REFINEMENT OF AN ENZYMATIC METHOD FOR ESTIMATING THE THEORETICAL DEGRADABILITY OF PROTEINS IN FEEDSTUFFS FOR RUMINANTS

Several enzymatic assays were examined to measure protein degradability. An alkaline protease from Streptomyces griseus (Type XIV) was used at pH 8 and pH 6.5, and a neutral protease from Bacillus subtilis was used at pH 6.5, a pH within the normal range of the rumen. Because many feedstuffs contain significant quantities of starch and occasionally beta-glucans and pectins, a mixture of bacterial alpha-amylase and fungal beta-glucanase (AFBG) was used to hydrolyse these components before the protease treatment. Using eight different feedstuffs (com, wheat, barley, wheat bran, com gluten meal, com gluten feed, soybean meal and sunflower meal), comparisons with and without AFBG showed that the addition of AFBG significantly increased (P<0.01) protein degradability by alkaline protease in starchy grains such as com, wheat, wheat bran and barley. With the same 8 feedstuffs, the enzymatic methods (4 h incubation time) were then compared with in situ theoretical degradability using least squares linear fitting. The in vitro methods at pH 8 appeared to be the less predictive: they explained 61% and 68% of the variation in theoretical degradability for alkaline protease without and with AFBG, compared with 91% and 92% for experiments conducted at pH 6.5 for alkaline and neutral proteases with AFBG, respectively. No difference was observed on this basis in the measurement of protein degradability between the two enzymes at pH 6.5. Student's t-test analysis of the regression equation slope and constant showed a small advantage for the alkaline protease method.