The herbicide 3’,4′-dichloropropionanilide (propanil) effectively controlled barnyardgrass [Echinochloa crusgalli (L.) Beauv.], brachiaria [Brachiaria platyphylla (Griseb.) Nash], crab-grass [Digitaria spp.], sesbania [Sesbania exaltata (Raf.) Cory], curly indigo [Aeschynomene virginica (L.) B.S.P.], spikerush [Eleocharis spp.], and umbrellasedge [Cyperus spp.] in the 1- to 4-leaf stages without significant injury to commercial varieties of rice in the 1- to 2-leaf stages. It was ineffective for control of ducksalad [Heteranthera spp.], redstem [Ammannia coccinea Rottb.], morningglory [Ipomoea spp.], sprangletop [Leptochloa panicoides (Presl.) Hitchc.], and red rice [Oryza sativa L.]. Mixtures of propanil and a blended surfactant, containing petroleum sulfonate, free and combined fatty acids, and petroleum oil, and mixtures of propanil and isopropyl N-(3-chlorophenyl)carbamate (CIPC) and methyl 2,4-dichlorocarbanilate (swep), controlled barnyardgrass better than propanil alone. Propanil sometimes injured the rice, but it recovered in 5 to 10 days after treatment and yield and quality were unaffected.

The fungus Colletotrichum coccodes (Wallr.) Hughes was isolated from diseased plants of eastern black nightshade (Solanum ptycanthum Dun. ♯ SOLPT) growing in the field. In greenhouse experiments, the isolate was highly pathogenic to eastern black nightshade and less so to seven other weedy species within the Solanaceae. No disease symptoms were observed on seedlings of tomato (Lycopersicon esculentum Mill.), potato (Solanum tuberosum L.), or 10 other species inoculated with the isolate. Dew periods of 16 h or longer were required for maximum disease development. This requirement could limit the practical use of this pathogen as a my coherbicide.

Short term incubations (4 d) were conducted to assess the effect of a wetting/drying cycle on atrazine sorption, as well as biodegradation, as a function of various atrazine concentrations (ca. 5, 10, and 25 μg g−1 soil) and levels of added crop residues (0, 5, and 10% cornstalks by weight), using a technique that allowed independent analysis of soluble and sorbed atrazine. Soil solution atrazine concentrations decreased, and KdS increased with increasing crop residues. The sorptive capacity of cornstalks for atrazine was estimated to be 860 μg g−1 vs 28 μg g−1 for unamended soil. Drying and rewetting resulted in lower soil solution concentrations and decreased extraction efficiencies (13 to 22%) for sorbed atrazine; the effect was most pronounced with added cornstalks. High recoveries of 14C from soils (combustion data) indicated that atrazine was not lost to volatilization. Rapid rates of biodegradation were observed in cornstalkamended soils shortly after rewetting; degradation was not observed in unamended soil. A longer incubation (6 wk) was conducted with ca. 10 μg g−1 atrazine and 5% cornstalks to assess metabolites and kinetics of biodegradation. Atrazine disappearance was observed after ca. 2 wk with concomitant production of deethyl- and deisopropyl-atrazine at a ratio of ca. 2:1. Dealkylated-atrazine accumulated after ca. 3 wk; there was no evidence for hydroxy-atrazine production. These data suggest that biodegradation may play an important role in atrazine losses in the field despite wetting/drying cycles. In addition, there may be apparent losses of atrazine due to decreased extraction efficiencies as a consequence of wetting/drying cycles, resulting in underestimation of field residues.

The combination of the herbicide 2-chloro-2′,6′ diethyl-N-(methoxymethyl) acetanilide (alachlor) with the insecticide 2,2-dimethyl-2,3-dihydrobenzofuranyl-7-N-methyl-carbamate (carbofuran) interacted synergistically to reduce barley [Hordeum vulgare L. ‘Larker’] but not corn [Zea mays L. ‘Michigan 400’] growth. Alachlor alone increased the respiration rate of barley. Accumulation of 14C from 14C-alachlor was greater in the plant roots than shoots. The increased accumulation of alachlor in the roots was accentuated by a reduced rate of alachlor metabolism. The basis for the observed interaction appeared to be greater alachlor uptake by barley plants grown from seed treated with carbofuran. A statistical procedure for pesticide combinations in experiments with a completely randomized design was developed for the calculation of interaction significance.

