Weed Control Efficacy of Herbicides in Soybean in Paddy Field and Their Residual Effect on Rice Growth

The experiments were conducted to determine weed control effect of several herbicide treatments in soybean and their residue effect on succeeding rice in rice-soybean rotated paddy field. In rice-soybean rotated soybean field, 24 weed species were occurred, and Echinnochloa crus-galli L., Persicaria pubescens, Ludwigia prostrata Roxb, and Aeschtynomene indica L. were dominant species. Short-term and slight phytotoxicity was observed in soybean 4～5 weeks after applications of flufenacet+metribuzin and metolachlor+prometryn. The weed control values were greater than 90% at flufenacet+metribuzin, metolachlor+prometryn, linuron+thiobencarb, and sequential application of s-metolachlor, alachlor, ethalfluralin, trifluralin or linuron followed by fenoxaprop-P-ethyl or fluazifop-P-butyl. However, weed control efficacy of s-metolachlor, ethalfluralin, trifluralin, and linuron without sequential application of graminicide herbicides were very low. Soybean yield was higher at alachlor followed by fenoxaprop-P-ethyl, alachlor followed by fluazifop-P-butyl and ethalfluralin followed by fluazifop-P-butyl treatments in comparison with that at s-metolachlor treatment. Consequently, flufenacet+metribuzin, metolachlor+prometryn, linuron+thiobencarb, and s-metolachlor, alachlor, ethalfluralin, trifluralin, and linuron followed by fluazifop-P-butyl were most effective to control weeds in soybean in rice-soybean rotation paddy field. Inhibition of early rice growth was observed by residues of s-metolachlor, linuron, and ethalfluralin followed by fenoxaprop- P-ethyl or fluazifop-P-butyl in soil at 4 months after soybean harvest