Summer and Rainy-Season Weed Control Effects on Weeds in Soybean and Succeeding Wheat

A field experiment laid out in a split plot design was undertaken during 2006–07 and 2007–08 to evaluate the effects of five summer treatments, namely, summer cowpea as smother crop; deep disking followed by (fb) glyphosate at 1.0 kg ha⁻¹ once, and twice; soil solarization fb glyphosate at 1.0 kg ha⁻¹; and farmers’ practice (one summer ploughing) supplemented with four rainy-season treatments, viz. unweeded control, imazethapyr at 75 g ha⁻¹ 20 d after sowing (DAS), in situ Sesbania grown and mulched and two hand weeding at 3 and 5 weeks after sowing (WAS) on weeds in soybean [Glycine max L. (Merrill)] and wheat (Triticum aestivum L. emend Fiori & Paol). Summer treatments were adopted during May-June, while rainy-season treatments were superimposed with them after soybean sown. Wheat was grown in succession without any weed control measure. Soil solarization fb glyphosate at 1.0 kg ha⁻¹ significantly reduced broad-leaved weed (BLW) and grass weed density and dry weight in soybean and wheat. Its carry-over effects, however, varied with weeds species in wheat: it reduced the density of Chenopodium album, C. murale, Coronopus didymus, Convolvulus arvensis, Spergulla arvensis, and Anagallis arvensis, but could not significantly reduce Melilotus indica. It gave 66% higher soybean seed yield and 28% higher wheat yield compared with those of the farmers’ practice. Deep disking fb glyphosate at 1.0 kg ha⁻¹ (twice) was the second best treatment for reducing the weed density and dry weight in soybean and wheat. Among rainy season treatments, Sesbania in situ mulch and imazethapyr at 75 g ha⁻¹ significantly reduced BLWs and grass weeds in soybean compared with unweeded control, and, thereby, caused an increase in soybean seed yield by 33% and 39%, respectively. But, rainy-season treatments could not produce significant effects on weeds in wheat and hence, wheat yields were comparable across these treatments. Thus, a combination of soil solarization fb glyphosate application during summer and Sesbania in situ mulch or imazethapyr at 75 g ha⁻¹ during rainy-season would reduce weed competition greatly and result in higher yields of soybean and wheat.