Three field studies were conducted over a 2-yr period to evaluate the persistence of fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl) aniline], and to determine whether DBCP (1,2-dibromo-3-chloropropane) affected persistence. Fluchloralin was applied to field plots at 1.1 kg/ha with and without DBCP at 20.5 kg/ha. In the first study, soil samples were taken periodically over a 1-yr period and assayed for fluchloralin by both gas chromatography (GC) and a sorghum (Sorghum bicolor L. Moench ‘AKS-516’) root-elongation bioassay. Both methods of analysis indicated that fluchloralin persistence was unaffected by DBCP. An oat (Avena sativa L. ‘Ora’) bioassay of soil from the field plots 41 weeks after treatment showed no residual herbicide activity. In the next two field studies, soil samples were taken periodically over a 32-week period and assayed by GC for fluchloralin. A greenhouse sorghum bioassay of soil samples taken from both tests 32 weeks after application showed residual activity of fluchloralin in one test, but differences were not attributable to DBCP. A two-phase process of fluchloralin dissipation in field soil was indicated from analysis of the data using a complex first-order regression, as opposed to a simple first-order regression. Half-life values describing fluchloralin persistence, using the complex first-order regression, ranged from 2.3 to 3.7 weeks for the first phase and 9.5 to 26.7 weeks for the second phase.

Adsorption of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] and fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] on a Taloka silt loam (Mollic Albaqualf) and a Roxana silt loam (Typic Udifluvent) from the 10- to 20- and 40- to 50-cm soil depths was measured. The order of adsorption of the herbicides was fluometuron = metolachlor > metribuzin. Adsorptivity of the soil from the 10- to 20-cm depth was greater than from the 40- to 50-cm depth. At 7, 15, 23, and 37C, all herbicides degraded more rapidly as temperature increased. Metribuzin was least persistent and fluometuron and metolachlor were similarly persistent. The time required for the initial herbicide concentration to decrease by 50% in the Taloka silt loam from the 10- to 20-cm depth at 23C was 2.6, 9.4, and 10.1 weeks for metribuzin, fluometuron, and metolachlor, respectively.

Field and laboratory studies were conducted in 1987 and 1988 on several Arkansas soils to determine the effects of residual levels of imazaquin on cotton. Imazaquin concentrations ranging from 0.007 to 0.024 μg/g and 0.00 to 0.015 μg/g were found in soil samples taken 12 and 24 months, respectively, following preplant-incorporated applications at rates of 140 g/ha. Cotton yields were reduced from 7 to 42% in 1987 as the soil concentration of imazaquin increased from 0.007 to 0.024 μg/g. Imazaquin persistence was greater in clay soils than loams or sandy loams. Imazaquin was weakly adsorbed to a clay, loam, and sandy loam soil in laboratory studies; the adsorption isotherms did not differ significantly by soil texture. A bioassay utilizing cotton as the indicator species was developed to determine imazaquin concentrations in soils ranging from 0 to 0.06 μg/g. Imazaquin concentrations determined by the bioassay method were found to be 75, 77, and 80% of those determined by chemical extractions for a clay, loam, and sandy loam soil, respectively.

Laboratory studies were conducted to determine the relative sorption, mobility, and degradation rates of triclopyr and 2,4-D on two surface soils and two subsoils from the rice-producing areas of Arkansas. Triclopyr sorption was slightly greater than 2,4-D sorption. However, mobility of the herbicides on a given soil did not differ. Sorption of both herbicides was greatest and mobility lowest on a subsoil with the lowest pH. Triclopyr degradation rates were lower than 2,4-D degradation rates in a dark incubator. The average half life was 138 d for triclopyr and 21 d for 2,4-D. High soil moisture content (0 versus 100 kPa water tension) increased the rate of 2,4-D degradation. Triclopyr degraded more rapidly at 30 C than at 15 C. The dissipation rates of both herbicides were lowest on the soil on which sorption was greatest.

Field studies were conducted on a Taloka silt loam (Mollic Albaqualf) to determine the effects of residual levels of metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] on rice (Oryza sativa L.). Yield reductions could be overcome by increasing rice seeding rate, by decreasing seeding depth, or by applying the safeners CGA-43089 [α-(cyanomethoxyimino)-benzacetonitrile] or CGA-92194 [α-(1,3-dioxolan-2-yl-methoximino)-benzacetonitrile] to the rice seed. Greenhouse studies with the safeners and reduced seeding depths indicated that CGA-92194 was better than CGA-43089 for protecting rice from metolachlor injury, but reduced seeding depths did not reduce metolachlor injury.

