On-farm assessment of mineral nitrogen and cultivar effects on rice productivity in Bhutan highlands

Abstract

In Western Bhutan, a traditional mixed farming system is practised with integration of both livestock and crops. Irrigated highland rice (Oryza sativa L.) is grown in terraced bunds by smallholder farmers for household consumption, with farmyard manure as the main source of nutrient input. The study objectives were to evaluate the rice productivity of the existing farming system, assess the potential of improved cultivars and nitrogen input, and their economic analyses. On-farm trials were carried out in 17 farmers' fields with one traditional and two improved cultivars of rice at 0 and 60 kg nitrogen ha−1 in two highland rice-growing districts. Across locations, environmental index varied from 2.9–6.3 Mg ha−1, exhibiting significantly wide differences in grain yield within and across village clusters. Soil properties were highly variable and soil organic matter was relatively high among the locations. Timely transplanting and small farm size had positive effects on grain yield. Significant effects of location, nitrogen, genotype, and environmental index×nitrogen were recorded on grain yield and total dry matter. Nitrogen fertilizer increased grain yield by 790–840 kg ha−1 in improved cultivars compared with the traditional cultivar (530 kg ha−1). Significant cultivar effects on grain yield were recorded with improved cultivars yielding 670–980 kg ha−1 more than the traditional cultivar due to higher harvest index (13–22%) and agronomic efficiency in nitrogen use (59%). Environmental index×nitrogen indicated significant difference in nitrogen response across the range of environmental indices. The highest yielding (Khangma Maap) cultivar was the most economical option due to high value/cost quotient (11.2) and marginal rate of return (Bhutan Ngultrum 16.77). Although yield levels were affected by both agronomic and socio-economic factors, Khangma Maap had higher grain and straw yield across the environments and, hence, the cultivar may be suggested for adoption.