Negative DCAD is Best Way to Minimize Hypocalcemia
Hypocalcemia affects way too many transition cows. While clinical hypocalcemia only affects 3% to 5% of postpartum dairy cows; the big culprit is subclinical hypocalcemia.
Subclinical hypocalcemia can affect 25% to 40% of primiparous cows and 45% to 80% of multiparous cows, explains Jose Santos, professor of animal science at the University of Florida. Cows with subclinical hypocalcemia often have reduced dry matter intake, suppressed immune function, compromised energy metabolism, increased incidence of other periparturient diseases and reduced milk yield.
“Subclinical hypocalcemia is a hidden ghost that you do not see until you perform biochemical assays,” explains Santos. “You just do not know how prevalent this issue is on farm until you look for it.”
But it doesn’t have to be that way. Hypocalcemia – both clinical and subclinical – is highly preventable. Santos and his team reviewed the existing research to examine the methods available to control and reduce the impact of hypocalcemia in dairy cows. They concluded that dietary manipulation remains the best choice to control and prevent both forms of hypocalcemia. Limiting the intake of sodium, potassium and phosphorus in the diet and then manipulating the remainder of the macro minerals to achieve a negative dietary cation-anion difference (DCAD) for prepartum cows is the best way to beat hypocalcemia, says Santos.
Three dietary strategies were evaluated on their ability to reduce the risk of clinical and subclinical hypocalcemia. Limiting dietary calcium is one strategy. Research has shown that when cows are fed diets to induce a negative calcium balance, they have increased concentrations of parathyroid hormone which helps stimulate bone resorption, intestinal absorption and renal reabsorption. However, the challenge is in our ability to induce a negative calcium balance. To do so a diet must contain no more than 0.25% calcium to assure that intestinal absorption is less than the 20 grams of calcium that the cow needs. Since most forage, grains and byproducts fed to dairy cows have at least 0.25% calcium it is not easy to achieve.
Another option is feeding Zeolite to sequester calcium in the gut and create a negative calcium balance. Zeolites are complex structural compounds with interlocking molecules of SiO4 and AiO4 in the form of a honey-comb. The aluminum silicate formed carries negative charges that attract positively charged ions such as Ca2+ and Mg2+ in the gut. However, while research shows feeding Zeolite during the last 2 weeks of gestation was effective at improving calcium metabolism at the onset of lactation; it also decreased dry matter intake, and reduced blood concentrations of magnesium and phosphorus.
The third and preferred option is feeding a negative DCAD diet prepartum. Salts containing strong anions such as Cl- and S2- are fed to induce a mild metabolic acidosis in the last weeks of gestation. Research conducted by Weich et al., 2013, shows that feeding a negative DCAD diet in the final 21 days of gestation improved calcium homeostasis and increased milk yield by 12.3 lbs/day during the first eight weeks of lactation included in the study. Based on the available research, feeding a negative DCAD diet prepartum remains the best way to minimize clinical and subclinical hypocalcemia.
Putting DCAD to work
To successfully feed negative DCAD diets to prepartum cows, you need to do more than just add a handful of salts.
First, you must limit the intake of the strong cations sodium and potassium. Always select feed ingredients with low potassium, sodium, and phosphorus (and high chloride if available). Doing so will help you formulate a diet with a relatively low DCAD, between — 100 to 150 mEq/kg is achievable.
“It makes little sense to feed diets that are high in potassium and sodium and then try to counteract their effects by feeding large quantities of strong anions,” explains Santos. By first minimizing the strong cations in the diet you minimize the amount of strong anions needed to achieve a negative DCAD. When feeding acidogenic salts, less is better.
Other keys include:
- Have current and continuous analysis of the mineral composition of feed ingredients fed prepartum. “Less than half of samples submitted to commercial labs request a mineral analysis,” he says. Use of book values for forages and byproducts will likely defeat your efforts to properly formulate a desirable DCAD target.
- Limit concentrations of phosphorus in the prepartum diet. Research has shown that a prepartum dietary intake of phosphorus above 50 g/day increases the risk of clinical hypocalcemia (Lean et al., 2006). Prepartum diets should not contain supplemental phosphorus.
- Magnesium enhances the cows’ ability to mobilize calcium from the bone at the onset of lactation and prevents hypomagnesemia. Most recommendations are for diets containing 0.4% to 0.45% Mg to result in intakes of 40 to 50 g/day.
- With proper feed selection, most diets generally require fewer than 2.5 equivalents (Eq) of strong anions fed to achieve a desirable negative DCAD. Feeding a small amount of acidogenic salts minimizes the risk of suppressed DMI and increases the efficacy of the program.
- The ideal DCAD to prevent hypocalcemia has not been identified. Current research suggests a DCAD range of -50 to -150 mEq/kg.
- Urine pH is a good indicator of the degree of metabolic acidosis. In Holstein cows receiving a negative DCAD diet prepartum, strive for a mean urine pH between 6.0 and 6.5; in Jerseys a mean urine pH between 5.8 and 6.3. The key is to have a majority of cows tested, 70% to 80%, fall within the desirable range, 5.8 to 6.5.
Make a change
Hypocalcemia is a costly disease. The estimated cost of clinical hypocalcemia is about $300 per case. But the incidence in most herds is low. With subclinical hypocalcemia the estimated cost per case is about $125, but the incidence is much higher.
Take for example, a 1,000 cow dairy with a 3% incidence of clinical hypocalcemia. The annual cost would be $9,000 (30 cows x $300 per case). If that same 1,000-cow dairy has a 25% incidence of subclinical hypocalcemia the annual cost would be $31,250 (250 cows x $125/case). Subclinical hypocalcemia quietly steals your profits.
When implemented correctly, producers see a lot of success with negative DCAD diets. Once they see the reduction in clinical and subclinical hypocalcemia, have healthier cows with better starts, producers embrace the concept. Use negative DCAD to improve the health and profitability of your transition cows.