Cow excretion big component of N pollution
Cow excretion big component of N pollution
CONTROLLING excretion could be one way of minimising the impact of dairying on the nitrogen cycle.
INRAs Raymond Vérité told delegates at the Paris conference that dairy animals are a big component of the nitrogen cycle. Cows can transfer protein into milk but in doing this, they create nitrogen-rich excretion in either faeces or urine. Cows cause pollution in two ways – nitrates in water and ammonia discharge to air, he said
According to Dr Vérité, each cow, on a diet based on 15% crude protein grass silage and producing 7500 litres of milk, results in 153kg of ingested N, of which 38kg is excreted in faeces and 75kg in urine. But urine is the biggest risk because it is fluid and quantity varies.
Two things affect N intake and excretion – the forage used and concentrate supplements. The impact of forage used depends on how much N is applied and how intensively land is stocked. The effect from supplementation is easier to control and is often used to improve milk output and breeding performance as well as feeding efficiency.
Concluding, Dr Vérité said there was a common value for improving performance and reducing nitrogen waste through supplementation. That value seems to be about 100g PDI/UFL. PDI is how the French express the amount of protein digested in the small intestine. UFL is a feed unit for lactation, and effectively reflects energy supply in the diet. A higher ratio of PDI than this results in small improvements in performance and sharp increases in N losses.
But control is harder in forage systems, with the amount of N excreted by cows varying from 80-125kg a year, he said. This depends largely on the amount of fertiliser applied and how much leguminous material is in forages.
When considering stocking rates and working in nitrogen excreted a hectare, Dr Vérité said increasing stocking rates and applying more fertiliser were big contributors to increasing pollution when cows were grazing. But increasing N inputs did boost milk production.