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The digestion of ruminant feeds in vitro
Jennifer Clark
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Abstract
Systems of feed evaluation require the determination of the digestibility of the components of the feeds. The determination of digestibility in vivo is laborious and expensive, thus considerable research effort has been directed towards the determination of techniques to measure digestibility in vitro. The development of laboratory methods for assessing the digestibility of ruminant feeds is discussed. A two- stage technique, composed of microbial and enzymatic digestion stages, has currently been applied to the analysis of herbages, mixed forages, wood, brassicas, concentrates and silages. The digestibility of substrates determined by means of the two-stage in vitro method may be affected by the method of preparation of the substrate, the pH and duration of the microbial digestion, and the ratio of the rumen fluid to the buffer solution. The source of the rumen fluid may also affect digestibility in vitro , since the activity of this. fluid varies between species, individuals within species and the diet of the donor animals. Variation due to the diet of the donor may be minimized by means of a diet with an adequate crude protein content. Regression analyses are presented relating in vivo digestibilities to values of digestibility estimated by various laboratory procedures. In vivo digestibility may be predicted more accurately by the in vitro digestibility technique than by the nylon bag technique or by chemical methods of evaluation. However, methods incorporating the digestibility of cell wall material, digestion by fungal cellulase or summative equations of chemical constituents may be used as alternatives to the two-stage digestibility technique. The review is concluded with a discussion of the relationships between in vitro digestibility values of feeds and the values of total digestible nutrients, index of nutritive value, starch equivalent, voluntary intake, digestible energy and metabolizable energy , the last two of which are currently used to formulate rations for ruminants.