Ammonia losses from urine and dung of grazing cattle
Nitrogen excretion by cattle during grazing is a significant source of atmospheric ammonia. In this study the relation between NH3 volatilization and N intake was investigated in wind tunnel experiments with simulated urine patches and dung pats. Excreta were collected from four groups of dairy cattle grazing continuously on either ryegrass fertilized with 300 kg N ha-1 or unfertilized white clover-ryegrass. The two groups of cattle in each grazing system received either 139 or 304 g N cow-1 d-1 in concentrates, corresponding to average total N intakes in the range of 500–700 g N cow-1 d-1. Ammonia losses from dung were insignificant, while total losses from urine, which were estimated by curve-fitting, ranged from 3 to 52% of urinary N. Urea-N in the urine applied in the experiments constituted, with one exception, 64–94% of urinary N. The fraction of urea-N increased significantly with total N concentration in subsamples from individual animals. In the soil, hydrolysis of urea to NH3 was almost complete within 24 h, and release of NH3 was indicated by scorching. Milk yield and the production of milk protein was not related to N intake or grazing system, while estimated NH3 losses were significantly reduced at the lower N intake level within the range of N intakes obtained.