Nitrogen balances for three strains of dairy cows and contrasting intensive grassland systems W. Ryan1,2, D. Hennessy1, J. J. Murphy1, T. M. Boland2 and L. Shallloo1, 1Animal and Grassland Research & Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork. Ireland 2School of Agriculture, Food Science and Veterinary Medicine, University College Dublin. Ireland Introduction Grass based milk production systems in Ireland and other temperate climates have a competitive advantage over confined systems due to the use of grass as a low cost feed source Grass based systems rely on inputs of N fertiliser and concentrate to ensure adequate feed supply for milk production N use efficiency is a key driver of environmental and economical sustainable, however, N use efficiency within animal production systems is low Factors which influence N use efficiency of individual animals include genetic strain and feeding system Objective To develop, evaluate and validate an annual dairy cow N balance model based on a range of Irish grass based dairy production systems Materials and Methods Three strains of Holstein-Friesian cows investigated - high-production North American (HP), high-durability North American (HD) and New Zealand (NZ). Three pasture based production systems - Moorepark Blueprint system (MP), a high concentrate input system (HC) and a high stocking rate system (HS). N inputs – fertiliser, feed, replacement animals N exports - milk, meat (calf and cull cow) Model outputs – N surplus and N se efficiency Two scenarios investigated: Scenario 1 (S1) examined N utilisation, N use efficiency and N losses of the contrasting dairy production systems, excluding the N required to rear replacement animals for the production system (from birth to first calving) Scenario two (S2) was similar to S1 but included the N required to rear replacement animals for the production system N leached or lost through gaseous emissions Recycled N used for grass growth N excreted and released from soil N Inputs Grazed Grass Winter Feed Concentrate N Outputs Milk Meat Exported feed Exported slurry Fig. 1 Schematic representation of the N balance model N immobilised/ mineralisation Table 1. N input, N output, N surplus and N use efficiency of three genetic strains of Holstein Friesian dairy cows managed on three grass based systems, including the rearing of replacement heifers (Scenario 2). Production system HS HC MP Strain of Holstein-Friesian HP HD NZ Total N input 197 179 169 206 190 198 182 171 Total N output 42 41 39 48 47 40 N surplus (kg N cow-1) 156 139 130 158 143 127 140 131 N use efficiency (per cow) 0.21 0.22 0.23 0.25 Results Replacement rate for the NZ, HD and HP genetic strains was 18%, 25% and 37%, respectively N input increased as replacement rate increased The N surplus per cow was greater for the HD and HP strains (141 and 157 kg N cow-1, respectively) than for the NZ strain (129 kg N cow-1). The HP genetic strain was the most efficient of the three strains in converting N to product (Table 1) Fig. 1. N surplus (kg N/cow) for three genetic strains of Holstein Friesian dairy cows managed on three grass based systems in Scenario 1 (rearing of replacement heifers not included) and Scenario 2 (including the rearing of replacement heifers). Conclusion High replacement rate reduces N use efficiency HP animals had the least efficient N utilisation rates per cow when the whole system including the rearing of replacement animals was evaluated. The importance of including the rearing of replacement heifers in the N balance Acknowledgement This Project was funded by the Research Stimulus Fund administered by the Department of Agriculture, Fisheries and Food (RSF 05-201)