THE start of each lactation challenges a dairy cow's ability to maintain normal blood calcium concentrations, and cows must quickly shift their metabolic priorities to adjust for this sudden calcium outflow, according to Drs. Garrett Oetzel of the University of Wisconsin School of Veterinary Medicine and Brian Miller with Boehringer Ingelheim Vetmedica Inc.
Average blood calcium concentrations noticeably decline around calving for cows in their second or greater lactation, with the lowest concentrations occurring about 12-24 hours post-calving, they said in a Boehringer Ingelheim "Technigram" article on hypocalcemia in fresh dairy cows.
Subclinical hypocalcemia can be defined as low blood calcium concentrations without obvious clinical signs, Oetzel and Miller said, adding that subclinical hypocalcemia affects about 50% of dairy cattle in their second or greater lactation that are fed typical pre-fresh diets. A cow does not necessarily have to become recumbent (down) to be negatively affected by hypocalcemia.
Subclinical hypocalcemia is more costly than clinical milk fever because it affects a much higher percentage of cows within a herd, Oetzel and Miller said.
For example, if a 2,000-cow herd has a 2% annual incidence of clinical milk fever and each case of clinical milk fever costs $300, the loss to the dairy from clinical cases is about $12,000 per year. If the same herd has a 30% annual incidence of subclinical hypocalcemia in second and greater lactation cows (65% of the cows in the herd) and each case costs $125 (an estimate that accounts for milk yield reduction and direct costs due to increased ketosis and displaced abomasum), then the total herd loss from subclinical hypocalcemia is about $48,750 per year, they noted.
Oral calcium supplementation is the best approach for hypocalcemic cows that are still standing, Oetzel and Miller said. Most cases of subclinical hypocalcemia go undetected on a dairy, so an intentional strategy for oral calcium supplementation is very cost effective, particularly due to increased milk yield in supplemented cows.
For herds with excellent observation of fresh cows, there may be subtle signs of hypocalcemia, such as being off-feed, a droopy appearance, decreased rumen motility, cold extremities, muscle quivering and weakness, they explained. These cows are able to stand but may be wobbly.
Oral calcium supplementation typically increases blood calcium concentrations rapidly enough to prevent these cows from going down. Other benefits of prompt oral calcium supplementation include prevention of injuries caused by musculoskeletal weakness, increased appetite and restoration of gastrointestinal motility, which ultimately improves milk yield, Oetzel and Miller said.
Transient hypocalcemia can occur in cows whenever they go off feed or have periods of decreased intestinal motility. They said it is unclear which comes first: hypocalcemia or gastrointestinal stasis. Whatever the case, the two problems make each other worse.
Intravenous (IV) calcium is not recommended for treating cows that are still standing, Oetzel and Miller emphasized, explaining that treatment with IV calcium rapidly increases blood calcium concentrations to extremely high and potentially dangerous levels, which may lead to fatal cardiac complications or will shut down the cow's own ability to mobilize calcium at this critical time. Even cows treated successfully with IV calcium often suffer a hypocalcemic relapse 12-18 hours later.
IV calcium is necessary for cows that have gone down due to hypocalcemia, they said. Downer cows can quickly suffer irreversible musculoskeletal damage. In these situations, the urgent need to get the cow up overshadows the potential complications of IV calcium administration.
To reduce the risk for hypocalcemic relapse, oral calcium is indicated following successful IV calcium treatment. Administer one oral supplement after the cow is standing, alert and able to swallow, followed by a second oral supplement about 12 hours later, Oetzel and Miller said.
Even herds with successful anionic salt programs and minimal clinical cases of milk fever will benefit from strategic use of oral calcium supplements, they noted.
Start by supplementing all standing cows that have signs of hypocalcemia and all down cows following successful IV treatment, they suggested. For herds with a high incidence of hypocalcemia, it may also be economically beneficial to strategically supplement all fresh cows (second and greater lactation) with oral calcium. Finally, cows with high milk yield in the previous lactation and lame cows have the best response to oral calcium supplementation.
Oral calcium products
Oetzel and Miller reported that oral calcium products use a variety of calcium sources. The source of calcium in an oral supplement greatly influences calcium absorption and blood calcium responses.
Calcium chloride has the greatest ability to support blood calcium concentrations, they reported, adding that its high calcium bioavailability and ability to invoke an acidogenic response in the cow causes her to mobilize additional calcium from her skeleton.
Oral calcium products are offered in a variety of physical forms; this has important implications for cow safety, Oetzel and Miller said. Thin liquid preparations may be aspirated instead of swallowed, leading to aspiration pneumonia. This is particularly problematic for preparations containing calcium chloride because it is very caustic.
Oetzel and Miller noted that another oral source is a combination of calcium chloride and calcium sulfate delivered in a fat-coated bolus (such as Bovikalc). These calcium sources are acidogenic, which causes the cow to mobilize more of her own calcium.
Calcium propionate is not acidogenic and is more slowly absorbed than calcium chloride, Oetzel and Miller said. It must be given at higher doses (usually 75-125 g of calcium) compared to supplementing with calcium chloride (usually 40-54 g of calcium). Calcium propionate is glucogenic, meaning it provides precursors to blood glucose, so it could be useful in cows after about two days in milk that are at risk for ketosis and may have hypocalcemia at the same time, they added.
Calcium carbonate mixed with water and given orally does not increase blood calcium concentrations at all, and in fact, it lowers them slightly, Oetzel and Miller said. This is not surprising due to the alkalogenic response it can invoke. Calcium carbonate (along with calcium oxide and calcium hydroxide) is generally regarded as insoluble and, therefore, unsuitable for use as an oral calcium supplement.
Oetzel and Miller suggested that strategies for giving oral calcium supplements around calving should include at least two doses: one at calving and a second dose the next day.
The expected low point in blood calcium concentrations occurs between 12 and 24 hours post-calving, so only giving one oral calcium supplement around calving leaves the cow without support when her blood calcium concentrations are naturally the lowest, they added.