UREA MANAGEMENT FOR LOWLAND TRANSPLANTED RICE AS AFFECTED BY APPLICATION OF FARMYARD MANURE

Farmyard manure (FYM) applied to rice-growing soils can substitute for industrial fertilizers, but little is known about the influence of FY M on the effectiveness and optimal management for industrial N fertili zers. A field experiment was conducted in northern Vietnam on a degrad ed sail in the spring season (February to June) and summer season (Jul y to November) to determine the effect of FYM on optimal timing for th e first application of urea. The experimental design was a randomized complete block with two rates of basal incorporated FYM (0 or 6 Mg ha( -1)) in factorial combination with two timings of the first applicatio n of 30 kg urea-N ha(-1) (basal incorporated before transplanting or d elayed until 14 to 16 d after transplanting). The FYM was formed by co mposting pig manure with rice straw for 3 months. Basal incorporation of FYM, containing 23 kg N ha(-1), increased rice grain yield in both seasons. The yield increase cannot be attributed to reduced ammonia lo ss of applied urea-N, because FYM did not reduce partial pressure of a mmonia (pNH(3)) following urea application in either season. Basal and delayed applications of urea were equally effective in the absence of FYM, but when FYM was applied rice yields in both seasons were higher for delayed (mean = 3.2 Mg ha(-1)) than basal (mean = 2.9 Mg ha(-1)) application of urea. Results suggest that recommendations for urea tim ing in irrigated lowland rice should consider whether farmers apply FY M.