Review: Problems in determining metabolisable protein value of dairy cow diets and the impact on protein feeding

Accurate determination of dairy cow's metabolisable protein (MP) requirements is essential for maximising milk yield and minimising N input in dairy production systems. The main objectives of feed evaluation systems are to predict accurately the nutritive value of feed resources and production responses to ingested nutrients. In recent years, protein evaluation systems have made progress in complexity but our analysis indicated that in comparison to simple MP models, or even models based on CP and metabolisable energy intake, new models have failed to improve milk protein yield predictions. Previous meta-analyses have indicated low marginal efficiencies for incremental CP intake even with high-quality feeds as protein supplements. Most current protein systems tend to overestimate MP supply from rumen undegradable protein and overlook the effects on rumen microbial protein synthesis. These findings suggest that there is scope for improving predictions of MP supply. This review highlights and discusses some evident problems in the current MP systems that may partly explain the modest progress achieved over the last few decades.