Nitrogen inputs and losses from clover/grass pastures grazed by dairy cows, as affected by nitrogen fertilizer application

Nitrogen (N) inputs and outputs were measured over 3 years in a trial with four farmlets teach with 16 randomly-allocated 0 4 ha paddocks) on permanen t white clover/ryegrass pastures which were grazed throughout the year by d airy cows near Hamilton, New Zealand. Three farmlets were stocked at 3.3 co ws/ha and received nominal rates of N fertilizer (urea in 8-10 split applic ations) of 0, 200 or 400 kg N/ha per year. A fourth farmlet with 4.4 cows/h a received 400 kg N/ha per year and was supplemented with maize grain durin g the first two years. Nitrogen balances were calculated, with Sigma N inputs approximate to Sigma N outputs. Annual inputs from N-2 fixation were 99-231 kg N/ha in the 0 N farmlet, but declined to 15-44 kg N/ha in the 400 N farmlets. The main N ou tputs tin kg N/ha per year) were in milk (72-126), nitrate leaching (20-204 ), and transfer of N via cow excreta from pastures to lanes and milking she d (54-92). Gaseous losses by denitrification (3-34) and volatilization (15- 78) were smaller than the other N outputs but increased significantly with N fertilizer application. In the maize-supplemented farmlet, N outputs in m ilk were 31% higher than in the corresponding non-supplemented 400 N farmle t, whereas leaching losses averaged 17% lower during the 2 years of supplem entation. In the N-fertilized farmlets, estimated N balances were influenced by inclu sion of the transitional N processes of immobilization of fertilizer N into the soil organic N pool (estimated using N-15 at 42-94 kg N/ha per year) a nd the contribution from mineralization of residual clover-fixed N in soil not accounted for in the current estimates of N-2 fixation (estimated at up to 70 % of measured N-2 fixation or 46 kg N/ha per year). However, these p rocesses were counteracting and together were calculated to have only a sma ll net effect on total N balances. The output of N in products (milk, meat and feed) relative to the total N i nput averaged 26 % in the 400 N farmlets, and is compared to that measured for commercial intensively-managed dairy farms in England and the Netherlan ds (14-20 %). The 0 N farmlet, which was reliant on N-2 fixation as the sol e N input, was relatively very N-efficient with the milk production being 8 3 % of that in the 400 N farmlet (at 3.3 cows/ha) and the N output in produ cts relative to total N input averaging 52%.