94 Clement et al. – Effect of grazing system on nitrogen partitioning in dairy cows Effect of grazing system on nitrogen partitioning in lactating dairy cows grazing irrigated pastures in Canterbury, New Zealand AR Clement a *, DE Dalley a , DF Chapman a , GR Edwards b and RH Bryant b a DairyNZ, Canterbury Agriculture and Science Centre, Gerald Street, PO Box 85066, Lincoln University7647, Canterbury, New Zealand; b Faculty of Agriculture and Life Sciences, Lincoln University, PO Box 85084, New Zealand *Corresponding author. Email: anna.clement@dairynz.co.nz Abstract Regulations currently being developed by Regional Councils throughout New Zealand to manage freshwater quality may require substantial reductions in nitrate leaching from dairy farms in many regions. The objective of this study was to quantify nitrogen intake, partitioning, and excretion in two irrigated farming systems in Canterbury, comparing different possible pathways for fu- ture industry development. A ‘higher input’ system (HI, n = 34 cows) was characterised by a stocking rate of 5 cows per/ha, ~300 kg/ha/year fertiliser N and ~1t DM/cow/year purchased supplement fed on the milking platform. The ‘lower input’ system (LI, n = 29 cows) used a stocking rate of 3.5 cows/ha, 150 kg/ha/year fertiliser N, and minimal purchased supplement. Urine, faeces, plasma, and milk samples were collected for N determination at consecutive afternoon and morning milkings once per month for four years. Estimated N intake did not differ between HI and LI (537 g N/cow/day and 557 g N/cow/day respectively, P = 0.29). No differences were recorded for N parameters and estimated milk and urine N output/cow was not affected by farm system. When scaled to per hectare, urinary N excretion was 33% greater for HI compared with LI (1.1 kg/ha/day and 0.8 kg/ha/day, respectively, P = 0.02). Keywords: nitrogen partitioning; pasture; dairy cows; grazing Introduction Urinary nitrogen (UN) excreted by grazing livestock is a significant source of environmental pollutants from the New Zealand dairy industry because nitrate (NO 3 - ) derived from UN contributes to ground and surface water contamination (Di & Cameron 2002). High-quality temperate forages, such as perennial ryegrass and white clover pastures, typically have metabolisable energy (ME) concentrations greater than 11.5 MJ/kg DM and crude protein contents between 20% and 30% of DM (Waghorn et al. 2007). This high-protein diet provides an excess of nitrogen (N) relative to animal requirements, and excretion of excess N increases the risk of environmental pollution. Only 10 to 40% of the N ingested by dairy cows is converted to animal product (Haynes & Williams 1993). Previous studies have suggested that about 50% of the total N eaten is excreted in the urine and around 25% in the dung (Pacheco & Waghorn 2008). The amount of N deposited in urine patches (average 613 kg N/ha for dairy cattle (Selbie et al. 2015)) is much greater than the capacity of pasture plants to assimilate, therefore, the N is eventually lost from the system via ammonia volatilisation, nitrate leaching and denitrification. Urinary nitrogen output is strongly correlated with N intake, with several studies (Kebreab et al. 2001; Huhtanen & Hristov 2009) reporting reduced UN excretion when the protein content of the diet is reduced. Kebreab et al. (2001) recommended several diet adjustments to reduce UN excretion e.g., grass grown with moderate fertiliser application and maize-based energy supplements formulated to provide slowly degradable protein. The objective of this paper is to quantify N intake, partitioning, and excretion of cows grazing two contrasting pasture-based dairy systems. The systems were designed to demonstrate the effect of management, particularly differences in levels of feed and nitrogen fertiliser input, on production, profit, and N leaching. Results are discussed in relation to the N mass balance of contrasting dairy systems and possible implications for the ability of farmers to meet environmental targets. Materials and methods Experimental design Two systems were compared, both with a strong focus on management efficiency: one based on the ‘traditional’ pathway of intensification, through increasing inputs of feed and fertiliser (HI), and one based on reducing those inputs to lower the amount of N imported to the farm (LI). The LI pathway results in less feed available and requires that feed supply and demand be re-balanced to maintain a sustainable system with high rates of pasture utilisation. In the LI system reported here, a reduction in stocking rate was required to achieve this, and tactical management decisions targeted greater per-cow production, through increased pasture intake/cow, to off-set some of the expected reduction in milk solids per hectare. The LI system also incorporated diverse pastures as an additional N mitigation option. Details of the two management systems are shown in Table 1. Both systems were irrigated, as required, from late spring to mid-autumn using a lateral sprinkler irrigator. Fifty-one Holstein-Friesian cows and 12 heifers were allocated to two farmlets in October 2011. The number of animals in each farmlet was 29 and 34 for the lower input (LI) and higher input (HI) farmlets, respectively (Table 1). All cows calved in spring with planned start of calving