Identifying the optimum concentration of RUP in diet DM is challenging. As a first step, we believe it is critically important that one expresses RUP as a percentage of diet DM (as one does for RDP) and that one change it as dictated by animal performance. There is no logical basis for expressing RUP as a percentage of CP. RDP provides peptides, AA and ammonia for rumen microorganisms and RUP supplies intestinally digestible AA for the cow. Too often, when RUP is expressed as a percentage of CP, "more RUP" in a diet results in less RDP in the diet because there is a targeted level of ration CP that the nutritionist is trying to maintain. This approach is not consistent with balancing diets for RDP, RUP and AA. As a second step for identifying the optimum concentration of RUP in diet DM, we suggest that you let your cows tell you how much they need. The nutritional model that you use can be used as a guide for determining RUP requirements, but it should not be used to provide the final answer. There are two reasons for this recommendation. First, there are too many factors that determine the cows' requirement for RUP to allow the model to be very accurate. Three important factors affecting RUP requirements are:
- Supply of microbial protein.
- RUP digestibility.
- AA composition of RUP.
Each of these factors can have a significant effect on how much RUP is needed. And second, current models do not adjust MP requirements, and thus RUP requirements, for changes in predicted concentrations of AA in MP. This is a serious deficiency and until models are designed to predict milk and milk protein yields from supplies of MP-Lys, MP-Met, or whichever AA is the most limiting, rather than MP per se, an effort is needed to modify models so that variable efficiencies of use of MP for milk protein synthesis are employed. Don't be surprised, as a result of balancing for Lys and Met in MP, how little RUP is actually needed in the diet. Field experience indicates that cows are more responsive to changes in diet RUP content when RUP has a good AA balance vs. when the balance is not good. This makes sense because the nutritional potency of the RUP is greater when it has a good AA balance vs. a poor AA balance.