Compared to corn (Zea mays L.) (resistant), oats (Avena sativa L.) (susceptible), and giant foxtail (Setaria faberii Herrm.) (susceptible), fall panicum (Panicum dichotomiflorum Michx.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) metabolized 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) at an intermediate rate. The order of tolerance of these five species (corn > fall panicum and large crabgrass > giant foxtail > oats) is identical to the order of their ability to metabolize atrazine. In 6 hr, corn, fall panicum, large crabgrass, giant foxtail, and oats metabolized 96, 44, 50, 17, and 2%, respectively, of the 14C-atrazine absorbed from a 10 ppm solution and translocated to the foliage, leaving concentrations of 2.2, 34.8, 30.1, 59.8, and 66.3 mμ moles, respectively, of atrazine per g of fresh weight of shoots. Hydroxyatrazine [2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine] was found in the shoots of corn and giant foxtail. Corn shoots also contained a more hydrophilic metabolite, presumably a peptide conjugate. Hydrophilic metabolites found in the shoots of giant foxtail, fall panicum, and large crabgrass were chromatographically identical to the hydrophilic metabolite found in corn.

Competitive relationships between johnsongrass, smooth pigweed, and soybean were examined in 1986 and 1987, and the adequacy of an additive response model (ARM) and product response model (PRM) in predicting yield reductions due to multispecies weed populations was assessed. A severe drought reduced soybean yields statewide in 1986, whereas ideal conditions for soybean production were experienced in 1987. Averaged over monospecific weed densities of 1, 2, 4, and 8 plants 4.6 m−1 of row, smooth pigweed intercepted 2.5 and 1.8 times more light than johnsongrass in 1986 and 1987, respectively. In multispecies populations having either 4 or 8 smooth pigweed plants 4.6 m−1 of row, light interception by johnsongrass was negligible. Averaged over monospecific weed densities, smooth pigweed produced 5160 and 1760 kg ha−1, while johnsongrass produced 1530 and 450 kg ha−1 dry weight in 1986 and 1987, respectively. In multispecies populations, smooth pigweed contributed more than 80% of the total weed biomass. As monospecific johnsongrass density increased, reductions in soybean seed yield were linear, whereas exponential response patterns adequately characterized reductions in soybean seed yield due to smooth pigweed interference. Based on calibrated monospecific responses, the ARM and PRM generally projected higher soybean seed yield reductions for multispecies weed populations than were observed. When crop production conditions were favorable and competitive effects of weeds were low, both models adequately predicted soybean seed yield reductions. When dry conditions unfavorable for crop production existed, the PRM best accommodated the interactive effects between johnsongrass, smooth pigweed, and soybean.

Forage quality was determined at vegetative, flowering, and fruiting stages for nine cool-season weed species and four cultivated forages. When compared at the vegetative stage of maturity, Carolina geranium (Geranium carolinianum L. # GERCA), Virginia pepperweed (Lepidium virginicum L. # LEPVI), Virginia wildrye (Elymus virginicus L. # ELYVI), wild oats (Avena fatua L. # AVEFA), cheat (Bromus secalinus L. # BROSE), and little barley (Hordeum pusillum Nutt. # HORPU) had high nutritive quality as indicated by in vitro dry-matter digestibility (IVDMD) values equal or superior to rye (Secale cereale L.), tall fescue (Festuca arundinacea Schreb.), and ladino clover (Trifolium repens L.). Cutleaf evening primrose (Oenothera laciniata Hill # OEOLA) and curly dock (Rumex crispus L. # RUMCR) had lower IVDMD values than the cultivated forages and all weed species except wild oats. However, the rate of decline in IVDMD as plants matured was generally greater for the weed species compared to cultivated forage plants. All weed and cultivated forage species had adequate levels of Ca and K for livestock. Nearly all weed and cultivated forage species had suboptimum levels of P for high-producing ruminant animals; Carolina geranium and cutleaf evening primrose had high Ca:P ratios, which can cause metabolic disorders. Carolina geranium, Virginia wildrye, wild oats, cheat, little barley, and all the cultivated forage species had inadequate levels of Mg and could be considered “tetany prone”. Most of the forbs had high Mg concentrations.

