Whole farm modelling the effect of grass silage harvest date and nitrogen fertiliser rate on nitrous oxide emissions from grass-based suckler to beef farming systems

The intensification of agricultural production systems to produce food for the growing world population is envisaged to result in the increase in nitrous oxide emissions (N2O). The goal of this study was therefore to assess the effect of different management practices on greenhouse gas (GHG) emissions from contrasting grass-based suckler beef farms with a particular focus on N2O emissions. The contrasting grass-based suckler beef systems evaluated were intensive (INT) and extensive (EXT) steer and heifer (SH) beef systems and bull and heifer (BH) systems. A whole farm model approach was taken to simulate GHG emissions from these baseline systems using data from a long-term research trial and a hybrid economic-GHG model. Several aspects of the hybrid model were updated. Default values for nitrogen (N) content of fresh and conserved grass were replaced with prediction equations. N excretion and partitioning prediction equations and emission factors (EF) for N2O from grazing cattle and fertiliser were also updated. The four baseline systems were simulated to harvest first cut silage on May 24. The pasture fertiliser rate for the EXT and INT systems were 77 kg N ha−1 and 205 kg N ha−1, respectively. To test the effect of changing management practices, the four baseline systems were simulated at earlier (May 5) and later (June 28) first cut silage harvest dates and 50% higher and lower pasture fertiliser application rates. In total, GHG emissions from four baseline systems and sixteen alternative scenarios were simulated. The carbon footprint of the baseline systems in kg CO2-equivalent (CO2e) per kg of carcass weight (kg CO2e CW−1) ranged from 17.7 for BH EXT to 19.4 for SH INT. This was lower than the latest published EU average of 22.2 kg CO2e CW−1. Across all scenarios, the increase in fertiliser application rate and earlier first cut silage harvest date increased the kg N2O kg CW−1 of the four production systems. Due to younger slaughter age facilitating higher stocking rates and thus higher productivity per hectare, systems finishing males as bulls at 16 months had lower N2O and total GHG emissions than production systems finishing males as steers at 24 months. Therefore, BH EXT with increased fertiliser application rate and earlier silage harvest date was the most sustainable suckler to beef production system while SH EXT with reduced fertiliser application rate and later silage harvest date was the least sustainable suckler to beef production system due to longer time to slaughter and consequently lower stocking rate.