The manner in which lactating dairy cow react to additional protein in the diet is certainl; at least partly dependent on the amino aci< (AA) proportions in that protein. The commonb held model follows the "uneven barrel staves" analogy of one clearlymost limiting AA in th( metabolizable protein (MP). In fact, this mode is clearly an oversimplification. Microbial protein production in the rumen responds to AA balance in the rumen degraded protein and does not always follow the simple single limitingAA model. Within the cow, the biological drive to produce milk protein is separable from the requirement for the most limiting AA needed to sustain that drive. This protein secretion drive is dependent on energy, endocrine, and even non-limiting AA facets of feeding. The presence of nearly co-limiting AA, variance among diets, and variation among cows, makes interpretation of experimental data based only on the single limiting AA model a bad idea. Since even well designed experiments do not, and maybecannot, include all necessary treatment combinations, other possible reasons for milk protein yield responses should be considered unless they are eliminated by the experimental design.
展开▼
