THE REGULATION AND PREDICTION OF FEED-INTAKE IN RUMINANTS IN RELATION TO FEED CHARACTERISTICS

Voluntary intake of ruminants is determined by two factors, the ingestibility of fed forage and the intake capacity of the animal. Only ingestibility is considered here. Ingestibility must be measured under standardized conditions, which is not always easy. Generally, ingestibility has been measured with sheep, but measurements with other standard animals like heifers (or bulls) and dairy cows are possible. In this review, factors of variation of ingestibility are discussed for fresh forages, hays and silages. Regulation of feed intake in ruminants is, primarily, controlled by short-term mechanisms. Physical, chemical and oropharyngeal regulation mechanisms are briefly described. For a given feed, two or three types of regulation are involved. For the purpose of prediction, it is therefore essential to group feeds into homogeneous categories. The prediction of feed intake is possible directly in the laboratory by chemical, biological, and physical methods. Prediction of intake is easier for fresh forages (and hays) than it is for silages and concentrates. It is also possible to predict ingestibililty of conserved forages by the known intake of the fresh forage. Despite the great amount of work already done on methods of prediction of forage ingestibility, difficult obstacles remain to be overcome: - to find a sufficiently general system - to define a standard feed, reference animals, and standard conditions of measurement - to find a good classification of the feeds allowing a better accuracy of prediction criteria. Systems of prediction will be improved by working together on prediction techniques and the mechanisms regulating intake.