At calving, cows have a sudden and urgent demand for colostrum and milk production that drives blood calcium down. During this critical period, a cow’s bones cannot respond quickly enough to replace the lost calcium. Through this, some cows will show clinical signs of milk fever and will require intravenous calcium supplementation. Others will not show clinical signs, but rather slip into subclinical hypocalcemia (SCH), which can be a silent robber of productivity and profitability.
A cow with SCH has a low blood calcium concentration without obvious clinical signs. SCH goes on undetected in a large portion of today’s dairy herds—a National Animal Health Monitoring System (NAHMS) study estimates as many as 54% of second-lactation and older cows and 25% of first-calf heifers succumb to SCH.
“Most cows will revert back to positive calcium balance by six to eight weeks post-calving, meaning the calcium intake now equals or exceeds calcium outflow,” says Garrett Oetzel, DVM, professor at the School of Veterinary Medicine, University of Wisconsin-Madison. “However, she can draw on her bone calcium for up to four months into lactation, including during times of stress or low feed intake.”
Like any disorder, certain cows are at higher risk of SCH than others, according to Brian Miller, senior professional services veterinarian with Boehringer Ingelheim. Age is one of the primary factors that can cause SCH, with the disorder more prevalent in cows in their second lactation and greater. High-producing cows, independent of age, are also at higher risk for SCH, Miller says.
According to several studies, the negative consequences of SCH include reduced milk yield, immune system suppression, higher concentrations of blood non-esterified fatty acids, increased risk for metritis, increased risk for coliform mastitis and an increased risk for early removal from the herd. All consequences result in lower milk production and higher health costs to the producer.
To reduce the impact of SCH, farmers must focus on prevention, Miller says. There are two approaches to prevention—dietary manipulation prior to calving and oral calcium supplementation after calving.
“Neither approach is sufficient by itself to prevent all of the negative impacts of SCH,” Miller says. “However, using both approaches together will minimize the impacts of SCH as much as possible.” Manipulating the precalving diet means feeding a ration with a negative dietary cation-anion difference (DCAD).
“Dairy producers have learned that feeding a diet at -8 to -12 meq/100g ration dry matter for three weeks prior to calving improves cow health and performance in the following lactation,” says Elliot Block, PhD, research fellow with Arm & Hammer Animal Nutrition.
Feeding a negative DCAD diet pre-partum positively impacts cow blood calcium status after calving, directly impacting SCH, he says.
One way to know if the cows in your transition pen are in the right metabolic state to prevent the damaging effects of SCH is through urine pH tests. According to Block, producers should take urine samples from cows that have been fed close-up diets for at least five days. Producers should collect urine samples at the same time post-feeding, recognizing urine pH will vary during the day. For Holsteins, urine pH levels should be between 6.0 and 6.8. For Jerseys, levels should be between 5.8 and 6.5. “If DCAD is adjusted to meet targeted levels, increase or decrease DCAD until 80% of the cows fall in this range,” he says.
Once cows have calved, oral calcium supplementation can help get cows back in balance. The best oral calcium sources are acidogenic salts such as calcium chloride and calcium sulfate, Oetzel says.
“These salts provide additional calcium for absorption from the gut, plus they extend the time period that the cow is in a relatively acidic state,” he says.
Being in an acidic state helps the cow mobilize calcium from her own bone.
At least two doses of oral supplementation should always be provided, Miller says, with the first bolus given at calving and the second bolus administered the next day at the first lockup in the fresh pen, approximately 12 to 24 hours post calving. New research has shown that persistent SCH is becoming a major issue, so for certain cows, Miller says supplementation beyond the standard two doses is necessary.
Several university studies show the positive impact of oral supplementation post calving. A study focusing on return on investment showed supplementing high-producing cows and lame cows had the highest net return on investment. Using a blanket strategy of supplementing all second and greater lactation cows had a lower, but still positive, return and required less management effort to identify lame and high producing cows.
Note: This story appears in the February 2018 magazine issue of Dairy Herd Management.