Differences in varietal response to nutrient management in rainfed lowland rice

In rainfed lowlands, both drought and soil infertility often constrain rice yield. In addition drought affects nutrient availability in the soil. The objective of this study was to evaluate varietal responses to nutrient management under irrigated and rainfed lowland conditions. Field experiments were established at the Cagayan State University (CSU) in Cagayan Valley and Mariano Marcos State University (MMSU) in Ilocos Norte, in the 2014 and 2015 wet seasons. In 2014, in both CSU and MMSU, 15 varieties were grown under four nutrient management (NM) treatments: 1) farmers' practice, 2) Rice Crop Manager (RCM), 3) RCM-Plus (additional nutrient input to RCM treatment), and 4) zero input. In 2015, the experiment was done only at MMSU. Ten varieties were grown under three NM treatments: 1) RCM, 2) RCM + 60P (additional 60 kg/ha of P2O5 to RCM treatment), and 3) zero input. Average yields under rainfed conditions were 1.2 t/ha (CSU), 3.7 t/ha (MMSU 2014), and 3.5 t/ha (MMSU 2015), which were lower by 11-17% than those under irrigated conditions (1.4 t/ha, 4.3 t/ha, and 3.9 t/ha, respectively). At CSU, yield increased by 15-24% with nutrient inputs under irrigated conditions and 12-28% under rainfed conditions compared with the zero input treatment (1.2 and 1.1 t/ha, respectively). At MMSU, yield responses to NM were 25-39% under irrigated conditions and 45-49% under rainfed conditions, compared with the zero input treatment (3.0-3.2 and 2.2-2.3 t/ha, respectively). Results showed that the yield response of rainfed lowland rice to nutrient input was as high as or even higher than that of irrigated rice. Genotypic variation among the varieties was observed in each location. Irrigated rice varieties was less responsive to nutrient input under rainfed conditions, whereas some drought-tolerant varieties (e.g., NSIC Rc282) performed well in both rainfed and irrigated conditions. Results indicate that both the use of locally adopted drought-tolerant rice varieties and appropriate nutrient management are important in the drought-prone rainfed lowlands of the Philippines.