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 Ž . 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 Ž . reduction in N fertilization leads to a decrease in organic matter OM digestibility by 0.02 units, Ž . 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 NE by 0.3 MJ L y1 Ž . y1 kg DM and Metabolizable Protein MP by 5 to 12 g kg 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 ) Corresponding author. Tel.: q33-2-99-28-50-94; fax: q33-2-99-28-51-01; e-mail: [email protected] 1 Present address: Universidad de la Republica, Facultad de Agronomıa, Montevideo, Uruguay. ́ ́ 0377-8401r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0377-8401 97 00191-0 ( ) J.L. Peyraud, L. AstigarragarAnimal Feed Science and Technology 72 1998 235–259 236 N losses in ruminants. The possible consequences on dairy cow nutrition are discussed. q 1998 Elsevier Science B.V. All rights reserved.