Previous research has shown that haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} and sethoxydim {2-[1-(ethoxyimino)butyl-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} give less control of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] when applied at the five- to seven-leaf stage compared to the three- to four-leaf stage. Current research indicates that reduced control of quackgrass in the eight-leaf stage, compared to the four-leaf stage, was not due to reduced herbicide retention on leaves or to changes in residual herbicide activity in the soil following postemergence applications. The efficacy of both herbicides on rhizome bud kill did not change between the four- and eight-leaf stages following application to the foliage. However, the efficacy of both herbicides on crown tissue bud viability was less at the eight-leaf stage. Rhizome sink intensity (the capacity to accumulate radiolabel) was similar at both stages of development following 14C-haloxyfop and 14C-sethoxydim application to either upper or lower leaves. Crown tissue sink intensity was significantly less at the eight-leaf stage compared to the four-leaf stage following 14C-sethoxydim application to either upper or lower leaves and following 14C-haloxyfop application to lower leaves. These data suggest that crown bud regrowth contributes substantially to reduced control of quackgrass in advanced stages of growth following applications of haloxyfop and sethoxydim.

Greenhouse studies were established in Fayetteville, AR, to investigate glyphosate resistance in Arkansas common ragweed populations. Common ragweed seed were collected from plants in Pope and Jackson counties in Arkansas. Plants grown from seed were sprayed with one of seven glyphosate rates. Populations in Pope and Jackson counties were 21-fold and 10-fold more tolerant to glyphosate, respectively, than a known susceptible population. Based on 14C-glyphosate absorption and translocation studies, reduced glyphosate absorption or translocation was not the resistance mechanism in Arkansas glyphosate-resistant common ragweed. Shikimate accumulation did not differ among the known susceptible and the two resistant populations at 3 d after treatment (DAT). However, by 5 DAT, shikimate accumulation in the two resistant populations was lower than the known susceptible population. Data indicate that glyphosate-resistant common ragweed is present in at least two locations in Arkansas, and the resistance mechanism is not an insensitive target site or reduced glyphosate absorption or translocation.

The relative adsorption and migration of four thiocarbamate herbicides were studied in five soil types. Pebulate (S-propylbutylethylthiocarbamate) was adsorbed to the greatest degree by any one of the soils or adsorbents used. EPTC (ethyl N-N-dipropylthiocarbamate) showed the least adsorption while ethyl N-ethyl-N-cyclohexylthiocarbamate (hereinafter referred to as R-2063) was intermediate. Charcoal and bentonite clay were very effective adsorbents while the soils varied in their ability to adsorb the herbicides. A correlation was found between the rate of adsorption and the amount of organic matter in the soil. Such correlation was not found with the clay content, but a multiple regression test showed that the clay content is also closely associated with adsorption. The leaching of EPTC, pebulate, R-2063, and S-2,3-dichloroallyl N,N-diisopropylthiolcarbamate (diallate) in soil columns was directly related to their solubilities in water and inversely related to the organic matter content of the soil. Incorporation of the herbicides into the upper 5 cm of the soil did not alter the pattern or depth of their leaching. The lateral diffusion of thiocarbamates in the soil was much more restricted than the leaching downward. Again, this movement was closely related to the water solubilities of the herbicides, to soil organic matter, and apparently to the vapor pressures of the herbicides. EPTC vapor loss was the greatest followed in order by pebulate and R-2063. The rate of vapor loss from soils of all of the herbicides tested was reduced when the soil properties enabled rapid infiltration and drying of the spray drops.

Transferability of lysimeter data to the field was investigated using the leaching and degradation behavior of 14C-labeled metamitron [4-amino-3-methyl-6-phenyl-2,4-triazin-5(4H)-one] and methabenzthiazuron [1,3-dimethyl-3-(2-benzthiazolyl)-urea]. Forty-one to 53 days after application about 4% of the applied metamitron was detected in the upper 20 cm of the field soil either as active ingredient or the major metabolite desamino metamitron, whereas about 8% was found in the lysimeter soil. Forty percent and 33% of the applied methabenzthiazuron were identified in the 0- to 10-cm soil layer in the field and lysimeter 127 to 133 days after application, respectively. It is demonstrated that lysimeter experiments using 14C-labeled compounds simulate actual field conditions and give further information about the amount of total 14C-residues in the soil and percolate.

