Feeding prepartum diets with a negative dietary cation-anion difference (DCAD) to close-up dry cows is a practice used frequently to reduce the risk of hypocalcemia postpartum. The DCAD is determined by the difference in chemical equivalents between the cations’ sodium and potassium and the anions’ chloride and sulfate. To obtain a negative DCAD, acidogenic ingredients containing more chloride and sulfate equivalents than sodium and potassium equivalents are typically fed to dry cows in the prepartum.

However, the success of obtaining diets with a negative DCAD is highly dependent on the concentrations of cations in the other ingredients included in the ration. In this regard, forages with high concentrations of potassium may demand greater inclusions of acidogenic ingredients to obtain the desired negative DCAD.

The alfalfa example

The relatively high dry matter yields and high concentrations of crude protein and energy make alfalfa one of the best forages to feed dairy cattle whether in the form of fresh grass, hay, or silage. A drawback of alfalfa when included in prepartum rations, however, may be the high concentration of potassium. This forage may challenge the possibility of obtaining a negative DCAD and, therefore, may be less desirable for inclusion in diets for close-up dry cows.

In a recent study from our laboratory, we evaluated the effects of feeding prepartum diets containing either grass hay or alfalfa hay to close-up cows. The grass hay contained 7.5% crude protein and 74.9% neutral detergent fiber, whereas the alfalfa contained 19.6% crude protein and 45.6% neutral detergent fiber.

Regarding minerals, the grass hay contained 0.36% calcium, 0.09% sodium, 1.88% potassium, 0.38% chloride, and 0.15% sulfur, while the alfalfa hay contained 1.52% calcium, 0.16% sodium, 2.50% potassium, 0.77% chloride, and 0.32% sulfur. These mineral concentrations led to forage cation-anion differences equal to 289 and 292 milliequivalents per kilogram of dry matter (mEq/kg DM) for grass hay and alfalfa hay, respectively, which are almost identical.

The similar forage cation-anion difference was somewhat unexpected, and this allowed us to include similar amounts of acidogenic salts when formulating prepartum diets with either grass hay or alfalfa hay. These data highlight the importance of the forage cation anion difference driven by all minerals, rather than simply the concentration of potassium.

If you need to use alfalfa hay in prepartum diets, do not look just to the potassium concentration of the forage but rather to the forage cation-anion difference. Soil quality and fertilization practices can elevate the concentration of anions, making alfalfa a suitable forage for formulating diets for close-up dry cows.