If you ask several nutritionists how much dietary calcium to feed prepartum cows on a negative DCAD diet, you’d probably get a wide range of answers. That’s because the topic is still actively debated and recommendations range from a diet with 0.5 to 0.6% (no additional dietary calcium) to as much as 2.4% of diet dry matter (DM), which requires a lot of supplemental calcium.

There have been very few controlled experiments that provide answers to the calcium question, explains José Santos, professor of dairy cattle nutrition and reproduction at the University of Florida. Some research had limited numbers of cows, and results were not conclusive to demonstrate evidence of a benefit from supplemental calcium in prepartum diets. In order to identify the amount of dietary calcium needed with a negative DCAD diet, experiments with large numbers of cows with titrated levels of calcium are necessary. Unfortunately, only a few experiments using titrated levels of calcium have been conducted. Funding for such experiments and the ability to individually feed 120 to 150 cows are two of the reasons why.

But there are some association studies and results from some key experiments that provide direction. Research by Goff & Horst (1997) clearly showed that manipulating dietary calcium prepartum did not significantly affect the incidence of milk fever or the degree of hypocalcemia experienced by multiparous Jersey cows. Another study by Gelfert & Staufenbiel (2008) concluded that when acidogenic products were fed there was no need to increase the dietary calcium concentration above the needs of the prepartum cow (9 to 12 grams/kg DM or about 100 grams/day).

A recent experiment by Glosson et al. (2020) compared a positive DCAD diet (60 mEq/kg of DM) with low dietary calcium (0.4% DM) with two negative DCAD diets (both -240 mEq/kg of DM) with either low (0.4% DM) or high (2% DM) dietary calcium. Results showed that both acidogenic diets, with or without additional dietary calcium, improved cows’ postpartum calcium status. Milk yield and milk components did not differ between treatments.

Meta-analysis provides a powerful tool to analyze data from multiple experiments and studies. It combines data from smaller experiments and studies into one large pool of data to create stronger statistical analytical capabilities and better understanding of how interventions behave under different conditions. In Santos et al. (2019), researchers examined the mineral composition and level of DCAD of prepartum diets and used meta-analytical methods to look for effects of DCAD on performance and health and how dietary calcium affected those responses. “For cows fed acidogenic diets, we observed that as the level of dietary calcium increased, so did the risk for milk fever,” explains Santos. In cows fed a diet with −100 mEq/kg, increasing dietary calcium from 0.6 to 1.6% increased the risk of milk fever from 2.0 to 7.7% in multiparous cows—that’s a 3.85-fold increase. Dietary calcium also influenced urine pH. As the level of calcium fed increased so did urine pH.

Cow Biology

In the past, the recommended target for dietary calcium for prepartum cows fed a negative DCAD diet was 1 to 1.2% of diet DM (about 120 grams of calcium), says Santos. However, the gestating cow only needs 20 to 25 grams of absorbable calcium per day to meet maintenance and the needs of the growing fetus. Calcium bioavailability from different dietary sources typically ranges from 30 to 80%. Using a bioavailability of 50%, the cow only needs to consume 40 to 50 grams of dietary calcium daily to meet those needs (50 x 0.5 = 25 grams/day). A cow with a DMI of 22 to 24 lbs/day of a diet containing 0.6 to 0.7% calcium takes in 70 to 80 grams of calcium per day. In fact, recent work by Santos’ group (Vieira-Neto et al., 2021) showed that cows fed diets with negative DCAD and 0.70% calcium had calcium retention of 15 to 20 grams per day, which is 1.5 to 2 times the amount needed for fetal growth in a Holstein cow delivering an 88 lb calf. Increasing the dietary calcium concentration to 2.0% or greater, an intake of about 200 grams per day, is unnecessary.

Calcium carbonate, or limestone, is 40% calcium. To increase the dietary calcium from 0.7 to 2.4%, you must add 1.7 percentage points more calcium to the diet. That means adding 400 grams of calcium carbonate to yield the 170 grams of calcium intended for the cow. When every bite counts, why waste dietary space with rock?

Calcium carbonate also has an alkalizing effect on the cow. Goff and Koszewski (2018) found that increasing dietary calcium from 0.46 to 0.72% with diets with the same DCAD increased urine pH from 7.0 to 7.4. The additional calcium fed mitigated the acidifying effect of the acidogenic supplement. And research from Kansas State University revealed a linear increase in urine pH as intake of calcium carbonate increased (ADSA Abstract M135, J. Dairy Sci 103 Suppl.1, p207). Therefore, to reach your target urine pH, more anions must be fed with higher calcium diets.

Bottom line: “Cows don’t develop hypocalcemia due to a lack of calcium in the diet,” explains Santos. It develops from the cows’ inability to optimize gastrointestinal calcium absorption or bone resorption promptly once colostrum production begins. Feeding an acidogenic diet that creates a mild metabolic acidosis helps “prime the pump” so that the cows’ own natural regulating mechanisms are already functioning by calving.

Use Common Sense

More does not equal better. This is especially true when it comes to the degree of metabolic acidosis as well as the amount of dietary calcium for prepartum cows. While the ideal level of negative DCAD and the ideal amount of dietary calcium to feed prepartum cows has not been identified, research has demonstrated what is safe and delivers beneficial results for the cows. A negative DCAD of -100 mEq/kg of DM meets both criteria. So, too, does a dietary calcium level of 0.6 to 1% of diet DM.

“We need to use common sense when feeding cows,” stresses Santos. When formulating a prepartum ration, you choose the level of DCAD, the amount of magnesium, phosphorus, energy and protein, to name a few nutrient variables. If the forages and feedstuffs included in the ration already contribute 0.8% of DM as dietary calcium, there is no need to add more calcium because the cow’s biological need has already been met.

“Until someone does the research with diets with negative DCAD using titrated levels of calcium with 120 to 150 cows in an experiment and shows benefits to cows with incremental calcium, I don’t see the value of extra calcium in the diet,” he says. “Current research provides no evidence that extra dietary calcium is beneficial to the prepartum cow.”

Your cows’ health and productivity should always be top of mind. Be prudent in your use of calcium in prepartum acidogenic diets. Don’t get led off track from one small study. And always look for evidence of benefits from the intervention based on sound interpretation of the results.

“Until I see solid data that I can quantify and clearly see better health and production, then I can’t justify feeding more calcium than 0.6 to 1.0% in the diet,” says Santos.