Do milk urea levels have an affect on the herd's fertility? (2025)

As the breeding season edges closer, it is no harm to understand the impact of milk urea levels on the fertility of your herd and the impact it may have on pregnancy rates.

To get a better understanding of how milk urea can affect the fertility of the herd, it is important to grasp what exactly milk urea is.

Milk urea indicates the crude protein (CP) in a cow’s diet and is formed from the metabolism of amino acids and body protein, sometimes described as a by-product of dietary protein.

Protein digestion releases ammonia and if there is an abundance, it is absorbed through the rumen and into the blood stream, which is then converted into urea in the liver.

The majority of the cow’s urea is excreted through their urine, with a small amount passing through the milk.

However, if a cow’s diet is lacking nitrogen (N), the urea will not be excreted, as the cow will recycle the urea back into the rumen and convert it back to ammonia.

When cows are out at grass, milk urea levels are usually higher than what they would be on an indoor diet due to the higher amount of crude protein (20-25%) in good quality pasture.

Milk urea levels will differ in early, mid, and late lactation, and will vary from a number of factors, including the cow’s diet and intake, water intake, and the condition of the cow, along with other contributing factors.

Understanding milk urea levels

A study was carried out by New Zealand dairy educational body, DairyNZ, which analysed the recommended CP requirements and the appropriate milk urea levels at different stages of the lactation.

The table below shows the requirements and the milk urea (MU) levels (measured in milligrams/deciliter) at the different stages of lactation:

New Zealand shares the same pasture-based systems as Ireland but in a total mixed ration (TMR) diet, where lower milk urea levels are present, as there is a large amount of CP in good quality pasture.

According to DairyNZ, when pasture makes up over 60% of the diet in a low-input system, the levels can often be greater than 30mg/dl.

When pasture is less than 60%, the levels may be used as a tool to help guide ration changes, whether to put in or pull out protein supplements.

Consequences of high/low milk urea levels

According to a Teagasc study, surplus protein can have a toll on energy as, in the study, they assumed a pasture crude protein of 22-23%, and the cow’s energy cost to excrete the surplus urea was 0.3-0.4 UFL which is 3-5% of the daily requirement and the equivalent to approximately 0.8L of milk.

Feeding excess protein to cows that do not need it is not only a costly practice, as you are getting no return for extra feed fed, but it also depletes the cow’s energy supplies and affects the amount of energy dedicated towards milk production.

There is always a concern with milk urea levels and what it does to a cow’s fertility and whether or not it affects conception rates and pregnancy rates.

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

Research across the world varies across different systems but, in general, studies show that high levels of milk urea of above 35mg/dl can hinder cows’ fertility and very low levels of milk urea – less than 15mg/dl – are associated with 5-9% lower conception rates.

High levels of over 35mg/dl after insemination have also been shown to decrease conception rates by 5-40% and are probably linked to a number of factors affecting this, such as dietary formation and body condition score.

In-vitro and heifer model Teagasc studies have shown that excess ammonia in the cow’s diet changes the uterine pH and can 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.

While many studies indicate that high and low milk urea levels do affect fertility, many other studies show no correlation between the two, but it should be a sure indication of diet formulation, effective feeding, and nitrogen use efficiency.

In general, under good management, high milk urea levels should not affect fertility which may involve applying nitrogen (N) in a little-but-often manner in the lead-up to breeding and during the breeding season.

N application should not be overloaded under drought conditions, and farmers should feed a 12% crude protein nut during the grazing season, especially if grass quality is good and cows are grazing consistently at optimal covers.

Feeding a high energy concentrate with 12-14% CP can help reduce nitrogen surplus in the diet and minimise environmental nitrogen losses, and will also further reduce the nutrient balance of the farm.

We know that under or over-feeding cows will have an affect on fertility and bulk tank milk urea will be an indication of this, which means that using milk urea levels as a guidance of diet formation and to optimise nitrogen utilisation for production will avoid possible negative affects on fertility.