CONTROL OF VOLUNTARY INTAKE OF PRECISION-CHOPPED SILAGES BY RUMINANTS- A REVIEW

The aim of this review is to provide a better understanding of the way s in which ruminants control their voluntary intake of finely-chopped silages. Silages with an excellent conservation quality can be ingeste d at similar levels to the corresponding fresh or dry forages. Intake levels decrease when conservation quality decreases. The implicated ph ysiological mechanisms for this phenomenon are discussed in this revie w. Poorly preserved silages may have low palatibility, decreasing the animal's motivation to ingest. At the ruminal level, the physical cont rol of intake is generally not involved. On the other hand the ferment ation products present in silage seem to induce a high and rapid level of satiation, out of proportion to their relatively low quantity. At the metabolical level some limiting factors may also exist (eg, a poor balance between amino acids and energy, and high levels of acids to b e metabolized). Fermentation products induce most of the observed reac tions. For well-preserved silages, high quantities of lactic and aceti c acids limit intake. Their effects seem to be additive, which explain s the observation that low pH often has a negative action. The negativ e effect of moisture is generally strengthened by that of the acids. F or poorly-preserved silages, products of protein decomposition must be considered in addition to volatile fatty acids. The effect of N-compo unds is less clearly explained than that of acids. For example, ammoni a alone generally seems to have no direct effect. However it is clear that N-compounds taken together have a negative effect on appetite. It can be concluded that the negative effects of poor quality silages ar e multifactorial; each fermentation product alone has a low effect, bu t the sum of all the components must be considered. Moreover, it is ap parent that some physiological mechanisms are used to control silage i ntake, which explains the complexity of the studies on this subject.