Four diets based on high quality pasture were formulated using additions of starch to contain 12.8, 19.6, 26.3, or 33% total nonstructural carbohydrate and were evaluated using four continuous culture fermenters during four 10 day periods. The true ruminal digestibility of organic matter was not changed by diet but the digestibility of neutral detergent fibre and crude protein linearly decreased, and the digestibility of total nonstructural carbohydrate linearly increased, as the level of starch inclusion increased. Production of NH3 N was reduced linearly from 35.2 to 14.2 mg/dl (P<0.001) and the flow of non-NH3 N as a proportion of total N flow increased linearly from 66% to 83% (P<0.001) as inclusion of starch increased. The quantity of microbial N and non-NH3 N flowing from continuous culture was unchanged. The experiment indicated that if the amount of organic matter digested is not appreciably increased by the inclusion of nonstructural carbohydrate then the linear increase in ruminal utilisation of pasture N arises more from a reduced pasture N intake than an improvement in microbial protein synthesis.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 106-109, 1998
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