Field experiments were established to evaluate weed control with AC 263,222 alone and in combination with imazaquin or imazethapyr. Addition of AC 263,222 to either imazaquin or imazethapyr resulted in additive or synergistic increases in control of johnsongrass, pitted morningglory, and entireleaf morningglory. Reducing the rate of imazaquin from 140 to 18 g ai ha−1 did not reduce control of johnsongrass or pitted morningglory when AC 263,222 was added at 18 g ai ha−1 and above, compared to 140 g ha−1 imazaquin alone, through 8 weeks after treatment (WAT). When 9 g ha−1 or more AC 263,222 was added to imazethapyr, reducing imazethapyr rate from 70 to 9 g ai ha−1 did not reduce control of johnsongrass or pitted morningglory through 8 WAT. Adding 9 g ha−1 or more AC 263,222 to various rates of imazaquin or imazethapyr 2 and 8 WAT additively or synergistically increased control of johnsongrass, pitted morningglory, entireleaf morningglory, and tall waterhemp compared to those herbicides applied alone. Soybean injury increased 2 WAT when 35 or 70 g ha−1 AC 263,222 was added to most rates of imazaquin or imazethapyr, but was less than 15% in all treatments by 8 WAT. Visual soybean injury did not translate into a yield loss, and an increase in weed control often resulted in increased yield.

The effect of soil disinfestation with methyl bromide (MB) or by soil solarization (solar heating) and the fungicides TMTD (tetramethylthiuram disulfide) and fentin acetate (triphenyltin acetate) on the degradation of terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-methylthio)-s-triazine] and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] were investigated. The degradation of terbutryn appeared to follow first-order kinetics with a half-life of about 2 weeks and was much slower in MB-treated or solarized soils, i.e. half-life of about 11 weeks. Suppression of terbutryn degradation in the MB-treated soil was still evident 8 months after soil fumigation, similar to that found in autoclaved soil. TMTD and fentin acetate at 20 μg/g soil strongly inhibited degradation of terbutryn in soil. Degradation of atrazine was affected to a lesser extent by soil disinfestation. Results from the present study suggest that biocidal soil treatments may slow herbicide degradation. Thus, herbicide dosages in disinfested soil should be adjusted in order to avoid phytotoxicity. Moreover, lower dosages might be sufficient to attain weed control, and combined disinfestation or fungicides with herbicide treatments might be intentionally used to extend herbicide activity.

The effect of two commonly used desiccant tank mixtures on Glycine max seed production and viability when applied at G. max growth stages R5 to R8 (R5 = beginning pod fill, R6 = full green pod, R7 = beginning maturity, R8 = full maturity) was evaluated. Two mid-Group IV G. max cultivars, one conventional (‘Northrup King 4260’) and one glyphosate-tolerant (‘Asgrow 4701RR’), were evaluated. Tank mixtures containing either glyphosate + sodium chlorate or paraquat + sodium chlorate + surfactant reduced yield, seed weight, and subsequent germination, emergence, and seedling growth when applied at R5 or R6 to either G. max cultivar. Glyphosate + sodium chlorate applied at R7 reduced germination of the glyphosate-tolerant cultivar. When applied at R6 or R7, glyphosate + sodium chlorate reduced the next generation's seedling length compared to paraquat + sodium chlorate. Glyphosate + sodium chlorate increased the number of abnormal seedlings produced in the conventional cultivar compared to paraquat + sodium chlorate. These results indicate that preharvest desiccant applications should not be made prior to G. max maturity (R7).

