Greenhouse studies were conducted to determine the response of 200 corn hybrids and 29 inbreds to metolachlor applied at 4.5 kg ai ha−1. Both hybrids and inbreds varied in their response to the herbicide. The distribution of injury revealed a normal distribution curve with most of the hybrids having a midlevel of tolerance. Some hybrids were very tolerant, while others were quite sensitive. Laboratory studies were conducted to evaluate absorption and metabolism of 14C-metolachlor for a subset of tolerant and sensitive hybrids. There was no observed difference in the product of metolachlor metabolism in the tolerant and sensitive hybrids. The observed variability in metolachlor tolerance among hybrids appeared due to differences in the amount of metolachlor absorption and metabolism and differences at the site of metolachlor action. The tolerant ‘Great Lakes 584’ hybrid absorbed significantly less 14C-metolachlor than did the sensitive ‘Pioneer 3744’, while the tolerant ‘Cargill 7567’ metabolized significantly faster more 14C-metolachlor than the other hybrids. The internal concentrations of available 14C-metolachlor were the same for the tolerant Cargill 7567 and the sensitive ‘Northrup King 9283’ after 8 h, indicating differences at the site of action of metolachlor for these two hybrids.

Spray solutions containing a cationic or a nonionic surfactant were applied to six plant species at a broad range of concentrations. The species investigated were three weeds (black nightshade, chamomile, and quackgrass) and three crops (winter wheat, pea, and tomato). The microroughness of the leaf surface as revealed by scanning electron microscopy appeared to be a relevant retention-determining factor. Plant species with crystalline epicuticular waxes (winter wheat, pea, and quackgrass) retained much less spray solution than the other species, which are characterized by a smooth cuticular surface. The two surfactants enhanced retention on species with a reflective surface, whereas retention on black nightshade, chamomile, and tomato was hardly influenced by addition of surfactants. The two surfactants had a similar influence on the retention. Surfactant at 1% (wt/v) enhanced retention on pea, winter wheat, and quackgrass by factors of twenty, six, and four, respectively, compared with retention without surfactant. A linear relation between retention and logarithm of surfactant concentration was observed. Retention of spray drops was related not to equilibrium surface tension of the spray solution but rather to dynamic surface tension.

Control of tall larkspur on mountain rangelands would substantially reduce cattle poisoning. Several herbicides were evaluated for their control of tall larkspur in subalpine and aspen vegetation types. Glyphosate (2.2 kg ai ha−1) and picloram (2.2 kg ae ha−1) killed more than 88% of larkspur plants in both vegetation types. Clopyralid and triclopyr were ineffective at comparable rates. Metsulfuron (88 and 138 g ai ha−1) provided variable control. Glyphosate is nonselective and killed all perennial vegetation, except for Thurbers fescue and mountain brome in the aspen type. Picloram applied at 4.5 kg ha−1 suppressed grasses on the subalpine site, but allowed grasses to increase at lower rates. All herbicides reduced forb cover.

Low soil water content for 10 to 14 days before and 7 days after herbicide application reduced control of green foxtail with fenoxaprop, fluazifop-P, haloxyfop, and sethoxydim. Withholding irrigation for 10 to 14 days before herbicide application did not reduce control of green foxtail with normal use rates of herbicides if plots were irrigated at the time of application. Low soil water content reduced the activity of sethoxydim less than the other herbicides.

Laboratory studies were conducted to characterize the adsorption-desorption and degradation of thiobencarb in soil. Thiobencarb adsorption on soil was correlated with soil organic carbon (r = 0.97) and values (ml g−1) for the following soils were: Pahokee muck 339, Everglades muck 169, and Immokalee sand 14. The adsorption values (ml g−1) per unit of organic carbon were Immokalee sand 1195, Pahokee muck 765, and Everglades muck 539. In desorption studies, thiobencarb Kf values increased 80 ml g−1 from the first to the third desorption step in the muck soils, but were similar in three desorptions from the sand. The half-lives of thiobencarb averaged 16, 18, and 24 days on surface samples of Pahokee muck, Everglades muck, and Immokalee sand, respectively, across the range of 25 to 35 C and 10 to 100 kPa soil water tension. Half-lives were generally longer with water contents equivalent to 100 kPa than 10 kPa soil water tension and incubated at 25 than 35 C. The relative percentages of extractable 14C, bound 14C, and 14CO2 from 14C-thiobencarb-treated soil were dependent on soil type, soil water content, incubation temperature, and time.

