Mathematical equations make a lot of people’s heads spin. Maybe you feel the same way when your nutritionist starts penciling out your cows’ dietary cation-anion difference, or DCAD, in an equation. The equation may look formidable, but it also could help you fight heat stress this summer.

The concept of balancing for a negative DCAD is best known for its role in close-up cow diets. However, as knowledge about balancing for a positive DCAD for lactating cows has grown, it has emerged as a tool to help alleviate some of the effects of heat stress.

Q: What’s the difference between a positive and negative DCAD?

When formulating rations, four compounds  are generally used to influence a cow’s DCAD. They include the elements sodium (Na), potassium (K), chloride (Cl) and sulfur (S). Sodium and potassium are positively charged cations. Chloride and sulfur are negatively charged anions.

Typically, nutritionists use an equation to adjust the amount of these minerals in the diet. Some nutritionists use all four elements in the equation. Some only use three. Here are two commonly used equations:

DCAD = Na+ + K+  -  Cl- – S-
Or
DCAD = Na+ + K+  -  Cl-

The units used in the equation are milliequivalents per 100 grams of feed dry matter.

During the dry period, your nutritionist manipulates the anions to lower a cow’s DCAD. The goal is to achieve a negative number. This improves the cow’s blood calcium status, explains Elliot Block, technical services nutritionist with Arm & Hammer. Ultimately, this helps her avoid milk fever.

During lactation, the DCAD concept still revolves around equations, Block says. However, the goal is to adjust the cations to deliver lactating cows a positive DCAD. Raising the DCAD is of particular benefit to the heat-stressed cow.

Q:  How does a positive DCAD help heat-stressed cows?

During lactation, the practice of raising the DCAD achieves the same result in heat-stressed and non-heat-stressed cows. Research shows the outcome is better feed intake and more milk. However, this technique is “even more important in hot weather,” explains Joe West, dairy scientist at the University of Georgia’s Tifton campus. That’s because the heat-stressed cow is more susceptible to acidosis.

Under heat-stress conditions, the cow pants more rapidly, causing her to exhale an increased amount of carbon dioxide. This lowers the body’s carbon dioxide reserves, which results in less bicarbonate available to buffer the blood, West explains. The end result is a heat-stressed cow that’s more susceptible to “summer acidosis.”

When your nutritionist raises the DCAD, it increases the bicarbonate levels in the blood. This improves the blood’s buffering capacity and offsets acidosis, West says.

The ultimate goal in doing that is to help the cow improve feed intake and sustain milk production during times of heat stress.

Q: What is the DCAD goal for lactating, heat-stressed cows?

Researchers still seek an optimal DCAD level for heat-stressed, lactating cows. However, most experts recommend a range, usually well above what the 2001 dairy NRC recommends for cool weather, West says.

If you use the four-mineral equation, a range of +35 to +40 milliequivalents (meq) DCAD per 100 grams of dry matter is a good goal. Don’t stray too far outside that range. Research shows a detrimental effect on milk production when DCAD exceeds +40 meq per 100 grams of dry matter. Levels below +25 meq may limit milk production, as well.

 Stage of lactation DCAD level (milliequivalents per 100 grams of feed) Early lactation +35 to +40 Mid-lactation +30 to +35 Late lactation +25 to +30

If you use the four-mineral approach, include potassium at the rate of 1.5 percent to 1.8 percent of diet dry matter. Keep sodium in the range of 0.4 percent to 0.6 percent. Maintain chloride a bit lower at 0.3 to 0.35 percent with recommended sulfur levels. These levels will give you a DCAD within the acceptable range of +35 to +40 meq, West says. High levels of dietary potassium can reduce magnesium absorption, so boost magnesium to about 0.35 percent, he adds.

Q: What feeding strategies can I use to raise DCAD?

During heat stress, cows must replace the potassium they lose in milk and through sweating and panting. They also experience reduced levels of bicarbonate, a natural buffer, in the blood.

Here are some nutritional strategies that help increase a cow’s DCAD:

1.         Obtain a complete mineral analysis of your forages. “Know where you’re at with sodium and potassium before you add more buffer,” West says.

2.         Add buffer back to the system. Supplement dietary buffers such as potassium carbonate or sodium bicarbonate.

3.         Avoid excessive levels of dietary chloride and sulfur.

Remember, work with your nutritionist before heat-stress season hits. Discuss which strategies best fit in your heat-stress arsenal.