In contrast to pigs, dairy cattle are as yet rarely fed protein-reduced diets. Studies show that there is also potential for protein savings in cattle, and thus for reducing ammonia emissions.
The Swiss Parliamentary Initiative Pa.Iv.19.475 on reducing the risk of pesticide usage calls for an appropriate reduction of nitrogen and phosphorus losses in the farming sector by 2030. As livestock husbandry is responsible for approximately 90% of agricultural ammonia losses, livestock management will need to make significant contributions to achieving the targets set on the reduction pathway.
Target attainment will require measures along the entire agricultural production chain. Feeding optimisation is particularly effective in this respect, as the potential for losses along the entire commodity chain can be automatically lowered by reducing nutrient inputs. In pig and poultry feeding, the use of nitrogen and phosphorus reduced (NPr) feed is widespread. In dairy cattle, however, the approach of feeding crude protein reduced diets has rarely been practised to date.
For this reason, the effects of protein-reduced rations on milk yield parameters were investigated in pilot trials on two Swiss commercial farms over a period of two years. In crossover trials, experimental groups (V) and control groups (K) were paired for comparison. In the experimental groups, the rations’ protein content was reduced by about 10 g/kg DM and the rations were supplemented with rumen-protected amino acids so that the amount of digestible lysine and methionine was at least equal to the amount contained in the control group’s ration.
Lower urea content leads to lower emissions
No significant effect on the energy-corrected milk yield and the milk fat and milk protein contents was detected. However, as in other studies, a slight numerical decrease in milk yield was observed. A possible reason for this decrease could be the reduced nitrogen supply for the rumen microbes and their growth. It is possible that the supplementation with the amino acids lysine and methionine did not compensate for the difference in available protein in the intestine. There are indications in the literature that under certain conditions histidine may be a limiting amino acid in dairying. For further trials, therefore, histidine would have to be taken into consideration in addition to the amino acids methionine and lysine.
Given the correlation between nitrogen excretion and milk urea content, the latter can be used to assess the environmental impact of the experimental ration. For all experimental groups, a significantly lower value (-3.2 to -4.6 mg urea per dl milk) was observed compared to the control group. Other studies show that a reduction in ammonia emissions of between 8 and 14% can be expected for the protein-reduced ration at the observed milk urea levels.
The present study concords with other studies in showing that a reduction in protein intake in dairy cattle feeding can be achieved without a loss in milk yield if it is specifically balanced with rumen-protected amino acids. This approach can thus contribute to reducing ammonia emissions.
- A reduction in the crude protein content in dairy cattle rations is possible without adverse effects on milk yield when compensated by rumen-protected amino acids.
- Reducing protein intake leads to significantly lower milk urea levels and nitrogen excretion.
- A reduction of ammonia emissions by 10% by means of feeding dairy cattle a protein-reduced diet appears to be realistic in future.
- Consideration of amino acids other than lysine and methionine, especially histidine, is likely to be necessary.