Simulating the response of maize to nitrogen fertilizer in semiarid Zimbabwe

Abstract

Crop yield responses to inorganic fertilizers are variable under low and erratic rainfall characteristic of the semiarid tropics. Risks are especially high on the infertile granitic sands in many smallholder communal areas of Zimbabwe. There is a need to develop fertilizer recommendations taking account of climatic variability, the price of maize and the cost of fertilizer, that are relevant to the low rates of fertilizer that smallholder farmers are willing to apply. The aim of this paper is to validate a cropping systems simulation model using experimental results of maize response to N, and then to apply the model to explore the risk associated with N fertilizer use and management strategies to minimize risk. Data for model validation came from a long-term maize N fertilizer rate trial (1991/92-1997/98) at Makoholi near Masvingo in semi-arid Zimbabwe. On average, maize growth and development in response to N was simulated with a degree of accuracy that justifies its use in analysis of risk associated with Nuse in these semi-arid regions of Zimbabwe. APSIM was then configured to simulate the response to N over a 46-year climate record in order to assess the long-term risks associated with N use. The simulated long-term distribution indicated that negative responses to N could be expected in 15% of years, whereas no negative response to N was recorded in the experiments at Makoholi. Median responses were 20-30 kg maize grain/kg N for observed and simulated results. In terms of return on fertilizer investment, the observed and simulated distributions were also similar: in about 20% of years, a negative return could be expected, while in the best 20% of years a return of $Z5000 or more could be expected, given the grain:fertilizer price ratio, which at March 2000 was about 1:7. The model analysis has suggested moderate rates (ca. 30 kg N/ha) of N fertilizer use would give greater responses per unit N applied that smaller rates (15 kg N/ha). There was no evidence that conditional fertilizer strategies based on early-season rainfall would offer significant benefits over fixed application strategies. Early sowing at recommended population densities gave higher responses to N than that for late sown and/or low density crops.