Review of the effect of nitrogen fertilization on the chemical composition, intake, digestion and nutritive value of fresh herbage: consequences on animal nutrition and N balance

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) fertilization on grassland. The aim of this review is to quantify the consequences of lowering N fertilization on dairy cow nutrition. For grass species harvested at the same age of regrowth, a reduction in N fertilization leads to a decrease in organic matter (OM) digestibility by 0.02 units, but the site of OM digestion is unaffected. Despite a much lower crude protein (CP) content in poorly fertilized 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, while the rumen degradability of CP is slightly decreased. Reducing N fertilization could reduce NEL by 0.3 MJ kg−1 DM and Metabolizable 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 average, N fertilization 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 fertilization might reduce the ease of prehension of herbage by reducing the green leaf mass per unit area. Lowering the levels of N fertilization would appear to be an efficient means of reducing N losses in ruminants. The possible consequences on dairy cow nutrition are discussed.