Feed & Additive Magazine Issue 2 March 2021

ARTICLE FEED & ADDITIVE MAGAZINE March 2021 69 to ensure a good feed mixing process to avoid the cow sorting the feed by preference, and consequently running the risk of acidosis and low N efficiency. Formulation is usually based on designing the least cost ration that provides the minimum level of re- quired nutrients for a certain level of milk production however, the feed cost can be up to 70% of the total production cost. Improving efficiency of N conversion has one of the highest impacts on farm profitability (Tozer, 2012, McGrath et al., 2018, Bach, 2012). Over the last few years, tremendous efforts and re- search have been made to refine the protein require- ments of dairy cows. Our growing understanding of cow requirements has led to recognising two sets of protein requirements, rumen degradable protein (RDP), and rumen undegradable protein (RUP). Metabolically the cow has specific requirements for individual amino acids (AA’s), rather than metabolis- able protein (MP). Together, the complex microbial metabolic activity in the rumen and intestinal pro- cesses make the study of N metabolism in ruminants more challenging than in the case of non-ruminants. AA’s are the building blocks of milk and body pro- teins and are considered one of the most important nutrients in dairy cow nutrition. Many of these AA’s need to be supplemented in the diet because they can’t be synthesised quickly enough to meet the require- ments of lactating cows. Therefore, these amino acids are known as essential AA’s. Inadequate supply of these essential AA’s can limit milk and milk protein yield. The essential AA’s that are present in MP is the smallest supply relative to the cow’s requirements and referred to as limiting AA’s. Methionine (Met) and lysine (Lys) have been recognised as the first limiting AA’s for lac- tating dairy cows under most feeding practices. This is fundamentally true because feed proteins have low- er concentrations of Met and Lys, when compared to their concentrations in milk and microbial protein. In European mid-range crude protein diets, it is not possible to meet the Met or Lys requirements with the use of dietary feed ingredients therefore, the use of rumen-protected supplements is needed. It is re- markable that Met and Lys work in synergy, as both AA’s are necessary in our diets for optimal and pre- cise feeding, to achieve maximum production perfor- mance without overfeeding protein. Balancing dairy rations for AA’s, rather than CP%, is the state-of-the- art approach when it comes to protein nutrition. Schwab et al. (2004) compared MP, Lys and Met