Monensin induced metabolic changes in cattle fed a restricted intake of lucerne hay

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Abstract
O'Kelly (1985) has presented evidence which suggests that at the same feed intake Brahman cross cattle synthesize more microbial lipid than British cattle with a resultant increased efficiency of utilization of metabolizable energy for maintenance. The lipid content of bacteria associated with both liquid and solid fractions of rumen contents is also higher in Brahman than in Hereford cattle (J. C. O'Kelly & W. G. Spiers, unpublished). The reasons for these breed differences are unknown. A step in investigating the problem could be to determine whether microbial lipid content in Hereford cattle can be increased by altering rumen fermentation patterns in association with changes in the microbial population. Ionophores alter rumen fermentation by increasing the production of propionic acid relative to other volatile fatty acids and also inhibit Gram-positive bacteria by facilitating the formation and transport across cell membranes of lipid-soluble complexes with various cations (Bergen & Bates, 1984). However, there are no reports on the effects of ionophores on microbial lipid metabolism in cattle on restricted feed intakes typical of those in tropical areas.