There is some correlation between high milk urea nitrogen (MUN) and reduced conception rate or greater days open, but the mechanism is still in question. Individual cows with high MUN are likely to have lower conception rates than the average for the herd. Herds with high MUN do not usually have lower conception rates. It may be that individual cows in the herd have some physiological stress that elevates MUN and decreases conception rate, but one may not cause the other. Elevated MUN for a herd or group of cows may indicate that feed protein is poorly utilized.
High herd MUN is often attributed to inadequate rumen fermentable energy or excessive rumen degraded protein; however, herd MUN can be high for many different reasons. Factors that contribute to high herd MUN include too much protein in the diet, too little energy, a poor quality of protein as well as a poor balance of ruminally degraded and undegraded protein, or too little rumen digestible energy. High herd MUN should be investigated to reduce the potential loss in milk production or increased feed cost. The ration and feed ingredients should be re-evaluated.
