In Vitro Ruminal Degradation and Synthesis of Protein on Fractions Extracted from Alfalfa Hay and Silage

¹ Agricultural Research Service, USDA, US Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706

Net release of degraded N as NH₃ and total AA plus microbial protein synthesis, quantified from incorporation of ¹⁵NH₃ into microbial protein, was used to estimate the rate and extent of in vitro degradation of protein fractions isolated from alfalfa hay and silage. Seven proteins (casein, alfalfa hay, alfalfa silage, extracts from alfalfa hay and silage, and residues from alfalfa hay and silage) were studied. Results from (NH₄) ₂SO₄ and SDS-PAGE fractionations suggested that soluble proteins in alfalfa hay and silage differed in susceptibility to proteolytic attack. Although the net release of NH₃ plus total AA N from alfalfa silage and alfalfa silage extract was twofold greater than that from alfalfa hay and alfalfa hay extract, net microbial protein synthesis on alfalfa hay and alfalfa hay extract was 33 and 43% greater. Despite greater NPN content in alfalfa silage, protein degradation rate and estimated escape were similar for intact alfalfa hay (0.103/h and 43%) and silage (0.067/h and 43%). This result might be explained by the less efficient microbial utilization of silage NPN, greater protozoal numbers on hay, greater soluble true protein in hay, or differences in molecular mass and stability of soluble proteins in hay versus silage. Use of a two-compartment model, based on water-soluble and insoluble CP fractions assumed to pass with the liquid and solid phases, respectively, yielded RUP estimates for alfalfa hay and silage that were similar to NRC estimates.

Key Words: alfalfa silage • alfalfa hay • microbial protein • protein degradation and yield

Submitted on May 8, 1995
Accepted on December 14, 1995

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