In bioassays, rice (Oryza sativa L.) recovery from metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] injury tended to be slower in flooded rice, but was not significantly different from the recovery rate in a nonflooded rice. In soils treated with 1 ppm (w/w) metolachlor and incubated in constant-temperature chambers, the half-life of metolachlor was shorter at 40 C than at 30 C. The degradation rate of metolachlor was not significantly correlated with declining moisture potentials in the range of −30 to −80 kPa. The CO2 evolution from metolachlor-treated soil was negatively correlated with incubation time and positively correlated to declining moisture levels. In a field study, metolachlor, as determined by bioassay, was mobile in a Taloka silt loam soil profile. After being incorporated to 7.5 cm, it became evenly distributed in the top 15 cm of the soil profile within 18 days. Metolachlor adsorption was positively correlated with clay and organic carbon content.

The effects of herbicide concentration, soil water content, and temperature on the fate and degradation of the monosodium salt of methylarsonic acid (MSMA) were evaluated in four soils. Available arsenicals were extracted, purified, and separated using reversed-phase high-performance liquid chromatography (HPLC). Residual MSMA was determined with a graphite furnace atomic absorption spectrophotometer at 193.7 nm. Initial loss of MSMA was rapid, and degradation followed first-order kinetics. The rate of change of the rate constant was temperature dependent only at soil water contents less than field capacity. MSMA dissipation was significantly faster in finer textured soils with continuous flooding and under controlled laboratory conditions (<180 days) than under field (ca. 350 days) conditions. Also, in finer textured soils, degradation of MSMA resulted in significant increases in arsenate and cacodylic acid (dimethyl arsinic acid) over native levels under flooded conditions. However, MSMA treatments contributed only a small fraction to total soil arsenic which dissipated to original concentrations by 120 days. The results indicate that under present recommended use patterns it is highly unlikely that MSMA will accumulate in the environment or carry over from one growing season to the next.

Field persistence of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} applied preemergence as high as 16 times the 0.14 kg ai/ha recommended rate was studied at two locations in Arkansas in 1984 and 1985. Rapid dissipation of these high rates occurred on the Taloka silt loam under hot, dry field conditions in 1984 when no rainfall occurred for 2 weeks after application. In 1985, when furrow irrigation was applied 7 days after application, imazaquin phytotoxicity was greater and dissipation was delayed. Imazaquin persistence and soil adsorption were greater on Sharkey silty clay than on Taloka silt loam. Following 3- and 8-cm irrigation, 84 and 78%, respectively, of the 14C-labeled imazaquin remained in the surface 5 cm, but in plots covered between rainfalls and receiving 8 cm rain, 74% leached out of this top layer. Since nonincorporated imazaquin dissipated rapidly under hot, dry field conditions, carryover into the next growing season or leaching below the plow layer would not be expected. However, in cool, wet conditions immediately after application, the weakly adsorbed imazaquin molecule was readily bioavailable and mobile in the soil profile.

Norflurazon applied for weed control in cotton, as well as some other herbicides, sometimes persists in soil from one growing season to the next at levels phytotoxic to the following crop. Dissipation modes of norflurazon were characterized by using 14C-labeled herbicides for adsorption, mobility, photolysis, and volatility studies to aid in the prediction of conditions influencing herbicide carryover problems. A direct soil-counting technique modified for use in these studies yielded a norflurazon recovery as high as 83% of the applied radioactivity. Relative mobility of the herbicides on soil thin-layer chromatography plates was fluometuron ≥ atrazine > norflurazon. Norflurazon adsorption increased and mobility decreased as soil organic matter and clay content increased. Considerable upward movement of norflurazon and atrazine occurred in subirrigated columns containing herbicide-treated Hebert silt loam. Upward movement in excess of 5 cm occurred in 10 days and 8 weeks for atrazine and norflurazon, respectively. Photolysis and volatilization studies with norflurazon and atrazine revealed low volatilization but significant photolytic losses for norflurazon when herbicide-treated soil-coated slides were exposed to ultraviolet or sunlight. After 98 h atrazine volatilization was greater and photolysis in sunlight less than that observed for norflurazon. Laboratory studies showing upward movement of norflurazon and atrazine, in conjunction with dissipation losses occurring at the soil surface, suggested that losses of norflurazon and atrazine are facilitated by movement in capillary water referred to as the “wick” effect.

