With the breeding season approaching, it is important to understand what milk urea is and how it could be affecting your herd's fertility.

Milk urea is formed from the metabolism of amino acids and body protein, but is generally referred to as a by-product of dietary protein in the cow.

It does come in useful at this time of the year when crude protein levels are up and down while cows are in and out between housing and pasture, as it actually indicates the crude protein in the cow’s diet.

Milk urea acts as a crude protein indicator because as protein digests, it releases ammonia. If there is an abundance of ammonia, it is absorbed through the rumen and into the blood stream, which is then converted into urea in the liver.

The majority of this urea is ultimately excreted through their urine - however, a small amount does make its way into the cow's milk.

If a cow’s diet is lacking nitrogen (N), the urea will not be excreted, but instead recycled back into the rumen and converted back into ammonia.

That is why we see milk urea levels typically rise when out at pasture, as lush grass can have a crude protein level of 20-25%.

However, milk urea levels will differ throughout the lactation, with the cow’s diet intake, water intake, and body condition all playing a part.

How milk urea can affect fertility

So how does milk urea actually affect fertility, and what other consequences are there?

In the weeks following insemination, any drastic changes in milk urea levels can have a significant impact on pregnancy, possibly having a negative impact on embryo survival.

Research suggests that milk urea (MU) levels (measured in milligrams/deciliter) should hover in or around the 25-40mg/dl mark during early lactation.

This will fall to 25-30mg/dl during mid-lactation and 20-25mg/dl during late lactation.

35mg/dl is considered a high level of MU, which can hinder a cow's fertility.

If MU is 35mg/dl during or after insemination, conception rates can decrease by anywhere between 5-40%, according to studies.

However, low MU levels (less than 15mg/dl) can also be attributed to a reduction of 5-9% in conception rates.

When we consider how milk urea is produced, these poor fertility figures may link back to factors such as dietary formation and body condition score.

However, studies have shown that excess ammonia in the cow’s diet changes the uterine pH, and can ultimately change the oocyte quality or the blastocyst formation rate.

Meanwhile, low milk urea levels are associated with low progesterone levels and again could possibly be an indication of other nutritional deficiencies that can affect fertility.

Despite the results of these studies, other research has found no correlation between low/high milk urea levels and infertility.

But at the end of the day, if urea levels are overly high or overly low, adjustments realistically need to be made to the diet.

It may also be an indication of poor nitrogen management, with a little-but-often approach needed in the lead-up to breeding and during the breeding season.