Greenhouse and field studies were conducted to determine the effect of the isooctyl ester of MCPA on the activity of the ethyl ester of fenoxaprop in four grass species. In the greenhouse, wheat tolerated fenoxaprop better than barley, oat, and wild oat. Among wild oat populations, SH 430 was the most tolerant to fenoxaprop followed by MON 73, CS 40, the natural population, and AN 51. MCPA reduced fenoxaprop activity in wheat and barley, protecting the crops from herbicide injury. However, MCPA did not reduce the activity of fenoxaprop in most wild oat lines. In field studies, wheat and barley treated with fenoxaprop at 150 g/ha plus MCPA at 300 g/ha were not appreciably different from weed-free controls while fenoxaprop applied alone at 150 g/ha damaged the crops. Fenoxaprop applied at 150 g/ha in combination with 300 g/ha MCPA resulted in at least 78% wild oat control.

Laboratory experiments were conducted to determine the physiological mechanism of tall morningglory resistance to the experimental cotton herbicide DPX-PE350. Tall morningglory, a resistant species, was compared with entireleaf morningglory, a sensitive species, to evaluate inhibition at the site of action, the acetolactate synthase (ALS) enzyme (E.C.4.1.3.18), by DPX-PE350 as well as uptake, translocation, and metabolism of DPX-PE350. No differences were found between species in the concentration required to inhibit the ALS enzyme by 50% (I50), or in uptake and translocation of the herbicide. Tall morningglory metabolized the herbicide more rapidly than did entireleaf morningglory. Tall morningglory contained 3.6 and 1.4 times more metabolites of DPX-PE350 than did entireleaf morningglory 6 and 24 h after treatment, respectively. Tall morningglory produced an O-desmethyl metabolite from the 3,5-dimethoxypyrimidine moiety of DPX PE350 that was not found in entireleaf morningglory. These data suggest that the ability of tall morningglory to more rapidly metabolize DPX-PE350, possibly through the production of the pyrimidinyldesmethyl metabolite, may be the mechanism of resistance to DPX-PE350.

Low pressure and reduced pressure fan and reduced pressure hollow cone nozzles were compared to conventional fan and hollow cone nozzles to determine their potential for reducing spray drift. Other comparisons included two different spray pressures for a low pressure fan nozzle, low and reduced pressure fan nozzles versus air-aspirating foam nozzles, low pressure versus reduced pressure fan nozzles, and reduced pressure hollow cone nozzles versus reduced pressure fan nozzles. Spray recoveries within 36 m downwind of the spray release point were significantly greater for the low pressure and reduced pressure fan and reduced pressure hollow cone nozzles than for the conventional fan and hollow cone nozzles. An air-aspirating nozzle producing spray without foaming adjuvant resulted in significantly greater spray recovery than a low pressure fan nozzle. The recovery from a reduced pressure fan nozzle having a rated flow of 12.6 cm3/s was significantly greater than that from a low pressure fan nozzle.

Trifluralin was evaluated at 1.1, 2.2, and 4.5 kg/ha in 1983 and 1984 at two locations in Iowa for residue carryover injury to corn the following seasons. Three methods of seedbed preparation (no-till, moldboard, and chisel plowing) for corn planting were also examined. There was no effect on corn growth at the 1.1 kg/ha rate of trifluralin. Averaged over the four experiments, reductions in corn height of 8 and 24% were observed 5 weeks after planting at 2.2 and 4.5 kg/ha, respectively. The relative degree of stunting due to trifluralin decreased as the growing season progressed. Early-season carryover injury was more severe in reduced tillage than in moldboard plow treatments in the 1983-84 Nashua experiment. Moldboard and chisel plowing reduced the concentration of trifluralin in the 0- to 7.5-cm zone of the soil profile by 62 and 31%, respectively, when compared to no-till. No yield reductions were observed at the 1.1 or 2.2 kg/ha rate of trifluralin. In 1984, grain yields were reduced by 8 and 16% at Ames and Nashua, respectively, by the 4.5 kg/ha trifluralin rate.

