AMMONIA LOSSES FROM URINE AND DUNG OF GRAZING CATTLE - EFFECT OF N INTAKE

Nitrogen excretion by cattle during grazing is a significant source of atmospheric ammonia. In this study the relation between NH3 volatiliz ation and N intake was investigated in wind tunnel experiments with si mulated urine patches and dung pats. Excreta were collected from four groups of dairy cattle grazing continuously on either ryegrass fertili zed with 300 kg N ha(-1) or unfertilized white clover-ryegrass. The tw o 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 int akes in the range of 500-700 gN cow(-1) d(-1). Ammonia losses from dun g were insignificant, while total losses from urine, which were estima ted 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 so il, hydrolysis of urea to NH3 was almost complete within 24 h, and rel ease of NH3 was indicated by scorching. Milk yield and the production of milk protein was not related to N intake or grazing system, while e stimated NH3 losses were significantly reduced at the lower N intake l evel within the range of N intakes obtained. (C) 1998 Elsevier Science Ltd. All rights reserved.