Ruminants grazing pasture rarely approach their genetic potential for meat, milk or wool production. There are wide differences also between plant species and between leaf, stem and inflorescence fractions in their capacity to sustain animal production. The relatively low productivity of grazing ruminants and the differences between plant components in nutritional value can be explained from an understanding of the factors that determine nutrient supply to the animal's tissue and the efficiency of metabolism. Nutrient supply to the tissues depends on Ii) the nutrient contents of the forage available, which varies between plant species, their component parts, their stage of maturity and the growing conditions; (ii) the pasture components actually selected by the animal which are influenced by ease of eating, taste, odour and tactile characteristics of the plant material available, its water content, the structure of the sward, the dimensions of the animal's incisor arcade and its previous experience; (iii) the amount of each selected component eaten, which is influenced by the potential of the animal to utilize nutrients, the capacity of the reticulo-rumen and factors that affect the rate of digestion and outflow of organic matter from the rumen, and the time spent in eating and ruminating activities; (iv) modifications of consumed nutrients by rumen microbes, particularly the extent of breakdown of plant protein and the relative proportions of volatile fatty acids produced; (v) the efficiency of biochemical reactions metabolising the absorbed nutrients. Although a great deal is known about the way each of these factors influence nutrient supply to grazing animals, the complexity of the interactions between them often makes it extremely difficult to identify the specific reasons for reduced performance in any particular situation. However, by integrating current concepts and knowledge using computer simulation techniques, the factors limiting nutrients supply to the animal's tissues can be determined and the value of alternative management strategies for improving animal productivity assessed.

RW, Moore, WH McMillan, G Dockrill, and BW Dow

Proceedings of the New Zealand Society of Animal Production, Volume 49, , 281-284, 1989
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