Michael E. McCormick, Angelica M. Chapa, J. Marcos Fernandez and James F. Beatty

Dietary protein is a key nutrient for high milk production in dairy cows. But determining how much protein a cow consumes and how well it is used is difficult, particularly at the farm level. A new tool being explored is the measurement of trace amounts of nitrogen in the milk. This analysis is known as milk urea nitrogen (MUN).

Excess dietary protein is converted to ammonia by microorganisms in the rumen of the cow and is quickly absorbed into the bloodstream and converted to the less toxic urea by the liver. The level of urea in the blood is equal to the nitrogen level in the milk, thus leading to the MUN measurement.

Normal levels of MUN range from 12 to 18 milligrams per deciliter. Values below 12 indicate insufficient dietary protein intake for maximum milk yield, and concentrations above 18 indicate excess crude protein intake and soluble protein in the diet. Feeding dairy cows excess dietary protein is costly, and recent research at the LSU AgCenter’s Southeast Research Station has shown a detrimental effect on reproduction. Yet, little is known about urea concentrations in the milk from Louisiana dairy herds and how they relate to season, feeding systems used and animal performance.

To answer some of these questions, a study was conducted to compare MUN levels with feeding practices, lactation performance and reproduction. Bulk-tank milk samples were taken monthly from 26 Louisiana dairy farms from March through August of 1998. All herds in the study used artificial insemination and the Louisiana Dairy Herd Improvement Association record keeping system.

MUN Levels Higher in Spring

Average MUN levels peaked in April at 17.9 and declined to 14.2 in August (Figure 1). Although these averages seem fairly moderate, some herds experienced monthly concentrations as low as 9; others contained MUN levels as high as 26, well outside the recommended range of 12 to 18. In fact, 58 percent of the herds evaluated had one bulk-tank MUN sample above 18 during the spring months of March through May, and only 29 percent of the herds exhibited elevated MUN during the summer.

During the spring, 85 percent of the herds used annual ryegrass pasture in their forage systems. Young ryegrass pasture often contains 30 percent or more crude protein, of which as much as 45 percent may be in the soluble form. These high concentrations of ryegrass protein, coupled with a relatively high protein concentration in grain supplements (80 percent of grain supplements contained 18 percent or more protein), likely provided more dietary protein than required by the lactating dairy cow. Only 15 percent of milk samples tested had MUN concentrations below 12, indicating that dietary protein shortage was not common among the herds evaluated.

Variations in MUN with Feeding System

The dairies in the study used several different forage feeding systems. In the spring, about 20 percent used pasture (ryegrass) alone, 35 percent used pasture plus some form of ensiled forage, 20 percent used pasture and hay and 15 percent used pasture supplemented with wet brewer’s grains. Fewer than 10 percent used total mixed rations (TMR), which is grain and forage mixed.

In summer, pasture (bahiagrass, bermudagrass or crabgrass) usage was similar to that recorded in spring, but the proportion of herds using TMR increased to nearly 20 percent of the total.

Milk samples from herds that relied on pasture and hay or TMR to meet forage needs generally had acceptable MUN concentrations (Figure 1). In contrast, bulk tank MUN samples from herds that received only pasture or pasture supplemented with wet brewer’s grains were high, averaging between 18.3 and 19.5. A few herds receiving ensiled forages had MUN concentrations below the recommended threshold of 12, suggesting a protein shortage. Most of the silage used in the dairy herd feeding systems was made from whole-plant corn, a low-protein forage.

MUN Levels and Herd Performance

In this study, milk production (average test day milk yield) was not closely correlated with bulk tank MUN concentration; however, average milk fat and protein percentages tended to be lower when MUN was higher than 18. A poor correlation between MUN and milk yield may be because most herds with MUN concentrations outside the range indicative of optimum protein nutrition were high, suggesting that more than enough protein was available for maximum milk production.

An examination of reproductive data revealed that estrus activity was not affected by milk urea nitrogen concentration, but pregnancy rates tended to be higher in herds with normal MUN levels. This supports recent research at the Southeast Research Station in which dairy cows within the recommended range of urea nitrogen in milk and plasma experienced 29 percent higher pregnancy rates than those cows with high urea nitrogen (25 milligrams per deciliter).

Implications for Dairy Producers