Does feeding less protein to cows over a longer period not only reduce nitrogen losses, but also affect methane emissions?
Researchers at Wageningen University & Research (WUR) investigated this in a multi-year study with dairy cows, funded by the Vereniging Diervoederonderzoek Nederland (VDN), the Dutch Ministry of Agriculture, Fisheries, Food Security and Nature (LVVN), and the Melkveefonds (Dairy Funds).
The dairy sector faces a major challenge as it attempts to reduce its environmental impact.
Researchers at WUR have said that lower protein levels in the ration can lead to lower nitrogen excretion, particularly via urine, and therefore to lower ammonia emissions.
Researcher AH (Axel) van Ruitenbeek stated that at the same time, it is important that this does not come at the expense of animal performance or lead to unwanted side effects, such as higher methane emissions.
This part of the research focused specifically on nitrogen metabolism and methane excretion when feeding low-protein rations over a longer period.
Study on protein, nitrogen and methane
In the study, 64 dairy cows were followed across two full lactations of at least 44 weeks each.
The cows received rations with different protein levels, with realized average crude protein (CP) contents of 133g, 143g and 154g CP/kg dry matter (DM) for the low, medium and high protein groups.
The protein levels of all three groups were therefore below the Dutch practical average of 161g CP/kg DM.
WUR researchers have said that the outcomes should therefore be interpreted in the context of a relatively low protein level.
Throughout the entire lactation, feed intake, milk production and methane emissions were measured continuously.
In addition, manure and urine samples were collected in early and mid-lactation to obtain a complete picture of nitrogen excretion alongside nitrogen intake.
Lower protein levels reduce nitrogen excretion
In previous research, WUR showed that reducing crude protein from 154g to 143g/kg DM is possible without a significant loss in milk production or feed intake, whereas a further reduction to 133g/kg DM clearly reduces both feed intake and milk production.
Regarding nitrogen intake, cows in the low-protein group consumed less nitrogen in both lactations than cows in the medium- and high-protein groups.
Over the entire first lactation, nitrogen intake averaged 462g/day for the low-protein group, compared with 548g and 590g/day for the medium- and high-protein groups respectively.
Nitrogen excretion via milk also decreased in the low-protein group.
As a result, nitrogen use efficiency for milk production was highest in the low-protein group (34.8%) and lowest in the high-protein group (31.9%).
According to the research, this pattern was repeated in the second lactation, indicating that the results were consistent across multiple lactations.
The largest differences were found in nitrogen excretion via urine.
In the first lactation, urinary nitrogen excretion averaged 105g, 146g and 185g/day for the low-, medium- and high-protein groups, respectively.
The same pattern was observed in the second lactation. In contrast, nitrogen excretion via manure did not differ significantly between the groups.
According to the researchers, this confirms that lower protein levels mainly lead to a strong reduction in nitrogen excretion via urine, contributing to lower ammonia emissions.
No additional methane?
Total methane excretion in the first lactation was lowest in the low-protein group (442g/day), compared with the medium- and high-protein groups (484g and 488g/day).
However, the research indicates that when methane was expressed per kg of milk or per kg of feed intake, these differences disappeared because the low-protein group also had reduced milk yield and feed intake.
As a result, methane production per unit of milk or feed remained comparable between the different protein levels.
The results paint a promising picture, according to the researchers: lowering the protein content of the ration leads to a clear reduction in nitrogen excretion, particularly via urine, without increasing methane emissions per kg of milk.
Reducing protein to 143g CP/kg DM can be achieved without reducing feed intake or milk production, whereas a further reduction to 133g CP/kg DM does lead to lower feed intake and milk production.
Long-term feeding of lower protein levels up to 143g CP/kg DM therefore offers prospects for a more sustainable dairy sector, allowing environmental benefits to be achieved without introducing new climate drawbacks or losses in productivity.