Grassland acreage: a key factor for estimating dairy cattle’s N-excretion

Animal nutrient excretion is a key factor in manure legislation and environmental monitoring. Because it can not be measured directly, total emissions are usually estimated using flat-rate coefficients. This is also the case for dairy cows, whose productivity has constantly increased over the past decade. It thus seems odd to use constant, flat-rate coefficients for estimating their N-excretion, as e.g. Flemish manure legislation has been doing (97 kg N /(cow x year)). N-excretion/cow is proportional to milk production/cow. For environmental monitoring, owever, it is not easy to get milk production data at farm level. As milk production in turn is losely correlated to feed uptake, estimating nutrient excretion indirectly, by means of fodder areage available, was explored. A significant relation was found between N-excretion per cow and the grassland and maize acreages per cow, in which the effect of grassland acreage is over 2,5 times larger than that of maize. Although average N-excretion has increased to 118 kg N/(cow x year) on specialised dairy farms, at the regional level, the cows’ increasingproductivity predominates, resulting in an increased eco-efficiency.