Rumen nitrate metabolism and the changes occurring in the composition of the rumen volatile fatty acids of grazing sheep

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Abstract
Adult sheep and weaned lambs grazing ryegrass and clover pasture, and sampled at approximately monthly intervals, exhibited marked variations in the molecular percentage composition of the individual rumen volatile fatty acids. These variations were characterised by comparatively large changes in the percentage of acetic acid and compensatory changes in an inverse direction in the percentages of propionic and, to a smaller extent, n-butyric and the branchedchain acids. It has been found that similar ch:mges in the relative amounts of acetic, propionic, and to a less extent n-butyric acids, can be induced by the oral administration of nitrate or its reduction product nitrite to sheep on pasture. Two factors may be operating in normal grazing sheep to alter the composition of the volatile fatty acids of the rumen; one the known variable nitrate content and the other the protein content of the pasture. It is postulated that a lowered molecular percentage of acetic acid may be due to a lack of acetic acid precursors usually present in the rumen; these probably being soluble carbohydrate or plant organic acids. Such a deficiency might be absolute, or brought on by the competitive removal of these substrates in their function as hydrogen donors required to bring about the reduction of nitrate to ammonia. Such conditions can apparently occur during periods of high protein levels in the pasture.