foxtail seed lots harvested in different years (1985, 1991, and 1992). Soil temperature fluctuations were manipulated by artificially shading the soil surface with plastic netting that provided 30, 60, and 90% of incident sunlight in addition to the full sunlight control (0% shade). Giant foxtail and shattercane seed was planted at depths of 1.25, 2.5, 5, and 10 cm under each shade, and emergence was monitored for 45 d. Soil temperature fluctuations were reduced by increasing depth and amount of shade. Increasing depth of seed significantly reduced percentage emergence by 63 and 54% for shattercane, and 71 and 73% for giant foxtail in 1992 and 1993, respectively. Decreasing light intensity significantly reduced percentage emergence of shattercane by 72 and 73% in 1992 and 1993, respectively, but did not decrease giant foxtail emergence. Seed lots of both species were different from one another both years. These results were consistent over years, even though 1993 precipitation was three times that for 1992. Therefore, shattercane seed uses diurnal fluctuations in temperature to sense proximity to the soil surface and covering vegetation to avoid germination when conditions are unfavorable for growth. In contrast, giant foxtail seed can germinate readily and grow when environmental conditions are within an acceptable range and when seed is not buried too deeply. Differences in percentage emergence between seed lots could play a major role in emergence patterns of shattercane and giant foxtail observed in the field and may be influenced by environmental conditions during seed development on parent plants.

The efficacy of current wild oat herbicides and their high cost have resulted in use rates that are less than those recommended. While acceptable weed control may be attained at less cost, this practice does not consider the potential for increased wild oat seed production. The objective of this experiment was to determine the interaction of wild oat density and reduced imazamethabenz rates on wild oat seed production in spring barley. As wild oat densities increased from 8 to 1,100 plants m−2, wild oat seed production increased from 180 to 9,950 seed m−2 without herbicide, and from 0 to 2,810 seed m−2 using 0.53 kg ai ha−1 imazamethabenz. This general pattern was modeled using a cumulative logistic function. Estimates from this model indicated that < 1 wild oat seed m−2 was produced at population densities of ≤ 20 plants at any imazamethabenz rate. Imazamethabenz rates of 0.26 kg ha−1 or greater at wild oat densities of less than approximately 190 plants m−2 did not result in wild oat seed production above the initial population density. As wild oat density increased, however, imazamethabenz rates below 0.40 kg ha−1 resulted in substantially greater wild oat seed production compared to the recommended rate.

To understand the role of the seed coat in regulation of houndstongue seed dormancy, the effects of manipulation of seed coat integrity on seed germination and O2 uptake were studied. The results suggest that the seed coat of this weed regulates dormancy in part by interfering with the diffusion of O2 to the embryo. Scanning electron microscopy showed a network of ridges on the seed coat surface that were partially dissolved following 1.5 min of sulphuric acid scarification. Mechanical scarification removed fragments of the seed coat surface. Both scarification treatments stimulated seed germination. Supply of an elevated level of O2 also stimulated seed germination. O2 uptake by seeds imbibed in O2-saturated water was 150% higher than that for seeds imbibed in air-saturated water. Although all treatments that stimulated seed germination also stimulated O2 uptake, there was a lack of a consistent, quantitative relationship between increases in O2 uptake and seed germination in various experiments. This suggests that limitation of O2 availability to the embryo is not the only factor involved in regulation of houndstongue seed dormancy by the seed coat. Mechanical restriction of embryo expansion by the seed coat may also be important. Methanol-insoluble phenolics constituted < 1% of the total phenolic pool in the embryo. Their potential oxidation could not account for more than a small fraction of the previously reported massive stimulation of O2 uptake by the embryo upon decoating. The present O2 uptake and seed germination studies indicate that not all of the large increase in O2 uptake following decoating is essential for houndstongue seed germination.

The purpose of this study was to investigate the role of sunlight perceived by weed seeds during and after soil cultivation with moldboard and chisel plows on induction of seed germination. Daytime cultivation with a moldboard plow increased weed seedling emergence by as much as 200% above the levels recorded following nighttime cultivation; however, the magnitude of this promotion varied depending on season and field history, and occasionally it was nil. In contrast, when a chisel plow was used in primary tillage, no differences in seedling emergence were observed between daytime and nighttime cultivation. The absolute germination level in plots cultivated with a chisel plow was at least 200% greater than in plots cultivated with a moldboard plow. This observation suggests that microenvironmental factors other than light played an important role in promoting seed germination in chisel-plowed plots. Covering moldboard-plowed plots with opaque plastic immediately after cultivation did not reduce weed seedling emergence compared to plots that were either covered with transparent film or left uncovered. Our results suggest that only the light stimulus perceived by the seeds during soil cultivation is effective in triggering germination.

