Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/17/1996
Publication Date: N/A
Citation: Interpretive Summary: High yields and quality make alfalfa the principal forage crop grown to feed dairy cattle in the U.S. Alfalfa also is a major protein source for dairy cows. However, extensive breakdown of alfalfa protein in the rumen, the first compartment of the cow's stomach, results in inefficient utilization and depressed production of milk and milk protein. This is because the rumen microbes, which can form as well as degrade protein, reduce the value of alfalfa by making less protein than they degrade. The protein in alfalfa silage is particularly subject to degradation in the rumen because the protein is "pre-digested" in the silo. Nevertheless, most alfalfa in the U.S. is fed as silage to dairy cows because harvesting silage requires much less manual labor than harvesting hay. Protein value of alfalfa hay and alfalfa silage were compared by determining the amount which would be degraded in the rumen when fed to the live dairy cow. This was done by incubating a non-radioactive isotope (Nitrogen-15) with rumen fluid in vitro (that is, outside the live animal) to estimate the amounts of protein converted to ammonia and free amino acids (protein breakdown products) or incorporated into microbial cells in fluid collected from the cow's rumen. Protein in alfalfa hay and silage was degraded at similar rates and to similar extents despite the extensive predigestion of alfalfa silage protein which occurs in the silo. The reason for this unexpected result was that the rumen microbes made more protein when incubated with alfalfa hay than with alfalfa silage. If this holds true inside the living cow, then this may explain why dairy cows make better use of protein in alfalfa hay than protein in alfalfa silage.
Technical Abstract: Net release of degraded N as microbial protein synthesis, quantified from **15NH3 incorporation into microbial protein, plus NH3 and total AA was used to estimate rate and extent of in vitro degradation of seven proteins (casein, alfalfa hay, alfalfa silage, alfalfa hay extract, alfalfa silage extract, alfalfa hay residue, and alfalfa silage residue) in four separate in vitro incubations. Twenty-four determinations were made on alfalfa hay and alfalfa silage, and eight determinations were made on each of the other five proteins. (NH4)2SO4 and SDS-PAGE fractionations suggested that soluble proteins from alfalfa hay and silage differ in ruminal stability, probably because of differences in susceptibility to proteolytic attack. Although net release of NH3 plus total AA N from alfalfa silage and alfalfa silage extract was 2-fold greater than from alfalfa hay and alfalfa hay extract, respectively, net microbial protein synthesis on alfalfa hay and alfalfa hay extract was 33 and 43% greater than on the comparable silage fractions. Despite greater NPN content in alfalfa silage, protein degradation rate and estimated escape were similar in intact alfalfa hay (.103/h and 43%) and silage (.067/h and 43%). This may be explained by less efficient microbial utilization of silage NPN, greater protozoal numbers on hay, greater soluble true protein in hay, and differences in molecular weight and stability of soluble proteins in hay versus silage. Use of a 2-compartment model, based on water soluble and insoluble CP fractions assumed to pass with the liquid and solid phases, respectively, yielded ruminal escapes for protein in alfalfa hay and silage which were similar to estimates reported by the NRC.