Weed Control in Soybean under Different Cropping and Tillage Systems

Studies were established in 1987 and 1988 near Brooksville and Poplarville, MS, to evaluate six weed‐control systems in monocrop soybean [Glycine max (L.) Merr.] and doublecrop wheat (Triticum aestivum L.)—soybean planted both with and without prior tillage. Greater than 80% broad‐leaf signalgrass [Brachiaria platyphylla (Griseb.) Nash.] control was achieved with all systems except metribuzin [4‐Amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2, 4‐triazin‐5(4H)‐one] plus alachlor [2‐Chloro‐2′‐6′‐diethyl‐N‐(methoxymethyl)‐acetanilide] preemergence (PRE) followed by either bentazon [3‐isopropyl‐1H‐2,1,3‐benzothiadiazin‐4(3H)‐one 2,2‐dioxide] plus acifluorfen [sodium 5‐[2‐chloro‐4‐(trifluoromethyl)phenoxy]‐2‐nitrobenzoate] postemergence (POE) or cultivation in the monocrop no‐till system. Southern crabgrass [Digitaria ciliaris (Retz.)Koel.] control was greater than 80% in all systems with metribuzin plus alachlor PRE followed by cultivation, pendimethalin [N‐(1‐ethylpropyl)‐3,4‐dimethyl‐2,6‐dinitro‐benzenamine] plus imazaquin [2‐[4,5‐dihydro‐4‐methyl‐4‐(1‐methylethyl)‐5‐oxo‐1H‐imidazol‐2‐yl]‐3‐quinoline carboxylic acid] PRE followed by imazaquin POE, or pendimethalin plus metribuzin plus chlorimuron [Ethyl 2‐[[[[(4‐chloro‐6‐methoxypyrimidin 2‐yl)amino]carbonyl]aminojsulfonyl]benzoate] PRE followed by chlorimuron POE in 1987. Pitted morningglory (Ipomoea lacunosa L.) and smallflower morningglory [Jacquemontia tamnifolia (L.)Griseb.] control varied, but was best with treatments containing imazaquin or metribuzin plus 2,4‐DB [4‐(2,4‐Dichlorophenoxy) butyric acid]. Sicklepod (Cassia obtusifolia L.) control was generally better in doublecrop than in monocrop systems, and in no‐till than tilled systems. Control of prickly sida (Sida spinosa L.) and prostrate knotweed (Polygonum aviculare L.) was greater than 90% with all weed control systems used in combination with tillage, but control was more erratic without tillage. Soybean yield at Poplarville was lowest in the monocrop no‐till system and highest in the doublecrop no‐till system in 1988. Soybean yield at Brooksville was better with tillage in both cropping systems in 1987, but in 1988 yields were very low in all cropping, tillage, and herbicide combinations, indicating the extremely dry conditions.