It's not 100% accurate and scores can vary. But Mike Hutjens believes the milk urea nitrogen (MUN) test can be a valuable tool to help dairy producers evaluate their feeding programs.

“I'm an enthusiastic supporter of the MUN test,” says Hutjens, University of Illinois extension dairy nutritionist. “There's controversy with the test because of its variability, but I'm excited about it.”

The test can signal over- or underfeeding of protein. Perhaps more importantly, says Hutjens, it can indicate whether cows are consuming the right combination of forage protein and rumen fermentable carbohydrates.

“Since 50-60% of the dry matter in most dairy rations comes from high-quality forages, MUN testing allows a producer or nutritionist to gauge how well that forage is working in the rumen,” he says.

If you're feeding wet haylage with high levels of soluble protein, for example, some of the protein may escape rumen microbes and end up as blood urea nitrogen in the bloodstream. That's likely to be reflected in high MUN values. Cutting back on haylage and substituting additional corn silage or dry hay should improve protein utilization.

Normal MUN values can vary from 10 to 14 milligrams/deciliter.

He recommends establishing a MUN profile or baseline for your herd. “You need to set a benchmark. Then, when that value changes by more than two or three units, you can analyze what happened.”

To build that profile, test the milk from 10 cows in each DHI production group for at least six consecutive months. Run all the tests at the same DHI milking.

“There can be differences between a.m. and p.m. DHI tests,” says Hutjens. “MUN values can change due to time of feeding, time of consumption and milking time.”

Don't be concerned if MUN values vary by two or three units between tests.

“If the MUN value of my high-producing cows is 14 this month and 13 next month, probably nothing happened,” says Hutjens. “But if it's 14 this month and 18 the following month, you have to ask yourself what happened.”

Consider the following:

  • High MUN values may simply mean that you're feeding too much total protein or too much degradable protein. Conversely, low values may signal that you need to increase ration protein levels.

  • Very high-quality alfalfa — especially haylage — tends to raise MUN levels

  • Corn silage tends to lower MUN levels because starch stimulates microbial protein production.

  • Look at your balance of forages. “If a producer is only feeding legume-grass forages, his chances of having an optimal MUN are low,” he points out.

  • Heat stress can also affect MUN levels. “Illinois MUN values have gone up two to three units on herds on DHIA/MUN testing during the summer,” says Hutjens.

  • “Look at fecal scores, milk protein content, ration protein and energy values to confirm MUN trends,” he adds.