A rainfall simulator was used to deliver the equivalent of 1.3 cm of water in 0.25 h at 0, 3, 6, 12, 24, and 48 h after POST applications of asulam to johnsongrass in greenhouse and field studies. Johnsongrass control responses were similar when asulam was applied with either a crop oil concentrate or a commercially blended organosilicone/crop oil concentrate premix. Rainfall 24 h after asulam application reduced johnsongrass control in greenhouse studies. Maximum visual johnsongrass control of 80, 69, and 69% was obtained in field studies when rainfall occurred 20, 14, and 8 h after asulam application, respectively. Based on reductions in johnsongrass dry weight, rainfree periods needed to insure maximum performance with asulam in field studies ranged from 3 to 16 h. Variation in critical rainfree periods was related to plant growth stage when asulam was applied and environmental conditions and is indicative of the inconsistency in johnsongrass control commonly observed with asulam.

Experiments were conducted to evaluate the effect of a 2.5-cm simulated rainfall 0.05, 0.25, 0.5,1, 3, 6, and 24 h after POST applications of AC 263,222 to sicklepod. Fresh weight reductions were directly related to increasing AC 263,222 rates and rain-free intervals. Fresh weight reductions with 9, 18, 36, 54, and 72 g ai/ha AC 263,222 were 32, 47, 55, 59, and 67%, respectively, with the rain-free treatment. A simulated rainfall 0.05 h after POST application lowered fresh weight reductions with 9, 18, 36, 54, and 72 g/ha AC 263,222 to 5, 7, 19, 23, and 28%, respectively, of the untreated control. As AC 263,222 rates increased, a simulated rainfall shortly after POST application was less detrimental. At 18 g/ha of AC 263,222, a 3-h rain-free interval was necessary for fresh weight reductions equivalent to the rain-free control. As the rate of AC 263,222 increased to 54 g/ha, the rain-free interval for fresh weight reductions equivalent to the rain-free control was reduced to 1 h. Radiolabeled studies indicated that at 3 h after application an equivalent amount of 14C was accumulated in the plant as compared with the rain-free control. The correlation coefficients for fresh weight reductions and 14C in the plant were r = 0.53, 0.75, 0.68, 0.74, and 0.70 at rates of 9, 18, 36, 54, and 72 g/ha AC 263,222, respectively.

Foliar absorption of imazaquin {ammonium salt of 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} following postemergence application was not necessary for control of sicklepod (Cassia obtusifolia L. # CASOB). Application of imazaquin to the soil resulted in control similar to application to the soil plus foliage. A 0.6-cm simulated rainfall 0.05 h after postemergence application of 140, 280, or 560 g ae/ha of imazaquin did not reduce sicklepod control. Foliar absorption was necessary for control with postemergence application of DPX-F6025 {ethyl ester of 2-[[[[(4-chloro-6-methoxypyrimidin-2-yl)amino]-carbonyl] amino] sulfonyl] benzoate}. Application of DPX-F6025 to the foliage resulted in control similar to application to the soil plus foliage. Sicklepod control resulting from postemergence application of 18 g ae/ha of DPX-F6025 was reduced when 0.6 cm of simulated rainfall was received 1 h after application but not when received 4 h after application. With application of 9 g/ha of DPX-F6025, simulated rainfall 24 h after application reduced control. Sicklepod control resulting from postemergence application of sublethal rates of imazaquin and DPX-F6025 was greater when plants were exposed to 3 or more days of 24/18 C day/night temperature than when grown at a continuous 32/24 C temperature. Exposure to low temperature for 3 days immediately before imazaquin application resulted in greater enhancement of control than did exposure for 3 days immediately after application. The reverse was found with DPX-F6025. Control obtained with both imazaquin and DPX-F6025 increased as the number of days of exposure to low temperature increased from 3 to 6. Enhancement of control with low temperature diminished as the herbicide application rate increased.

The objectives of this research were to compare the weed control efficacy of liquid, granular, and microencapsulated formulations of preemergence herbicides in moldboard plow, chisel plow, ridge tillage, and no-tillage corn and soybean production systems, and to determine whether herbicide formulation can influence herbicide interception and retention on surface corn residue. Common lambsquarters populations were threefold higher in corn than in soybeans. A mixed population of giant foxtail and green foxtail was highest in the chisel plow and lowest in the ridge tillage system as were total weed numbers. Percent weed control was not influenced by tillage when considered across all herbicide treatments. Weed control was not influenced by herbicide formulation in the moldboard plow, chisel plow, or ridge tillage systems, but granular herbicide applications provided better weed control than liquid applications in the no-tillage system and across various rates of corn residue in an experiment with no tillage variables. Two- to threefold less granular-applied herbicide was intercepted by surface corn residue at the time of application compared to liquid-applied herbicide. Increasing amounts of postapplication rainfall decreased the difference among formulations with regard to both total soil reception of the herbicide and resultant weed control. There was no consistent advantage for the microencapsulated formulation over the other herbicide formulations. Surface corn residue controlled many weeds without the aid of a herbicide and actually contributed to overall weed control even where herbicides were applied. This suggests that the binding of preemergence herbicides on surface crop residue may not be the cause of weed control failures in reduced-tillage systems as is often assumed to be the case.

Bispyribac is registered for postemergence control of broadleaf, sedge, and grass weeds in rice. Bispyribac inhibits the acetolactate synthase enzyme in sensitive plants. Herbicides in this class of chemistry require a spray adjuvant to achieve optimal efficacy, often achieve different levels of weed control according to the spray adjuvant used, and typically have rainfast periods of at least 6 to 8 h. Efficacy and rainfastness of bispyribac can be affected by spray adjuvant and the addition of urea ammonium nitrate (UAN). Greenhouse experiments were conducted to investigate the effect of spray adjuvant type, addition of UAN, and soil moisture on bispyribac efficacy on barnyardgrass. Control of barnyardgrass was improved when UAN was added to bispyribac at 0.4 or 0.8 g ha−1 plus an organosilicone-based nonionic surfactant (OSL/NIS) or methylated seed oil/organosilicone (MSO/OSL) spray adjuvant. The type of adjuvant added to the spray solution affected bispyribac efficacy on barnyardgrass. The addition of UAN decreased the rainfast period from 8 h (registered rainfast period) to 1 or 4 h (99 to 100% control) when either the OSL/NIS or MSO/OSL adjuvant was applied with bispyribac, respectively. Applying UAN and OSL/NIS or MSO/OSL adjuvant with bispyribac enhanced efficacy and reduced the time period required between bispyribac application and washoff during a rainfall event. Increasing soil moisture conditions resulted in greater efficacy from bispyribac when applied with and without UAN.