The Effects of System Changes in Grazed Dairy Farmlet Trials on Greenhouse Gas Emissions

Abstract

Simple SummaryDairy farm system practices aimed at reducing nitrate leaching can also reduce emissions of the greenhouse gases methane and nitrous oxide. A study comparing ‘current’ and ‘improved’ grazed dairy system practices showed that ‘improved’ systems generally produced lower greenhouse gas emissions while milk production was maintained. The amount of feed eaten per hectare was the key driver of total greenhouse gas emissions per area, with ‘improved’ systems generally exhibiting lower total enteric methane and less N flowing through the herd.AbstractAn important challenge facing the New Zealand (NZ) dairy industry is development of production systems that can maintain or increase production and profitability, while reducing impacts on receiving environments including water and air. Using research ‘farmlets’ in Waikato, Canterbury, and Otago (32–200 animals per herd), we assessed if system changes aimed at reducing nitrate leaching can also reduce total greenhouse gas (GHG) emissions (methane and nitrous oxide) and emissions intensity (kg GHG per unit of product) by comparing current and potential ‘improved’ dairy systems. Annual average GHG emissions for each system were estimated for three or four years using calculations based on the New Zealand Agricultural Inventory Methodology, but included key farmlet-specific emission factors determined from regional experiments. Total annual GHG footprints ranged between 10,800 kg and 20,600 kg CO2e/ha, with emissions strongly related to the amount of feed eaten. Methane (CH4) represented 75% to 84% of the total GHG footprint across all modelled systems, with enteric CH4 from lactating cows grazing pasture being the major source. Excreta deposition onto paddocks was the largest source of nitrous oxide (N2O) emissions, representing 7–12% of the total GHG footprint for all systems. When total emissions were represented on an intensity basis, ‘improved’ systems are predicted to generally result in lower emissions intensity. The ‘improved’ systems had lower GHG footprints than the ‘current’ system, except for one of the ‘improved’ systems in Canterbury, which had a higher stocking rate. The lower feed supplies and associated lower stocking rates of the ‘improved’ systems were the key drivers of lower total GHG emissions in all three regions. ‘Improved’ systems designed to reduced N leaching generally also reduced GHG emissions.

Benefits and Trade-Offs of Dairy System Changes Aimed at Reducing Nitrate Leaching

“…The 'Improved' farmlets were designed by applying a suite of strategic changes to the Current for each region to reduce nitrate leaching (Table 1). Farm, animal, and feed management practices for the farmlets in each region are described by Clark et al [8] for Waikato, Chapman et al [9] for Canterbury, and Van der Weerden et al [11] for Otago. A summary of the main features is given in Table 2.…”

“…Table 1. System changes applied to typical regional dairy farms in developing nitrate leaching mitigated farms as part of the Pastoral 21 farmlet trials in Waikato, Canterbury, and Otago, New Zealand [11].…”

“…Annual average GHG emissions for each system were estimated for all the monitored years using calculations based on the NZAI methodology (http://www.mfe.govt.nz/node/23304/) [15], but included key farmlet-specific activity data from the P21 farmlet systems as well as farmlet-specific emission factor values determined from targeted regional experiments (see [11] for more detail).…”

“…The five key changes used to design the P21 Improved farmlets were using lower nitrogen (N) fertiliser inputs; fewer, but higher producing cows; lower herd replacement rate; greater use of high-energy/low-N feeds; and using off-paddock facilities to reduce the time cows spend on pasture (or on forage crops). In all regions, the Improved systems could reduce nutrient losses to water [8][9][10][11] while GHG emissions were estimated to be reduced in most of the Improved systems [11]. The total annual GHG emissions were strongly related to total feed eaten, and the lower feed supplies and associated lower stocking rates of the Improved systems were the key drivers of lower total GHG emissions in all three regions [11].…”

“…In all regions, the Improved systems could reduce nutrient losses to water [8][9][10][11] while GHG emissions were estimated to be reduced in most of the Improved systems [11]. The total annual GHG emissions were strongly related to total feed eaten, and the lower feed supplies and associated lower stocking rates of the Improved systems were the key drivers of lower total GHG emissions in all three regions [11]. These findings align with international studies where the general trend was that increased farming intensity within a system (more input and more animals) may decrease the GHG intensity of milk (kg emissions/kg milk), but absolute emissions (kg emissions/ha) will increase [12][13][14].…”