Protein nutrition of dairy cows : Some relevant further aspects of protein nutrition of dairy cattle, when reducing protein nutrition in dairy cows in view of societal demands

Summary

The aim of the current desk study is to provide an overview of various aspects of protein metabolism in dairy cows that do receive little or no attention with protein evaluation in current practice. However, a number of aspects may be relevant, in particular under feeding conditions with a reduced dietary crude protein content and supply of intestinal digestible protein (i.e. metabolizable protein) to the cow in order to limit nitrogen excretion. Firstly, a fluctuating or oscillating supply and temporary deficiencies for a few days seems to have little impact on the cow's performance. However, it is unclear whether there are no negative side-effects regardless of the protein content of the ration, and whether these temporal deficiencies can be sustained in the longer term. Secondly, the choice of glucogenic instead of lipogenic nutrients seems to be interesting in particular to stimulate the fixation of nitrogen in milk protein. But with a low supply of intestinal digestible protein, fat sources can also increase the conversion of amino nutrients to milk protein. The cow appears to have great flexibility in the production of milk protein, fat and lactose from the supply of lipogenic, glucogenic and aminogenic nutrients that could be better used to make maximum use of intestinal digestible protein when the cow has a low supply. Thirdly, the few long-term studies on the effect of a low crude protein content in the ration indicate that this is a very effective way to improve the efficiency of nitrogen utilization and to reduce nitrogen excretion and emissions, without major consequences for the energy balance and health status of the cow. A level of 14% to 16% crude protein in the dry matter ration for cows producing 10,000 kg of milk per year seems to minimize nitrogen excretions. However, refinement of the ration to the different stages of lactation is needed. Fourthly, it seems advisable to take into account the need for extra intestinal digestible protein for the growth of visceral organs during the start of lactation. Although only an indirect indicator of this need, research results so far provide different insights into the duration of the negative nitrogen balance. Fifthly, there seems to be only limited potential for improving the efficiency of rumen microbial protein synthesis and of the transport of urea from blood to the rumen environment, as options to improve nitrogen utilization. Only a marginal gain can be made from rumen defaunation (reduction of predation on bacteria by protozoa) to improve the efficiency of nitrogen utilization, as well as from switching between non-protein-nitrogen and protein-nitrogen sources in the ration. Finally, a targeted supplementation with rumen-bypass essential amino acids seems to be an important solution for improving nitrogen utilization towards a ration with a low supply of intestinal digestible protein and a crude protein content between 12 and 14% (provided it contains an appropriate profile of essential amino acids). A level of nitrogen utilization efficiency of ~35% seems achievable, compared to a ~25% at a content of 16% crude protein, which is roughly average current Dutch practice. For this development / innovation in dairy farming, it is important that a broad profile of rumen-bypass essential amino acids becomes available.