Abstract/Summary

This paper examines the principal nutritional factors that influence the content of protein in cow's milk and offers possible strategies for ensuring acceptable levels, suitable for both the liquid market and processing. Rather than provide a review of the numerous experimental studies, with the principal aim of developing predictive relationships, this paper has considered a number of key studies where in many cases it has been possible to offer some interpretation of the results obtained. The paper concludes that underfeeding of protein is rarely a major problem with respect to feeding dairy cows and the use of additional protein sources, and especially protected amino acids, does not appear to be the most promising approach to improving milk protein levels. In contrast, attention to the intake of metabolisable energy, and the form of energy does offer real opportunities for improving milk protein content. Strategic use of different and better quality forages appears to be fundamental as well as the nature of the carbohydrate component of the concentrates. In this respect, high quality starch offers considerable benefit with both wheat and maize being highly attractive feed sources. In contrast, while digestible fibre sources will improve metabolisable energy intake and milk yield, they seem to be less effective with respect to improving milk protein content and in this respect, the excessive use of lupins needs to be questioned. Finally, the paper considers the quantitative metabolism of amino acids by the host animal following absorption and concludes that both mammary supply and mammary uptake are important in the synthesis of milk protein. Increasing the supply of amino acids without the ability of the mammary gland to increase uptake will however lead to modest and sometimes zero responses to additional protein supply.