Most dairy nutrition specialists agree the cut point for beta-hydroxybutyrate (BHBA) levels is about 1.2 to 1.4 mmol/L. If more than 15% of your cows test above that level, alarm bells typically go off and nutritionists scramble to find the cause.
That’s because cows with high BHBA and non-esterified fatty acids (NEFA) may have depressed dry matter intakes, decreased immune function, decreased pregnancy rates, increased risk of displaced abomasum, and development of fatty liver and ketosis among other ailments.
But Rick Grummer, an emeritus professor of dairy nutrition at the University of Wisconsin, say levels of 1.2 to 1.4 mmol BHBA/L might actually be normal—especially if you have high genetic merit cows.
First, a bit of biology. “Beginning at calving and through early lactation, cows will mobilize energy from fat tissue,” says Grummer. “The triglyceride within fat stores is broken down to glycerol and non-esterified fatty acids (NEFA). NEFA leaves fat tissue and enters the blood stream. We know that approximately 25% of these NEFA are taken up by the liver.
“Ideally, all NEFA would be completely oxidized in the liver to provide energy for the cells or be transferred to the udder of the cow to support lactation. However, during times of elevated NEFA uptake, some NEFA are converted back to triglyceride and stored in the liver or are incompletely oxidized to ketones such as beta-hydroxybutyrate (BHBA).”
Epidemiological studies that looked at elevated levels of NEFA and BHBA used very large animal and herd numbers. “Consequently, the conclusions of these studies probably apply to the average cow or herd,” Grummer says. But they might not apply to all cows and all herds.
Two studies suggest high genetic merit cows might be different. In one study done nearly 30 years ago, cows of high genetic merit produced 12 lb/day more milk than cows with average genetic merit. These high genetic cows did so without eating more feed during the first four weeks of lactation, suggesting they were able to mobilize more fat tissue to support that milk production and would thus have higher levels of BHBA circulating in their blood. Another study done in Wisconsin in 2017 showed that cows testing above 1.2 mmol BHBA/L produced more milk than lower testing cows.
“If cows are milking like crazy and testing above the alarm level, and they do not have other issues such as low reproductive performance or a high incidence of DAs, RELAX,” says Grummer.
That’s not to say you shouldn’t test early-lactation cows for BHBA, because it can be useful for detecting other problems such as low-quality forage, increased stocking density and an elevation of internal body fat that isn’t detected by condition scoring.
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