Dairy Nutrition: Dietary Cation-Anion Difference Update

Dietary cation-anion difference, or DCAD, is a measure you should be using in both dry and lactating cows. In close-up dry cows, a negative DCAD can help prevent metabolic problems and in lactating cows, a positive DCAD can help increase milk production and milk components. The most common equation to determine DCAD is based on the dietary concentration of the cation minerals sodium (Na) and potassium (K), and the anion minerals chloride (Cl) and sulfur (S). The DCAD formula is as follows:

DCAD mEq (milliequivalents)/100g (grams) dietary DM = [(%Na × 43.5 + %K × 25.6) − (%Cl × 28.2 + %S × 62.5)] (Mineral % are on a dry matter (DM) basis).

DCAD for close-up dry cows:

* Blood calcium at calving is needed to meet the calcium requirements for colostrum and milk production. When blood calcium levels decrease, milk fever results and other metabolic diseases (i.e., retained placenta, displaced abomasum) are more likely. Negative DCAD diets are fed to close-up cows to help maintain blood calcium levels through release of calcium from bone and increased absorption of calcium from the intestine.

* Recommended DCAD for dry cows less than 3 weeks prior to calving is -10 to -15 mEq/100 g dietary DM.

* To measure whether your negative DCAD diet is effective, determine urine pH on a regular basis. If adequate, urine pH should be 6.0 to 6.5.

DCAD for lactating dairy cows:

* A recent summary by Dr. Dave Beede from Michigan State University indicates a positive DCAD of 25 to 40 mEq/100 g DM is effective and adequate to maximize feed intake and milk production.

* Increasing the dietary DCAD to the recommended range has been shown to improve milk yield and DM intake of lactating dairy cows in hot or cool environmental conditions.

* Feeding a positive DCAD diet is useful in heat stress conditions. Cows under heat stress experience losses of bicarbonate and potassium. Based on the summary of DCAD research during heat stress, Dr. Beede found that 25 to 30 mEq/100 g DM DCAD is adequate to meet the increased needs during heat stress.

Recent lactating cow DCAD research:

Two research articles recently appeared in the Journal of Dairy Science (Wildman et al. 2007) on the impact of DCAD on lactating dairy cow performance during heat stress. A summary of these research articles follows.

Effect of Dietary Cation-Anion Difference and Dietary Crude Protein on Performance of Lactating Dairy Cows During Hot Weather

Thirty-two late lactation Holstein cows were utilized in a 42-day study. This study had four treatments; two levels of DCAD (12 and 40 mEq/100 g dietary DM) and two dietary CP levels (15 or 17% of the diet DM). Temperature averaged 89 degrees and relative humidity ranged from 58 to 96%. There was no impact of DCAD level or dietary protein concentration on DM intake averaging 48.9 lb/day across treatments. With 15% CP in the diet, there was only a slight decline in milk yield (0.7 lb/day) going from 12 to 40 DCAD but at 17% CP, increasing the DCAD to 40 decreased milk yield by 7.9 lb/day compared to the 12 DCAD diet. Increasing the DCAD to 40 did result in a higher milk fat % by about 0.4% units. The higher protein diet supported a higher milk fat % also.

Effects of Dietary Cation-Anion Difference and Potassium to Sodium Ratio on Lactating Dairy Cows in Hot Weather

The impact of a ratio between cation minerals (K:Na) and the effect on dairy cow performance had not been evaluated. Forty-two Holstein dairy cows in mid-lactation were utilized in this 56-day study. The study included two levels of DCAD (41 and 58 mEq/100 g dietary DM) and three K:Na ratios (2:1, 3:1 or 4:1). Dry matter intake was similar across treatments averaging 50.4 lb/day. For unknown reasons, the 3:1 K:Na ratio lowered milk yield by 3.7 lb/day compared to the other two ratios (2:1 or 4:1) at both the 41 and 58 mEq DCAD levels. Although not statistically significant, cows on the 3:1 K:Na treatment averaged the lowest milk fat percentage, but the highest milk protein percentage.

The research is rather clear in indicating a diet with a positive DCAD is beneficial to the performance of lactating cows. The optimum practical range in DCAD appears to be between a 25 and 30 mEq/100 g of DM. Forages are the primary source of K with legume or grass forages being higher in K than corn silage. Most forages and other common feeds are low in sodium and, therefore, salt and sodium bicarbonate buffer are the primary sources of Na in diets. When alfalfa or legumes are two-thirds or more of the forage DM in a diet, supplementing salt to the requirement of 0.5% of the diet DM should achieve the optimal DCAD level and keep the ratio of K:Na to greater than 3. In diets where corn silage is greater than two-thirds of the forage DM, supplemental K may be needed to achieve a minimum of 1.3% K in the diet DM. Buffers should be fed with high corn silage diets and they will increase DCAD levels through sodium. A ratio of 2:1 for K:Na with a DCAD between 25 to 30 mEq/100 g DM appears to be adequate based on current research.

By Jim Linn and Mary Raeth-Knight - University of Minnesota Dairy Extension