Tropical highland conditions in Mwea Kenya, ensure the high radiation and the large day-night temperature differences. Such conditions are generally believed to promote rice growth and yield, but the current grain yield is lower than the expectation. In the current standard N fertilizer practice in Mwea, 75 kg nitrogen (N) ha −1 is applied in three splits at fixed timing. The effects of increases in N fertilizer amount (125, 175, and 225 kg N ha −1) on rice growth and yield were evaluated to test the hypothesis that unachieved high rice grain yield in Mwea is due to insufficient amount of N fertilizer. Two popular lowland varieties in Mwea (Basmati 370 and BW196) and two varieties reported as high yielding at other countries (Takanari and IR72) were used. Shoot dry weight (DW) increased with increases in the amount of N fertilizer applied in three splits at fixed timing, irrespective of variety. It reached approximately 20 t ha −1 under increased N conditions (>75 kg N ha −1) in several cases, indicating that high biomass production could be achieved by increasing N application rate. However, the increased biomass did not increase grain yield, due to decreased grain filling under high N conditions in all varieties. Thus, N amounts above 75 kg ha −1 were ineffective for increasing grain yields in Mwea, where N fertilizer was applied in three splits at fixed timing. Increasing influence of low temperature under high N conditions may be one of the reasons for the decreased grain filling in Mwea.

Cropping calendar optimization contributes to an increase in rice yield. Information on the seasonal variation in grain yield and climate conditions is necessary to determine an appropriate cropping calendar. We sought to find the optimal cropping calendar in Mwea, Kenya, in a tropical highland in equatorial East Africa. We conducted a series of 58 experiments using a local popular rice variety, Basmati 370, between 2013 and 2016, using a secured water supply and adequate blast control, sowing every 15 days. The grain yield was 0-2 t ha −1 when the variety was sown between March and June. This poor grain yield was attributable to the low temperature and low solar radiation from May to September. In contrast, the grain yield was always more than 3 t ha −1 when the variety was sown between July and February. Sowing Basmati 370 between March and June is not recommended, because it may lead to a suboptimal yield due to cold stress. The current cropping calendar (July-December or August-January) is acceptable even under abundant year-round water supply, but sowing between October and February is a good alternative sowing period for single rice cropping. Rice production per year is expected to increase to >100% with the introduction of double cropping by adding cultivation from between January and February before the current cropping calendar. These findings serve as useful references for considering and determining the appropriate calendar options for single and double cropping of rice in tropical highlands in equatorial East Africa.

Apart from nitrogen (N) rates, N use efficiency (NUE) (yield N/total input N) is affected by seasons, crop developmental stages, and varieties. Knowledge of how these factors affect NUE in rice production in Kenya is limited. Therefore, field experiments were conducted with ‘low rates’ of N (simulating farmers’ practices) of 0, 26, 52 and 78 kg N ha−1 with five varieties (MWUR1, MWUR4, IRAT109, NERICA4 and NERICA10) and higher rates of N (125, 175, and 225 kg N ha−1) simulating researchers’ doses with two lowland varieties (Basmati 370 and BW 196) and IR 72. Another experiment on NUE responses to sites, N rates and dose (split or full dose) was undertaken with the IR97 variety. With the ‘low rate’, yields increased with incremental N rates up to 52 Kg N ha−1 and declined (during cold periods, for some varieties). In this scenario, the N agronomic efficiencies (AEN) declined with increasing N but depended on sites and seasons. However, most AEN values were above 100, implying nutrient mining. In most cases (except at the Mwea site), the N utilization efficiency (NUtE) ranged from 16 to 22kg kg−1 and were not significantly affected by sources and methods of N application. In all cases, an increase in N elicited declining trends in NUtE. Moreover, N uptake efficiency ranged between 22 and 90kg kg−1 without significant variation among varieties. For the ‘high N rates’, high biomass yield resulted in higher grain yields in BW 196 and IR 72 but yield declined beyond 75 kg ha−1 N rates due to poor grain filling, particularly when a cold period coincided with booting and grain filling. We conclude that N rates, doses and rice varieties are key determinants of AEN and NUtE in contrasting rice growing seasons in Kenya. Cropping seasons and rice varieties are therefore potential key determinants of sustainable rice productivity and improved NUE in rice-based systems in the studied regions of Kenya.

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Scarcity of nitrogen fertilizer is a major constraint to rice production particularly in developing countries. Low soil fertility prevalent in farmer's fields has led to low rice yields and the ever escalating fertilizer prices have made this important input unaffordable to most smallholder farmers who have limited resources for purchasing the required inputs. The Mwea Upland rice (MWUR) varieties were bred under low fertilizer input environment while other authors have indicated that the New Rice for Africa (NERICA) gives high yields under low input conditions. There is therefore need to identify the superior rice varieties that are adaptable to low nitrogen levels. Thus, the objective of this study was to investigate the effects of different rates of nitrogen fertilizer on improved upland rice varieties and identify the low input adaptable varieties. Field studies were conducted at Alupe in Western Kenya under rainfed upland conditions between August 2012 and April 2013. The experiment layout was split plot factorial in a Randomized Complete Block Design with three replicates. The main plot treatments were four rates of nitrogen fertilizer levels which were; 0 (control), 40, 80 and 120 kg ha-1 applied as calcium ammonium nitrate (26% N) in two equal splits; 21 days after sowing (DAS) and at panicle initiation (46 DAS). Sub-plots consisted of Original Research Article

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