Reducing Content of N and P in Dairy Nutrition

Rationale, mechanism of action A close relationship exists between the excretion of N and P by dairy cattle and the amounts consumed with feed. Hence, there are good options to reduce P excretion by nutritional measures. In many regions, grass delivers the main fraction of dietary P, whereas soil P status strongly determines P content of grass. This means that changes in whole farm management which lead to a lower soil P status will directly affect P excretion and the efficiency of P retention in milk. Trade-offs with changes in roughage quality and net energy value always need to be taken into account. Alternatively, grass may be replaced by maize which immediately leads to a substantial reduction in P intake while stimulating feed intake and milk yield per cow. Also, changes in the choice of concentrates composition may lead to a reduced P content of concentrates and hence less P intake by the cow. Adopting the first two options (grass and maize) requires analysis of the combination of nutritional effects and effects of farm nutrient management. The latter option requires consideration of nutritional effects only, including feed costs. From a nutritional perspective, there is a lot of room for reducing the N and P content in diets [1]. Making use of the high capacity of dairy cattle to recycle urea-N and P from blood to the rumen ensures that rumen fermentation will be maintained, even in the low range of dietary N and P content. However, critical limits to N and P availability to rumen micro-organisms needs to be taken into account, if known. Also analysis is required from an animal metabolic perspective. The N and P metabolism in dairy cows differs strongly, with emphasis on rumen function and protein digestibility in the case of N, and emphasis on the dynamic aspects of P metabolism and P storage in bone in the case of P (P absorbability is less important).