Factors Affecting Asulam Activity on Sugarcane (Saccharum Sp.) Cultivars and Johnsongrass (Sorghum Halepense).

Studies were conducted to evaluate several factors that may account for the variability in johnsongrass (Sorghum halepense (L.) Pers.) control and sugarcane (Saccharum sp.) cultivar sensitivity observed with postemergence application of asulam, methyl ((4-aminophenyl)sulfonyl) carbamate, herbicide. Johnsongrass at boot stage to 70% seedhead emergence was exposed to simulated rainfall (1.3 cm of water in 15 min) 0, 3, 6, 12, 24, and 48 h after application of asulam at 3.7 kg/ha. Predicted critical rainfree period at which greatest johnsongrass control (65 to 80%) was obtained ranged from 8 to 20 h after treatment, dependent on johnsongrass growth stage and environmental conditions following application. Compared with the 48 h washoff treatment, fresh weight 42 d after treatment was reduced 58, 46, and 64% at the predicted critical rainfree periods of 3, 8, and 18 HAT, respectively. Asulam was applied at 3.7 kg/ha to rhizomatous johnsongrass 3 d before fertilization (DBF) or 0, 3, 7, 10, or 14 d after fertilization (DAF) to evaluate possible interactions. Liquid fertilizer was injected 15 cm deep with knives spaced 70 cm apart on each side of the row. Johnsongrass control based on biomass reduction was 60 to 78% when asulam was applied 3 DBF, which was greater than for applications 0, 3, 7, or 10 DAF. Reduced johnsongrass control appeared to be related to root pruning and subsequent stress associated with the fertilizer operation. In plant cane with seedling johnsongrass, highest control (75%) and lowest johnsongrass panicle counts were observed with asulam application May 15 compared with April 15 or May 1. Cane and sugar yields were 18% higher when asulam was applied May 15 rather than June 15, but were equivalent to application April 15 or May 1. With rhizome johnsongrass in plant cane, johnsongrass control and cane and sugar yields were equivalent with asulam application April 15, May 1, or May 15 and greater than for June 15. Sugarcane cultivars were most sensitive to asulam application in June and injury ranking was \u27CP 72-370\u27 $\u3e$ \u27LCP 82-89\u27 $\ge$ \u27CP 70-321\u27. Leaf uptake of $\sp{14}$C-asulam was greater for CP 72-370, but translocation within the plant was less than for CP 70-321, which may account for the differential sensitivity observed under field conditions. In none of the studies conducted was johnsongrass controlled more than 80%. Results of these studies help to explain the inconsistency in johnsongrass control with asulam and provide information applicable to its efficient use in a sugarcane production system