MILK-PRODUCTION AND RUMINAL METABOLITES FROM COWS OFFERED 2 PASTURE DIETS SUPPLEMENTED WITH NONSTRUCTURAL CARBOHYDRATE

The effects on milk solids production, ruminal metabolites, and microb ial protein synthesis of four pasture-based diets were investigated. T wo pastures (high or low nitrogen content (HN, LN) each with and witho ut nonstructural carbohydrate supplementation (+NSC, -NSC, at 10% of m etabolisable energy intake) were offered to eight stall held, rumen-fi stulated Friesian cows during four 14-day periods in early to mid lact ation in a Latin square design. HN and LN pastures contained 2.8 and 2 .1% N, respectively. Total daily dry matter intakes averaged 15.6, 14. 5, 15.3, and 14.3 kg on HN+NSC, HN-NSC, LN+NSC, and LN-NSC, respective ly. NSC supplementation had no effect on milk and milk fat yields but increased milk protein yield by 0.04 kg/day on HN pasture. Yields of m ilk, fat, and protein were higher on HN than LN pasture but microbial protein synthesis (MPS, g N/day) was not affected by type of pasture. MPS and efficiency of MPS (g N/kg digestible organic matter intake) we re increased by NSC supplementation on HN pasture but not on LN pastur e. Ruminal ammonia concentration and total tract N digestibility were reduced by NSC supplementation on both pastures and were lower on LN c ompared with HN pasture. The results indicated that pasture type affec ted response to NSC supplementation and that on HN but not LN pasture NSC supplementation improved N utilisation in the rumen and increased milk protein output.