THE MYTH: You should feed supplements to change your milk urea levels.
BUSTED. Milk urea is only a rough indicator of the amount of protein in the diet and values are of limited use in pasture-based systems.
Milk urea (MU) is formed when excess dietary protein is converted to ammonia in the rumen. The ammonia is absorbed through the rumen wall into the blood stream and then converted to urea in the liver. Most of the urea is excreted in urine, but some passes into the milk.
High milk urea levels
Due to the crude protein content of high quality pastures, MU values are typically higher in cows grazing pasture compared with cows fed a total mixed ration or high levels of supplement. Cows grazing pasture can reach MU levels of 50mg/dl or more.
Contrary to some advice within our pasture-based dairy industry, high MU levels are not detrimental to milk production, cow health or reproduction. The process of the cow converting ammonia to urea is not energetically expensive.
The reduction in MU through feeding high starch supplements is almost exclusively through the reduction of dietary protein intake and not through increased “capture” of more protein in the rumen – as is often claimed.
The only time that high MU levels (approximately greater than 25mg/dl) should be reviewed is when supplements make up a large proportion of the diet. In this scenario, high MU levels may indicate the cow is being fed more protein than required. If the dietary crude protein is excess of requirements, this may allow expensive protein supplements to be removed from the diet.
Low milk urea levels
In contrast, if MU levels are lower than usual (approximately less than 20-25mg/dl) this may indicate there is not enough protein in the diet. However, as evident this spring, bulk milk MU levels are not a precise measure of dietary protein. There were numerous reports of MU levels in the low teens and even reaching single figures, when dietary crude protein was not limiting.
Therefore, as with high MU levels, laboratory analysis of feed ingredients and an assessment of the complete diet for protein and amino acid availability should be undertaken before any nutritional changes are made.
Where dietary protein levels are lower than recommended, the total cost of feeding the additional protein must be considered and compared with the expected milksolids response before supplements are purchased and fed.
Milk urea levels are of limited use in a pasture-based system and should not form the sole basis for nutritional decisions. Where high levels of supplements are fed, the protein content of the diet needs to be assessed, and the financial implications of any nutritional changes must be scrutinised.
This article was originally published in Inside Dairy December 2014