A biotype of rigid ryegrass (Lolium rigidum Gaudin biotype VLR69) resistant to some ALS inhibitors was characterized to determine the mechanisms of resistance to the sulfonylurea herbicides. The biotype had a high level of resistance to chlorsulfuron (20×) and triasulfuron (25×), and an intermediate level of resistance to imazaquin (7×) and sulfometuron (7.5×) but exhibited a low level of resistance to imazapyr (2.5×). At 60 to 90 g ai ha-1 sulfometuron, 4% of the population survived without apparent herbicidal effect The same response to sulfometuron was also observed when seeds of the resistant biotype VLR69 were germinated on agar containing sulfometuron. At 27 nM sulfometuron, 4% of the seeds germinated and grew normally while the growth of the bulk of the population was retarded. This differential response to sulfometuron at the seedling stage allowed selection of sulfometuron-resistant individuals from the population. Activity of ALS extracted from these sulfometuron-resistant plants was less sensitive to inhibition by chlorsulfuron than ALS extracted from plants considered sulfometuron susceptible in the same system. Unselected plants from the VLR69 population were able to detoxify chlorsulfuron more rapidly than susceptible VLR1 plants. It is apparent that there are at least two mechanisms of sulfonylurea resistance in the resistant biotype VLR69 which occur at different frequencies within the population. These data show that more than one mechanism or resistance may develop in response to herbicide selection pressure and that the resulting populations are not necessarily homogeneous.

The rate and final germination percentage and emergence of green foxtail [Setaria viridis (L.) Beauv.] was found to be very dependent on the soil temperature and moisture conditions at the time of seeding. In contrast, the germination and emergence of wheat (Triticum aestivum L. ‘Sinton’) was only slightly affected over the temperature and moisture range of the study. In laboratory studies, a temperature decrease from 20 to 15 C caused a 53- and 12-h delay in the time to reach 50% germination of green foxtail and wheat, respectively. Soil moisture was found to have a greater effect than soil temperature on green foxtail germination. At −5.3 bars water potential, the final germination percentage of green foxtail was less than at the higher water potentials. A further decrease in the water potential to −6.5 bars effectively reduced green foxtail germination to zero. Wheat germination was delayed only slightly, and the final germination percentage remained unchanged over the range of 0 to −15.3 bars water potential. In field studies, a soil temperature decrease from 22 to 14 C caused a 6-day delay in the time of 50% emergence of green foxtail, and the emergence of wheat was delayed only 1 day by a similar decrease in temperature. Field studies showed that when the soil was moist (0 to −4 bars) and soil temperatures were warm (20 to 25 C), green foxtail emerged within a few days of wheat. However, when the soil was dry (-4.0 to −6.5 bars) and soil temperatures were low (15 to 20 C), green foxtail emerged 7 to 14 days after wheat. The intensity of green foxtail competition was greater when green foxtail emerged within a few days of wheat than when green foxtail emerged 2 weeks after wheat.

Greenhouse and field studies demonstrated that plants compete for soil-applied atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Oats (Avena sativa L. ‘Neal’) and soybeans [Glycine max (L.) Merr. ‘Clark 63’] were grown in greenhouse bioassay studies to study the effects of plant population on bioassay sensitivity. High plant populations were not as effective as low plant populations in detecting low levels of atrazine in soil. Conversely, by increasing plant populations or decreasing soil volumes quantitative measurement of higher atrazine concentrations can be determined. A 14C-labeled atrazine study showed that by increasing soybean populations from one to six per pot, atrazine uptake per plant was decreased 50%. When soybeans were planted at a rate of six plants per 100 cm2 in a field treated with atrazine at 1.1 kg/ha, soybean dry weight production was 97% of the untreated control; when planted at a rate of one plant per 100 cm2, dry weight was only 26% of the comparable untreated check. Thus, herbicide rates for optimum weed control may have to be increased with increasing weed populations or increased crop seeding rates or both.

