High and Low milk urea is linked to lower pregnancy rates

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Milk urea (MU) is a by-product of the normal breakdown of dietary protein and nitrogen. The digestion and processing of the dietary protein and nitrogen by the cow requires energy.

When there is an imbalance between the protein and energy in the cow’s diet, MU can rise or fall outside the optimal range of 25 – 35 mg/100ml. High MU is typically seen in situations where the energy provided by the diet is insufficient to process the proportionately high level of protein or nitrogen in the diet. Low MU is the reverse picture with insufficient protein and relatively excess energy in the diet. When cows are at pasture, MU levels are usually higher than during the indoor period. This is due to the greater amount of crude protein (20-25 %) in good quality pasture.

International research on the impact of high and low MU on cow fertility is not always consistent across different dairy production systems. However, for seasonal calving systems it important to consider that both high and low MU levels in the weeks following insemination can be detrimental for the pregnancy. Sudden increases in MU are undesirable and believed to have a negative impact on embryo survival. Increasing MU levels above 35 mg/100ml can decrease dairy cow fertility. Very low MU levels (< 15 mg/100ml) after breeding are associated with 5 – 9% lower conception rates and high MU levels (>35 mg/100ml) after breeding have been reported to decrease conception rates by 5 – 40% compared to MU levels in the optimal range. The reasons for this effect on the pregnancy are not fully understood and probably multi-factorial.

High MU seems to play a role in ovarian cysts, creating a toxic environment in the cow’s reproductive tract unsuitable for the embryo and poor embryo development. Low MU is associated with low progesterone (the pregnancy hormone) levels and is possibly an indication of other nutritional deficiencies that can affect fertility.

A specific cause of high MU is grass being grazed too soon post the spreading of N fertilizer. This can easily happen when blanket spreading or when grass is scarce and the rotation shortened. Days 14-17 post spreading are usually worst but the timing of rainfall, uptake and grass growth are all influencing factors.

Maintaining and monitoring MU in dairy herds provides an opportunity to formulate a balanced diet that optimizes nitrogen utilization for milk production and avoids possible negative effects on herd fertility. For more information on any of the topics covered please contact your local Milk Quality Manager.

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