Weed distribution maps can be developed from remotely sensed reflectance data if collected at appropriate times during the growing season. The research objectives were to determine if and when reflectance could be used to distinguish between weed-free and weed-infested (mixed species) areas in soybean and to determine the most useful wavebands to separate crop, weed, and soil reflectance differences. Treatments included no vegetation (bare soil), weed-free soybean, and weed-infested soybean and, in 1 yr, 80% corn residue cover. Reflectance was measured at several sampling times from May through September in 2001 and 2002 using a handheld multispectral radiometer equipped with band-limited optical interference filters (460 to 1,650 nm). The spatial resolution was 0.8 m2. The reflectance in the visible spectral range (460 to 700 nm) generally was similar among treatments. In the near-infrared (NIR) range (> 700 to 1,650 nm), differences among treatments were observed from soybean growth stage V-3 (about 4 wk after planting) until mid-July to early August depending on crop vigor and canopy closure (76-cm row spacing in 2001 and 19-cm row spacing in 2002). Reflectance rankings in the NIR range when treatments could be differentiated were consistent between years and, from lowest to highest reflectance, were soil < weed-free < weed-infested areas. Increased reflectance from weed-infested areas was most likely due to increased biomass and canopy cover. Residue masked differences between weed-free and weed-infested areas during the early stages of growth due to high reflectance from the residue and reduced weed numbers in these areas. These results suggest that NIR spectral reflectance collected before canopy closure can be used to distinguish weed-infested from weed-free areas.

A Shubata soil was modified by the addition of organic matter or montmorillonite clay. Three annual applications of fluometuron [1,1-dimethyl-3-(α,α,α-trifluro-m-tolyl)urea], prometryne [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine], and trifluralin (α,α,α-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine) were made and soil samples analyzed at the end of the third year. Less than 0.05 ppm prometryne was found in the subsoils. Prometryne present (by chemical analysis) in the surface soils ranged from 2% (of that originally applied) in the soil without additives to 20% in the soils modified with organic matter and clay. Approximately 12% of the prometryne present in the soil modified with organic matter and 32% of that in the soil modified with clay was detectable by bioassay. Trifluralin concentration (by chemical analysis) ranged from 2% in the soil without additives and soil modified with clay to 11% in the soil modified with organic matter. Approximately 100% of the trifluralin in soil without additives and soil modified with clay was detectable by bioassay while only 15% of that present in soil modified with organic matter was biologically detectable. Fluometuron was presented in the soils in amounts ranging from 0.6 to 1.9% of that applied. Prometryne and fluometuron greatly decreased the large crabgrass [Digitaria sanguinalis (L.) Scop.] population and increased the population of yellow nutsedge (Cyperus esculentus L.) and crowfootgrass [Dactyloctenium aegyptium (L.) Richter]. Trifluralin greatly decreased large crabgrass and crowfootgrass populations, but the plots became completely infested with yellow nutsedge. Horseweed [Conyza canadensis (L.) Cronq.] became the overall dominant species the first year after the field was taken out of production. Yellow nutsedge, bermudagrass [Cynodon dactylon (L.) Pers.], and other grasses were also prevalent.

Tests with 18 growth regulators and chemicals revealed that the cytokinins [zeatin, kinetin, and 6-benzylaminopurine (BAP)], ethylene, and sodium hypochlorite stimulate germination of cowpea witchweed seeds at various concentrations. Witchweed seeds preconditioned in different concentrations of sodium hypochlorite for 3 wk had germination rates of 25 to 89% after incubation in distilled water, 10−5M solution of kinetin, 50 mg L−1 of 2-chloroethylphosphonic acid, and exudates from cowpea roots. Witchweed seeds preconditioned in distilled water and subsequently incubated in distilled water did not germinate.

Tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] inhibited growth of seedling corn (Zea mays L. ‘DK T 1100’), wheat (Triticum aesetivum L. ‘Arthur’), and sorghum (Sorghum vulgare Pers. ‘DK 42Y’) in growth chamber experiments. These inhibitions were partially circumvented by simultaneous treatment with OTC (L-2-oxothiazolidine-4-carboxylic acid). Tridiphane, atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], and OTC each increased levels of low molecular weight thiols (glutathione) in intact roots of treated corn seedlings, but only OTC did in excised roots. Tridiphane and atrazine caused a decrease in thiol content of excised roots. Tridiphane treatments reduced the amount of glutathione S-transferase activity extractable from corn roots, and this reduction was circumvented partially by OTC applied in combination with tridiphane. These data present evidence that tridiphane interference with cysteine or glutathione metabolism can be reversed by increasing cellular content of cysteine.

Sequential application of cycloate (S-ethyl N-ethylthiocyclohexanecarbamate) at 3.4 kg/ha, phenmedipham (methyl m-hydroxycarbanilate m-methylcarbanilate) at 1.1 kg/ha, and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0.6 or 0.8 kg/ha, plus certain mechanical measures, controlled weeds during all periods of the growing season without significant injury to sugarbeets (Beta vulgaris L.). Early-season weed control was complete enough that the new selective mechanical thinners could have replaced labor for thinning. The additional labor required to bring the crop to harvest free of weeds was limited to one late-season hoeing, which required less than 10% of the total labor required in traditional weed control schedules without herbicides.

