Nitrogen is a key nutrient in milk production systems, but is also of primary environmental concern because of losses of ammonia in the air and potential nitrate contamination of surface and groundwaters. In order to reduce the potential for water pollution by nitrate from agricultural sources, the European Union introduced the Nitrates Directive in 1991 with mandatory limits on the maximum levels of manure N that can be applied to grassland. Manure N excretion from dairy cattle is primarily a function of N intake, and consequently from an environmental point of view there is a major impetus to reduce protein concentrations in dairy cow diets. The most important nutritional challenge is to identify opportunities to reduce dietary protein concentration whilst maintaining animal performance, thereby improving the overall efficiency of N use.

An important dietary strategy to improve N efficiency is balancing amino acids (AA) in dairy diets. Our current understanding of AA nutrition in ruminants supports the concept that when essential AA are absorbed in the profile as required by the animal, the requirement for total protein is reduced and the efficiency of AA use for protein synthesis is maximized. In situations where the supply of one AA limits protein synthesis by the mammary gland, dietary supplementation of this AA in a reliable metabolizable form would improve the profile of absorbed AA, resulting in additional protein synthesis. It has been shown that Methionine (Met) and Lysine (Lys) are the main AA limiting milk protein synthesis. Supplementation of Met or Met and Lys postruminally has had positive effects on milk yield, milk protein concentration and milk protein quality like casein. It has also been shown to play a preventative role for certain metabolic disorders with positive effects on energy balance and reproduction. Due to these economically important effects feeding diets balanced for AA in order to reduce N excretion can improve profitability as well.

Plasma urea nitrogen (PUN) and milk urea nitrogen (MUN) are helpful indicators to determine improvement of AA efficiency for protein synthesis when a reliable metabolizable source is added into the diet. St-Pierre and Sylvester (2005) observed that PUN was numerically and MUN was significantly lower (P=0.01) for diets balanced in AA and supplementing a highly metabolizable source of Met.

This is another illustration of the overall benefits of feeding rations balanced in AA.