For dairy cows, a blood calcium level of 2.0 mM (8.0 mg/dL) or higher has been the diagnostic standard for normal for many years. Blood calcium levels less than that were characterized as too low or hypocalcemic. However, recent research reports have used several different and higher thresholds for blood calcium levels to differentiate between normal and subclinically hypocalcemic status. Higher blood calcium thresholds increase the apparent prevalence and incidence of subclinical hypocalcemia reported. At first glance it may appear that the diets with negative DCAD (dietary cation-anion difference) fed prepartum in the studies are not working as well as they have in past research. But that’s not the case.
The change in blood calcium thresholds used in research is about examining negative outcomes, explains Thomas R. Overton, professor of dairy management at Cornell University. Negative outcomes include less milk production, reduced reproduction efficiency and the common metabolic diseases that often plague just-fresh cows. Researchers are trying to determine if certain blood calcium levels can predict specific negative outcomes.
“The cut-off for subclinical hypocalcemia remains unclear and the exact value that this threshold should be remains greatly unanswered at this point,” says José Santos, professor of animal sciences at the University of Florida. “We still do not know if the issue is a single day with blood calcium levels below a particular threshold or a combination of threshold and duration that determines the subsequent negative outcomes on the cow.”
Traditionally, blood calcium has been tested on day 1 or day 2 after calving to diagnose hypocalcemia. But prevalence rates for subclinical hypocalcemia drop quickly from day 1.
“We just don’t know the best time to test blood calcium relative to calving—day1, day 2, or day 3,” says Overton. “Some cows bounce right back despite low blood calcium and others remain sluggish for several days.” It may be more important to know which cows' blood calcium levels stay low, as opposed to how many were subclinical on day 1. It appears that when trying to predict negative outcomes, it may be better to test blood calcium levels two or three days after calving.
Several recent studies that have used higher blood calcium thresholds indicate that different blood calcium levels may indeed be able to predict negative outcomes. Research by Rodriguez et al., 2017 looked at the associations between subclinical hypocalcemia and several common fresh-cow diseases. While more research is needed, they did identify several different blood calcium thresholds that indicated different problems ≤1.93 mM for ketosis, ≤2.05 mM for retained placenta and metritis and ≤2.10 mM for displaced abomasum.
The information on blood calcium thresholds available right now is incomplete, it’s not definitive, explains Santos. More research is needed to evaluate the consequence of a particular threshold on health, survival, production and reproduction. But one thing is certain, feeding a diet with negative DCAD prepartum increases milk production, increases dry matter intakes, increases blood calcium levels and helps cows transition better.
Negative DCAD Works
Feeding a diet with negative DCAD helps cows prepare for the transition to lactation. It “primes the pump” for calcium mobilization by the cow once she freshens, says Overton. This speeds her recovery and helps her have a successful transition with fewer negative outcomes.
“Diets with a negative DCAD have clearly been shown to reduce the risk of milk fever and to reduce the incidence and prevalence of subclinical hypocalcemia,” says Santos. This has been widely reported in the literature.
To determine if a diet with negative DCAD reduces subclinical hypocalcemia or not, you must compare diets with a negative DCAD to diets with a positive DCAD. These new studies are not testing whether or not negative DCAD works. They are using diets with negative DCAD as part of the study design because it is a proven concept. The next step is now looking at different blood calcium thresholds as predictors of negative outcomes, or to evaluate a protocol that may help further reduce the incidence of subclinical hypocalcemia and the problems that often go along with it.
Take, for example, a study recently completed by Vieira-Neto et al., 2017. Researchers were testing if a single injection of calcitriol (the active form of vitamin D3) after calving could reduce the prevalence of subclinical hypocalcemia and influence immune function. The blood calcium threshold used in the study was 2.125 mM. (It was selected because Martinez et al., 2012 showed that 2.125 mM was the best predictor of uterine diseases in dairy cows.) In the study the daily prevalence rate of subclinical hypocalcemia during the first three days in milk averaged 56.6% for control cows fed a diet with negative DCAD compared to 6.8% for cows that received an injection of calcitriol after calving in addition to a diet with negative DCAD. If the current diagnostic standard for blood calcium of 2.0 mM had been used to determine the daily prevalence rate of subclinical hypocalcemia during the first three days in milk, it would have been 33.3% compared to 6.7%. This experiment was proof of concept for calcitriol used in addition to a diet with negative DCAD fed prepartum. More research is needed to determine exact dose and benefits of the protocol.
Don’t get distracted by the higher rates of subclinical hypocalcemia reported in new research when higher blood calcium thresholds are used. The threshold value used in any experiment provides a reference point and guide so fair comparisons can be made across treatments within that experiment, says Santos. The key for nutritionists and producers to remember is that implementing dietary and management strategies, such as feeding negative DCAD, that have been proven to reduce hypocalcemia in dairy cows, works.
DCAD on Your Dairy
The amount of acidification, or negative DCAD, needed will vary by farm says Overton and Santos. There is no one size fits all. Diets with negative DCAD must be tailored to each dairy based on the feeds available, the mineral content of those feeds, mineral content of water and the management available.
Monitoring urine pH on farm is the best way to know if your diet with negative DCAD is working. Your goal is to have the majority of cows within the expected pH range of 5.5 to 7.0. Cows with a higher level of acidification will have a lower pH and vice versa. Remember, the level of negative DCAD needed to acidify cows will vary by farm. And some dairies that choose to feed a less negative DCAD can get similar results.
For best results, feed a consistent diet, test the mineral content and tailor the level of negative DCAD to get the results you want, and aim for a DMI of 22 to 24 lbs/day for nulliparous cows and 25 to 28 lbs/day for multiparous cows. Test urine pH and track health records on your farm. Monitor these results and look for deviations from normal for your dairy. Use that information to decide when to tweak diets.