Lowering the greenhouse gas and ammonia emissions from grassland-based dairy production

CONTEXT: Lowering greenhouse gas (GHG) and ammonia emissions from ruminant production systems is critical to mitigating climate change and enrichment and acidification of vulnerable habitats. Quantifying emission reductions from the implementation of best practices (BP) on grassland-based dairy systems is essential to guide farmers and policy towards wider adoption of best practices. OBJECTIVE: The aim was to evaluate the extent to which the adoption of BP lowered the GHG and ammonia emissions of grassland-based dairy systems per kg of fat protein corrected milk (FPCM) and per hectare (ha). METHODS: Life cycle assessment was used to calculate emissions from systems of dairy production adopting BP. Data were collected from systems-scale experiments conducted at Solohead Research Farm, Co. Tipperary, Ireland (52°51′N, 08°21′W) between 2011 and 2022. There were three systems that had an average of 27 cows per system and an average annual stocking rate of 2.53 cows ha⁻¹. INT was the control and included average annual fertiliser N input of 265 kg ha⁻¹, applied as calcium ammonium nitrate (CAN 27.5% N) and urea (46 % N) and average annual clover content of grassland dry matter was 110 g kg⁻¹, and slurry was applied by splash plate. BPN included average annual fertiliser N input of 99 kg ha⁻¹ applied as urea or protected urea, clover content was 230 g kg⁻¹ and slurry was applied by trailing shoe. BPO received minimal (<5 kg ha⁻¹) inputs of fertiliser N, clover content was 280 g kg⁻¹ and slurry was applied by trailing shoe. INT encompassed each of 7 experimental years, BPN 7 years and BPO 4 years. All relevant farm activity data was modelled on the basis of a 59 ha farm. RESULTS AND CONCLUSIONS: GHG emissions averaged 1.05, 0.80 and 0.73 kg CO₂e kg FPCM⁻¹ (SEM = 0.035, P < 0.001) for INT, BPN and BPO respectively. GHG emissions per hectare were 15.0, 11.5 and 10.7 t CO₂e (SEM = 0.485, P < 0.001). Ammonia emissions were lower from BPN and BPO compared with INT (P < 0.001). Replacing artificial fertiliser N with biologically fixed N (BFN) had the largest impact on GHG mitigation. Low emissions slurry spreading (LESS) in the form of the trailing shoe and replacing artificial fertiliser N with BFN had the greatest impact on ammonia emissions. SIGNIFICANCE: Adopting BP lowered GHG emissions by up to 29 % and ammonia emissions by up to 37 % compared with a conventional intensive system of grassland-based milk production. These results can aid farmers to contribute to emissions reduction targets.