Dairy Sense: Pairing Milk Urea Nitrogen (MUN) with Milk N Efficiency

March 2023

Production perspective:

Dairy producers and their consultants have many data sources to utilize when monitoring performance benchmarks. The challenge is putting the numbers into perspective and to not overlook the importance of the big picture approach. Milk urea nitrogen (MUN) is one metric that is associated with how cows are utilizing nitrogen, which can impact milk production and reproduction. It's also important as an environmental measure since an ideal MUN range should equate to improved nitrogen efficiency and theoretically less being excreted. However, using MUN alone without examining the big picture can lead to some flawed interpretations.

The Extension dairy team has been conducting feeding system assessments on dairy farms. In herds utilizing a total mixed ration, the goal is to evaluate mixing order of ingredients, mixing times, feed delivery, TMR analysis to compare to the formulated ration, and calculating actual dry matter intake. Financial and environmental topics are discussed with a focus on the benefits of precision feeding. Table 1 shows results from nine farms, which include MUNs and milk nitrogen efficiency (MNE).

An observation made from MUN data collected from DHIA and milk cooperatives spanning over ten years is that a large percentage of dairy farms are within the ideal range of 8 to 12 mg/dl. On the surface, this appears to be a good thing. However, like farm 6's results (Table 1), a low MUN might mean missed opportunities. If the cows were averaging over 85 pounds of milk, then a MUN of 8 mg/dl would be acceptable. That is not the case with farm 6; production is below the benchmark for a 2x milking herd and due to higher dry matter intake, MNE is not ideal. Farm 1 also is not meeting the benchmark for production and even with a decent MUN, MNE is not stellar.

If MNE was used from the formulated rations, then all farms except for number 3 would appear to be excelling. Using the actual results, this is not the case. Farms 2, 5, and 8 match closely to the formulated ration in terms of milk production and dry matter intake with the other farms showing discrepancies. Using actual results for farm 3, MNE is ideal, but when MUN and actual production are factored in, this farm has major problems.

One metric alone is not enough to validate that a farm is correctly implementing good feeding management practices. Several metrics are needed to determine that a herd is fundamentally sound. Making assumptions that dairies with low MUNs or high MNE are good examples of precision feeding has its limitations. Feeding and financial assessments are recommended to investigate opportunities for improvements, which can benefit the bottom line and address environmental issues, such as minimizing N excretion.

Table 1. Milk nitrogen efficiency and milk urea nitrogen comparisons on various dairy operations.

Economic perspective:

Monitoring must include an economic component to determine if a management strategy is working or not. For lactating cows, income over feed cost is a good way to check that feed costs are in line with the level of milk production. Starting with July 2014’s milk price, income over feed cost was calculated using average intake and production for the last six years from the Penn State dairy herd. The ration contained 63% forage consisting of corn silage, haylage, and hay. The concentrate portion included corn grain, candy meal, sugar, canola meal, roasted soybeans, Optigen, and a mineral vitamin mix. All market prices were used.

Also included are the feed costs for dry cows, springing heifers, pregnant heifers, and growing heifers. The rations reflect what has been fed to these animal groups at the Penn State dairy herd. All market prices were used.

Income over feed cost using standardized rations and production data from the Penn State dairy herd.

Note: February's Penn State milk price: $23.03/cwt; feed cost/cow: $8.09.; average milk production: 85 lbs.

Feed cost/non-lactating animal/day.