The influence of wheat straw cover, tillage, and irrigation on metribuzin and alachlor movement and dissipation in an Appling coarse sandy loam (Typic Hapludult) and the influence of soil depth on their rate of dissipation were measured in field and laboratory experiments conducted in 1987 and 1988. Overall, metribuzin moved more than alachlor in both years. Alachlor movement was greater in tilled plots compared to no-till plots in 1988. Metribuzin movement was greater in no-till plots in 1987. Straw cover had little effect on the movement of alachlor, but the presence of 2800 kg ha−1 of straw on the soil surface increased the downward movement of metribuzin in both years compared to soil with no straw cover. The rate of alachlor dissipation in the soil was faster in straw-covered and no-till plots in both years and in the high irrigation level in 1988. Metribuzin dissipation was not affected by any of these factors. Alachlor rate of dissipation did not differ among depths in the field; however, dissipation differed in the laboratory in the order 0 to 20 cm > 20 to 45 cm = 45 to 68 cm. Metribuzin dissipated faster at the 8-cm depth in the field compared to the 58-cm depth. In the laboratory, metribuzin dissipation rates followed the order 0 to 20 cm > 20 to 45 cm > 45 to 68 cm. At the greatest depth, the dissipation of metribuzin was faster in the field than in the laboratory.

Laboratory experiments were conducted to evaluate butylate and EPTC degradation in two butylate-history soils collected in 1983, 1984, and 1985, and stored at 4 C. No significant differences in biodegradation of butylate and EPTC were found due to length of cold storage of Varina or Wagram soils. Enhanced biodegradation of butylate and cross-adaptation for EPTC were not affected. Storage of soils up to 2 yr should not adversely affect results.

Two nonionic surfactants altered photolysis rates of chlorimuron and metsulfuron in aqueous solutions and on glass slides exposed to sunlight. Photolysis of both herbicides followed first-order kinetics over an 8-day exposure period. Chlorimuron half-lives in solution were 5.8, 3.7, and 2.7 days in the presence of no surfactant, oxysorbic, and octoxynol, respectively. The extrapolated half-life of metsulfuron in solution with no surfactant was 15.7 days, compared to half-lives of 2.9 and 1.5 days in solutions containing oxysorbic or octoxynol, respectively. With the exception of metsulfuron in the absence of a surfactant, rates of chlorimuron and metsulfuron photolysis on glass slides were approximately two- to fourfold slower, and surfactants had little or no effect on herbicide photolysis rates compared to those in aqueous solution. Extrapolated half-lives on glass ranged from 9.9 to 12.5 days for chlorimuron and from 7.6 to 11.7 days for metsulfuron. Presence of oxysorbic or octoxynol did not greatly alter rates of riboflavin-sensitized herbicide photolysis on glass but did increase photolysis rates of metsulfuron in aqueous solution containing riboflavin.

Laboratory and greenhouse studies were conducted to determine the mobility and bioactivity of thiobencarb on a Pahokee muck, Everglades muck, and Immokalee sand. On all soils > 93% of thiobencarb remained in the upper 1 cm after leaching during unsaturated flow. During saturated flow the retardation factor (RT) of thiobencarb on an Immokalee sand was 68, 20, 2, and 1 with 0, 25, 50, and 75% by volume methanol in 0.01 N CaCl2, respectively. There was a significant log-linear relationship (r2 = 0.97) between the Freundlich adsorption values (Kf) estimated from RT values and the percentage by volume methanol of the leachate. In general, thiobencarb in combination with overhead irrigation reduced lettuce vigor and plant dry weight on an Immokalee sand more than with subsurface irrigation.

