Department of Agronomy, Annamalai University, Annamalainagar-608 002 (Tamil Nadu), India.
Field experiments were conducted on rice-blackgram sequences consecutively over a period of three years at Annamalai University Experimental Farm, Annamalainagar, India. In the base crop of rice, the treatments compared included the herbicide butachlor at 1.0, 1.5 and 2.0 kg ha−1 doses independently and in combination with a hand weeding on 40 days after transplanting (DAT) alongwith a twice hand weeding treatment and an unweeded control. For the succeeding blackgram crop, the same set of treatments with alachlor replacing butachlor was compared (the treatments were being taken up in the same plot every year). Butachlor 2.0 kg ha−1 and hand weeding on 40 DAT gave highest grain yields in all the three years with an increase of 26.39, 41.58 and 51.05% over the un weeded control, respectively, whereas twice hand weeding recorded highest grain yields which were 58.39 and 61.55% higher than the unweeded control in first and second year blackgram, respectively. Rice weed control was observed not to have been carried over to blackgram. Dependence on herbicides solely, in the cropping system, resulted in a shift towards perennial weeds like nutsedge, whereas hand weeding alone led to the dominance of grassy weeds and unweeded rice culture caused the preponderance of broad-leaved weeds. Integration of herbicide with hand weeding restricted the dominance of individual weed species.
