Modelling nitrogen excretion, elephant grass growth and animal production in a stall-feeding dairy system

This study reports on a simulation model of livestock-forage with excreted nitrogen (N) as a source of N for elephant grass (Pennisetum purpureum) growth. It is shown that nitrogen partitioning between urine and feces can be estimated using feed characteristics when elephant grass is the sole feed in stall-fed dairy heifers. The percentage N excreted in feces decreased with increasing dietary N, while N excreted in urine increased with increasing N intake. The simulation results indicate that at stocking level of 5 heifers ha-1, the application of excreted N supported the animals for up to 700 days but an additional heifer led to the depletion of forage within 90 days. It was observed that one hectare without N fertilization would support only 3 heifers for the same duration as 5 heifers on a fertilized hectare. It is concluded that N excretion can be predicted in stall-feeding dairy system, and it is possible for farmers to improve the forage biomass yield and thus animal performance by not only applying manure, but also by using the most appropriate method to minimize excreted N losses.