Variations in the acetolactate synthase (ALS) gene sequence were determined from 28 populations of corn poppy resistant (R) to tribenuron and from 6 populations susceptible (S) to this herbicide. The ALS gene fragment (634 bp) sequence revealed in R populations five point mutations at the codon Pro197, and among them the substitution of Pro197 by Ala was the most common. The sequencing chromatograms revealed that nine R individuals had only the mutant ALS gene and were homozygous (RR), 18 R individuals had both the wild type and the mutant ALS gene and were heterozygous (RS), whereas one R individual was heterozygous but with two different mutant ALS alleles (R1R2). The use of restriction digestion profile analysis to verify the DNA sequence results by detecting the existence of point mutations at the codon 197 managed to distinguish the R and S alleles and confirmed the results obtained from the sequencing chromatograms analysis. The secondary protein structure prediction suggested the formation of novel β-strands for each of the five mentioned amino acid substitutions that was not present in wild type ALS around the mutant site. These findings support the hypothesis that the substitution of Pro197 by Ser, Thr, Ala, Arg, or Leu resulted in altered secondary structure, which stabilizes an ALS tertiary conformation that prevents tribenuron binding and thus confers resistance to this herbicide.

A significant limitation in using sorption coefficients (Kd ) to predict solute transport through natural soils is the spatial variability of soil properties over large field areas. Spatial variability in Kd for imazethapyr was determined on representative samples from a 31.4-ha field, covering a pH range from 4.9 to 7.6 and an organic carbon (OC) range from 1.45 to 5.80 g kg−-1. Kd varied from 0.18 to 3.78 across the field, with an average value of 1.56. The analysis of Kd variability showed two distinct patterns in spatial distribution: areas in which pH > 6.25 and Kd < 1.5, where Kd variation is based primarily on pH, and areas in which pH < 6.25 and Kd > 1.5, where other soil properties, i.e., OC content, have a significant influence on Kd variation. Based on soil pH distribution, an easily measured property, the field was divided into two potential management areas. This separation allowed identification of portions of the field where herbicide sorption would be minimal, with a relatively higher potential for leaching (i.e., areas with Kd < 1.5), and provided a rationale for site-specific imazethapyr application.

The apparent solubility in water of 2-ethylamino-4-isopropylamino-6-methylmercapto-s-triazine [ametryne], 3-(3,4-dichlorophenyl)-1,1-dimethyl urea [diuron], and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine [atrazine] was increased when a surfactant was added to the water. At 0.5% surfactant concentrations, considerable variation in solubility was found, both as regards the herbicide and surfactant used, as measured by ultraviolet absorption spectrophotometry. The two triazines were more soluble in cationic surfactants, followed by anionic and nonionic; diuron was more soluble in cationic followed by nonionic and anionic types.

Minimum lethal levels of surfactant in water for 2 week old cucumber plants varied from about 0.1% for the cationic to 0.2% for most nonionic and 1.0% or more for anionics of the alkyl aryl sulfonate type.

An important factor in controlling invasive plant infestations is frequently the acceleration of the deterioration of their persistent seed bank, which is often associated with physical dormancy mechanisms. We hypothesized that breaking dormancy by heat would enhance the vulnerability of the nondormant seeds to hydrothermal stresses. The aim of the present study was to examine the effect of soil solarization treatments (heating the soil by means of polyethylene mulching) on buried Australian Acacia seeds, with emphasis on Acacia saligna L. The results of three field experiments indicate that soil solarization treatments caused an almost complete eradication of buried seeds of Acacia saligna and two other Australian Acacia species, Acacia murrayana and Acacia sclerosperma. The killing mechanism of solarization was further studied in laboratory experiments. We observed two phases of the heat-induced deterioration of seed persistence: breaking the dormancy of the seeds and exposing the “weakened seeds” to lethal temperatures. From an ecological perspective of conservation, the present study shows for the first time the possible utilization of solar energy, by means of soil solarization, for reducing persistent seed banks of invasive woody plants.

In response to farmer complaints of poor triallate performance, wild oat seed was collected from 34 fields in Alberta in the fall of 1990. Screening trials in the greenhouse indicated that 15 of the populations were highly resistant to triallate applied at the equivalent of the recommended field rate (1.7 kg ha-1), whereas the other 19 populations were adequately controlled. All triallate-resistant populations were also highly resistant to difenzoquat applied at 1.7 kg ha-1 (equivalent to twice the recommended field rate). The effect of increasing rates of both herbicides on dry weight of five of the resistant and two of the susceptible populations was determined in greenhouse experiments. Triallate applied up to 3.4 kg ha-1 had little or no effect on the resistant populations, whereas the susceptible populations were controlled at 1.7 kg ha-1. At rates of 6.8 kg ha-1 or higher, there were differences among the resistant populations and among individuals within the populations in the response to triallate. Response of the resistant populations to increasing difenzoquat rates was variable between experiments, but in all cases the effect of difenzoquat on wild oat dry weight was considerably less in triallate-resistant than triallate-susceptible populations. Effects of increasing rates of triallate and difenzoquat on resistant and susceptible wild oat populations growing with barley in field experiments were generally similar to the responses in the greenhouse.

