Decreasing the dietary cation-anion difference (DCAD) in pre-fresh rations is a successful strategy to reduce the incidence of subclinical and clinical milk fever (hypocalcemia) in fresh cows. Even though the success of this practice was first described in 1971, adoption of this technology varies widely.

Many nutritionists and dairy producers fear that this technology takes considerable monitoring for success and that failure is common. This may be true to some extent, but it doesn’t have to be. In most herds, routine monitoring of urine pH and wet chemistry mineral analysis of the forages are necessary to properly balance an anionic ration.

How does it work?

When lactation begins, there is significant drain of calcium from the blood. To limit calcium loss and prevent milk fever, the cow must adapt to absorb more ration calcium and make calcium-stores in bone more available for absorption. Two hormones, parathyroid hormone and vitamin D, cause the release of calcium from bone and the increased intestinal absorption of calcium, respectively. Both of these hormones work more efficiently when the blood pH is more acidic. This is the key to anionic rations. Lower blood pH can make the calcium factory more efficient.

A simple chemistry lesson explains how we achieve this. 

Blood pH is largely determined by the number of positively and negatively charged particles absorbed from the diet. By feeding more negative charges (sulfur and chloride) and fewer positive charges (sodium and potassium), we can lower blood pH. The following equation shows how we can achieve a negative DCAD:

DCAD = (sodium + potassium) – (sulfur + chloride)

More complex equations that take into consideration the absorption rate of each of these minerals and the additional effects of calcium, magnesium and phosphorus are also used, but this is the simple explanation.

Take the edge off

Tom Overton, animal scientist at Cornell University, recently suggested a “take-the-edge-off dose” of anionic salts. (Go to for more.) We know that this is not an all-or-none response. So, why can’t we feed a ration with a lower level of anions, which requires less monitoring and no pre-fresh cow-urine collection?

The suggestion is to start feeding 0.25 pounds per cow per day of a commercially available product. This can be increased to 0.5 pounds per day if the desired effect is not achieved. 

It is important to feed calcium at 100 grams per cow per day. If you care to catch urine, we are looking for a urine pH in the 7.5 range (vs. the 6.2-6.8 target range for conventional anionic rations).

Keep in mind that this strategy may not work in all situations. Remember, if urine pH is too low, we can experience decreased intake and sick cows. This level of anions can be mixed directly in a pre-fresh feed formulation and fed as a top-dress.

Subclinical benefits?

Even though milk fever appears to be relatively easy to treat — as long as there are not any secondary issues like paralysis — we know that both clinical and subclinical milk fever puts cows at a greater risk for other disorders like mastitis, ketosis, retained placenta and displaced abomasum. Even if you don’t see clinical milk fevers, the subclinical form may cause other issues on your dairy.

As always, work closely with your herd veterinarian and nutritionist to correctly balance transition-cow rations. The guidelines suggested here just cover the basics, and there are many other strategies for a successful transition period. Remember that full anionic rations are successfully fed in many herds and may be necessary if this low-dose approach does not solve milk fever problems. 

The “take-the-edge-off dose” of anionic salts is just another tool to potentially help you reach your goals for healthy, productive cows.  

Mark J. Thomas is a veterinarian and owner of Countryside Veterinary Clinic, LLP in Lowville, N.Y.