The influence of ultraviolet-B (UV-B) radiation on seedling growth and leaf surface characteristics of important rangeland weeds Cynoglossum officinale L. (houndstongue), Centaurea diffusa Lam. (diffuse knapweed), and Tragopogon pratensis L. (meadow salsify) was investigated in a greenhouse by exposing seedlings to 0, 7, or 11 kJ m−2 d−1 of biologically effective UV-B radiation. UV-B radiation reduced leaf, stem, and root fresh weights, leaf area, and leaf: shoot ratio and increased shoot dry matter content, specific leaf weight, and leaf greenness of C. officinale seedlings. Leaf, stem, and root fresh weight, and leaf area of C. diffusa seedlings decreased, whereas shoot dry matter content, specific leaf weight, leaf: shoot ratio, and leaf greenness were unaffected. T. pratensis seedlings exposed to UV-B radiation had diminished leaf greenness, but no change was observed in leaf or stem fresh weight, leaf area, or leaf: shoot ratio. Uniseriate trichome abundance in C. officinale seedlings increased and trichome orientation changed in response to UV-B exposure. UV-B treatments decreased uniseriate trichome abundance in C. diffusa; glandular trichome abundance and the ratio of glandular: uniseriate trichomes were not affected. T. pratensis seedlings showed no change in the amount of epicuticular wax on the adaxial leaf surface in response to UV-B irradiation. Shoot dry weight was not influenced by removal of epicuticular wax prior to UV-B exposure. Results suggest that epicuticular wax is not an important factor in acclimation of T. pratensis seedlings to UV-B radiation. Susceptibility to UV-B–induced damage was greatest for C. officinale seedlings and least for T. pratensis seedlings. Modification of plant morphology or reduction in growth induced by enhanced UV-B radiation may influence competitive relationships between rangeland weeds and their associated forage species.

Dyers woad (Isatis tinctoria L. # ISATI) has greatly expanded on rangelands in the Intermountain region. Herbicidal and tillage controls are not feasible on rangelands. Better knowledge of the biology of this species could help in the development of biological controls. We examined characteristics that could assist in this effort. Seed viability remained high and relatively stable, but germination decreased over a 10-month period. The seed dispersal pattern of dyers woad was best described by a negative exponential model (log10 y = 1.92-0.02x; r2=0.60), where y equals seeds/m2 and x = distance from mother plant (cm). The root system of dyers woad is dominated by a taproot with some laterals in the upper 30 cm of the soil profile. Survivorship of experimentally established populations monitored over 2 yr showed constriction at two stages: 1) germination and establishment, and 2) young rosette. The latter stage should be targeted for biological control.

Simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) at 0.12 to 1.0 ppmw in nutrient cultures of common duckweed (Lemna minor L.), elodea (Elodea canadensis Michx.), and parrotfeather (Myriophyllum brasiliense Camb.) inhibited oxygen evolution within 24 hr. Of the plants studied, the submersed form of parrotfeather exhibited the greatest reduction in apparent photosynthesis as measured by dissolved oxygen in the water. Simazine in nutrient culture without plants remained relatively stable during the treatment period; a slight, but not significant, diminution of chemical was detected after 4 days. Each species, elodea or emersed parrotfeather, reduced the concentration of simazine in solution within 48 hr after treatment.

Eleven herbicide systems were evaluated from 1981 to 1983 for cotton (Gossypium hirsutum L. ‘Stoneville 825’) planted no-till directly into cover crops or winter fallow and for cotton planted following conventional tillage. Herbicide systems consisted of paraquat (1,1’-dimethyl-4,4’-bipyridinium ion) or glyphosate [N-(phosphonomethyl)glycine] and/or residual herbicides applied prior to crop emergence. Some systems also included early postemergence or postemergence-directed applications. Cover crops were crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), and rye (Secale cereale L.). Soil cover in fallow treatments was comprised mainly of cotton stalk residue. Conventional tillage and seedbed preparation included fall moldboard plowing and spring disking/smoothing. Treatments were maintained in the same site each year. Order of cover crop susceptibility to herbicides applied prior to crop emergence was rye>clover>vetch. In 1981, all residual treatments except cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} alone prior to crop emergence provided better than 80% control of annual weeds. In subsequent years only systems that included applications prior to crop emergence and postemergence-directed herbicides provided acceptable control. From 1981 to 1983, annual grasses increased 20- to 100-fold for systems in which control was poor. Weed control was generally superior in conventional tillage. Vetch adversely affected cotton stands in all 3 yr. Also, in 1982, reduced cotton stands resulted from cyanazine treatments applied prior to crop emergence in clover, vetch, and fallow. Cotton yields were affected by cover dessication, annual weed control, and cotton stands. Buildup of annual weeds reduced yields to near zero for some treatments in 1983.

