Dissipation of S-metolachlor, a soil-applied herbicide, on organic and mineral soils used for sugarcane production in Florida was evaluated using field studies in 2013 to 2016. S-metolachlor was applied PRE at 2,270 g ha−1 on organic and mineral soils with 75% and 1.6% organic matter, respectively. The rate of dissipation of S-metolachlor was rapid on mineral soils compared with organic soils. Dissipation of S-metolachlor on organic soils followed a negative linear trend resulting in half-lives (DT50) ranging from 50 to 126 d. S-metolachlor loss on organic soils was more rapid under high soil-moisture conditions than in corresponding low soil-moisture conditions. On mineral soils, dissipation of S-metolachlor followed an exponential decline. The DT50 of S-metolachlor on mineral soils ranged from 12 to 24 d. The short persistence of S-metolachlor on mineral soils was likely attributed to low organic matter content with limited adsorptive capability. The results indicate that organic matter content and soil moisture are important for persistence of S-metolachlor on organic and mineral soils used for sugarcane production in Florida.

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.

This study examined the response of corn to clomazone, chlorimuron, imazaquin, and imazethapyr the year following their application to soybeans. Herbicides were surface-applied from one-half to three times the labeled application rates. Soybeans were planted the year of application and crop tolerance was evaluated. Corn was planted in rotation the following season. Soybeans were tolerant of all four herbicides. The highest rates of clomazone, chlorimuron, and imazaquin injured the corn early in the season. Imazethapyr did not influence corn growth. Visual estimates of clomazone injury were as high as 39% chlorosis. Seedling dry weight reductions at the highest chlorimuron and imazaquin rates were 32% and 24%, respectively. Although corn was injured by higher rates of clomazone, imazaquin, and chlorimuron at the 3-leaf stage, none of the herbicides significantly reduced grain yield. This study suggests that these herbicides can carry over and injure corn, especially if labeled application rates are exceeded. However, low to moderate early season injury to corn may not affect grain yield.

One year after application under irrigated conditions, atrazine at 1.2 kg/ha injured oat but there was no effect after two years. On dryland, wheat and barley were injured for up to two years after atrazine was applied at 1.5 kg/ha. Atrazine concentrations in the soil were related to accumulated rainfall in an exponential equation. The equation predicted that, under the most severe drought conditions ever recorded for southern Alberta, injury to cereal crops could occur for at least three years after atrazine application at 1.2 to 1.5 kg/ha. Cyanazine did not injure subsequent crops under either irrigated or dryland conditions.

Field experiments were conducted to determine wheat injury following clomazone application in soybeans and fallow in North Dakota. Clomazone at recommended rates generally caused 10% or less visible chlorosis in spring wheat planted 11 to 12 mo after application or winter wheat planted 11 mo after application, although greater chlorosis was observed in two of seven location/year environments. Tillage preceding wheat planting increased chlorosis from clomazone residues in some environments. Clomazone residues reduced wheat grain yield only in three of seven location/year environments and usually at application rates of 1.4 kg ai ha-1 or greater. Severe drought prevailed during the study and probably increased clomazone persistence and wheat chlorosis. Drought also may have limited expression of grain yield reductions attributable to clomazone residues.

The overall degradation of chlorimuron was very similar at −0.1 and −1.5 MPa and slightly less in air-dry soil. Degradation rates increased with increasing temperature. The primary 14C-labeled compounds observed in moist-soil extracts were desmethyl chlorimuron and saccharin, while the primary 14C-labeled compound observed in air-dry soil extracts was saccharin. Saccharin is formed quantitatively from ethyl 2-(aminosulfonyl)benzoate (phenylsulfonamide) during extraction and therefore represents phenylsulfonamide formed in the soil as a result of chemical hydrolysis of the sulfonylurea bridge. These degradation products suggest that chemical hydrolysis of the sulfonylurea bridge is the primary mode of degradation in air-dry soil, while microbial degradation and chemical hydrolysis both occur in moist soil. These laboratory results demonstrate that chlorimuron will degrade in air-dry soil at a temperature-dependent rate by chemical hydrolysis.