Field research was conducted to characterize hemp dogbane crown root carbohydrates, to quantify crown root lipids, and to determine seasonal fluctuation of each. The effect of day length on the release of hemp dogbane crown bud dormancy was studied in growth chambers. Total nonstructural carbohydrate (TNC) levels fluctuated with seasonal lows occurring during bud to mid-flowering and seasonal highs in fall predormancy periods. Starch was the primary storage carbohydrate, comprising up to 39.6% of crown root dry weight at the highest seasonal level. Ethanol soluble reducing sugar levels were negatively correlated with seasonal starch levels, and appeared to be maltose and glucose accumulated before conversion to sucrose for translocation. Most lipids were in ethanol soluble fractions and were inversely correlated with seasonal TNC patterns. The maximum level of total lipids in crown roots was 6%. Lipids appeared to be by-products of metabolism in nonarticulated lacticifers and not a significant carbon energy source. An 18-h day length resulted in a 10-fold increase in intact, 2-yr-old crown root dormancy release compared with 10- or 14-h day lengths in growth chamber studies. Seasonal carbohydrate fluctuations in hemp dogbane suggest the most effective control with mowing or tillage would occur when applied at mid- to full flower before root carbohydrates begin to recover. The most effective translocation of phloem mobile herbicides to crown roots would occur after mid- to late flower through leaf senescence when carbohydrates are being stored in the roots.

Field experiments were conducted at five locations in Colorado, Kansas, and Wyoming in 1994–1995 and 1995–1996 to compare the effects of MON 37500 rate and application timings on downy brome control and winter wheat tolerance. MON 37500 at 18 to 35 g ha−1 applied preemergence or fall postemergence reduced downy brome density 71 to 92% in 1995. Spring-applied MON 37500 suppressed downy brome growth but did not reduce plant density. No application reduced downy brome density in 1996. At each location, downy brome was controlled best by MON 37500 applied preemergence or fall postemergence at 35 g ha−1. MON 37500 did not affect wheat height at Archer or Torrington, WY, and Burlington or Stratton, CO, but wheat treated preemergence or fall postemergence was taller than untreated wheat at Hays, KS, in 1995. Spring-postemergence-treated wheat at Hays in 1995 was shorter than untreated, preemergence-, or fall-postemergence-treated wheat. Wheat head density did not differ among treated and untreated plots at Torrington, but herbicide treatment increased wheat yields. Wheat head density increased with all MON 37500 treatments at Hays in 1995, as did yield. However, preemergence and fall-postemergence applications resulted in the highest wheat yields. No herbicide treatment affected head density or yield at Hays in 1996.

Experiments were conducted in 1993 and 1994 to determine if seed treated with disulfoton at 3, 6, or 12 g ai kg−1 reduced bleaching of rice foliage caused by clomazone applied delayed preemergence at 0.56, 1.1, 1.7, or 2.2 kg ai ha−1 on silt loam and silty clay soils. Clomazone injured rice more on silt loam soil than on silty clay soil. Disulfoton reduced bleaching caused by clomazone. In additional field experiments conducted during 1995 and 1996, clomazone at rates as high as 2.2 kg ha−1 did not affect grain yield in seven of 10 experiments. Clomazone at rates below 1.7 kg ha−1 did not reduce grain yield in any of the experiments.

Atrazine is the most widely used herbicide in the U.S. and has been detected in surface water and groundwater. Technologies are needed for onsite and in situ remediation of water and soil containing atrazine. We investigated the potential of using fine-grained, zero-valent iron (Fe0) to remove atrazine and promote its degradation in contaminated water and soil. Atrazine loss from aqueous solution increased with increasing Fe0 concentration (w/v). Agitating 20 μg 14C-ring-labeled atrazine L−1 with 10% Fe0 (w/v) removed 92% of the 14C from solution within 48 h. Only about 4% of the 14C lost from solution was extractable from the iron with 3 mM CaCl2 (readily available pool), 81% was extractable with CH3CN (potentially available pool), and 11% was unextractable residues. A companion experiment indicated that most of the 14C extracted from the iron with 3 mM CaCl2 after the 48-h Fe0 treatment was unaltered atrazine, while the CH3CN extract contained approximately 33% atrazine and 48% was unidentified atrazine transformation products. Treating a highly contaminated solution (20 mg atrazine L−1) with 20% Fe0 (w/v) removed 88% of the 14C (added as 14C-ring-labeled atrazine) from solution within 48 h. Deethylatrazine was the main atrazine transformation product detected in solution after treatment, but small amounts of deisopropylatrazine, didealkylatrazine, and hydroxyatrazine were also found. Treating Sharpsburg surface soil containing 1 mg atrazine kg−1 with Fe0 (2%, w/w) increased atrazine mineralization from 4.1 to 11.2% after 120 d. Pyrite (4% FeS2, w/w) also increased atrazine mineralization in surface soil, but was less effective in the presence of NO3 − or SO4 2− (100 mg kg−1 soil). Adding 2% Fe0 (w/w) and 100 mg NO3 − kg−1 to contaminated subsurface soil increased atrazine mineralization from 0.4 to 8.2% within 120 d, and unextractable residues increased from 4.6 to 9.8%. These results indicate iron can sorb atrazine and promote its transformation in water and soil.