Ammonia volatilisation from grazed, pasture based dairy farming systems

Context: A key pathway for nitrogen (N) loss, which is poorly quantified in livestock farming systems with grazing, is the ammonia (NH3) volatilisation that occurs from fertilisers, soils, fresh and stored animal excreta. Objective: This study used farm systems modelling to understand, the likely losses of N via NH3 volatilisation from different farming systems in diverse locations throughout the geographical extent of the dairy industry in Australia. Methods: The systems model DairyMod was used to simulate daily water, energy and N cycles in representative farming systems. DairyMod simulated the flows of N through soils, pastures, feed, animals throughout the whole farm system in order to sensibly simulate the volatilisation from fertilisers, soil and animal excreta across 18 locations. Results and conclusions: Average annual NH3 volatilisation for the years 2000-2019 was 40 kg N/ha but varied greatly from 10 to 97 kg N/ha depending on year, location and farming system. Across all locations, volatilisation was higher in Spring > Winter > Autumn > Summer. Across the 18 locations annual volatilisation was, on average, 51% greater from the livestock excreta source than from the fertiliser N source, which was 22% greater than from soil N sources over the long term assess how N losses volatilised. Heterogeneity was a feature as the amount of volatilisation changed between seasons and N sources but with no obvious trends across all locations or systems when averaged over all years. Supplemental forage feeding with grass silage made up between 10 +/- 6% of total intake on a dry matter basis, but by substituting this for low protein maize silage, the overall diet N concentration was reduced from 3.0% to 2.4% which in turn caused a 47% reduction in volatilisation, from 56 to 30 kg N/ha/year, improvement in whole farm nitrogen use efficiency (NUE) by 65% from 31 to 60 g milk MS/g N-NH3 volatilisation and milk production was uncompromised overall. Significance: This is the first study to produce realistic losses of ammonia losses from pasture based dairy farming systems in Australia. The findings indicate if addressed at the whole farm scale, farm NUE can be improved significantly but this needs to be understood in the broader context of the farming system. Due to the highly heterogenous nature and responses of farming systems that include grazing animals, a pervasive and inflexible one-size-fits-all approach by policy to modifying dairy systems to improve environmental air quality would be implausible.

Automated summary

This study used farm systems modelling to investigate ammonia volatilisation-induced nitrogen losses in different dairy farming systems across Australia. Results showed that ammonia volatilisation varied significantly across seasons and nitrogen sources, with higher levels observed in Spring. By substituting low protein maize silage, overall diet nitrogen concentration was reduced, resulting in a significant decrease in volatilisation and improvement in whole farm nitrogen use efficiency.