Immediately following postemergence application, similar quantities of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] were retained on the foliage of downy brome (Bromus tectorum L. # BROTE) and winter wheat (Triticum aestivum L.). Three days following treatment, more metribuzin was present on the surface of the foliage of downy brome than of winter wheat and similar quantities of metribuzin had been absorbed into the interior of the foliage of both species. Fourteen days after treatment, metribuzin had disappeared from the surfaces of the foliage of both species but more metribuzin was present in the interior of the foliage of downy brome than of winter wheat. Results from petri dish studies in the field suggested that the amount of unaltered metribuzin available for absorption by leaves was influenced primarily by volatilization and secondarily by photodecomposition. In addition, small quantities of rainfall may have washed metribuzin from leaf surfaces. In 1984, yield of winter wheat was greater in metribuzin-treated plots than in nontreated check plots.

The loss of metribuzin and ethyl-metribuzin from soil and glass surfaces was studied to determine if volatilization and/or photodegradation could explain inconsistent weed control with these herbicides. In growth chamber experiments, phytotoxicity of ethyl-metribuzin decreased as the time between surface application and initial watering increased. Approximately 50% of the applied metribuzin and ethyl-metribuzin volatilized from soil within 24 h, with an additional 15 to 20% lost during the next 11 days, except about 5% additional ethyl-metribuzin was lost from Chehalis sandy loam. Both herbicides volatilized more rapidly from glass than from soil, with 75 to 90% of the loss occurring within 24 h and as great as 99% loss after 12 days. Loss by photodecomposition from soil or glass surfaces was minimal.

Losses of picloram (4-amino-3,5,6-trichloropicolinic acid) in water exposed to sunlight ranged from 50 to 80% after 1 days' exposure (14 h sunlight) and 95% or more after 4 days exposure (56 h of sunlight) at four locations in Southern Arizona. Samples exposed at the highest elevation (2800 m) consistently lost picloram more quickly than samples at the lower elevations. Dry, recrystallized picloram losses were 11, 47, and 67% after 1, 2, and 4 days of sunlight exposure, respectively. At the same times, picloram losses in water were 65, 89, and 99%, respectively. These results indicate that photodecomposition is an important factor for the use and residue longevity of picloram on high-altitude rangelands of the western United States.

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.

The volatilization, photolysis, microbial degradation, and field persistence of imazethapyr were studied using formulated and 14C-labeled imazethapyr. Volatilization losses from soil were less than 2%. Photodecomposition losses of up to 8% occurred from soil and up to 52% from a glass slide with no soil. Significantly greater photodecomposition occurred with chain-labeled than ring-labeled 14C-imazethapyr. The amount of 14CO2 evolution from soil treated with either ring- or chain-labeled 14C-imazethapyr was not significantly different. The total 14CO2 evolved from the soils ranged from 2.4 to 3.6% of the total 14C-imazethapyr applied to the soil. However, degradation of imazethapyr from the same soils, as determined by high-pressure liquid chromatography, indicated that 62 to 82% of the applied 14C-imazethapyr had been degraded. The degradation rate increased as soil moisture was increased from −100 to −33 kPa. Imazethapyr was more persistent in soil with the higher clay and organic matter content.

The behavior and dissipation of 26 dinitroaniline herbicides in soils are reviewed. Some of the compounds also are referred to as dinitrotoluidines, -benzen(e)amines, -benzenediamines, -cumidines, -benzenesulfonamides, -acetophenones, and sulfonylsulfilimines. All dinitroanilines are sorbed to soil particles, particularly to organic or humic substances, and are nearly immobile in soils. Soil K values range from 7 to 117 and KOC values range from 80 to 471 000. Compounds with vapor pressures greater than 50 × 10-6 mm of Hg at ambient temperatures were reported to volatilize and diffuse through, and out of, the soil in significant amounts, depending on temperature and moisture conditions. Greater losses occurred from warm, moist soils than from cool, dry soils. Photodecomposition of dinitroanilines on soil surfaces was low but occurred when the chemicals were present in the vapor state or in aqueous solutions. Bioavailability of the chemicals decreased as organic matter contents of the soils increased. Dinitroaniline herbicides were degraded primarily by soil microorganisms, and fungi were the major organisms involved. Degradation proceeded faster under anaerobic conditions than under aerobic conditions and faster under warm, moist conditions than under cool, dry conditions. Half-life values for dinitroanilines ranged from 7 to 27 days under anaerobic conditions and from 19 to 132 days under aerobic conditions. The chemicals did not affect most soil microflora; and any significant effects reported were of short duration.

Microbial degradation of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was monitored by measuring 14CO2 evolution for 7 months under controlled laboratory conditions. Up to 10% of the 14C chain-labeled imazaquin that was applied to a Crowley silt loam was evolved as 14CO2 in 7 months. Less evolution of 14CO2 occurred on a Sharkey silty clay, a soil with higher clay and organic matter content, than on silt loam soils. The loss of 66 to 100% of the imazaquin applied to a Crowley silt loam incubated for 8 months at 18 C or 35 C, respectively, suggested that metabolic changes in addition to CO2 evolution were occurring. Rapid loss of imazaquin phytotoxicity occurred when soils were held at warm-moist (35 C and −33 kPa) conditions conducive to microbial growth. Imazaquin was more persistent in soils stored under cool, dry (18 C and −100 kPa) conditions. Imazaquin on a soil surface dissipated rapidly when exposed to ultraviolet light or sunlight. Photodecomposition could be a major mode of imazaquin dissipation if this herbicide is allowed to remain on the soil surface.