Effect of grazing system on nitrogen partitioning in lactating dairy cows grazing irrigated pastures in Canterbury, New Zealand

摘要

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 future 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 ~lt 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). Nodifferences 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).