Urea nitrogen levels in milk can be used to monitor optimal protein levels in lactation cow diets, say Michael Wattiaux and Sanjeewa Ranathunga, both dairy scientists with the University of Wisconsin.

“Well balanced diets of approximately 16.2% crude protein results in milk urea nitrogen (MUN) of 11.3 mg/dL, which correspond to maximum protein yield (of about 2.64 lb. of protein per day) and income over feed cost (IOFC) of $9 per cow per day,” they say.

At this level of crude protein, the expected urinary urea-N excretion would be 25% less than cows fed an 18% CP ration with a MUN of 14 mg/dL and IOFC of less than $8 per cow per day, they add.

But finding the best MUN level for your herd should be done in a systematic way. Here are six steps to determine your best MUN level.

1. Make sure your milking ration is balanced for both protein and energy, then pull MUN samples for at least three weeks to obtain a reference level for your herd.

2. Lower the rumen undegraded protein (RUP) content of the diet 0.25% to 0.5% while holding the energy and rumen degradable protein (RDP) constant. This diet should be fed for two to three weeks to determine if dry matter intake and milk production decrease with the lower RUP level being fed.

3. If no production loss occurs, continue to decrease the RUP until production declines.

4. Add back the last increment of RUP to see if production bounces back.

5. Once the best RUP level is found, continue the same process with RDP.

6. Once milk production declines, add back the last RDP increment to see if milk production bounces back.

Once it stabilizes, you’ll have a better sense of where your herd’s optimal RUP, RDP and MUN levels should be. While time consuming and not totally foolproof, such a systematic approach should allow you to better feed cows without wasting dollars or nitrogen.