A field study was conducted for 3 yr to evaluate residual levels of fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) for their effects on injury and yields of rice (Oryza sativa L.). All three herbicides consistently injured rice and reduced yield at application rates above 0.1 kg/ha. These herbicides were similar in their effects on rice, but trifluralin was slightly more phytotoxic than fluchloralin or profluralin. Bioassay showed fluchloralin and trifluralin to be more inhibitory than profluralin of root and shoot elongation. The order of toxicity of the vapors of these herbicides arising from treated soil was found to be trifluralin > profluralin > fluchloralin. Grain sorghum (Sorghum bicolor L. Moench) was slightly more sensitive to these herbicides than was rice when seedlings were grown in treated soil and over 10 times more susceptible than rice to injury from their vapors. Both plant species were less affected by the herbicides in silty clay than in silt loam soil.

Field studies were conducted in 1987, 1988, and 1989 to evaluate the effects of freshly applied residual levels (simulated residue carryover) of imazaquin and norflurazon on the growth and yield of corn, cotton, grain sorghum, rice, and wheat. Studies ware conducted on five soils, and concentrations of herbicides were determined at crop planting. Crop injury ratings were recorded 4 wk after emergence, and yields were measured at the end of each growing season. In simulated residue carryover studies, corn and grain sorghum injury occurred and yields were reduced when norflurazon concentrations were 450 ng g-1 soil or greater. Rice yields were reduced at norflurazon concentrations of 710 ng g-1. Corn, cotton, and wheat heights and yields were reduced when imazaquin concentrations were 13 ng g-1, 13 ng g-1, and 45 ng g-1 soil, respectively for crops. Comparisons between simulated carryover and actual carryover were made for corn and cotton grown in imazaquin treated fields as well as for corn grown in a norflurazon treated field.

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.

Experiments were conducted from 1989 to 1991 on two silt loam and two clay soils to determine the effect of herbicides applied to the previous crop on growth and yield of rice. All herbicides were applied preplant-incorporated at recommended rates adjusted as needed for soil texture. Rice was planted the following year. Imazaquin, imazethapyr, alachlor, metolachlor, clomazone, trifluralin, and atrazine did not injure rice the year following application. Norflurazon was the only herbicide to injure rice on silt loam soils, with injury at one silt loam location in one of two years. Norflurazon and fluometuron residues caused rice injury on clay soils, and chlorimuron residues caused injury in one year on a day soil. This chlorimuron carryover injury was from August-planted soybean but did not occur from June-planted soybean. Norflurazon, fluometuron, and chlorimuron temporarily reduced rice dry matter early in the season. No herbicide reduced either rough rice or percent head rice yield on any of the soils.

In a Crowley silt loam, between 30 kPa, continuous flood, and alternate flood laboratory treatments at 25 C, dissipation of incorporated oxadiazon [2-tert-butyl-4(2,4-dichloro-5-isopropoxyphenyl)-Δ2-1,3,4-oxadiazolin-5-one] varied little, with an average of 59% remaining after 20 weeks. In the greenhouse, subsurface application of oxadiazon reduced its phytotoxicity but increased its persistence up to four times more than with surface-applied oxadiazon. In the field, 50% of the surface-applied oxadiazon dissipated from the soil within 6 to 11 days when the soil was flush irrigated and then flooded, compared to 15 to 17 days when the soil was irrigated but not flooded in two rice (Oryza sativa L.) management systems. Oxadiazon dissipation in the field was greater during the first 2 or 3 weeks after application than in the laboratory study. This is explained at least partially by the lack of herbicide incorporation in the field.

A field study was conducted with fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)aniline], profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine], and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 1× and 2× rates in soybeans [Glycine max (L.) Merr.] for 2 consecutive yr to evaluate their persistence in soil and to determine their residual effect on rice (Oryza sativa L.). In another field study that was conducted to evaluate the effect of winter flooding on the persistence of 4× rates of the same herbicides, gas chromatographic analysis and bioassay procedures both revealed increased dissipation of these herbicides because of winter flooding. Flooding rice for 2 or 4 months during winter reduced the adverse effect from herbicidal residues. Laboratory studies evaluated the persistence and volatility of these herbicides at 4 and 25 C in two soils at three moisture contents (air-dried, field capacity, and flooded). The persistence of the three herbicides was similar in the field and in the laboratory and usually followed the order fluchloralin > profluralin = trifluralin. Volatility was most rapid with moisture at field capacity, but overall dissipation was most rapid in flooded soil. Significant dissipation and volatility losses of all three herbicides occurred at 4 C in flooded soil and in soil at field capacity. These studies indicated that winter flooding is effective in reducing the concentration of dinitroaniline herbicides in soil.