Estimating digestibility of oak browse diets for goats by in vitro techniques

Abstract

Predicting digestibility of shrubs is important to evaluating many of the world's rangelands. We examined laboratory procedures for predicting in vivo digestion of browse-alfalfa (Medicago sativa) mixed diets and how drying temperature and inoculum source affect digestibility. In addition, we considered the effect of oak tannin on pepsin activity and dry matter digestion. The commonly used Tilley and Terry (1963) two-stage in vitro digestion technique was a precise (r2=0.97) but inaccurate predictor of in vivo apparent digestibility of mixed oak (Quercus gambelii) and alfalfa diets for goats. The Van Soest et al. (1966) neutral detergent method for predicting true digestibility was less precise (r2=0.76). Estimates from the Goering and Van Soest (1970) summative equation were not correlated (P is lesser than or equal to 0.05) with in vivo digestion. Separate regression equations are necessary if in vitro methods are to predict accurately in vivo digestibility of browse diets. In vitro digestibility was inversely related to percentage of oak in the diets and the amount of oak in the inoculum donors' diets. High drying temperatures depressed digestibility of oak browse and this effect was greater for immature than for mature forage.