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

Quantifying rice yield gaps and their causes in Eastern and Southern Africa

The demand for rice in Eastern and Southern Africa is rapidly increasing because of changes in consumer preferences and urbanization. However, local rice production lags behind consumption, mainly due to low yield levels. In order to set priorities for research and development aimed at improving rice productivity, there is a need to characterize the rice production environments, to quantify rice yield gaps —i.e. the difference between average on-farm yield and the best farmers’ yield— and to identify causes of yield gaps. Such information will help identifying and targeting technologies to alleviate the main constraints, and consequently to reduce existing yield gaps. Yield gap surveys were conducted on 357 rice farms at eight sites (19-50 farmers per site) across five rice-producing countries in Eastern and Southern Africa —i.e. Ethiopia, Madagascar, Rwanda, Tanzania and Uganda— for one or two years (2012-13) to collect both quantitative and qualitative data at field and farm level. Average farm yields measured at the eight sites ranged from 1.8 to 4.3 t ha–1 and the average yield gap ranged from 0.8 to 3.4 t ha–1. Across rice growing environments, major causes for yield variability were straw management, weeding frequency, growth duration of the variety, weed cover, fertilizer (mineral and organic) application frequency, levelling and iron toxicity. Land levelling increased the yield by 0.74 t ha–1, bird control increased the yield by 1.44 t ha–1, and sub-optimal management of weeds reduced the yield by 3.6 to 4.4 t ha–1. There is great potential to reduce the current rice yield gap in ESA, by focusing on improvements of those crop management practices that address the main site-specific causes for suboptimal yields