Milk production and ruminal metabolites from cows offered two pasture diets supplemented with non-structural carbohydrate

The effects on milk solids production, ruminal metabolites, and microbial protein synthesis of four pasture-based diets were investigated. Two pastures (high or low nitrogen content (HN, LN) each with and without nonstructural carbohydrate supplementation (+NSC, -NSC, at 10% of metabolisable energy intake) were offered to eight stall held, rumen-fistulated Friesian cows during four 14-day periods in early to mid lactation 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, respectively. 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 milk, 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) were increased by NSC supplementation on HN pasture but not on LN pasture. Ruminal ammonia concentration and total tract N digestibility were reduced by NSC supplementation on both pastures and were lower on LN compared with HN pasture. The results indicated that pasture type affected 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.