Basically, MU is an indicator of the amount of protein in the diet of the cow; generally, the more protein in the diet the higher the MU.
"MU is however only an indicator and is not a sensitive measure of dietary protein. This is particularly true when dietary protein exceeds 20%, as often occurs in pasture," says Dr Jane Kay, DairyNZ Senior Scientist.
"Urea is a non-toxic compound that animals produce to get rid of surplus ammonia and having urea in cow's milk is perfectly natural" says Jane.
Rumen microorganisms digest protein and produce ammonia. Excess ammonia crosses from the rumen into the bloodstream and is transported to the liver where it is converted to urea. The urea is then released into the bloodstream and is either excreted in urine or milk.
In New Zealand pasture-based systems, MU levels are naturally much higher than in systems where cows are fed a total mixed ration.
"When cows are grazing spring pasture, MU levels can be greater than 50 milligrams per decilitre (mg/dl) and, contrary to advice being given to New Zealand farmers, high MU concentrations are not detrimental to milk production, cow health or fertility" says Jane.
The high MU is because cows grazing high quality pasture are eating more protein than they require.
"To reduce MU, farmers would have to feed a lower protein diet. This requires the feeding of low protein supplementary feeds, which are not profitable if there is already enough pasture for the cows" says Jane.
According to Dr John Roche, Principal Scientist at DairyNZ, farmers should be mindful of any advice on feeding supplements containing starch or sugar to 'capture' more dietary protein, or that a low MU means that more protein is being 'captured' by the cow.
"Such advice shows a lack of consideration for post-ruminal digestion and post-digestive physiology.
The conclusion from New Zealand data and data from all over the world is that the reduction in MU through feeding supplements is almost exclusively through the reduction of dietary protein and not some magical 'capture' of protein by the cow", says John.
Although, high MU is not a problem and does not need to be corrected, low MU (less than 20-25 mg/dL) may indicate that dietary protein is limiting milk production.
Care is however needed when interpreting MU values says John.
"As MU is not a sensitive indicator of dietary protein it should be followed up with a laboratory analysis of feed ingredients as well as an assessment of the complete diet for adequacy of protein. Only then should any decision be made on whether to purchase high protein feeds."
Although MU values are associated with the concentration of urinary N, lowering the MU value will not necessarily reduce environmental N loading. Other management factors, such as stocking rate, pasture utilisation, effluent management and nitrogen fertiliser, have a much larger impact on environmental N loading.
For further information see DairyNZ's Milk Urea page.
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