Grassland Management with Emphasis on Nitrogen Flows

This chapter deals with the challenge to reduce nitrogen (N) excretion and the amount of N able to leach under pasture. N inputs need to be adjusted while limiting adverse effects on animal nutrition and milk production per cow and per hectare. The chapter starts with 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 due to urinary N and can be influenced by fertilization and nutrition management. At the animal level, lowering N fertilization improves the N utilization and N balance without greatly affecting the nutritive value of grasses. Only when the grass crude-protein content falls below 120 g kg dry matter (DM), dry-matter intake and consequently animal performance may be reduced. The negative effects of lowering N fertilization on animal performance can be compensated by the inclusion of (white) clover in the sward, due to its higher intake and digestibility. Because of higher ruminal N losses, white clover may again increase the amount of N excreted, despite of higher milk yields per cow on mixed swards. At the paddock level, decreasing N fertilizer always reduces N surplus and the risk of nitrate leaching, because N input is reduced to a far greater extent than the N exported to milk. Replacing high N fertilization 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 of sward DM. Increasing the stocking rate for a given level of N fertilization only marginally reduces N surplus per ha. Supplementation with either cereal-based concentrates or maize silage reduces the N excretion per cow, but because it enables an increased stocking rate it may increase the N surplus per ha.