THE REGULATION AND PREDICTION OF FEED-INTAKE IN RUMINANTS IN RELATIONTO FEED CHARACTERISTICS

Voluntary intake of ruminants is determined by two factors, the ingest ibility of fed forage and the intake capacity of the animal. Only inge stibility is considered here. Ingestibility must be measured under sta ndardized conditions, which is not always easy. Generally, ingestibili ty has been measured with sheep, but measurements with other standard animals like heifers (or bulls) and dairy cows are possible. In this r eview, factors of variation of ingestibility are discussed for fresh f orages, hays and silages. Regulation of feed intake in ruminants is, p rimarily, 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 homogeneo us categories. The prediction of feed intake is possible directly in t he laboratory by chemical, biological, and physical methods. Predictio n of intake is easier for fresh forages (and hays) than it is for sila ges and concentrates. It is also possible to predict ingestibililty of conserved forages by the known intake of the fresh forage. Despite th e 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 an imals, and standard conditions of measurement - to find a good classif ication of the feeds allowing a better accuracy of prediction criteria . Systems of prediction will be improved by working together on predic tion techniques and the mechanisms regulating intake.