Influence of degradability of supplemental protein and time post-partum in early lactation dairy cows. 2. Kinetics of rumen ingesta turnover and whole tract digestibility

Abstract

Eight lactating dairy cows with rumen cannulae were fed either high or low concentrate totally mixed diets in two blocks of four cows. The forage source was a low quality (9.1% crude protein, 61% neutral detergent (ND) fibre) whole crop oat silage. Degradability of supplemental protein was varied within each block by substituting corn gluten meal, a slowly fermented protein, for canola meal, a rapidly fermented protein. Measurements were made in early lactation (weeks 4 to 13 inclusive) to determine effects of diet concentrate level, degradability of supplemental protein, and time opost-partum on whole tract digestibility, rumen organic matter (OM) and nitrogen (N) pool sizes, and rates of rumen ND fiber passage. Whole tract digestibility of dry matter (DM) and OM was higher for cows fed high concentrate diets, reflecting the higher dietary inclusion of starch, Rumen ND fibre and bacterial OM pool sizes did not differ between dietary concentrate levels. Total non-ammonia N (NAN) pool was higher for cows fed high concentrate diets and this was primarily non-bacterial N, as bacterial N as a proportion of total rumen NAN was higher for cows fed low concentrate diets. Rumen OM and NAN pool sizes were little influenced by dietary protein degradability or time post-partum, although pool sizes of most components tended to increase as time post-partum increased. Rate of rumen ND fibre digestion was lower for low concentrate fed cows. Rate of rumen passage of ND fibre and liquids decreased with increasing time post-partum for low concentrate fed cows.