Crop yield loss–weed density relationships critically influence calculation of economic thresholds and the resulting management recommendations made by a bioeconomic model. To examine site-to-site and year-to-year variation in winter Triticum aestivum L. (winter wheat)–Aegilops cylindrica Host. (jointed goatgrass) interference relationships, the rectangular hyperbolic yield loss function was fit to data sets from multiyear field experiments conducted at Colorado, Idaho, Kansas, Montana, Nebraska, Utah, Washington, and Wyoming. The model was fit to three measures of A. cylindrica density: fall seedling, spring seedling, and reproductive tiller densities. Two parameters: i, the slope of the yield loss curve as A. cylindrica density approaches zero, and a, the maximum percentage yield loss as A. cylindrica density becomes very large, were estimated for each data set using nonlinear regression. Fit of the model to the data was better using spring seedling densities than fall seedling densities, but it was similar for spring seedling and reproductive tiller densities based on the residual mean square (RMS) values. Yield loss functions were less variable among years within a site than among sites for all measures of weed density. For the one site where year-to-year variation was observed (Archer, WY), parameter a varied significantly among years, but parameter i did not. Yield loss functions differed significantly among sites for 7 of 10 comparisons. Site-to-site statistical differences were generally due to variation in estimates of parameter i. Site-to-site and year-to-year variation in winter T. aestivum–A. cylindrica yield loss parameter estimates indicated that management recommendations made by a bioeconomic model cannot be based on a single yield loss function with the same parameter values for the winter T. aestivum-producing region. The predictive ability of a bioeconomic model is likely to be improved when yield loss functions incorporating time of emergence and crop density are built into the model's structure.

Ten corn inbred lines were field evaluated for their tolerance to DPX-M6316 at 32 and 64 g ai/ha. Inbreds ‘A619′, ‘A641′, and ‘ND246’ were highly susceptible to DPX-M6316; inbreds ‘A671′, ‘A632′, and ‘B73’ were highly tolerant; and inbreds ‘A654′, ‘CM105′, ‘W153R’, and ‘M017’ were intermediate in their response. The basis for differential tolerance was studied by comparing the susceptibility of acetolactate synthase (ALS) to inhibition by DPX-M6316 in tolerant A671 and susceptible A619, and by examining the absorption, translocation, and metabolism of DPX-M6316 in both genotypes. I50 values for DPX-M6316 inhibition of ALS activity in extracts from etiolated shoots of A671 and A619 were similar, 15.6 and 17.4 nM, respectively. There was little difference in 14C-DPX-M6316 absorption by the two inbreds, but twice as much of the absorbed 14C was translocated out of the treated leaf of A619 (13%) compared to A671 (6%). Differences in translocation may have been due to much more rapid DPX-M6316 metabolism in A671 than in A619. Extracts from treated leaves of A671 had only 23% DPX-M6316 remaining 5.5 h after treatment (HAT) compared to 78% DPX-M6316 remaining in extracts from A619 leaves. Therefore, rate of metabolism was the major factor involved in the tolerance of A671 and the susceptibility of A619 to DPX-M6316.

Sunflowers (Helianthus annuus L.) produced maximum seed yields when kept weed free for a period of 4 to 6 weeks after planting. This was accomplished with one cultivation anytime between 2 and 6 weeks after the early planting in 1969, with two cultivations 2 and 4 weeks or 4 and 6 weeks after the late planting in 1969, and with one cultivation at 4 weeks after either the early or late planting in 1970. The average height of sunflower plants was not influenced by weed competition if weeded at either 2 or 4 weeks after planting, whereas, head and seed size were not affected if weeded at 4 weeks after planting. The oil content of seed was higher in plots with the least amount of weed competition.

