Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/23/1996
Publication Date: N/A
Citation: Interpretive Summary: Dairy cows obtain their protein needs both from synthesis by the microbes living in the rumen, the first compartment of the cow's stomach, plus feed protein that escapes breakdown by these same microbes. Dietary protein that escapes rumen degradation is very important because the rumen microbes often degrade more protein than they synthesize and thus waste protein. Therefore, it is important to know how much protein from common feedstuffs will be degraded and how much will escape the rumen. Our objective was to develop a method for use in feed testing laboratories to estimate protein degradation in the rumen. Two mixtures of commercial enzymes were formulated to have the same protein degrading activity as rumen fluid (which contains the protein degrading microbes) obtained from live cows. The two enzyme mixtures had protein digestion activity that was similar to rumen fluid when assayed with the standard compounds normally used to test for this activity; however, testing the enzyme mixtures on actual feedstuffs indicated that they degraded proteins much more slowly than rumen fluid. Two other commercial enzymes also were compared with rumen fluid from the live cow and found to degrade feed proteins to a lesser degree. These results are important because they indicate that the commercial protein digesting enzymes that are commonly used by feed testing laboratories do not degrade feed proteins to the same extent as the rumen microbes and will be unreliable for estimating protein degradation in the rumen. Assays used by feed testing laboratories will have to be based on tests using the actual rumen microbes.
Technical Abstract: The objective of this research was to formulate a mixture of commercial proteases that would mimic the rate and extent of protein degradation obtained using strained ruminal fluid. The proteolytic activity of strained ruminal fluid and several commercial proteases was characterized using 13 L-amino acid p-nitroanilides as artificial substrates. A mixture of Streptomyces griseus protease, chymotrypsin and proteinase K, at .042, 2.5 and .5 enzyme units/ml, respectively, was similar to the activity of strained ruminal fluid against the same artificial substrates. However, degradative activities were different in incubations with feed proteins as substrates. The rates of degradation of expeller soybean meal, solvent soybean meal and casein by the enzyme mixture were .08, .05, and .08/h, while those obtained using strained ruminal fluid were .03, .15, and .24/h, respectively. In another experiment, S. griseus enzyme at .066 enzyme units/ml, ficin at .5 enzyme units/ml, or a mixture of trypsin, carboxypeptidase B, chymotrypsin, and carboxypeptidase A at 116.6, .5, 2.5 and .5 enzyme units/ml, respectively, were compared. Protein degradation rates obtained with strained ruminal fluid were 2 to 6 times faster than those obtained with the enzyme mixtures. A third experiment compared the activity of the mixture of trypsin, carboxypeptidase B, chymotrypsin and carboxypeptidase A with that of strained ruminal fluid in incubations with 15 feeds. Degradation rates using strained ruminal fluid ranged from .008 to .250/h. However, results using the enzyme mixture as the inoculum source detected no differences in degradation rates among feeds. Overall, these experiments indicated that the commercial enzymes employed did not mimic the protein degradative activity of strained ruminal fluid.