A review of efficiency of nitrogen utilisation inlactating dairy cows and its relationship withenvironmental 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 rivers and ground water resources. A quantitative analysis of the contribution of dairy cows to pollution at the farm level is given and the effect of different types of carbohydrate and protein supplementation discussed. The relationship between nitrogen intake and nitrogen balance was investigated using data from 580 dairy cows and 90 treatments published in the literature. Regression analysis described the relationships between nitrogen intake and output in faeces, urine and milk. Inefficient utilisation of nitrogen by dairy cows indicates that about 72% of consumed nitrogen is excreted in faeces and urine. There were positive linear relationships between nitrogen intake and output in faeces, urine and milk up to an intake of 400 g N/d. However, above 400 g N/d, excretion in urine increased exponentially while the rate of increase in nitrogen excretion in faeces and milk declined linearly. 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 compared 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%. Management practices with respect to silage making and the choice of supplements need to be considered with the aim of reducing total nitrogen in excreta and if possible shifting nitrogen excretion from urine to faeces.