Lowland Rice Response to Urea Following Three Cowpea Cropping Systems

The integrated use of legume green manure or legume residue with inorganic N fertilizer could offer rice (Oryza sativa L.) farmers an opportunity to reduce their expensive inorganic fertilizer inputs. A 2-yr field experiment was conducted on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems on response of a subsequent rice crop to applied urea and effectiveness of several urea management practices for rice. The pre-rice cropping systems including fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with either grain and pods removed or all aboveground vegetation removed prior to soil puddling for rice. The mean aboveground N content was 66 kg N ha⁻1 for green manure and 54 kg N ha⁻1 for mature cowpea without grain and pods. Rice yield responses following fallow and cowpea cropping with removal of all aboveground cowpea vegetation were similar, but mean rice grain yields were 0.9 and 0.7 Mg ha⁻1 higher following incorporation of green manure and residue, respectively. The green manure substituted for 34 and 54 kg urea-N ha⁻1 in 1986 and 1987, respectively. Residue substituted for 44 and 50 kg urea-N ha⁻1 in 1986 and 1987, respectively. Early application of urea (two-thirds basal and one-third at 5 d before panicle initiation) was superior to delayed application (one-half at 14 d after transplanting and one-half at 10 d after panicle initiation), even when supplemental N as green manure or residue was incorporated 15 d before transplanting. Cowpea grown for green manure or for grain, with incorporation of residues remaining after harvest, made similar positive contributions to rice yield, but growing cowpea to maturity offered the advantage of attaining approximately 1.0 Mg ha⁻1 cowpea grain production. Joint contribution from IRRI, IFDC, and IARI.