Chemical weed control in soybeans planted in nontilled wheat stubble

A study was made to (1) determine whether soybeans (Glyclne max (L.) Merr.) could be grown successfully in wheat stubble as part of a double cropping system, (2) test different combinations of herbicides for weed control, (3) determine the effects of burning versus non burning of straw on soybean vigor and weed population, and (4) determine by bioassay if herbicide residues persisted at the end of the growing season. At Knoxville herbicide treatments effectively controlled weeds throughout the growing season but at Springfield only through the sixth week. Weed control at Springfield failed completely by the end of the growing season. The best weed control ratings were recorded on the DCPA (dimethyl 2,3,5,6-tetrachloroterephthalate) plus paraquat (1,1'- dimethyl-4,4'bipyridinium ion) treatment, linuron [3-(3,4-dichlorophenyl)- 1-methoxy-1-methylurea] plus paraquat treatment and the alachlor [2-chloro-2’,6'-diethyl-N-(methoxy ethyl) acetanilide] plus linuron plus paraquat treatment. The poorest weed control was recorded on the amiben (3-amino-2,5-dichlorobenzoic acid) and the amiben plus paraquat treatments. At Knoxville, percent weed control was higher on the burned plots than on the non burned plots. At Springfield, soybeans showed an extreme loss of vigor by the end of the growing season; however, at Knoxville, soybeans remained vigorous throughout the growing season. Extreme loss of vigor at Springfield was attributed to increasing weed competition. Soybean yields on herbicide treated plots averaged only 4 bushels per acre as compared to 18 bushels on the conventional seedbed plus cultivation treatment. The low yields on herbicide treated plots were due to weed competition and insufficient rainfall. At Knoxville, yields on herbicide treated plots averaged 38 bushels per acre as compared to 45 bushels on the conventional seedbed plus cultivation treatment. At Knoxville, a bioassay with crimson clover (Trifolium incarnatum L.) and wheat (Triticum aestivum L.) seedlings indicated that no toxic herbicide residues remained in the soil at the end of the growing season.