Subclinical hypocalcemia shows no symptoms but can affect half of all second-lactation or greater fresh cows.
by Abby Huibregtse, Hoard's Dairyman Associate Editor
Dairy cows struggle to maintain normal blood calcium levels at the start of each lactation. Colostrum and milk both use a large amount of calcium, and the cow must suddenly adjust for this rapid nutrient outflow. Blood calcium drops more noticeably in second lactation or greater cows, and the lowest concentration occurs 12 to 24 hours after calving.
Hypocalcemia (or low blood calcium) results from inadequate calcium metabolism in fresh cows, leading most often to milk fever. While the number of milk fever cases has declined noticeably the past twenty years (affecting only 3.5 percent of cows in North America (DeGaris and Lean, 2008)), recent research shows that hypocalcemia, and especially subclinical hypocalcemia, remains a metabolic concern for dairy cows.
Cows with subclinical hypocalcemia have low blood calcium levels but don't show the visual signs of milk fever. Subclinical hypocalcemia affects about half of all cows second lactation or greater. Gary Oetzel, D.V.M., University of Wisconsin-Madison School of Veterinary Medicine, explained the negative impact the subclinical form of this disease can have at the Professional Dairy Producers of Wisconsin's (PDPW) "Fine Tuning the Transition Cow" workshop last week.
A large multi-farm study collected data from 2,365 cows in 55 herds in the U.S. and Canada. The recently published findings showed that the biggest impacts from subclinical hypocalcemia come from lost milk yield and a higher risk of displaced abomasums. Cows were at greater risk of developing fever, metritis and ketosis, as well. The research found that cows with subclinical hypocalcemia also had reduced pregnancy rates and longer days open.
Subclinical hypocalcemia is more costly than clinical milk fever because it affects a much higher percentage of the herd. Oetzel calculated that, for example, a 2,000 cow herd with a 2 percent incidence of milk fever would lose about $12,000 per year. (This is assuming each case of clinical milk fever costs $300. (Guard, 1996)) If the same herd had a 30 percent incidence rate of subclinical hypocalcemia in older cows (and 65 percent of the herd was second or greater lactation cows), they would lose $48,750 per year. (Assuming $125 per case of subclinical hypocalcemia, accounting for lost milk production and direct costs due to other resulting diseases.)
Subclinical hypocalcemia is a quiet but costly disease on most farms. Diagnosis is difficult without visual symptoms, but treating every cow for milk fever whether she shows signs or not is impractical. The best method of control is prevention through diet and oral supplementation, according to Oetzel.
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