INTERACTION BETWEEN INOCULUM DONOR DIET AND SUBSTRATE IN IN VITRO RUMINANT DIGESTION STUDIES
Six wethers with rumen fistula were each fed a different combination of alfalfa hay, wheat straw, and barley grain. The same mixtures were fermented for periods of 12 and 48 hours by inoculum from each wether, using a two-stage in vitro digestion technique. The in vitro digestibility of dry matter and organic matter was determined on each treatment combination.The digestibility data revealed interactions among inoculum donor diets, in vitro substrate mixtures, and fermentation periods. When the diet lacked one or more of the substrate components, a 12-hour artificial rumen fermentation resulted in digestibilities that decreased as the composition of the substrate deviated increasingly from that of the donor diet. This effect was eliminated by extending the fermentation period to 48 hours. When all the substrate ingredients were included in the donor diet, the absolute amount of each ingredient, within 10 to 80%, was of less importance.Indications were that alfalfa possessed a specific growth-promoting effect on the microflora.Increasing the proportion of alfalfa in the substrate increased the difference between the in vitro digestibility of dry matter and organic matter. This was suggested to result from a higher concentration of soluble ash in the alfalfa fraction of the mixtures.