RUMEN FERMENTATION CHARACTERISTICS AND DUODENAL NUTRIENT FLOW IN LACTATING COWS RECEIVING 2 TYPES OF GRASS-SILAGE WITH 2 LEVELS OF CONCENTRATES

Four lactating dairy cows were used, each equipped with simple cannulae in the rumen and duodenum. Each cow was given, in a 4 X 4 Latin square arrangement, four diets consisting of an early cut (EC) or late cut (LC) grass silage ad libitum each with either 3 (low) or 9 (high) kg dry matter (DM) of concentrates daily. Total and silage DM intake were significantly (P<0.01) higher on LC silage. Total DM intake was higher but silage DM intake was lower with the higher level of concentrates. Acid detergent fibre and neutral detergent fibre digestion in the rumen were significantly (P<0.05) higher on EC silage diets compared with LC silages. Mean rumen pH was significantly affected by silage type and concentrate level, being lower on EC silages and on 9 kg concentrates. Mean ammonia nitrogen and total volatile fatty acid (VFA) concentrations were similar on all diets although the concentrations of individual VFA (except propionic) were significantly affected by silage type. Molar proportions of acetic acid were significantly (P<0.01) lower on high concentrate diets compared with low concentrate diets and those of propionic acid were significantly (P<0.01) higher. Proportions of butyric acid were significantly (P<0.01) lower on EC diets. Non-ammonia nitrogen and microbial nitrogen flows at the duodenum were significantly higher (P<0.05) on EC compared with LC silages and with diets containing high compared with low levels of concentrates (P<0.01). Efficiencies of microbial protein synthesis were significantly higher (P<0.05) on EC compared with LC based diets but there were no effects due to concentrate level. Milk yields were significantly (P<0.05) higher on EC diets and with higher levels of concentrates with both silages (P<0.01). Milk fat concentrations were significantly (P<0.05) lower on EC compared with LC diets and on 9 kg concentrate compared with 3 kg concentrate containing diets (P<0.01). Constituent yields were significantly increased with the higher concentrate level on both silages but only lactose yield was influenced by silage type.