Soils used for cropping of rainfed lowland rice are frequently rather poor, and nutrient limitation is known to be a major constraint for grain yields. However, actual fertilizer use by most rainfed rice farmers is still very low even if input-responsive rice varieties are widely adopted. To address this contradiction, this study, conducted in central and southern Laos, intended to test existing fertilizer recommendations in farmers' fields and under their crop management, to evaluate the effect of topographic field position on fertilizer response, and to conduct an economic evaluation of fertilizer use based on farmers' input and output prices. Trials were conducted for two seasons in three different agroecological environments (rainfed rice in lower fields and in middle/upper fields, and irrigated rice in lower fields). Tested were five different fertilizer treatments in four farmers' fields per site and season. Commercial organic fertilizer increased yields but resulted in negative economic returns in almost all cases because of its unfavorable nutrient/price ratio. In lower fields, farmyard manure and inorganic fertilizer gave good yield responses and positive economic returns but the average value/cost ratios of 1.3-1.6 were at the lowest level required for widespread adoption of fertilizer use. Water limitation reduced attainable yields in middle/upper fields and in this environment good fertilizer response was observed only for farmyard manure and half the recommended rate of inorganic fertilizer. The actual benefit of fertilizer use depended strongly on the input/output price ratios at each site. Thus, site-specific fertilizer management options, including economic parameters, are required to make fertilizer use in rainfed lowland rice more profitable. The necessary information for such guidelines is available but needs to be prepared in the form of decision tools to help farmers choose the optimal nutrient management option for their agroecological and socioeconomic production environment. (C) 2010 Elsevier B.V. All rights reserved.
