Nitrogen fertilisation of grasslands and dairy cow nutrition. Consequences on N losses

Abstract

The changes in agricultural policy and the increased concern about the effects of intensive production systems on the natural environment may lead to a reduction in the levels of nitrogen (N) fertilisation on grassland. The aim of this review is to quantify the consequences of lowering N fertilisation on dairy cow nutrition.
For grass species harvested at the same age of regrowth, a reduction in N fertilisation leads to a 0.02 unit decrease in organic matter (OM) digestibility, but the site of OM digestion is unaffected. Despite a much lower crude protein (CP) content in poorly fertilised grass, the amount of non-ammonia nitrogen entering the intestine is decreased by only about 5 % since the efficiency of microbial protein synthesis is unaffected. The rumen degradability of CP is, however, slightly decreased. Reducing N fertilisation could reduce NE_L by 0.3 MJ kg^-1 DM and Metabolisable Protein (MP) by 5 to 12 g kg^-1 DM. These moderate effects can be attributed to the fact that any decrease in CP content is compensated for by an increase in water-soluble carbohydrates, which are completely digestible and provide a readily available source of energy for ruminal proteosynthesis. Conversely, cell wall content remains unchanged. On the average, N fertilisation has no effect on the quantity of dry matter voluntarily ingested by stall-feeding animals, but herbage intake at grazing may be reduced since low amounts of N fertilisation might reduce the ease of prehension of herbage by reducing the green leaf mass per unit area. Lowering the levels of N fertilisation would appear to be an efficient means of reducing N loss in ruminants. The possible consequences on dairy cow nutrition are discussed.