Efficiency of phosphorus and calcium utilization in dairy cattle and implications for the environment

Phosphorus (P) is an essential element involved in numerous metabolic processes in the body of cattle. Efficiency of P utilization (the ratio of P excreted in milk to dietary P intake) is highly variable but in dairy cattle it is usually about 0.30 (Wu et al., 2000). The excess of P may be detrimental to the environment.
Of particular concern is the amount of P surplus per hectare and the limited potential of soils to further accumulate P. Some soils are saturated with P as a result of intensive fertilization for many decades. Hence, a considerable fraction of manure P may leach from soils and contributes to water eutrophication. Legislation on P management is, or may become, a more important constraint on farming practice than legislation on N management (e.g. EU Water Framework Directive, http://eur-lex.europa.eu). For this reason, it is expected that the efficiency of utilization of P resources in intensive dairy farming has to increase substantially in the near future.
Besides environmental constraints, there are also economic or political arguments for improving P
efficiency on dairy farms. World P resources for artificial fertilizer are becoming scarce and may become more expensive at a rapid rate (Cordell et al., 2009). There is no atmospheric loss of manure P such as for
manure nitrogen (N). However, atmospheric N can be bound again by root systems of certain plant species and enrich soil N. Also, atmospheric N is used to produce artificial N fertilizer. Such recycling is not possible for P, which to a large extent is permanently lost from watersheds and rivers eventually to oceans. For this reason, P is lost much more permanently than N.
Scarcity of P stocks and more stringent future legislation to preserve water quality may lead to an increased pressure to maximize P utilization by cows and minimize P losses from manure. This can be achieved by lowering farm P import from purchased feeds and artificial fertilizers, and by making more efficient use of feed and manure P. A more efficient use of feed P can be achieved by reducing the amount of P ingested by dairy cows per unit milk yield. This chapter discusses the implications of nutrition on the efficiency of P utilization by cows, and
puts this in the perspective of P balance at a farm level. To a lesser extent, efficiency of calcium (Ca) utilization is also discussed because the vast majority of P and Ca are stored in bones in an almost constant ratio of 1:2. We have addressed Ca as required for proper P utilization but not from a perspective of milk fever in early lactation.