While treating a cow with IV calcium has become a rarer occurrence on many farms these days, hypocalcemia is still a big deal, says Jesse Goff, veterinarian and professor emeritus, Iowa State University. When 50% of multiparous cows develop subclinical hypocalcemia and nearly 3% of multiparous cows require treatment for milk fever, that’s a big deal, and it impacts your bottom line.
The day before calving a cow needs about 18 grams of dietary calcium for maintenance and fetal development. On the day of calving a cow needs about 50 grams of dietary calcium. That’s an additional 32 grams of calcium needed to produce colostrum and avoid hypocalcemia. The cows’ endocrine system, based on parathyroid hormone (PTH), signals the body to restore calcium homeostasis. As a result, the kidneys stop excreting calcium in the urine, vitamin D is converted into a hormone that enhances diet calcium absorption and bones release calcium into the bloodstream. In cows fed an alkaline diet, blood and urine pH is elevated (urine pH > 7.75), which means bone and kidney tissues are less responsive to PTH, and many cows fail to mobilize the additional 32 grams of calcium needed and develop hypocalcemia.
But when cows are fed an acidogenic diet, the cows move into a state of mild metabolic acidosis which increases tissue responsiveness to PTH signals. That’s why acidified cows can pull more calcium immediately from their bones compared to non-acidified cows. They also begin absorbing dietary calcium faster. Properly acidified cows generally mobilize the required calcium to meet the 32 gram increase in demand imposed by colostrum and milk production the day after calving. The level of DCAD fed determines blood pH which is reflected in urine pH. Research has shown that the optimal urine pH needed to provide benefits for the transition cow is 5.8 to 6.8. Within that range, “My sweet spot for urine pH is 6.2 to 6.3,” says Goff. This is an average for the group, not for each cow. Dry matter intake varies within a group of cows, and it only takes a 1 lb difference in DMI to alter urine pH.
With a group average for urine pH of 6.2 to 6.3 all cows are in a state of compensated metabolic acidosis. If you aim for a lower group average for urine pH, 5.5 to 6.0, some cows will become over acidified. These cows reduce intake because eating less of an extremely acidified diet is the only way cows can avoid developing life threatening acidosis. From all of the research and on-farm experience so far, “I can’t see any benefit to push cows’ urine pH below 6.0,” stresses Goff. “I don’t want to run the risk of over acidifying the cows and have them stop eating.”
During the transition period keeping cows eating and maximizing DMI is so important. Feeding an anionic product instead of traditional anionic salts also helps increase DMI. The palatability and consistency of anionic products, such as SoyChlor which utilizes chloride as anion source rather than sulfate, boosts prepartum cows’ DMI by 2.5 to 3 lbs/day over traditional anionic salts.
Calcium and Magnesium
The amount of dietary calcium fed also impacts urine pH. Calcium is a cation and therefore alkaline. So, if you feed anions to produce a mild metabolic acidosis and feed high levels of dietary calcium in the form of limestone, greater than 1% of diet DM, that extra calcium works against the anions. When aiming for a target urine pH of 6.0 or less, feeding high levels of dietary calcium may rescue cows from over acidification. However, it also means you are paying more to feed extra calcium that your cows don’t need and paying more to feed additional anions to reach your desired urine pH, says Goff. That is not his preferred strategy. Current research recommends a range for dietary calcium between 0.7 and 1.3% of diet DM. In diets Goff formulates he prefers to keep calcium in the lower end of that range with a group urine pH of 6.2 to 6.3.
Magnesium is another important aspect of hypocalcemia prevention. Inadequate magnesium in the prepartum diet impedes calcium mobilization from the bone. In the transition diet very soluble sources of magnesium, such as magnesium chloride or magnesium sulfate, should be fed at 0.3% of diet dry matter (0.4% if MgO is primary Mg source). After calving, when cows are switched to the lactating diet, a cheaper and less soluble magnesium like magnesium oxide is often fed. After the switch it can take cows 7 to 10 days to get their magnesium levels back up to normal. Hypomagnesemia can decrease DMI and lead to secondary milk fevers during lactation. If feeding magnesium oxide look for one that is finely ground with small particles that can be quickly absorbed by the cow.
Another key point is your dry cow pen. If you have the ability to separate multiparous cows and first-calf heifers during the close-up period, do so. First-calf heifers don’t benefit from negative DCAD diets like multiparous cows do, and they actually lose a little bit of milk production that first lactation if fed anions. The compromise in a mixed pen, Goff says, is to not push urine pH so low. Instead feed for the upper end of the optimal range for urine pH. This still provides the health and production benefits to multiparous cows but can minimize the negative impact on heifers.
“It still surprises me when I visit a farm that has no control measures in place to prevent hypocalcemia,” says Goff. “They IV cows for milk fever, but none of the proven control measures to prevent hypocalcemia are in place—feeding a negative DCAD diet, space for all cows to eat at once, good cow comfort and cow management.”
When you have a good transition cow management program that includes a negative DCAD diet it is money well spent. The payback is easily 3:1, says Goff. By using production data from the Santos et al. (2019) meta-analysis and rounding for easy math, a milk production increase of 3 lbs/day for multiparous cows yields an additional 900 lbs of milk per cow for a 305-day lactation. At $17/hundredweight that’s about $153 in extra milk income from feeding a negative DCAD diet. You could spend $50 per cow on prevention and still have a 3:1 return. Anionic products, such as SoyChlor, cost about $15 to $20 per cow. Even if you give multiparous cows a couple of calcium bolus after calving at $6 each that’s an additional $12 per cow. Total spent on prevention is $32. That’s a profit of $121 per multiparous cow. Add in the value of improved cow health including fewer milk fevers, fewer retained placenta and displaced abomasa and it makes feeding a negative DCAD diet to prepartum cows a home run.