Low efficiency of nitrogen use by maize on smallholder farms in sub-humid Zimbabwe

Nitrogen (N) is the major nutrient limiting maize production in Zimbabwe. Surprisingly, little information is available about the maize yield response to N that smallholder farmers obtain on their fields under their own management. On-farm trials were conducted in five sub- humid smallholder areas of northern Zimbabwe during 1996/97 and 1997/98 to establish the response of maize to N. N fertilizer, at rates from zero to 125 kg N/ha (at 25 kg intervals), was applied to farmers' maize crops and to researcher-managed crops planted on farmers' fields. In twelve researcher-managed trials, three maize genotypes: SC401 (short season hybrid), SC601 (medium maturity hybrid) and ZM607 (an open- pollinated experimental population) were used. In 22 farmers' maize crops, N was applied by the farmer to the crop selected by the farmer using the farmers' normal methods and timing. During the very wet 1996/97 season, maize yields in the researcher-managed trials averaged 708 kg/ha without fertilizer and up to 2261 kg/ha with 125 kg N/ha, and from 506 kg/ha without fertilizer up to 1076 kg/ha with 125 kg N/ha in the farmers' crops. In 1997/98, low to medium rainfall led to higher yields. Maize yields in 1997/98 averaged 1205 kg/ha without fertilizer up to 3199 kg/ha with 125 kg N/ha in researcher managed crops, while those in farmers' fields averaged 1096 kg/ha without fertilizer and up to 2210 kg/ha with 125 kg N/ha. In farmers' crops in 1996/97, the highest amount of maize grain (kg) produced for every kg of applied N averaged across the sites was only 3.8 kg, and 12.4 kg in researcher- managed crops. Corresponding values for 1997/98 were 8.9 kg, and 16.0 kg. During 1996/97, it was uneconomic to apply any N at 46% of the sites. In 1997/98, modest economic gains were realised by applying N at up to 100 kg N/ha. The low efficiency of N use demonstrated in this study emphasises the need to develop and use technology and management that will allow farmers to get better returns from N fertilizer on maize.