Effect of herbicides and their combinations on weeds and productivity of direct-seeded rice (Oryza sativa)

A field experiment was conducted during the rainy (kharif) seasons of 2014 and 2015 at the Bidhan Chandra Krishi Viswavidyalaya (BCKV), West Bengal, to evaluate the effect of different herbicides and their combinations on growth of composite weed flora and productivity of direct-seeded rice (DSR) (Oryza sativa L.). The experiment was laid out in a randomized block design with 3 replications. The experiment comprised 8 weed-control treatments, viz. T₁, pyrazosulfuron ethyl @ 30 g/ha as post-emergence (PoE); T₂, pretilachlor @ 0.75 kg/ha as pre emergence (PE); T₃, pyrazosulfuron ethyl 0.75% + pretilachlor 30% WG (pre-mix) @ 1.50 kg/ha (commercial product) as PoE; T₄, pyrazosulfuron ethyl 0.75% + pretilachlor 30% WG (pre-mix) @ 1.75 kg/ha (commercial product) as PoE; T₅, pyrazosulfuron ethyl 0.75% + pretilachlor 30% WG (pre-mix) @ 3.50 kg/ha (commercial product) as PoE; T₆, Almix @ 20 g/ha (commercial product) as PoE; T₇, bispyribac-sodium @ 25 g/ha as PoE + 1 hand weeding (HW) at 40 days (farmers’ practice); and T₈, unweeded control (season-long weedy condition). All weed-control options significantly influenced the weed growth and crop yield. Monocot weeds, particularly sedges, were the most dominant among all the species. Grain yield losses due to weeds would amount to 59.0–65.6%, if weed-control measures were not undertaken. Combined applications of ready-mix herbicides provided better weed control with higher weed-control index (WCI) in DSR than those observed with sole application of herbicides. Post-emergence application of pyrazosulfuron ethyl + pretilachlor as ready-mix (3.5 kg/ha) resulted in significantly lower weed density and dry matter with higher WCI (77.25%) at 30 days after application (DAA) than the other sole and combined applications of herbicides. Application of herbicides initially reduced microbial activity slightly up to 3 DAA, but recovered well at subsequent stages owing to improved crop growth. A significant negative correlation was found between weed growth (density and biomass) and yield of rice. Ready-mix application of pyrazosulfuron ethyl + pretilachlor (3.50 kg/ha) being at par with pyrazosulfuron ethyl + pretilachlor @ 1.75 kg/ha (ready-mix) and bispyribac-sodium (25 g/ha) + 1 hand weeding (farmers’ practice) resulted in the highest grain (2.99 t/ha) and straw yield (4.29 t/ha) of rice. This weed-control treatment resulted in a mean grain yield increase of 190.3% compared to uncontrolled weed growth.