Greenhouse experiments determined differences in imazaquin bioavailability over time under various soil moisture regimes. All soils were initially fortified with 63 ppb (w/w) of commercially formulated imazaquin. Treatments consisted of maintaining at field capacity for 15 wk (FC), maintaining air-dry for 15 wk (AD), soil that was maintained air-dry for 2 wk and then wet to field capacity at 2 wk intervals (2WAD), and soil that was wet to field capacity at the initiation of the experiment and at 2 wk intervals (2WFC). Little dissipation of imazaquin occurred in the AD soil over the 100 d of this study. Rapid dissipation over the first 35 d occurred for the other three treatments. Imazaquin half-life ranged from 17 to 18 d for the FC, 2WAD, and 2WFC soils. Imazaquin concentration increased or only slightly decreased in samples taken after the second watering in both the 2WAD and the 2WFC soils. Due to differences in adsorption/desorption of imazaquin with changing soil moisture levels, bioassays may not determine the dissipation of the actual concentration of imazaquin contained in soil.

Herbicides can persist in the soil and injure sensitive crops planted in the years following herbicide application. The response of sugarbeet to clomazone, imazaquin, imazethapyr, and chlorimuron residues in soil was examined. Clomazone at 1.1 or 2.2 kg ai ha-1 did not reduce yield of sugarbeet planted 1 or 2 yr after application. Imazaquin at 0.07 to 0.28 kg ai ha-1, and imazethapyr at 0.09 to 0.14 kg ai ha-1, can reduce yield of sugarbeet planted 1 yr after application. Imazaquin at 0.07 to 0.14 kg ha-1 caused 28 to 47% sugarbeet injury and imazethapyr at 0.09 kg ha-1 caused 54% injury 2 yr after application in soybean. A 25% yield loss occurred from the imazethapyr treatment. Sugarbeet injury 2 yr after application was greatest when sugarbeet also was planted the year following soybean. Chlorimuron at 0.02 to 0.08 kg ai ha-1 plus linuron at 0.4 to 0.9 kg ai ha-1 reduced yield of sugarbeet planted 1 yr after application when compared with linuron alone, regardless of soil pH. Visible sugarbeet injury of 38 to 66% was still evident from chlorimuron plus linuron treatments 2 yr after application. Sugarbeet should not be planted less than 25 mo following imazaquin, imazethapyr, or chlorimuron application.

Experiments to determine effects of chlorimuron, clomazone, and imazaquin applied the year before planting corn, cotton, and sorghum were conducted at three locations in Georgia. Corn injury was <20% in 1987 and 1988 and yield was not affected. In 1988, imazaquin applied at a high rate or sequentially the previous year to late-planted soybeans injured corn more than applications to the early-planted soybeans. Cotton was the most sensitive crop to herbicides applied the previous year. However, seed cotton yield was not decreased due to herbicides applied the previous year. Sorghum growth and yield 8 weeks after planting was not affected by soybean herbicide application the previous year. Sicklepod, corn, and wheat bioassays did not detect any of the herbicides in soil samples obtained the season after treatment. Bioassays accurately predicted potential injury in corn and sorghum but not cotton.

Degradation of flurtamone in a Greenville sandy clay loam, a Cecil loam, and a Dothan loamy sand with 0, 1, or 2 yr of previous flurtamone field use was evaluated under controlled conditions. Soil sterilization by autoclaving significantly reduced flurtamone dissipation rate in all soils. Enhanced degradation of flurtamone was observed in a Greenville sandy clay loam after 1 yr of previous flurtamone field use and in a Cecil loam after 2 yr of previous flurtamone field use. No enhancement of flurtamone degradation was observed in a Dothan loamy sand. Flurtamone degradation kinetics in these studies was described as a first-order process. Microbial populations in each soil showed no major changes in total bacterial numbers due to preexposure to flurtamone in the field.

The availability and persistence of imazaquin, imazethapyr, and clomazone were studied in a Cisne silt loam (1.3% organic matter) and a Drummer silty clay loam (5.8% organic matter). Availability of all three herbicides to bioassay species was greater in the Cisne soil than in the Drummer soil. Corn root growth was more sensitive to imazaquin and imazethapyr than corn shoot growth. Shoot and root growth of wheat was inhbited by similar clomazone concentrations. In field experiments conducted in 1984, 1985, and 1986, all three herbicides were more persistent in the Drummer silty clay loam than in the Cisne silt loam. Clomazone and imazethapyr were detected by liquid or gas chromatographic analysis in the Drummer soil 3 yr following application. Crop injury occurred 5 months after application of imazaquin and clomazone to the Drummer soil. In the Cisne soil, only imazethapyr caused crop injury 5 months after application. Herbicide residues found below 7.5 cm were greater in the Drummer soil than in the Cisne soil.