The animal Article of the Month for February is entitled ‘Reducing dietary protein in dairy cow diets: implications for nitrogen utilisation milk production, welfare and fertility’

The combined effects of an increased cost of protein and legislation introduced in several countries on the storage and application of cattle manure has resulted in renewed interest to reduce protein levels in dairy cow diets. As only between 25-35% of dietary protein is captured and secreted in milk, with the majority of the remaining nitrogen being lost in urine and faeces, our article in the February edition of animal considered the feasibility of reducing dietary protein levels by conducting an extensive review and meta-analysis on the subject.

It is evident from several studies that reductions in dietary protein significantly increase the efficiency of nitrogen (N) utilization. Negative effects of reduced dietary crude protein (CP) (to around 140 g/kg DM) on dry matter intake and milk yield are less pronounced when introduced during the second half of lactation; although benefits for N capture are also less. The negative effect of reduced dietary protein on yield varies little with forage source (i.e. maize vs grass silage). There may be scope to mitigate any negative effects by increasing the proportion of starchy concentrates in the diet in order to increase the yield of microbial protein. In this respect the nature of forage (e.g. lucerne vs maize vs grass silage) and starch (e.g. maize vs wheat) may influence the extent to which this can be achieved.

Level and balance of intestinally absorbable amino acids, in particular methionine and lysine, may become limiting at lower protein concentrations. Consequently, a systematic review and meta-analysis of responses to rumen protected forms of methionine and lysine was conducted for high yielding cows fed diets containing 150 g CP per kg dry matter. This analysis revealed a small but significant increase in milk-protein yield when cows were supplemented with these rumen protected amino acids. Variation in milk and milk protein yield responses between studies was not random but due to differences in diet composition. Cows fed low protein diets can respond to supplemental methionine and lysine so long as intake is not limiting, metabolisable protein is not grossly deficient and other amino acids such as histidine and leucine do not become rate limiting.

Whereas excess dietary protein is known to impair reproduction and can contribute to lameness, there is no evidence to indicate that reducing dietary CP levels to around 140 – 150 g/kg DM will have any detrimental effect on either cow fertility or health, although this requires confirmation in contemporary high-yielding genotypes.