Methodology for evaluation of lowland rice genotypes for nitrogen use efficiency

Rice is a staple food for more than 50% of the world's population. Based on land and water management practices, rice ecosystem is mainly divided into lowland, upland, and deep water or floating rice. However, major area and production at global level comes from lowland or flooded rice system. In rice growing regions nitrogen (N) is one of the most yield-limiting nutrients for rice production. Adaptation of cultivars or genotypes with high N use efficiency is a potential strategy in optimizing N requirements of crops, lowering the cost of production and reducing the environmental pollution. The objectives of this paper are to discuss rate and timing of N application, define N-use efficiency, discuss mechanisms involved for genotypic variation in N-use efficiency and present experimental evidence of genotypic variations in N-use efficiency in lowland rice. Evaluation methodology and criteria for screening N-use efficiency are also discussed. Significant variation in N use efficiency exists in lowland rice genotypes. Nitrogen use efficiency parameters (grain yield per unit of N uptake, grain yield per unit of N applied and recovery of applied N) are useful in differentiating lowland rice genotypes into efficient and non-efficient responders to applied N. Such an evaluation could assist in identification of elite genotypes that could be used in breeding program to produce cultivars with high N use efficiency and capable of producing high yields.