Cotton plants were treated with C14-ring-labeled 2-methylmercapto-4,6-bis(isopropylamino)-s-triazine (prometryne) which was soil-incorporated or placed in solution cultures. C14-labeled material accumulated in the lysigenous glands of roots, stems and leaves. No evolution of C14-carbon dioxide could be detected. Unaltered C14-prometryne was present in stems and leaves 23 weeks after treatment, but no radioactive materials could be detected in the seed. Small amounts of hydroxypropazine were isolated from the stems and leaves of plants grown in soil, but none was detected in plants grown in solution culture. Both water-soluble and acetone-insoluble radioactive materials accumulated in the leaves, stems and roots of all treated plants.

Horseweed can be a problematic weed in no-till soybean fields and populations can vary in their response to 2,4-D. The objective of this study was to evaluate the growth and seed production of four horseweed populations after exposure to 2,4-D. 2,4-D amine was applied at 0, 140, 280, and 560 g ae ha−1 to 5- to 10-cm-tall horseweed plants. An additional treatment of 280 g ha−1 of 2,4-D + 840 g ae ha−1 of glyphosate was included in the study. At 2 wk after treatment (WAT), injury ranged from 47 to 98%, but by 6 WAT the injury ranged from 89 to 100% for all four populations. Between 6 and 12 WAT some individual horseweed plants started to recover. No differences in dry weights were observed between the four populations in the untreated checks at 0, 2, 6, and 12 WAT. At 280 g ha−1 of 2,4-D amine, seed production was reduced by greater than 95%. However, three of the four horseweed populations had plants that survived and produced seed after exposure to 840 g ha−1 of glyphosate + 280 g ha−1 of 2,4-D. One plant produced seed after exposure to 560 g ha−1 of 2,4-D. These results suggest that horseweed can evolve resistance to 2,4-D and no fitness penalities were observed in populations that had higher levels of tolerance to 2,4-D.

The effect of reduced light intensity on the growth and development of three common grass weeds, blackgrass, silky windgrass, and annual bluegrass, was studied. Two identical greenhouse experiments displaced in time were performed with six light levels aiming at 0%, 20%, 50%, 80%, 90%, and 95% shade corresponding to a mean daily light integral (DLI) of 12.4, 9.63, 7.13, 2.74, 0.95, and 0.69 mol m−2 d−1 in experiment 1 and 21.2, 18.0, 10.7, 3.71, 1.64, 1.20 mol m−2 d−1 in experiment 2. Climate screens of acrylic fabric were used to create the light levels. A DLI of 0.69 to 3.71 mol m−2 d−1 substantially reduced the plant height, the number of leaves, leaf chlorophyll content index, stomatal conductance, maximum photochemical efficiency of photosystem II, and dry matter of blackgrass. It also reduced plant height, the number of leaves, and dry matter and delayed flowering of windgrass and annual bluegrass. Annual bluegrass reacted most rapidly when light levels increased from the lowest levels by producing more leaves. DLI thresholds for blooming were estimated to be about 7.13 mol m−2 d−1 for windgrass and 1.64 mol m−2 d−1 for annual bluegrass. Annual bluegrass was able to bloom and sustain biomass even at a DLI of 1.64 mol m−2 d−1. This ability may contribute to an explanation of why annual bluegrass is among the most common weed species in highly competitive and well-fertilized crops even though it is much smaller than the two other grass species.

Common lambsquarters is highly competitive in many cropping systems and has demonstrated resistance to several herbicide mechanisms of action. However, predicting the spread of resistance is difficult due to limited information about gene flow. We conducted research to determine the potential for movement of resistance alleles in common lambsquarters under field conditions. Chenopodium giganteum (a member of the C. album aggregate) that has a dominant magenta phenotypic marker was used as a pollen parent in gene flow experiments. A wild-type accession of common lambsquarters was used as a seed parent. Seed parents were grown in a soybean field and arranged in concentric circles 2 to 15 m from a center which contained 24 pollen parents. The concentric circles were divided into eight directions. Pollen movement was estimated by determining the percentage of progeny with the magenta phenotype from seed parents. Average cross-pollination across directions was greatest (3.0%) at 2 m and decreased to low levels (0.16%) 15 m from the center, consistent with observations of other primarily self-pollinated species. Cross-pollination was greatest (P < 0.10) in the south-southwest, west-southwest, and west-northwest directions, approximately 180° from the prevailing wind direction during the time of pollen shed. Since common lambsquarters does not have an active dispersal mechanism for seeds, pollen-mediated gene flow may play an important role in the transfer and frequency of resistance alleles within and between populations.

