Urea nitrogen concentration in milk (MUN) is related to crude protein intake, the percentage of rumen degradable and undegradable protein and the ratio of protein to energy in the ration.
Normal or target values for MUN should fall into the range of 10 milligrams per deciliter to 15 milligrams per deciliter of milk.
High values usually indicate inefficient protein utilization, either due to feeding excess protein or inadequate energy relative to the amount of protein.
Low MUN values, however, may indicate inadequate protein feeding.
Therefore, MUN can be used as a practical monitoring tool to evaluate ration protein and energy balance.
Monitoring. Monitoring MUN could be an important dairy herd management tool for the following reasons:
* Protein supplements are costly ration ingredients;
* Consumption of excess protein increases the requirement for energy in the ration;
* Excess protein or nitrogen in the ration may impair reproductive performance; and
* Excess nitrogen in the manure and urine has a negative environmental impact.
Random study. In 2001 and 2002, a study of 24 randomly selected Ohio Holstein herds was conducted by professors in the Ohio State University Department of Preventive Veterinary Medicine to collect and evaluate data on MUN concentrations over a one-year period.
In cooperation with Ohio Dairy Herd Improvement Association, levels were measured in individual cow monthly test day milk samples (1681 total cows).
Half the herds were defined as low producers (rolling herd average below 16,000 pounds) and half as high producers (RHA above 23,000 pounds).
Overall unadjusted average MUN concentration in high producing herds was 13.9 milligrams per deciliter and 11.3 milligrams per deciliter in low producing herds.
The range for herd level MUN concentrations in the low producing herds was from 5.0 to 15.1 milligrams per deciliter.
The lowest average MUN concentration in a high producing herd was 10.1 milligrams per deciliter with a RHA of 24,750 .
The highest MUN in a high producing herd was 19.2 milligrams per deciliter with a RHA of 24,050.
Highs and lows. Concentrations were lowest during the first month of lactation, then increased gradually toward the end of lactation.
Concentrations also varied with season of the year. The concentrations did not differ significantly between first lactation and older cows.
Concentrations did not vary a lot between cows within herds, especially when month of lactation was considered.
Two thirds of the variation between herds in the low producing group was between test days. High producing herds exhibited less variation between test days, possibly indicating more consistent day-to-day feeding management.
Variability. The great variability of MUN readings between test days, observed within both high-producing and low-producing herds may indicate that feeding management can be improved further, even in high-producing herds.
Further, the observation that herds producing over 24,000 pounds can have overall MUN as low as 10 milligrams per deciliter suggests that using measurements as a practical monitoring tool for ration balancing may provide an opportunity to reduce feed costs, improve protein feeding efficiency and improve overall herd profitability.
And fertility. The association between MUN and fertility of dairy cows was also evaluated in this study.
Increasing MUN concentrations were negatively correlated to fertility and were associated with lower pregnancy detection rate at herd checks.
Cows with MUN levels below 10.0 were more than twice as likely and cows with MUN levels between 10.0 and 12.7 were 1.4 times more likely to be confirmed pregnant than cows with MUN levels above 15.4 milligrams per deciliter.
Negative effects on fertility have previously been reported when MUN are above 19 or 20 milligrams per deciliter.
The results of this Ohio study would suggest that the levels of MUN which are adversely associated with fertility may be lower than previously reported.
(The author is an agricultural extension agent in Columbiana County. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem, OH 44460.)