Crop yields can be similar in organic and conventional systems even when weed biomass is greater in organic systems. Greater weed tolerance in organic systems may be due to differences in management-driven soil fertility properties. The goal of this experiment was to determine whether soil collected from a long-term organic cropping system with a diverse crop rotation and organic fertility inputs would support higher soil nitrogen (N) resource partitioning, as indicated by overyielding of corn–weed mixtures, than a cropping system with a less diverse crop rotation and inorganic N inputs. A replacement series greenhouse experiment was conducted using corn : smooth pigweed and corn : giant foxtail proportions of 0 : 1, 0.25 : 0.75, 0.5 : 0.5, 0.75 : 0.25, and 1 : 0 and harvested at 29, 40, or 48 d after experiment initiation (DAI). The monoculture density of corn was 4 plants pot−1 and the monoculture density of each weed species was 36 plants pot−1. Corn was consistently more competitive than both weed species at 40 and 48 DAI when soil inorganic N was limiting to growth. Corn–smooth pigweed mixtures had greater shoot biomass and shoot N content than expected based on the shoot biomass and shoot N content of monocultures (i.e., overyielding) at the onset of soil inorganic N limitation, providing some evidence for N resource partitioning. However, soil management effects on overyielding were infrequent and inconsistent among harvest dates and corn–weed mixtures, leading us to conclude that management-driven soil fertility properties did not affect corn–weed N resource partitioning during the early stages of corn growth.

Horseweed has become increasingly difficult to control in the San Joaquin Valley (SJV) of California. Resistance to glyphosate may not fully explain the quasi-invasive behavior of this native species. We contrast glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed biotypes for vigor during the vegetative stage and for resistance to ozone (O3). The SJV is impacted by O3 air pollution, which could be a factor in competitiveness of GR vs. GS. Both biotypes were exposed during the seedling and vegetative stages of rosette development to a range of O3 concentrations in greenhouse exposure chambers. Leaf injury was evaluated visually and biomass production and allocation destructively. In O3-free air, the GR biotype exhibited fewer foliar lesions, more vigorous growth, and 40% greater biomass than the GS biotype. The slope of the response to O3 was greater in the GR than in the GS biotype, implying greater relative sensitivity to O3. This was due to greater vigor at low O3, as the biotypes performed similarly at high O3. The competitive advantage of the GR biotype may be reduced in polluted environments. There appeared to be no linkage between the evolution of resistances to O3 and to glyphosate.

Resistance to atrazine (a photosystem II [PSII] inhibitor) is prevalent in waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] across the U.S. Midwest. Previous research suggests that target-site mutation or rapid metabolism of atrazine mediated by glutathione S-transferase (GST) conjugation confers resistance in A. tuberculatus from Illinois. The distribution and mechanism of resistance to atrazine in A. tuberculatus populations from Nebraska (NE) are unknown. In this research we (1) evaluated the response and frequency of resistance in NE A. tuberculatus to soil-applied PSII (metribuzin and atrazine) and protoporphyrinogen oxidase (sulfentrazone) inhibitors, as well as POST-applied atrazine; and (2) determined the mechanism of atrazine resistance in NE A. tuberculatus. The chloroplastic psbA gene, coding for a D1 protein (the target site of atrazine) was sequenced in 85 plants representing 27 populations of A. tuberculatus. Furthermore, 24 plants selected randomly from four atrazine-resistant (AR) populations were used to determine the metabolism of atrazine via GST conjugation. Results from the soil-applied herbicide evaluation suggest that metribuzin (0.56 kg ai ha−1) and sulfentrazone (0.28 kg ai ha−1) were effective on A. tuberculatus management. PRE and POST screenings against atrazine in the greenhouse indicate that atrazine (1.345 kg ai ha−1) was not effective on 39% and 73% of the A. tuberculatus populations evaluated (total of 109 and 85 populations, respectively), suggesting the prevalence of atrazine resistance in A. tuberculatus in NE. Sequence analysis of the psbA gene found no known point mutations conferring atrazine resistance. However, the AR plants conjugated atrazine via GST activity faster than the known atrazine-susceptible A. tuberculatus. Overall, the outcome of this study demonstrates the predominance of metabolism-based resistance to atrazine in A. tuberculatus from NE, which may predispose this species to evolve resistance to other herbicides. The use of integrated management strategies for A. tuberculatus is crucial for the control of this troublesome species.

The depth of leaching of 3(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) in a Yolo sandy clay loam soil was altered by the various types—anionic, cationic, or nonionic—of surfactants. The effects of surfactants Aliquat 204 and Aliquat 221 on the leaching of 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), 1-butyl-3-(3,4-dichlorophenyl)-1-methylurea (neburon), and diuron may reflect a sorption complex of the substituted urea herbicide, surfactant, and the soil. This sorption appears to be closely associated with the dilauryl portion of the surfactant molecule. Other surfactants of the Aliquat series containing lauryl, dioctyl, and distearyl substitutions had little or no effect on this sorption phenomenon.

