Farm System and Diet Changes to Decrease Methane and Nitrogen Outputs on New Zealand Dairy Farms

Whole farm system modelling assessed the impacts of multifactorial farm system changes on greenhouse gas (GHG) emissions, nitrogen (N) losses, productivity, and profitability. Variability in stocking rate, cow size, genetic merit, and supplement inputs were assessed, whilst keeping milk yield constant. Modelling software Udder, Red Sky, Cornell Net Carbohydrate and Protein System (CNCPS), and Overseer were used to predict the productive, economic, and environmental outcomes of a baseline scenario, constructed to represent the average Waikato dairy farm for the 2018/2019 season. In four independently modelled scenarios (1, 2, 3, 4) cow size, genetic merit, and concentrate feed input (up to 18.5% of the diet) were progressively increased from the baseline, and a fifth scenario (5) reduced the stocking rate by 15% without the use of concentrate supplements and without changes to cow size or genetic merit. Compared to the baseline, larger cows of higher genetic merit and fed the greatest supplement level (scenario 4) reduced GHG emissions and N losses by 15.5% and increased profitability by 22.2%. Farm feed demand reduced by 13.2% in this scenario, which was the primary mechanism for reducing GHG emissions. Land area required also reduced by 8.5%, stocking rate declined from 2.94 to 2.06 cows per hectare, and the number of replacement heifers required was concomitantly reduced. Total methane emissions estimated using CNCPS declined by 13.9%, and increased feed efficiency reduced methane output per unit of fat corrected milk by 15.1%. Scenario 5 reduced GHG emissions (14.4%) and N losses (9.7%), but also milk production (10.9%) and operating profit (7.4%) compared with the baseline. Operating profit for scenario 4 was 32.0% higher than scenario 5. This study suggests that a whole systems approach to reducing GHG emissions may be more effective than only reducing cow numbers, as integrating diet changes with altered genetics and lower stocking rates can enhance farm efficiency, reduce environmental impacts, and improve profitability.