Effect of diets with different crude protein levels on ammonia and greenhouse gas emissions from a naturally ventilated dairy housing

Less crude protein (CP) in the diet can reduce nitrogen excretion of dairy cattle and lower their ammonia (NH 3 ) and nitrous oxide (N 2 O) formation potential. The diet composition might also affect emissions of methane (CH 4 ) and carbon dioxide (CO 2 ). However, previous studies did not investigate the effect of diets with different CP levels that are customary practice in Switzerland on NH 3 and greenhouse gas emissions on a practical scale. In a case-control approach, we quantified the emissions (NH 3, N 2 O, CH 4, CO 2 ) in two separate but identical compartments of a naturally ventilated cubicle housing for lactating dairy cows over six days by using a tracer ratio method. Cows in one compartment received a diet with 116 g CP per kilogram dry matter (DM), in the other compartment with 166 g CP kg −1 DM. Subsequently, diets were switched for a second 6-day measurement phase. The results showed that the diet, aside from outside temperature and wind speed in the housing, was driving NH 3 and N 2 O emissions. NH 3 and N 2 O emission reduction per livestock unit (LU) was on average 46 % and almost 20 %, respectively, for the diet with low CP level compared to the higher CP level. In addition, strong relationships were observed between the CP content of the diet, N excretion in the urine and the milk urea content. An increased temperature or wind speed led to a clear increase in NH 3 emissions. Differences in CH 4 and CO 2 emissions per LU indicated a significant influence of the diet, which cannot be attributed to the CP content. Our herd-level study demonstrated that a significant reduction in NH 3 and N 2 O emissions related to LU, energy-corrected milk as well as DM intake can be achieved by lowering the CP content in the diet.