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 ways in which ruminants control their voluntary intake of finely-chopped silages. Silages with an excellent conservation quality can be ingested at similar levels to the corresponding fresh or dry forages. Intake levels decrease when conservation quality decreases. The implicated physiological mechanisms for this phenomenon are discussed in this review. Poorly preserved silages may have low palatibility, decreasing the animal's motivation to ingest. At the ruminal level, the physical control of intake is generally not involved. On the other hand the fermentation 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 be metabolized). Fermentation products induce most of the observed reactions. For well-preserved silages, high quantities of lactic and acetic acids limit intake. Their effects seem to be additive, which explains the observation that low pH often has a negative action. The negative effect of moisture is generally strengthened by that of the acids. For poorly-preserved silages, products of protein decomposition must be considered in addition to volatile fatty acids. The effect of N-compounds is less clearly explained than that of acids. For example, ammonia 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 are multifactorial; each fermentation product alone has a low effect, but the sum of all the components must be considered. Moreover, it is apparent that some physiological mechanisms are used to control silage intake, which explains the complexity of the studies on this subject.