Competition for nutrients by rice and weeds.
In trials in 1976, rice cv. Triveni (short duration) seedlings were transplanted into plots given 70 kg N, 35 kg P and 35 kg K/ha into which Echinochloa crus-galli seeds had been broadcast and left unweeded, weeded to provide weed-free periods of 10, 20 or 30 days beginning at various times between transplanting and 60 days later or kept weed-free. Crop N, P and K uptake was lowest in unweeded plots and in those weeded 1-10 or 51-60 days after transplanting and highest in weed-free crops and those weeded for 20 or 30 days up to 40-50 days after transplanting. Weed N, P and K uptake followed a reverse pattern. Weeding before the 20th and after the 40th day after transplanting did not reduce weed nutrient uptake which was negatively correlated with crop grain yield. Crop and weed nutrient demand was in the order K > N > P. Weeds competed with the crop for N and K during days 11-50 and for P during days 21-40.