Modelling of nitrogen transactions in the dairy cow and their environmental consequences

This paper reviews the fate of nitrogen in the feed of lactating dairy cows and considers the consequences of the various transactions for urinary N excretion. A simple computer model was established and used to investigate the major factors influencing urinary N. In setting the model up it was clear that a number of areas of uncertainty about nitrogen transactions in microbes or host tissues remain. The basal animal and area efficiencies for a herd producing 5000 litres of milk per cow per annum are 0.68 g milk N per g urinary N and 97.7 kg urinary N per hectare per annum. Feeding concentrates at 0.3 kg per litre of milk to obtain yields of 6700 litres per cow per annum increased these to 0.74 g and 191.0 kg respectively. Two major areas of uncertainty remain, the efficiency of capture of rumen degraded N by rumen microbes and the efficiency of utilisation of amino acids by host tissues. Simulations of the possible ranges in these variables illustrate the potential effects on nitrogen waste and the dietary crude protein required to support a given level of production. A major practical problem is that whilst it is very difficult to assess these variables under experimental conditions it is impossible to identify them at the farm level. This can lead to incorrect feeding decisions which merely exacerbate the problem. Given the simple assumptions of this model, the strategy that results in the lowest return of urinary N per forage hectare is an extensive system of production, particularly if this can be achieved using animals which have a high efficiency of transfer of amino acids into milk protein. Nevertheless, there is an urgent need to provide more consistent predictive relationships, and/or diagnostic tests for farm-use, in order to advance models and reduce urinary N excretion.