When offered a free choice between different forage species presented in a pasture association, ruminants will choose a mixed diet, even when one dietary component could meet all of their nutritional needs. Thus, preference and selection cannot be explained simply by the common measures of species nutritive or feeding value. The question then arises, what is the nutritional basis of the dietary choices that animals make? The objective of this paper is to review the role of synchronization of forage composition factors and nutrient release patterns on the processes controlling preference in grazing ruminants. The satiety theory is used as a model system to explore outcomes of changing the physico-chemical attributes of forages on grazing behavior of sheep and cattle. The review will examine further the biological basis for the alteration in meal pattern, duration and extent in ruminants offered clover only (relatively high rumen degradable protein content) compared to animals eating only grass (with relatively low rumen degradable protein content), or a mixture of grass and clover. One theory that has been proposed to explain the induction of satiety in grazing ruminants is the rate of release of ammonia from the soluble protein fraction of the forage, and subsequent uptake in the blood. By mixing grass with the clover, the animal is able to increase the duration of the meal potentially reflecting a “better” dietary balance of energy to soluble protein that controls the rate of accumulation of ammonia in rumen fluid. This concept is evaluated in light of recent data from in vitro studies examining digestive efficiency. From this analysis, it is clear that direct, real-time information on the relationships between forage physico-chemical factors, rumen condition, meal initiation and cessation, and dietary switching is needed to further develop propositions about the control of dietary choices of grazing ruminants.
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Vol. 62 • No. 5