Nitrogen (N) inputs and outputs were measured over 3 years in a trial with four farmlets (each with 16 randomly-allocated 0n4 ha paddocks) on permanent white clover\ryegrass pastures which were grazed throughout the year by dairy cows near Hamilton, New Zealand. Three farmlets were stocked at 3n3 cows\ha and received nominal rates of N fertilizer (urea in 8-10 split applications) of 0, 200 or 400 kg N\ha per year. A fourth farmlet with 4n4 cows\ha received 400 kg N\ha per year and was supplemented with maize grain during the first two years.Nitrogen balances were calculated, with ΣN inputs $ ΣN outputs. Annual inputs from N # 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 outputs (in 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 shed (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 milk were 31 % higher than in the corresponding non-supplemented 400 N farmlet, whereas leaching losses averaged 17 % lower during the 2 years of supplementation.In the N-fertilized farmlets, estimated N balances were influenced by inclusion of the transitional N processes of immobilization of fertilizer N into the soil organic N pool (estimated using "&N at 42-94 kg N\ha per year) and the contribution from mineralization of residual clover-fixed N in soil not accounted for in the current estimates of N # fixation (estimated at up to 70 % of measured N # fixation or 46 kg N\ha per year). However, these processes were counteracting and together were calculated to have only a small net effect on total N balances.The output of N in products (milk, meat and feed) relative to the total N input averaged 26 % in the 400 N farmlets, and is compared to that measured for commercial intensively-managed dairy farms in England and the Netherlands (14-20 %). The 0 N farmlet, which was reliant on N # fixation as the sole N input, was relatively very N-efficient with the milk production being 83 % of that in the 400 N farmlet (at 3n3 cows\ha) and the N output in products relative to total N input averaging 52 %.

Introduction

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In this publication we review recent research and understandings of nutrient flows and losses, and management practices on grazed pastoral farms in New Zealand. Developments in nutrient management principles in recent years have seen a much greater focus on practices and technologies that minimise the leakage of nutrients, especially nitrogen (N) and phosphorus (P), from farms to the wider environment. This has seen farm nutrient management planning shift from a relatively small set of procedures designed to optimise fertiliser application rates for pasture and animal production to a comprehensive whole-farm nutrient management approach that considers a range of issues to ensure A07001;

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Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha -1 yr -1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha -1 yr -1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha -~ yr -1 by ~SN dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a 15N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.

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