PSX-1 Nitrogen balance in beef cattle offered mid-season grass herbage fertilised with two rates of nitrogen.

摘要

Nitrogen (N) excretion from cattle contributes to water and air pollution. The objective was to determine the effects of inorganic fertiliser N application rate to mid-season grass (July-August) on herbage chemical composition, and intake, digestion and N-balance in beef cattle. Sixteen Charolais steers (live weight 568 kg, S.D. 22.1) were used in a randomised block design experiment. Treatments were zero-grazed grass herbage (predominantly ) harvested 21 d after receiving either 15 (LN) or 80 (HN) kg N/ha. Herbage was offered daily and dry matter (DM) intake measured during a 12-d diet adaptation phase and at 0.9 of this DM intake during the subsequent 10-d period in metabolism stalls. Total faeces and urine were collected on a 48-h and 24-h basis, respectively, for 8 d. Blood samples were obtained by jugular venepuncture immediately before, and 3 and 6 h post-feeding on d 12 and 22; plasma urea concentrations for the six sampling times were averaged giving one value per treatment. Data were analysed using AVOVA with terms for treatment and block in the model. Herbage DM concentration was 138 vs. 149 g/kg, and crude protein concentration was 206 vs. 156 g/kg DM, respectively, for HN and LN. Herbage DM intake (7.8 kg/d) and DM digestibility (816 g/kg) did not differ (P>0.05) between treatments. Compared to LN, N intake, N digestibility and faecal N loss was 75 g/d, 34 g/kg and 5 g/d greater, respectively, for HN (P<0.05). Plasma urea concentration, total N and urine N loss was 1.7 mmol/l, 26 g/d and 20 g/d greater, respectively, for HN compared to LN. Nitrogen retained (63 vs.112 g/d) and, therefore, N-use efficiency was greater (33 vs. 42 %) for HN (P<0.05). Reducing fertiliser N application is a means of lowering N excretion in beef cattle consuming mid-season grass herbage.