Protein-Reduced Dairy Cattle Feed Supplemented with Rumen-Stable Amino Acids

Various studies demonstrate the potential of protein-reduced feed to reduce ammonia emissions. A one-year trial on two Swiss dairy farms shows how this approach performs in the long term under practical conditions.

Agriculture accounts for 94% of ammonia emissions, with 77% of emissions from livestock production attributed to cattle farming. Under Article 10a of the Ordinance on the Assessment of Sustainability in Agriculture (VBNL), agricultural nitrogen losses are to be reduced by at least 15% by 2030 compared with the average for 2014–2016.

To achieve this target, the dairy sector, among others, must also play its part. One possible approach is to reduce the protein intake of dairy cows. Reducing the crude protein content of the diet, combined with targeted supplementation of the limiting amino acids methionine and lysine in rumen-protected form, can increase nitrogen-use efficiency and reduce ammonia losses. Short-term studies indicate such potential, but robust data collected over longer periods under practical conditions are lacking.

Therefore, a one-year field trial was conducted on two Swiss dairy farms to investigate how a protein-reduced diet, supplemented with rumen-protected methionine and lysine, affects milk yield, estimated ammonia emissions and nitrogen-use efficiency. On each farm, the cows were paired based on their lactation number and lactation stage; from each pair, one cow was then assigned to the control group and the other to the treatment group. For one year, the treatment groups were fed a diet reduced by 10 g of crude protein/kg of dry matter, supplemented with 10 g of methionine and 15 g of lysine in rumen-protected form. Control groups were fed the farm’s standard diet.

The long-term trial confirms the potential of protein reduction to cut emissions

In terms of milk yield, one farm showed a significant decrease in energy-corrected milk yield of 2.8 kg per day, while the other showed a non-significant reduction of around 1.0 kg per day. This is consistent with earlier studies, in which declines in yield were also observed with protein-reduced diets. One possible explanation is a limited supply of other essential amino acids besides lysine and methionine. Histidine, in particular, can be a limiting factor in low-protein, grass-silage-based diets. Regarding milk composition, no significant differences were observed in fat and protein contents or daily fat yield. Only the daily protein yield was significantly reduced on one farm, mainly due to the lower milk yield despite an unchanged protein content.

Studies show a positive correlation between milk urea content and urinary nitrogen excretion. Therefore, the reductions in milk urea content of 1.7 and 3.8 mg/dl, respectively, observed in this study point to a decreased N excretion which can contribute to a reduction in ammonia emissions. The estimated reduction in NH₃ emissions was 4.3–13.8%, depending on the calculation method used. At the same time, the lower N intake increased nitrogen-use efficiency by 3.0% and 0.4%, respectively.

Conducted over the course of a year under practical conditions, the study shows that dietary protein reduction supplemented with rumen-stable amino acids can improve nitrogen-use efficiency and help lower ammonia emissions. However, potential losses in milk yield must also be taken into account.