Effect of nitrogen supply on the comparative productivity of maize and sorghum in a semi-arid tropical environment III. Grain yield and nitrogen accumulation

Differences in grain yield and grain nitrogen content due to variable N supply in maize and sorghum, grown under irrigation in the semi-arid tropics, were associated with differences in their rates of accumulation and in the extent of mobilization of pre-anthesis dry matter and N. Increases in harvest index, nitrogen harvest index, and grain size during grain-filling were essentially linear, and the rates were constant under variable N supply. Differences in harvest index at maturity at different levels of applied N were small relative to differences in biomass and grain yield, and differences in grain size were small relative to differences in grain number. In effect, variation in harvest index and grain size at maturity was associated with variation in the length of the effective grain-filling period. Nitrogen supply had a greater effect on the effective duration of grain-filling in maize than in sorghum.Maximum grain yields at high levels of N uptake were greater in maize (10.0 t ha⁻¹ at 15.5% moisture) than in sorghum (7.2 t ha⁻¹ at 14% moisture). These differences were related to differences in biomass production, rate of increase in harvest index and the effective duration of grain-filling. The rate of increase in harvest index was greater, but biomass production was less and the duration of grain-filling was shorter, in sorghum than in maize.