Growth and Productivity of Irrigated Rice (Oryza sativa L.) for a Tropical High Altitude Environment in Rwanda

Yield components, grain yield, biomass and plant N accumulation, and N fertilizer responses of irrigated rice (Oryza sativa, L.) were evaluated for a tropical inland valley environment in the high altitudes in Rwanda. Effects were measured for nitrogen (N) fertilizer rate (0, 60, 90,120,150,180 kg N ha-1), season, growth stage and cultivar of different subspecies and plant types during 2012 dry season (DS) and 2013 wet season (WS) at Cyili Rice Research Farm (1380 m above sea level). Variations in irradiance (17.2+/- 0.32 MJ m-2 day-1 in DS and 9.4 +/- 0.66 MJ m-2 day-1in WS) were linked to seasonal differences in biomass and closely related to N responses, yield components and the grain yields which were greater in the DS (9.0 t ha-1) than in the WS (5.3 t ha-1). Total plant N uptake was in the same range (200 to 250 kg N ha-1) for both seasons at maturity. Grain yields were highly correlated (r2 = 0.89) to yield components and mostly with the number of spikelets per panicle (r2 = 0.70) and spikelets per m2 (r2 = 0.80). In most cases, yield components were equivalent or greater to the potential under irrigated lowlands in the tropics of Asia and the highest farm yields (8 to 10 t ha-1) in the temperate. Rice genotypes of Indica x Japonica subspecies combinations were overall greater in yield, harvest index and in important yield components. Crop growth rate (CGR) over seasons and cultivars was 11g m-2 day-1 at maturity. The average total crop duration of 145 days may be considered as the current optimal for achieving the yield potential obtained in the irrigated lowlands of the tropics in Asia. Responses to N fertilizer addition were generally minimal with most measured attributes maximized when the lowest N rate of only 60 kg N ha-1 was applied. This suggested a large contribution from the native soil N reserves linked to ample amounts of total soil N (2.80 +/- 0.34 g kg soil-1). The Indica x Japonica combinations may represent a cultivar improvement strategy for greater adaptation and grain yield and quality in Rwanda.