Theoretical and applied aspects of voluntary feed intake by ruminants, with special reference to the kinetics of rumen digestion.

Abstract

The aim of these studies was to examine the factors which determine voluntary feed intake and feed quality in ruminants. In the first experiments, the concepts of ruminal digestion kinetics were conceptualised and measured in animals. These concepts were applied practically in feed evaluation studies which followed. In vivo studies on alkali treated wheat straw explained why voluntary intake of ruminants increased when roughages are treated with alkali. The effect of washing the treated feed to remove excess sodium was also studied. The explanation was found in terms of ruminal digestion kinetics, showing that the mean rate of digestion was not changed, but chemical treatment improved the potential digestibility, thereby increasing the active pool size in the rumen which resulted in a faster clearance rate from the rumen. A study of the effect of starch fermentation on the kinetics of roughage fermentation in the rumen, revealed that the fermentation of different diets were affected in a different manner. The paramount factor was found to be a reduced rate of forage fermentation in the presence of starch fermentation in the rumen. A study of Pennisetum clandestinum revealed the reasons why animal performance on kikuyu pasture is often lower than what would be expected from the digestibility and chemical composition of the material. It was shown that a high soluble nitrogen content of the material was the most likely reason for low voluntary intakes, low ruminal fill and therefore poor animal performance on lush kikuyu pasture. A method was developed by which the concepts of ruminal digestion kinetics (MRT method) are used to determine voluntary feed intake with grazing animals. The method gave a mean intake that was similar to the mean obtained when intake was calculated from faecal collections, but had the advantage of a clearer pattern of intake. The accuracy obtained when using the MRT method to estimate voluntary feed intake was confirmed in a second experiment where actual intakes were known, and predicted intake was very close to actual intake. Indirect methods were developed by which two important determinants of voluntary intakes, i.e. rate and extent of digestion may be estimated. The Tilley & Terry in vitro method was adapted to allow the estimation of fermentation rates from rates of gas production. Digestion rates obtained with in vitro gas production agreed well with in sacco estimates. In vivo digestion rates were much slower than those obtained in vitro or in sacco. This discrepancy is yet unexplained, and is in contrast with the results of a previous experiment where in sacco and in vivo results were in good agreement. Increasing the mean particle size of the fennenting forages resulted in a small but statistically significant decrease in fennentation rate. Stirring the fennentation vessels did not have any positive effect on fennentation rate. Total volume of gas produced was not a good indicator of in vitro digestibility because gas production measures ruminal digestion, while in vitro digestibility includes both a ruminal and an acid pepsin phase. The rate of in vitro gas production, as measured by pressure changes in the fennentation vessels, is a practical method that was easily automated by using a data logger. The automated measurement of rate and ex'tent of digestion allows their inclusion into routine analyses for feed evaluation and the results obtained so far indicate that the system is sufficiently accurate to give useful estimates of voluntary feed intake and animal production.