Four early-generation backcross populations (BC1F2) derived from one common recipient parental background, Weed Tolerant Rice 1 (‘WTR1’), and four different donor parents (‘Y134’, ‘Zhong 143’, ‘Khazar’, and ‘Cheng Hui-448’) were tested to identify suitable donor and recipient parents for weed competitiveness and to standardize evaluation of the weed-competitive ability in rice. ‘GSR IR2-6’ (G-6) derived from a backcross of WTR1/Y134//WTR1 was selected as the best population and was advanced for phenotypic experiments in the 2014 dry season. The introgression lines (ILs) derived from the G-6 population were evaluated for seed germination and seedling vigor in greenhouse conditions and for weed-competitive ability under field conditions (upland weed-free, upland weedy, and lowland weedy). Parents and checks were included for comparison. Selection pressure for weed competitiveness was relatively stronger in upland conditions than in lowland conditions. After three rounds of selection and based on their relative grain yield performances across conditions, a total of 21 most-promising introgression fixed lines showing superior traits and weed-competitive ability were identified. G-6-L2-WL-3, G-6-RF6-WL-3, G-6-L15-WU-1,G-6-Y16-WL-2, and G-6-L6-WU-3 were the top ILs in lowland weedy conditions, whereas G-6-Y7-WL-3, G-6-Y6-WU-3, G-6-Y3-WL-3, and G-6-Y8-WU-1 were the highest yielding in upland weedy conditions. The use of weed-competitive rice cultivars in African and Asian countries will be a highly effective strategy to reduce production costs and provide alternative solutions to the unavailability of herbicides. Competitive rice varieties will also significantly improve grain yields in aerobic rice systems and can become an important strategy for successful upland rice production.

Achene germination of common crupina (Crupina vulgaris Cass. ♯ CJNVU) was 86% or greater at day/night temperatures ranging from 15/4 to 29/25 C. Germination was greatly reduced at a day temperature of 36 C, regardless of the night temperature. The time required for maximum germination (9 to 19 days) increased with decreasing night temperature. Maximum leaf area, rosette diameter, and dry-matter accumulation during vegetative growth occurred at 22/18 to 29/25 C, 22/18 C, and 22/18 C, respectively. The plants attained 50% or more of maximum dry-matter accumulation at day/night temperatures ranging from as low as 22/4 and 15/11 C to as high as 36/25 C. Net assimilation rate was relatively insensitive to growth temperature. Rosettes of common crupina grown at 22/11 or 29/18 C exhibited almost complete photosynthetic and respiratory temperature acclimation. Increasing the photoperiod from 10 to 12, 14, or 16 h accelerated flowering. Maximum achene production occurred in the 16-h photoperiod, but viable achenes were produced in all photoperiods. Temperature during vegetative growth influenced subsequent responses to photoperiod. Plants grown for 83 days in a 14-h photoperiod at 15/4 or 22/11 C and transferred to 26/20 C flowered and produced seed in photoperiods of 8 or 16 h. Plants grown at 29/18 or 36/25 C subsequently flowered only in the 16-h photoperiod at 26/20 C. Common crupina has a wide tolerance of variations in temperature and photoperiod. Its range in the United States can be expected to continue to increase.

Infestations of common milkweed (Asclepias syriaca L.), in descending order of incidence in Nebraska, occurred on areas occupied by soybean [Glycine max (L.) Merr.], oats (Avena sativa L.), sorghum [Sorghum bicolor (L.) Moench.], railroad rights-of-way, fallow land, roadsides, corn (Zea mays L.), winter wheat (Triticum aestivum L.), pasture, and alfalfa (Medicago sativa L.). Average crop yield reductions, within common milkweed communities of 11100 to 45 200 plants/ha, ranged from 2 to 10% for corn, 4 to 29% for sorghum, and 12 to 19% for soybeans. Reductions in crop yields tended to increase with increasing common milkweed populations. Germination percentage of common milkweed and sorghum were not significantly affected by growing medium conductivities below 7 and 13 mmho/cm2, respectively. Growth media at pH 2 significantly reduced germination percentage, hypocotyl length, and radicle length of both common milkweed and sorghum as compared with pH 4 to 10. Undiluted aqueous extracts of common milkweed shoot and root material significantly reduced germination percentage, hypocotyl length, and radicle length of sorghum.