Effect of combined cattle manure and mineral nitrogen on maize N uptake and grain yield

The effectiveness in increasing N recovery by maize (Zea mays L.) of N fertiliser (0, 60, 120 kg N ha-1 annum-1) and aerobically composted cattle manure (0, 12.5 Mg ha-1 annum-1 or 37.5 Mg ha-1 applied only in the first year) was determined when the two N sources were applied separately or in combination. A field experiment was established on a moderately leached sandy loam soil (Typic Kandiustalf) over three seasons, and N uptake was determined fortnightly from 4 weeks after planting (WAP) until harvest. Net N recovery during plant growth increased with N application rate (up to 90 kg N ha-1 in first season and up to 60 kg N ha-1 in second and third seasons). Net N recovery from manure during the growing period was relatively poor. There was no net recovery (up to 12 WAP) from annual application of manure during the first season and from first year application of the large rate of manure in the second season. Combined application of manure and N fertiliser increased net N recovery in all growing seasons (up to 120 kg N ha-1) above that achieved by separate applications of both N sources. There was a manure by N fertiliser interaction that enhanced N recovery, in two treatment combinations during the first season (low manure rate) and in the second season, and this was attributed to some non-N nutrient effects of the manure. Highest percentage increases in total N recovery at harvesting were observed when manure was combined with the low N fertiliser rate (60 kg N ha-1) (av. 58% in first season, 63% in third season) and limited further increases were recorded when the N fertiliser rate was doubled (120 kg N ha-1) (av. 15% in first season, 32% in third season). It was concluded that aerobically composted cattle manure from the smallholder farming areas of Zimbabwe was a poor source of N for maize growth in the short-term, even at high application rates. Combined application of manure with judicious use of N fertiliser can be positively exploited by smallholder farmers in Zimbabwe and other countries of sub-Saharan Africa, to increase yields through enhanced efficiency of use of scarce nutrient resources