Nutrient uptake by direct seeded rice and associated weeds as influenced by sowing date, variety and weed control

Nutrient uptake by direct seeded rice and associated weeds was studied in a field experiment during kharif 2008. Twenty four treatment combinations viz. six sowing dates (direct seeding on 0 (June 5), 7,14, 21 and 28 days after nursery sowing (DANS); transplanting 28 DANS); two varieties (short duration PR 115 and medium duration PAU 201) and two weed control treatments (3 hand weedings at 20, 40, 60 days and pendimethalin 0.75 kg ha−1 pre-emergence followed by bispyribac-sodium 0.030 kg ha−1 as post emergence) were evaluated in a split plot design with three replications. Among sowing dates and establishment methods, rice grains in manual transplanting utilized significantly higher amount of nitrogen and phosphorus than all the direct seeding dates 0, 7, 14, 21 and 28 DANS; potassium uptake was at par to direct seeding on 0 DANS. The rice straw utilized the highest amount of potassium in transplanted treatment and was at par to direct seeding 0 DANS; nitrogen and phosphorous uptake did not show any specific trend. The nitrogen and potassium removal by the weeds was the lowest in transplanted check which was at par to direct seeding on 0–7 DANS and for phosphorous removal it was at par to 0–14 DANS. Nitrogen and potassium uptake by rice grain and straw did not vary among varieties, however, phosphorous uptake by grain and straw was significantly higher in case of PR 115 than PAU 201; weeds also removed significantly higher amount of potassium in PR 115 as compared to PAU 201. Among weed control treatments, rice grains in the three hand weeding treatment utilized significantly higher amount of nitrogen as compared to sequential application of herbicides; nitrogen uptake by straw was non-significant. The weeds in herbicidal plot removed significantly higher amount of nitrogen, phosphorous and potassium as compared to three hand weedings. The nutrient uptake by crop was directly related to crop dry matter accumulation and grain yield while nutrient removal by weeds was directly related to the weed dry matter accumulation under different treatments.