Traditional weed control systems based on soil-applied herbicides and cultivations were compared with systems utilizing postemergence grass herbicides for Texas panicum (Panicum texanum Buckle. # PANTE) control in peanut (Arachis hypogaea L. ‘Florunner′) production. Maximum net returns ($150/ha) and peanut yield were achieved with traditional systems utilizing benefin [N-butyl-N-ethyl-2,6-dinitro-4-(trifluoromethyl)benzenamine] applied preplant incorporated plus alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] applied at ground cracking and two cultivations. The postemergence system using alachlor plus dinoseb applied at ground cracking and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-1-one} applied postemergence also provided maximum net returns ($146/ha) and peanut yield.

Grain sorghum (Sorghum bicolor (L.) Moench ‘RS 671’) was grown during 1969 and 1970 in competition with two other members of the same species, wild cane and Kansas Orange forage sorghum, transplanted at various densities along grain sorghum rows. All densities of wild cane and Kansas Orange significantly reduced both grain and fodder yields of grain sorghum. Grain yield was highly and positively correlated with leaf area, culm length, culms per plant, panicles per culm, seeds per panicle, and amount of light received at the grain sorghum canopy; and grain yield was negatively correlated with plant factors that benefited wild cane and Kansas Orange. Grain yield did not correlate with either plants per hectare or seed weight. Panicle size was most important in determining grain yield. Kansas Orange reduced grain yields more than did wild cane in 1969. During 1970, with less favorable rainfall, grain sorghum responded similarly to both transplanted weed types.

A 6-h imbibition in sodium azide solution stimulated nearly 100% germination of dormant wild oat (Avena fatua L.) seeds in petri dishes. Sodium azide stimulated germination in soil in both field and growth chamber experiments. Disc-incorporated sodium azide in soil at 11.2 kg/ha caused a 4-fold increase in wild oat emergence as 50% of the wild oat seeds in a natural infestation germinated compared to 12% in control plots. Wild oat seed germinated from greater soil depths following sodium azide application. Sodium azide appears to need a deep, thorough soil incorporation and adequate soil moisture for maximum stimulation of germination.

Field sudies on the persistence of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] showed less persistence under coulter than chisel or conventional plow tillage in the year of application. The residue on the coulter system from the previous corn (Zea mays L.) crop initially prevented as much as 30% of the atrazine from reaching the soil surface. After five annual applications, the atrazine residue was generally higher in the coulter than the chisel or conventional tillage systems, but below levels considered to be biologically active.

Transformation of N-(1,1-dimethylpropynyl)-3,5-dichlorobenzamide (compound I) in soil occurs readily and two products are produced, initial cyclization giving 2-(3,5-dichlorophenyl)-4,4-dimethyl-5-methyleneoxazoline (compound II) followed by subsequent hydrolysis to N-(1,1-dimethylacetonyl)-3,5-dichlorobenzamide (compound III). These transformations can be brought about in vitro, the first step by means of acid or base, and the second by extended treatment with acid. The rate of cyclization and hydrolysis of compound I varies directly with soil temperature, being rapid at high temperature (37 C) and very slow at low temperature (5 C). The rate of chemical change of compound I in soil is influenced to a much greater degree by temperature than by soil moisture content. The effect of soil type on transformation of compound I was studied and compounds II and III were present in five of the six soils examined. The herbicidal activity of compounds II and III was negligible in comparison to compound I.

‘Tracy′, ‘Tracy-M′, and ‘Centennial′, soybean [Glycine max (L.) Merr.] cultivars were evaluated under greenhouse and field conditions for their tolerance to metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5 (4H)-one]. The I50 (50% injury) concentration of metribuzin, as determined in nutrient culture, for Tracy-M was more than three times the I50 for Tracy, but was equivalent to the I50 for Centennial. Data from greenhouse and field studies on Sharkey clay and Dundee silt loam soils indicated that Tracy-M was more tolerant to metribuzin than its parent selection, Tracy, but no more tolerant than Centennial when metribuzin was applied preplant soil incorporated or surface preemergence alone or in combination with several dinitroaniline herbicides.