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.

Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), dinitramine (N4,N4-diethyl-α,α,α-rifluoro-3,5-dinitrotoluene-2,4-diamine), profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro 2,6-dinitro-N-propyl-p-toluidine], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], and USB 3153 (N3,N3-di-N-propyl-2,4-dinitro-6-trifluoromethyl-m-phenylenediamine) were applied to evaluate weed control, cotton (Gossypium hirsutum L. ‘Dunn 119′) tolerance, and effect on rotational crops. Each herbicide was applied at 1-month intervals for 6 months prior to planting at 1, 1.25, 1.5, and 2X the recommended rates. Pigweed (Amaranthus hybridus L. and A. retroflexus L.) population counts and visual control ratings showed that weed control improved when treatments were applied closer to planting. Applications in November and December at 1.25X profluralin, 1.5X trifluralin, fluchloralin, and pendimethalin, 1X USB 3153, and 2X butralin were required to give effective weed control. Spring applications of 1X rates of profluralin, trifluralin, fluchloralin, and USB 3153, 1.25X rates of pendimethalin and dinitramine and 1.5X rate of butralin were required for adequate weed control. USB 3153 at all rates and dates of application severely limited growth and yields of wheat (Triticum aestivum L. ‘Funk 332′) planted after harvest and also the following May. Spring applications of profluralin at 1.5 and 2X rates and trifluralin at the 2X rate resulted in significant injury of rotational sorghum [Sorghum bicolor (L.) Moench ‘Dekalb E-59+’].

Producers of food have always been troubled with weeds. Hand labor was the primary source of weed control for centuries, and selective weed control was not achieved until copper sulphate was used to control mustard (Brassica spp.) in wheat (Triticum aestivum L.) in 1896 (4, 42). Adequate crop selectivity was usually difficult to obtain with herbicides during the first 40 yr of the 20th century and the cost of the early herbicides was high.

The effects of various densities and durations of wild oat (Avena fatua L.) competition in soybean [Glycine max (L.) Merr. ‘Evans’] were determined in the field during a 2-yr period. Season-long competition by densities of 1, 3,9, and 30 wild oat plants/m of row reduced soybean seed yield an average of 6, 17, 32, and 51%, respectively. An infestation of 30 wild oat plants/m of row did not reduce soybean yield if the period of competition was limited to 4 weeks after crop emergence; however, yields were reduced 29, 50, 63, 58, and 63% when wild oat competed for 5, 6, 7, and 8 weeks, or season long, respectively. Wild oat competition reduced soybean pods per plant and seeds per plant more than seeds per pod or seed weight.

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.

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.

Absorption and translocation of the methyl ester of haloxyfop (haloxyfop-methyl) in sorghum were reduced by presence of bentazon in the treatment emulsion. After 6 h, 18% of applied haloxyfop-methyl was recovered from the leaf surface in the absence of bentazon, compared to 43% of that applied in combination with bentazon. An average of 18% of applied haloxyfop-methyl was recovered as haloxyfop + haloxyfop-methyl from leaf tissues outside the treated zone in the absence of bentazon, compared to 8% of that applied in combination with bentazon. Haloxyfop-methyl absorption was similar whether applied with or without formulations of imazaquin or chlorimuron. However, the addition of imazaquin or chlorimuron to treatment emulsions decreased the translocation of haloxyfop + haloxyfop-methyl as only 13 and 12%, respectively, of the total amount of haloxyfop-methyl applied was translocated out of the treated zone. Mixtures of all herbicides were chemically stable; formulations of bentazon, imazaquin, or chlorimuron did not alter the ester or free acid levels of haloxyfop in an aqueous mixture over an 8-h period.

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.

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.

Greenhouse studies were initiated to identify potential interactions of the postemergence herbicide acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} with the postemergence grass herbicides sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio) propyl)-3-hydroxy-2-cyclohexen-1-one}, diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid}, and RO-13-8895 [acetone 0-(D-(2-(p-[(α,α,α-trifluoro-p-tolyl)oxy) phenoxy)propionyl)oxime}. Greatest control of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] was obtained with the combination of acifluorfen and sethoxydim plus a crop oil concentrate. This combination increased foliar absorption of 14C-acifluorfen by barnyardgrass but not by soybean [Glycine max (L.) Merr. ‘Corsoy’]. The combination of acifluorfen with diclofop appeared to reduce phytotoxicity to barnyardgrass and increase it to soybean. Little or no interaction was observed in combinations of acifluorfen and RO-13-8895.

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.

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.

The current widespread use of ester and amine formulations of 2,4–dichlorophenoxyacetic acid (2,4–D) and/or 2,4,5–trichlorophenoxyacetic acid (2,4,5–T) has made apparent the potential vapor hazard of these herbicides in their injury to susceptible crop plants growing nearby. A method used for evaluating the volatility of proprietary formulations was reported previously (1,2) using temperatures of 70° to 75° F. At this “normal” temperature eighty commercial ester preparations representing thirty-one different formulations of 2,4–D and/or 2,4,5–T showed, conclusively, that esters such as the isopropyl, butyl, ethyl, amyl, and pentyl individually or in mixture were volatile; and the iso-octyl, butoxyethanol, tetrahydrofurfuryl, polypropylene glycol, and propylene glycol butyl ether esters were low volatile. These formulations do not comprise all the commercial preparations tested but are given only to indicate the extent of volatility found for 2,4–D and 2,4,5–T ester herbicides commonly available on the market.

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 fate of 2,4-D [(2,4-dichlorophenoxy)acetic acid] in the soil under winter wheat (Triticum aestivum L. ‘Nugaines’) and fallow cropping schemes was studied under the field conditions of eastern Washington in 1973 and 1974 using formulated dimethylamine salt and isooctyl ester of 2,4-D. Soil samples taken 1 hour after herbicide application showed that amine-treated plots retained considerably more applied 2,4-D than ester-treated plots. The rapidity of 2,4-D breakdown decreased gradually with time, and at the end of 6 months, an average of 0.04 ppm of 2,4-D remained in the sampled soil profile regardless of formulation, application rate, or cropping scheme. Loss of 2,4-D from the soil surface in runoff occurred when the plots were irrigated heavily one day after the herbicide application. The herbicide was also leached into the soil profile by both irrigation and natural precipitation. Herbicide concentrations in the sampled portion of the upper soil profile decreased during the summer and then increased slightly in the fall.

Combinations of 3-chloro-2-methyl-p-valerotoluidide (solan) with 1,1′-dimethyl-4,4′-dipyridylium cation (paraquat) were used postemergence in transplanted tomatoes (Lycopersicon esculentum Mill.). Addition of .05 lb/A of paraquat to 2 or 4 lb/A of solan resulted in more effective control of crabgrass (Digitaria sanguinalis (L.) Scop.) than did doubling the rate of solan. The combinations were synergistic on crabgrass but only additive or more often antagonistic on tomato plants. The herbicides dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA) and sodium 2,4-dichlorophenoxyethyl sulfate (sesone) were applied for preemergence weed control in strawberry beds. Velvetleaf (Abutilon theophrasti Medic.) was resistant to both DCPA and to sesone, but was quite susceptible to the combination. Combinations of 4,6-dinitro-o-sec-butylphenol (DNBP) with 2-methoxy-4,6-bis(ethylamino)-s-triazine (simetone) were highly synergistic on tomato plants. This synergism was more pronounced at low rates of herbicides.

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.

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.

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.