Field persistence of butachlor and 2,4-D in rice soil under continuous and rotational use: Effect of nitrogen sources and seasons

Field experiments were conducted from kharif 2000 to rabi 2008 to study the influence of continuous application of herbicides on their fate and persistence behavior under transplanted rice-rice cropping systems. Treatments included hand weeding, combined application of butachlor plus 2, 4-D for both seasons and the rotational use of butachlor plus 2.4- D and pretilachlor plus 2,4-D for kharif and rabi seasons, respectively under two sources of nitrogen viz., 100% N as inorganic and 75% organic plus 25% organic N sources during kharif seasons. The soil samples at periodic intervals were collected and analyzed for residues using GC-ECD/HPLC. Degradation of herbicide was described using first order kinetics. Mean half life of the butachlor and 2,4-D during kharif was found to be 9 and 7 days respectively and 13 days during rabi for both the molecules. Comparing pooled data of 2000–2004 with 2005–2008, butachlor build up was 31% high irrespective of continuous or rotational application and was also influenced by the N sources. 2,4-D build up was also influenced by the N sources and method of herbicide use. Under continuous usage, 19 per cent more 2,4-D build up was recorded when compared to 10 per cent under rotational use. Similarly mean 2,4- D build up in rice soil during 2005–2008 was 16 and 14 per cent under 100 per cent inorganic and 75 per cent organic N + 25 per cent inorganic plots respectively when compared to 2000–2004.