Systems for sicklepod control in soybeans

Experiments were conducted at Vonore and Savannah, Tennessee, to evaluate various sicklepod (Cassia obtusifolia L.) control systems in soybeans. Soil types were a Congaree loam and a Wolftever silt loam. Organic matter for the respective soils was 3.4 and 2.5%. A split-block design was used with four replications. Four preemergence treatments were applied across the soybean rows. Six postemergence treatments were applied along rows crossing the pre emergence treatments at right angles. The resulting subplot units consisting of a combination pre-emergence treatment and a postemergence treatment were four rows wide and 9.1 meters long. The four pre-emergence treatments were: (1) alachlor (2-chloro-2',6'-ethyl-N- (methoxymethyl)acetamide) at 2.28 kg/ha; (2) metribuzin (4-amino-6- tert-butyl-5-(methylthio)-as-triazine-5(4H)one) at 0.42 kg/ha; (3) a tank mix of alachlor (2.28 kg/ha) + metribuzin (0.42 kg/ha), and (4) no herbicide treatment. The six postemergence treatments were: (1) chloroxuron 3-P-(P-chlorophenoxy)phenyl-1,1- dimethylurea) at 1.12 kg/ha applied overtop; (2) toxaphene at 2.8 kg/ha applied overtop; (3) metribuzin at 0.42 kg/ha applied as a directed spray; (4) paraquat (1,1'dimethyl-4,4’-bypyridinium ion) at 0.14 kg/ha applied as a directed spray; (5) mechanical cultivation four and six weeks after planting; and (6) no treatment. 'Williams' and 'York' soybeans were planted on June 8 and 10, and preemergence treatments were applied immediately following planting. Overtop postemergence applications were made four weeks after planting and directed spray applications were made six weeks after planting. Visual ratings of preemergence treatments were made three to four weeks after planting. Metribuzin controlled 90% of the sicklepod at Vonore but only 15% at Savannah. Visual ratings of postemergence treatments were made two and four weeks following their application. Overtop applications of chloroxuron or toxaphene controlled 10 to 50% of the sicklepod. Postdirected applications of metribuzin or paraquat controlled 65 to 98% of the sicklepod but also caused slight crop damage when a 10 to 15 cm height differential between crop and weed was maintained. Moderate (40 to 60%) crop injury occurred in areas of shorter soybeans. Best sicklepod control was obtained in those treatments containing metribuzin as a preemergence treatment and where metribuzin or paraquat were applied as a postdirected spray. Severe crop injury can be expected if metribuzin or paraquat is applied to the top half of the soybean plant.