A review of efficiency of nitrogen utilisation in lactating dairy cows andits relationship with environmental pollution

The objective of this paper is to review the literature concerning nitrogen utilisation in lactating dairy cows with an emphasis on their contribution to environmental pollution. Nitrogen, as oxides or ammonia, is one of the green houses gases contributing to air pollution and through leaching to ri vers and ground water resources; A quantitative analysis of the contributio n of dairy cows to pollution at the farm level is given and the effect of d ifferent types of carbohydrate and protein supplementation discussed. The r elationship between nitrogen intake and nitrogen balance was investigated u sing data from 580 dairy cows and 90 treatments published in the literature . Regression analysis described the relationships between nitrogen intake a nd output in faeces, urine and milk. Inefficient utilisation of nitrogen by dairy cows indicates that about 72% of consumed nitrogen is excreted in fa eces and urine. There were positive linear relationships between nitrogen i ntake and output in faeces, urine and milk up to an intake of 400 g N/d. Ho wever, above 400 g N/d, excretion in urine increased exponentially while th e rate of increase in nitrogen excretion in faeces and milk declined linear ly. To reduce nitrogen pollution, it is recommended to decrease the amount of crude protein in the total diet to approximately 150 g/kg DM which compa red with levels of 200 g/crude protein/kg DM consumption can reduce annual nitrogen excretion in faeces by 21% and more importantly in urine by 66%. M anagement practices with respect to silage making and the choice of supplem ents need to be considered with the aim of reducing total nitrogen in excre ta and if possible shifting nitrogen excretion from urine to faeces.