Agriculture is aiming to reduce greenhouse gas and ammonia emissions. Agroscope showed that for dairy cattle housing, feed composition plays a role in these emissions as well as wind and temperature.
Apart from carbon dioxide (CO2), the most powerful anthropogenically emitted greenhouse gases are methane (CH4) and nitrous oxide (N2O). In Switzerland, around 64% of N2O emissions and 83% of CH4 emissions were from agriculture in 2020. Ammonia (NH3) emissions accounted for around 94% of agricultural emissions in 2019, primarily from livestock farming. The Swiss agricultural sector aims to reduce NH3 emissions by around 40% and greenhouse gas emissions by at least one-third. Measures for feed optimisation start at beginning of the production chain and can be implemented in practice without the need to convert existing housing.
Greenhouse gas and NH3 emissions under practical conditions
Our study quantifies the emissions (NH3, N2O, CH4, CO2) in two separated but structurally identical compartments of a naturally ventilated cubicle housing system for lactating dairy cows. The cows in one compartment were fed a diet with 116g crude protein (CP) per kilogram of dry matter (DM), in the other compartment with 166g CP kg−1 DM. The diets fed in both compartments were subsequently switched around for a second six-day measurement phase.
Protein-reduced diet leads to lower ammonia and nitrous oxide emissions
The results showed that, in addition to outside temperature and wind speed, diet is a relevant factor influencing NH3 and N2O emissions. Higher temperatures or wind speeds led to a significant rise in NH3 emissions. Lower levels of CP in the diet can reduce dairy-cow nitrogen excretion and hence ammonia and nitrous oxide formation potential. On average, there was a 46% reduction in NH3 and an almost 20% reduction in N2O emissions per animal when the lower rather than the higher-CP diet was fed. DM intake and milk yield decreased slightly in cows fed the lower-CP diet. Regardless of whether the emissions related to livestock unit (LU), DM intake or energy-corrected milk yield, the effects of diet, temperature and wind speed in the housing on NH3 emissions were highly significant. Furthermore, a strong correlation between dietary CP content, urinary N excretion and milk urea content was observed.
CH4 emissions are also affected by diet
The composition of the ration can also influence CH4 emissions. The differences in CH4 emissions point to a significant influence of the feed which is actually attributable to crude fibre content rather than crude protein content. In terms of CH4 emissions, therefore, both dietary crude fibre content and DM intake must be taken into account.
- Lower dietary CP content led to a significant reduction in ammonia and nitrous oxide emissions. The optimisation of dietary CP content occurs at the beginning of the nitrogen chain and can be widely implemented in agricultural practice, since no structural changes to buildings or additions are required.
- Dietary CP content, urinary nitrogen excretion and milk urea content are strongly correlated. Accordingly, milk urea content can be used as an indicator for nitrogen excretion, and hence for ammonia emission potential.
- Methane emissions are also influenced by feed, with inter alia crude fibre content playing a role.