EFFECT OF FORAGE AMOUNT AND PARTICLE SIZE IN DIETS OF LACTATING DAIRY COWS ON SITE OF DIGESTION AND MICROBIAL PROTEIN SYNTHESIS

Lactating Holstein cows fitted with ruminal and duodenal T-type cannulae were used in two studies to determine the effect of forage: concentrate ratio and forage particle size on site of nutrient digestion and microbial protein synthesis. In exp. 1, cows were fed alfalfa hay at 24, 38, 58 and 80% of total dry matter intake. Organic matter (OM) digestion in the total tract (OMD) increased with increasing concentrate level but apparent ruminal OM digestion (AROMD) was unaffected by diet. Digestion of acid detergent fiber (ADF) was similar among the higher forage diets, but was depressed at the 24% forage level. Efficiency of microbial protein synthesis (MPS) was depressed by high concentrate diets and was positively correlated (P < 0.05) to turnover rate within the rumen. In exp. 2, cows were fed 20% concentrate and 80% alfalfa hay in long, chopped or ground and pelleted form. AROMD was lower with ground hay but OMD was unaffected by diet. Digestibility of ADF in the rumen was lower with ground hay, but was partially compensated for by increased hindgut digestion of ADF. Flow of feed plus endogenous nitrogen (N) at the duodenum was 37% and 47% on N intake with long and ground hay, respectively. Efficiency of MPS increased 15% and postruminal N digestion increased 36% when ground hay replaced long hay. Efficiency of MPS was directly related to ruminal solids turnover rate and inversely related to liquid dilution rate. These results demonstrate improvements in efficiency of MPS with either increasing amounts of forage in the diet or increased ruminal passage of solids. Key words: Digestion, microbial protein, rate of passage, cattle, rumen forage