The hidden Costs of High MUN

Elevated milk urea nitrogen (MUN) levels are established indicators of inefficient crude protein utilization at the rumen level and are recognized as a waste of crude protein, often at a cost. Beyond the direct financial implications associated with unused crude protein, high MUN levels are also linked to broader issues, including inefficient utilization of digested amino acids, altered energy balance and potentially compromised reproductive performance.

Urea is generated by the liver from ammonia and released into the bloodstream, where it is measured as blood urea nitrogen (BUN). Once produced, BUN circulates in the blood for elimination. Since urea is highly soluble in body fluids, it equilibrates with the water found in milk.

High MUN is often attributed to excess rumen-degraded protein (RDP) in a cow’s diet. However, this may not be the source, or at least the only source, of elevated MUN. Another key factor relates to a cow’s ability to utilize absorbed amino acids. When a cow absorbs an imbalanced mixture of amino acids, such as a diet limiting in lysine or methionine, protein synthesis becomes less efficient, resulting in reduced protein production. The extra amino acids that cannot be used to produce tissue or milk are instead broken down and used as an energy source. The nitrogen component of the unused amino acids is removed and routed to the liver to form urea, increasing BUN and MUN even when RDP is not excessive. This represents a significant limitation for milk protein production. However, it is not the only problem resulting from high MUN.

The conversion of ammonia to urea is not free; it requires energy from the liver and kidneys to excrete the urea. In practical terms, every gram of protein catabolized to urea not only represents lost nitrogen that could have been used for milk protein but also costs metabolic energy that otherwise could have supported production1.

A growing body of evidence also links high MUN levels to lower conception rates and poorer pregnancy outcomes. Contributing factors include an altered uterine environment resulting from shifts in follicular fluid composition, and hormonal disturbances around conception, all of which can reduce embryo viability and conception success2. Elevated ammonia levels have also recently been associated with reduced sperm viability3.

Balancing diets for essential amino acids is of paramount importance. An example is the substitution of canola meal for soybean meal in dairy diets. Canola meal has an amino acid profile that closely matches that of milk, aligning with a cow’s need for milk protein synthesis. Numerous trials have shown that canola meal lowers MUN when it replaces solvent-extracted soybean meal. In one large study, results showed that average MUN values were 0.8 mg/dL lower when canola meal replaced soybean meal across different forage ratios4. At the same time, canola meal diets increased milk and milk protein yield. This has been shown to be related to the flow of essential amino acids that match a cow’s need for milk protein synthesis 5, 6.

Elevated MUN levels may not necessarily indicate excessive RUP. Enhancing the supply of metabolizable protein and balancing limiting essential amino acids through the strategic use of various feed ingredients can improve amino acid profiles and reduce the catabolism of amino acids. Precisely formulating diets to meet the essential amino acid requirements can help lower

MUN concentrations, potentially resulting in energy savings and improvements in both production and reproductive performance.

Key takeaways:

· Excess ammonia entering the liver requires energy to convert toxic ammonia to less toxic urea.

· Excess rumen ammonia production is not solely responsible for high BUN and MUN concentrations; poor amino acid balance can be a major contributor.

· High MUN levels reduces herd fertility.

· Balancing diets for amino acids and supplying them in amounts that a cow needs can improve protein efficiency and lower MUN levels.

 

 

References

1Reed, K.F., Bonfá, H.C., Dijkstra, J., Casper, D.P. and Kebreab, E., 2017. Estimating the energetic cost of feeding excess dietary nitrogen to dairy cows. Journal of Dairy Science, 100(9), pp.7116-7126.

2Raboisson, D., Albaaj, A., Nonne, G. and Foucras, G., 2017. High urea and pregnancy or conception in dairy cows: A meta-analysis to define the appropriate urea threshold. Journal of Dairy Science, 100(9), pp.7581-7587.

3Abdelli, A., Besbaci, M., Al-Kass, Z., Iguer-Ouada, M. and Morrell, J.M., 2025. In Vitro Effect of Elevated Ammonia and Urea Levels on Post-Thawed Bull Semen Sperm Characteristics. Veterinary Sciences, 12(10), p.997.

4Paula, E.M., Broderick, G.A. and Faciola, A.P., 2020. Effects of replacing soybean meal with canola meal for lactating dairy cows fed 3 different ratios of alfalfa to corn silage. Journal of Dairy Science, 103(2), pp.1463-1471.

5Mutsvangwa, T., 2017. The True Value of Feeding Canola Meal What is Canola Meal (CM). In Western Canada Dairy Seminar Proceedings (Vol. 29, pp. 109-124).

6Holtshausen, L., Benchaar, C., Kröbel, R. and Beauchemin, K.A., 2021. Canola meal versus soybean meal as protein supplements in the diets of lactating dairy cows affects the greenhouse gas intensity of milk. Animals, 11(6), p.1636.