Milk Urea Nitrogen & Read Your Feeding Balance

The widely used target window is about 10–14 mg/dL, read together with a milk-protein percent of roughly 3.0–3.4%. Within that box the ration's rumen-degradable protein and fermentable energy are in balance. Values outside it point to a specific imbalance — high MUN to excess protein or short energy, low MUN to a protein deficit.

A MUN above ~14 mg/dL usually means the rumen is receiving more degradable protein (RDP) than its fermentable energy can capture, so the surplus nitrogen is converted to urea and excreted. If milk protein is also low it points to an energy (starch) shortfall as well. The fix is to trim soluble/RDP protein and/or add fermentable energy, then re-test.

A MUN below ~10 mg/dL generally signals a protein deficit — not enough rumen-degradable protein to feed the microbes for peak microbial-protein yield. If milk protein is also low, the whole ration is short of protein. Adding degradable protein (e.g. soybean or canola meal, or a measured amount of urea) typically lifts both MUN and milk yield.

Plot MUN on one axis and milk protein on the other to land in one of four quadrants. High MUN with low protein is excess RDP plus an energy shortfall; high MUN with adequate protein is simply protein over-fed; low MUN with low protein is a protein deficit; and on-target MUN with good protein is well balanced. The calculator places your herd and captions the quadrant with its fix.

Every mg/dL of MUN above the upper target is protein the cow was fed and then excreted. Urinary nitrogen rises about 12.54 g per cow per day for each mg/dL of MUN (Jonker, Kohn & Erdman 1998); converted back to dietary crude protein that is roughly 0.08 kg of CP per cow per day per excess mg/dL. Multiply by herd size and your protein-feed cost to see the daily waste.

Both. Producing and excreting excess urea costs the cow energy and has been associated with poorer reproduction at very high levels, and the excreted nitrogen loads slurry and increases ammonia and nitrate emissions. Trimming MUN back into the target window saves feed money and reduces the farm's nitrogen footprint at the same time.

Sometimes. A low MUN with high milk protein indicates very efficient nitrogen capture — the cow is turning most of its dietary nitrogen into milk rather than urine. That is generally good, but you should still confirm that rumen-degradable protein is not so low that it caps microbial growth, feed intake or peak yield.

Yes. MUN rises after feeding and varies with sampling time, diet changes, heat stress and forage quality, so single readings can mislead. Use bulk-tank or DHI averages and look at the trend across test days rather than reacting to one figure. Sampling consistently (same time relative to feeding) makes the trend reliable.

MUN is milk urea nitrogen, measured in the milk; BUN is blood urea nitrogen, measured in plasma. They track each other closely because urea diffuses freely between blood and milk, so MUN is a convenient, non-invasive proxy for the cow's protein-energy balance that you get for free on every milk test.

Reduce the most soluble, rapidly degradable protein sources first and replace some with rumen-undegradable (bypass) protein or fermentable energy so microbial yield holds. Improving the synchrony of energy and protein release in the rumen — for example pairing degradable protein with starch — captures more nitrogen as microbial protein and lowers MUN while protecting milk yield.

It means the rumen is fermenting protein and energy together efficiently: degradable protein is being captured as microbial protein rather than lost as urea, and there is enough energy and metabolizable protein to support milk-protein synthesis. That quadrant gives the best combination of milk output, feed efficiency and low nitrogen waste.

Yes — the MUN target window and the quadrant interpretation apply across breeds and forage systems, because they reflect rumen protein-energy balance rather than a specific feed. Enter your bulk-tank MUN and milk protein and the diagnosis holds; only the magnitude of the ration tweak changes with your particular feeds.