Seeds of plants naturalized in the United States were examined for the presence of elaiosomes. Seeds of 47 species belonging to 13 families (Asteraceae, Boraginaceae, Dipsacaceae, Euphorbiaceae, Fabaceae, Fumariaceae, Lamiaceae, Liliaceae, Poaceae, Polygonaceae, Resedaceae, Rosaceae, and Solanaceae) were found to have elaiosomes, indicating that these species are probably mymecochorous, i.e., dispersed by ants. These include important rangeland weeds such as bull thistle, Canada thistle, musk thistle, diffuse knapweed, spotted knapweed, and leafy spurge. Myrmecochory in naturalized species may enhance their weediness in areas where they are established and assist their colonization of new and relatively closed communities.

Laboratory experiments were conducted to determine the effects of nonionic surfactants and reaction media (water, glass, and corn leaf residue) on photolysis and subsequent phytotoxicity of chlorimuron and metsulfuron residues. Oxysorbic and octoxynol enhanced rates of chlorimuron and metsulfuron photolysis in aqueous solution and on glass slides compared to controls with no surfactant. Enhanced photolysis of chlorimuron by surfactants was greatest on glass, where 93 and 89% loss occurred after 48 h exposure to ultraviolet light in the presence of oxysorbic and octoxynol, respectively, compared to 38% loss with no surfactant. Similarly, surfactant-enhanced metsulfuron photolysis was greatest on glass with 37 and 67% loss after 48 h exposure in the presence of oxysorbic and octoxynol, respectively, compared to 9% loss with no surfactant. Photolysis of herbicides deposited on corn leaf residue was significantly slower than that on glass or in aqueous media at all exposure times and metsulfuron photolysis on corn residue was enhanced by surfactants only after 144 h exposure. Bioassays confirmed that phytotoxicity of photolyzed herbicide residues was negatively correlated (r=-0.94 for chlorimuron and r=-0.92 for metsulfuron) with loss of parent herbicide as measured by liquid chromatography.

Light reflectance from soybean foliage was compared to visual evaluations for assessing soybean injury caused by postemergence-applied paraquat and glyphosate. Coefficients of variation and of determination from multiple linear regressions for both herbicides indicated that reflectance was better correlated with injury measured by changes in leaf number, leaf weight, and stem weight than visual evaluations. Correlation coefficients among radiometer readings taken by different individuals were ≥0.98 for both herbicides. Correlation coefficients among visual evaluators ranged from 0.69 to 0.96 and from 0.74 to 0.94 for plots treated with paraquat and glyphosate, respectively. Light reflectance measured with a radiometer may be useful in evaluating injury from herbicides that cause chlorosis, necrosis, or defoliation of the target species.

Fall, spring, or late-summer applications of glyphosate, paraquat, sethoxydim, and HOE-39866 were investigated in field trials for no-tillage renovation of endophyte fungus-infected tall fescue pastures. Only spring or late-summer applications of glyphosate at 1.68 and 2.52 kg ae ha−1 consistently provided greater than 80% visual tall fescue control. However, none of the herbicide treatments significantly reduced final percent endophyte fungus infection levels after the pasture was reestablished. High final infection levels were due to regrowth from underground rhizomes of old tall fescue sod that survived the herbicide treatments.

Experiments were conducted to determine the interaction between cupric hydroxide and paraquat on the control of American black nightshade. Three cupric hydroxide applications lowered paraquat toxicity with no further decrease in toxicity with six or nine cupric hydroxide applications when paraquat was applied 3 days after the last cupric hydroxide application. The interval between the last cupric hydroxide application and paraquat application was more important in decreasing paraquat toxicity than the number of cupric hydroxide applications. Three cupric hydroxide applications with a 10-day interval before paraquat application decreased paraquat toxicity less than three cupric hydroxide applications with a 3-day interval before paraquat application. Plants collected from three American black nightshade populations (Naples, Bradenton, and Gainesville) differed in their sensitivity to paraquat. I50 values, based on dry weight, showed Naples plants to be 12 times more tolerant to paraquat than Gainesville plants without cupric hydroxide, and 14 times more tolerant when cupric hydroxide was applied. More plants of the Naples biotype regrew from apical buds after paraquat treatment following cupric hydroxide application, and from axillary buds with paraquat but not cupric hydroxide treatment, than did the Bradenton biotype. Plants of the Gainesville biotype did not regrow after paraquat application, regardless of cupric hydroxide treatment.