Influence of fat-to-protein ratio and udder health parameters on the milk urea content of dairy cows.

The urea nitrogen excreted by animals with their urine, which is subsequently degraded by the microbial enzyme urease, is a major source of ammonia emissions. In case of an imbalance between protein and energy supply in the feed, the surplus of ammonia is detoxified to urea in the liver and enters the bloodstream, where it diffuses or is transported to other fluid pools, such as milk or rumen liquid. The majority of urea is excreted through the urine.

Milk urea content is influenced by a large variety of factors, including the quantity and quality of protein fed and its balance with energy, diurnal fluctuations, management, season, and analysis method, as well as individual cow factors, which include the health status of the cow. In the latter context, metabolic disorders and udder infections are the most common diseases of dairy cows, and they can be detected by parameters in the milk. The consequences of metabolic disorders such as acidosis and ketosis for dairy cows are well understood. Nevertheless, their influence on the milk urea content has been less studied. The connection between udder infections and milk urea in particular has rarely been the focus of research to date. Hence, the objective of the study cited was to examine the influence of metabolic disorders and udder diseases on the milk urea content of cows.

The use of five large datasets with a wide range in management, geographical, and farm-level characteristics in Germany provided a strong database to investigate the factors mentioned and fill the identified research gaps. For this purpose, five independent datasets were analysed, containing information derived from monthly milk recordings (dataset A, 6,140,342 test day records in 2015; dataset D, 439,767 test day records in 2020–2023; dataset E, 399,279 test day records in 2019–2020) in combination with the differential SCC in datasets D and E, or individual recordings of daily feed and energy intake and milk analysis (dataset B, 58,235 test day records in 2014–2017; dataset C, 352,346 test day records in 2018–2021).

The group of cows with severe energy deficiency showed a 11.0 to 20 mg/L higher milk urea content than cows with a demand-oriented energy supply. The results for the effect of a very high energy supply were inconsistent across the five datasets. Furthermore, the milk urea content of cows with the highest SCC were observed to be 9.0 to 13.0 mg/L lower in comparison to cows with a healthy udder. Moreover, the milk urea content was 14 mg/L lower in cows diagnosed with mastitis compared with those without a diagnosis.

In conclusion: Milk urea content is influenced by a large variety of factors, including the health status of the cow. Consequently, milk urea content should not be employed as an indicator for an individual cow, but rather as a tool for evaluating the nutritional status of protein supply of a herd. In groups of cows, individual incidences of a disease will not have a significant impact on the average milk urea content. However, potential shifts should be taken into account for herds with high prevalence of sick cows. Further studies are required to identify the veterinary and physiological relationships between the milk urea content and rumen acidosis as well as mastitis. If the observed correlation is a consequence of an elevated protein demand in the context of an acute immune response, it is anticipated that a lower milk urea content would be observed in cows with any acute disease.