Potential to Improve Nitrogen Fertilizer Use Efficiency and Mitigate Pollution in Tropical Pastures Grazed by Sheep

We hypothesize that increasing N application rates will affect N use efficiency (NUE) for lambs grazing warm‐season annual grasses. We evaluated the effect of N fertilizer application rates (50, 100, 200, and 400 kg N/ha) on forage pearl millet ( Pennisetum glaucum (L.) R.) and animal responses: N balance and NUE per animal and per area basis. The experiment was conducted in a randomized complete block design with three replicates over a 70‐day grazing period. Pastures were grazed under continuous stocking using variable stocking rate (SR) to keep pasture sward height at 30 cm across treatments. Nitrogen application rate did not affect ( p > 0.05) animal ADG (g/day) and his N balance but increased ( p < 0.01) forage production resulting in an increase in SR from 850 to 1560 kg live weight (LW)/ha. This also resulted in an increase of N excretion ( p < 0.01; 55–124 kg N/ha), particularly via urine (34 to 91 kg N/ha). Animal NUE (kg LW gain/kg of N applied) decreased with increasing N application rates. These results demonstrate that the major impacts of N application occur on pasture growth and its ability to support greater SR, which increases N deposition via excreta.