Nitrogen is one of the major essential plant nutrients and a key input required for better crop yields and therefore scarcity of nitrogen fertilizer has been 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. There has been concerted efforts to identify rice varieties that are tolerant to low soil nitrogen since varieties differ in their ability to impact productivity and some varieties can perform well under low nitrogen input.The Mwea Upland rice (MWUR) varieties have been 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 soil 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 to 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 experimental 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 four MWUR varieties namely MWUR 1, MWUR 2, MWUR 3, MWUR 4; and four NERICA varieties namely NERICA 1, NERICA 4, NERICA 10 and NERICA 11. The parameters measured included plant height, tiller number, filled grain ratio percentage and yield components. In the study, nitrogen treatment showed significant effect on plant growth and the measured parameters increased significantly with increase in nitrogen level. MWUR varieties studied were more adaptable to low nitrogen conditions as compared to NERICA varieties. The NERICA varieties recorded higher yield at high nitrogen levels as compared to MWUR varieties. However, NERICA 4 gave higher yield as compared to other NERICA varieties regardless of the nitrogen level. Results from our study suggest that MWUR 1 and 2 and NERICA 4 were more tolerant to low nitrogen as compared to MWUR 3 and 4 and NERICA 1, 10 and 11, because of higher height, more tiller number, higher filled grain ratio percentage and higher yield component as compared to the other studied varieties and may be suitable for soils low in nitrogen.
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