Effects of water-quality management mitigations on greenhouse-gas emissions from deer farms

Context: Red deer farming in New Zealand has increased in intensity, increasing the emissions to water and air. Outdoor wintering systems pose a significant threat to water quality through sediment loss and nitrate leaching. Changing wintering systems to bring animals indoors shifts emphasis to greenhouse-gas emissions. Aims: To investigate the relative potential emissions to water and air when red deer are wintered outdoors on forage crops or indoors on supplements. Methods: The impacts of wintering red deer on forage or indoors were calculated for five farms, involving 32 herds containing 2167 deer over 2 years, in southern New Zealand. Animal classes included weaners, hinds and stags. Potential losses to water included sediment, nitrogen and phosphorous, while losses to air included methane, nitrous oxide and ammonia. Losses to air were calculated using current New Zealand greenhouse-gas inventory calculations and revised calculations recognising published forage, soil and bedding emissions factors not yet included in the inventory. Key results: Calculated outdoor winter feed intake was 9.5% greater than indoor measured feed intake. The average herd size of 115 deer wintered indoors for an average of 87 days would have needed 1.8 ha of winter forage crop. Potential losses of sediment, nitrogen and phosphorus were calculated to be 5362, 106 and 5.2 kg per herd respectively, if wintered on crop. Total greenhouse-gas (GHG) emissions calculated using current inventory emission factors were higher if deer grazed a forage crop than when wintered indoors (2.58 vs 2.41 kg CO2-e/head/day respectively). When revised emission factors were used, indoor wintering produced greater GHG emissions than did wintering outdoors (2.61 vs 2.28 kg CO2-e/head/day respectively). Implications: Variability may occur both in contaminant loss to water and emissions to air. Trade-offs between the two need to be recognised in decision-making. As the science of GHG develops, the relative ranking of different systems may change.