Symposium on Factors Influencing the Voluntary Intake of Herbage by Ruminants: Voluntary Intake in Relation to Chemical Composition and Digestibility

TLDR

The nutritive value of forages is largely governed by chemical composition, with higher lignin or fiber reducing digestibility, yet the link between voluntary intake and composition is less clear due to animal variability and competing theoretical arguments. Forage composition effects on intake and digestibility are categorized into three classes: influencing intake alone, promoting a positive intake–digestibility relationship, or fostering a negative relationship.

Abstract

IT is commonly assumed that chemical composition determines nutritive value of forages. The negative association of digestibility with lignin or fiber content is well established and easy to rationalize, since the problem involves the nutritive availability of different chemical fractions of forages and factors affecting this availability, such as lignification. When considering the problem of comparing voluntary intake (VI) with chemical composition, expected relationships are more difficult to rationalize, because the individuality of the animal plays a larger role, and also because one can develop separate lines of argument that lead to different conclusions regarding the role of the diet. Classification of the effects of forage composition upon nutritive value may be made according to how chemical constitution affects intake, digestibility, and the relationship between them. Three classes can be distinguished: (1) the factor affects intake, but has no direct or reliable effect on digestibility, (2) positive relationship between intake and digestibility is promoted, and (3) negative relationship between intake and digestibility is promoted.