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Ruminal and Intestinal Digestion in Brahman Crossbred and Hereford Cattle Fed Alfalfa or Tropical Pasture Hay

CSIRO, Division of Animal Production³, Rockhampton, Queensland 4701 Australia

Abstract

Three Brahman crossbred (BX) and three Hereford (H) steers, fitted with rumen and abomasal cannulae, were given alfalfa hay or low quality pasture hay during two experiments. Digestion in the reticulorumen was measured by reference to the content in abomasal digesta of the digesta markers, ruthenium - phenanthroline and chromium EDTA. Apparent digestibilities of organic matter (OM), cell wall constituents (CWC) and N were lower for steers fed pasture hay than for steers fed alfalfa, but there was no difference between breeds. Brahman crossbred steers digested more (90%) of the digestible OM intake in the reticulorumen than did H steers (83%) fed pasture hay. Differences in the apparent digestion of OM and of CWC in the reticulorumen were related to the retention time of digesta in the rumen as measured by reference to microbial and lignin retention times. Efficiency of microbial synthesis in the rumen, calculated from flow of OM and microbes at the abomasum, was inversely related to the retention time of microbes for steers fed alfalfa. Apparent digestibility of nonammonia N entering the intestines was 52 to 67% for steers given alfalfa, but only 38 to 43% for steers given pasture hay. In a third experiment, a greater proportion of an iv dose of ¹⁴C-leucine escaped oxidation in fasting BX steers than in H steers. The results suggested that differences between animals in site of digestion could be attributed to retention time of digesta in the rumen and that the ability of BX crossbreds to maintain a heavier weight than H animals given a fixed amount of feed was related to more extensive ruminal digestion and more efficient protein synthesis in BX crossbreds.

¹ Present address: Dept. of Anim. Sci. The Univ. of Alberta, Edmonton, Alberta, Canada T6G 2P5.

² Assistance from Mr. C. R. Holmes, Mr. J. Dickfos and Mr. W. Sim is gratefully acknowledged.

³ Tropical Cattle Res. Centre, P. O. Box 545.