BHB (beta-hydroxybutyrate)

What is BHB?

BHB stands for beta-hydroxybutyrate, also called beta-hydroxybutyric acid. Together with acetoacetate and acetone it is one of the three ketone bodies, and the main one in a cow’s blood. Ketone bodies are formed in the liver when body fat is broken down and cannot be fully used.

After calving, milk yield rises faster than feed intake. The cow fills the gap from her fat reserves. In small amounts ketone bodies are normal and even serve as a source of energy. If they rise too far, the cow has ketosis. The BHB value is therefore the standard measure for detecting ketosis, including the subclinical form without visible signs.

Threshold values in blood

• below 1.2 mmol/l: normal
• from 1.2 mmol/l: subclinical ketosis
• from about 3.0 mmol/l: clinical ketosis, the cow usually shows clear signs

Some sources set the threshold for subclinical ketosis at 1.4 mmol/l. More important than the exact threshold is that you always measure the same way and compare the values over time.

How is BHB measured?

There are three ways:

• Blood: with a handheld meter and a test strip, one drop of blood from the tail vein is enough. You have the result within seconds. This is the most accurate measurement in the barn.
• Milk: test strips for milk or values from milk recording. Some milking robots measure BHB or acetone in the milk automatically.
• Urine: test strips here usually measure acetoacetate, not BHB. They are sensitive but tend to show a trend rather than an exact value.

When should you measure?

The highest risk is in the first two to three weeks after calving. Measuring fresh cows between day 3 and day 14 of lactation makes sense. It pays to target cows at risk: overconditioned cows, older cows, cows with milk fever, twins or a retained placenta, and cows that are eating less.

For a view of the whole herd, you can test a sample group of fresh cows. If a large share of them is above the threshold, the problem usually lies in the feeding of dry and fresh cows, not in individual animals.

Other signs of ketosis

Even without a meter there are signs. A fat to protein ratio above 1.4 in milk recording in the first weeks of lactation points to heavy fat mobilisation. So does a loss of more than one BCS point after calving. Sometimes the breath or the milk smells sour, of acetone.

What to do when BHB is high

A high value shows that the cow is getting too little energy. The aim is to supply energy quickly and encourage feed intake. Glucogenic substances such as propylene glycol are used for this. With high values or a sick cow, talk to your vet.

In the long run the solution lies in the dry period: not letting cows get too fat, good close-up feeding and high feed intake after calving. Read more in Milk fever or ketosis? What every dairy farmer needs to know. How earlier field trials assessed the BHB content of milk is described in Give boluses before or after calving? This is what robot data shows.

Frequently asked questions

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