Growth and productivity of lowland rice (Oryza sativa L.) under different meteorological conditions imposed by varying the dates of planting in Agusan del Norte, Philippines

Changing the seasonal cropping calendar to avoid heavy rainfall, floods and low solar radiation is being considered for improving rice (Oryza sativa L.) productivity in Agusan del Norte, Philippines. An experiment was conducted to determine the growth and productivity of lowland rice grown under different meteorological conditions imposed by varying the dates of planting. Four lowland rice varieties: PSB Rc82, NSIC Rc146, NSIC Rc122 and PSB Rc18 were grown in six different planting dates such as the early or regular planting dates (Jan 15 and 30) and late planting dates (Feb 14 and 28; Mar 15 and 30). For early planting dates, high amount of total rainfall during the vegetative stage resulted in less tillering, decreased leaf area index (LAI) and dry matter yield (DMY), while high amount of cumulative solar radiation (CSR) and cumulative temperatures (CTemp) enhanced the growth parameters. In the later dates of planting (Feb 14 to Mar 30), high CSR did not result in high grain yields as shown by low DMY, sink size (SS), solar radiation-use efficiency (SRUE), and harvest index (HI). The growth and yield-determining parameters that significantly contributed to grain yields of rice grown in six planting dates were: filled grains per panicle, total spikelets per panicle and HI. Parameters such as panicles per sq m and 1000-grain weight had minimal contribution to the improvement of grain yield. Declining grain yields in the later planting dates could also be attributed to negating effect of high CTmax as CSR increases. When total pest damage due to rodents, WSB, ShB and BLB were added in the set of factors, the contributions of meteorological elements were overridden by such factors. Although pest management was improved using PalayCheck guidelines, damages caused by pests and diseases were apparent in the very early and late plantings. Intermittent rainshowers and sunny period coupled with high relative humidity and favorable temperature encouraged pest build-up. Among the six planting dates, regular planting at Jan 30 produced high grain yields (5.4-6.7 t/ha) for all varieties, and surpassed the target grain yield of 9 t/ha by 8-26%. High grain yields were associated with high crop growth rate (CGR) and net assimilation rate (NAR) and solar-radiation use efficiency (SRUE) due to more number of spikelets per panicle and filled grains per panicle, reflecting greater SS, grain filling, DMY [dry matter yield] and HI [harvest index]. The meteorological conditions in the planting date could be considered optimum for rice crop growth due to declining amount of rainfall towards reproductive stage (106.8 mm) and maturity (40.9 mm), optimum CSR from vegetative to maturity of 919.0 MJ/sq m, and favorable mean air temperatures of 27.3-29.0 deg C. With the regular planting dates (Jan 15 and 30), CSR had the greatest contribution to grain yield (R square = 56**). Among the varieties, PSB Rc18 was the best performing variety, having the consistently high grain yields (5.23-6.69 t/ha) across different planting dates, and being the only variety that surpassed the target yield (5t/ha) by 0.23-1.59 t/ha in all planting dates due to wide-range of adaptability and tolerance to pest infestation. On the other hand, varieties such as PSB Rc82, NSIC Rc146, and NSIC Rc122 performed when planted during the regular planting dates.