Doubts over meaning of dairy data

DairyNZ says feed supplements are not cost effective when cows have enough pasture. Photo / File

Milk urea (MU) values are now supplied to New Zealand dairy farmers and there is conflicting information in the industry about what the values mean.

Basically, MU is an indicator of the amount of protein in the cow's diet; generally, the more protein in the diet the higher the urea value.

DairyNZ scientist Jane Kay says: "MU is only an indicator and is not a sensitive measure of dietary protein. This is particularly true when dietary protein exceeds 20 per cent, as often occurs in pasture.

"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."

Rumen micro-organisms 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 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," Kay says. The high MU is because cows grazing high-quality pasture are eating more protein than they need.

"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," Kay says.

DairyNZ principal scientist John Roche says 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 the cow is "capturing" more protein. The reduction in MU through feeding supplements is almost exclusively through reduction of dietary protein, Roche explains.