Relative time of weed and crop emergence is crucial for managing weed seed production: A study under an aerobic rice system

Managing weed seed production may help in reducing weed emergence and biomass in successive seasons in dry-seeded rice. Field experiments were conducted in 2012 and 2013 to ascertain the effect of delayed weed emergence, i.e., 0, 14, 28, 42, 56, and 70 days after sowing (DAS) on seed production ability of Digitaria ciliaris and Echinochloa crus-galli in two rice cultivars (PR115 with 125 days duration and PR114 with 145 days duration). Emergence of E. crus-galli was similar in both the cultivars, except at 42 DAS, where the emergence was higher in PR114 than PR115 during 2012. In PR114, D. ciliaris continued to emerge up to 56 DAS, although the increase was significant up to 28 DAS; the increased emergence of this weed was noticed up to 14 DAS in PR115. Weed biomass was also higher in PR114, although the rate of the decrease with respect to emergence time was similar in both the rice cultivars. D. ciliaris, when emerged with the crop, produced 11,820 seeds per plant in PR114, but seed production reduced by 25% in PR115. However, differences in seed production of E. crus-galli were noticed when this weed emerged between 14 and 42 DAS. An exponential model predicted that, for a 99% reduction in seed production of D. ciliaris, 42-45 days of crop advancement, while, for E. crus-galli, 21-33 days of crop advancement was required. Our study suggests that delaying the weed emergence in the crop and/or adopting better competitive rice cultivars can reduce the weed seed production substantially.