Greenhouse and growth chamber studies were conducted to determine the effect of drought, flooding, and cold stress on the efficacy of glyphosate for velvetleaf control, and the interaction between these stresses and adjuvant and posttreatment temperature. Glyphosate activity on velvetleaf decreased when plants were stressed with drought ≥ flooding > cold. Leaf blades of environmentally stressed velvetleaf angled downward, which increased tolerance to glyphosate but was not as great a cause of tolerance as the stress effects. Glyphosate applied to 6- and 12-leaf velvetleaf was two and eight times more phytotoxic on nonstressed compared with drought-stressed plants, respectively. Glyphosate was most effective on nonstressed plants, followed by plants recovering from stress, and least effective on plants still under stress. None of the adjuvants completely overcame the adverse affects of stress on glyphosate efficacy; use of a nonionic surfactant and ammonium sulfate resulted in a 9–13 percentage point improvement in control of stressed plants compared with glyphosate applied without an adjuvant. Low temperatures (5 or 12 C) maintained for 48 h after herbicide treatment enhanced glyphosate phytotoxicity to stressed and nonstressed velvetleaf. Glyphosate at a low rate stressed velvetleaf, which made them more tolerant to subsequent glyphosate application compared with velvetleaf not pretreated with glyphosate.

Greenhouse experiments were conducted to evaluate growth and reproduction of Benghal dayflower in response to daily (nondrought stress) and weekly (drought stress) irrigation. With daily irrigation, Benghal dayflower plants added one leaf per plant each week during the initial 6 wk of growth and then increased leaf number eightfold between the intervals of 6 and 10 wk after planting (WAP) and 10 and 15 WAP. By 15 WAP each plant had in excess of 400 leaves. Benghal dayflower plant height increased 2.4 cm wk−1 between 5 and 14 WAP, increasing eightfold during this interval, while plant width increased 20-fold. Aerial spathe formation began between 7 and 8 WAP, with 26 spathes maturing (containing seeds ready for dispersal) each week beginning at 11 WAP. In another study, the influence of duration of drought stress at intervals between 7 and 56 d on early growth and development of cotton and Benghal dayflower was evaluated. Benghal dayflower aboveground biomass was 3.5 times greater than cotton. There was an inverse linear relationship between aboveground biomass and duration of drought stress for cotton and Benghal dayflower, though there was a more rapid decline for Benghal dayflower. A final study evaluated Benghal dayflower response to weekly moisture regimes that approximated 13, 25, 50, and 100% of soil field capacity. Benghal dayflower aerial spathes were 4.6 times more numerous than subterranean spathes. Rate of seed production decreased in a linear manner with decreasing water volume, however, rate of subterranean seed production was less affected by water volume than was aerial seed production. These data indicate that Benghal dayflower thrives under high soil moisture regimes, but that drought stress inhibits growth and reproduction. Cotton appears to be more drought tolerant than Benghal dayflower. Judicious water use in cotton cropping systems in the southeastern United States could be an important component of multiple-tactic Benghal dayflower management program.

The herbicides, 3–(p–chlorophenyl)–1,1–dimethylurea (monuron) and 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron), have been widely accepted as selective soil applied herbicides; they are now used in a number of crops and under a wide range of soils and climatic conditions. Their extreme residual herbicidal activity contributes to their success as residual pre-emergence herbicides and as semipermanent soil sterilants.

The forage selection and grazing behavior of sheep in leafy spurge (Euphorbia esula L. ♯3 EPHES)-infested pastures was examined in a 3-month grazing study conducted during the summer, 1981. No definite preference for or avoidance of leafy spurge was detected. An initial 1- to 3-week avoidance was observed, followed by a steady increase in leafy spurge consumption. A maximum intake of 40 to 50% of the diet per animal was observed. Weight gain by ewes in pastures infested with leafy spurge was not significantly different from weight gain by ewes in pastures free of leafy spurge. Results from these studies indicate that leafy spurge may be classified as a forage for sheep. Sheep will consume a significant amount of vegetation while suffering neither harmful internal effects nor a significant loss of body weight. Sheep can be considered an effective biological control agent for leafy spurge.

The effects of environmental factors on seed germination and seedling emergence of Oriental mustard were studied in laboratory and field experiments. Seed germination was stimulated by light; however, some germination occurred in the dark. In light/dark, potassium nitrate at 0.02 M stimulated germination of 1-mo-old seed to a maximum level of 45% as compared with the control (18%). Oriental mustard germination was inhibited by high levels of salt and osmotic stress. Seed germination was greater than 50% over a pH range of 4 to 10. Seedling emergence of 4- to 5-mo-old seeds was highest when present on the soil surface (70%) but declined with depth, and no seedlings emerged from a soil depth of 10 mm. In the field, seedling emergence was greater under no-till than minimum tillage systems.