Maîtrise des flux d'azote dans la gestion des prairies et du pâturage en systèmes laitiers intensifs

Abstract

This paper deals with the challenge to reduce N excretion and the amount of N able to leach under pasture by adjusting N inputs while limiting adverse effects on animal nutrition and milk production per cow and per hectare. The paper starts by discussing the origin and variation of N flow in dairy cows. N intake is divided into N exported in milk and N excreted in faeces and urine. Variation in N excretion is mainly in urinary N and can be influenced by fertilisation and nutrition management. At the animal level, lowering N fertilisation improves N utilisation and N balance, without greatly affecting the nutritive value of grasses, except when the crude grass protein content falls below 120 g kg-1 DM that may reduce dry matter intake and consequently animal performance. The negative effects of lowering N fertilisation on animal performance can be compensated by the inclusion of (white) clover, due to its higher intake and digestibility. Because of higher ruminal N losses, white clover may again increase the amount of N excreted despite higher milk yield per cow on mixed swards. At the paddock level, decreasing N fertiliser always reduces N surplus and the risk of nitrate leaching because N input is reduced to a far greater extent than N exported in milk. Replacing high N fertilisation by grass clover mixtures also has the potential to reduce N surplus and nitrate leaching but to avoid high levels of N2 fixation, the clover content should not exceed 0.3 to 0.4. Increasing stocking rate for a given level of N fertilisation only marginally reduces N surplus per ha. Supplementation with either cereal based concentrates or with maize silage reduces N excretion per cow but because it enables an increased stocking rate it may increase N surplus per ha.