During the transition period, the cow goes from a pre-calving period in which her need for calcium is low, quickly to a time, as she calves and produces milk, when her demand for calcium is high. She is equipped to handle that through mobilization of calcium from her bones for a short time until her dietary intake catches up with her lactational demand. Certain biological factors can cause sufficient mobilization to be delayed, resulting in hypocalcemia.
Dry cow diet
The most common way to try to prevent milk fever is to provide feeds low in calcium, sodium and potassium during the dry period. Alfalfa, with higher levels of calcium and potassium than grasses or straw, is typically eliminated from dry cow diets. Even so, to be effective in preventing subclinical hypocalcemia, those mineral levels need to be very low, and that is not practically achievable in many cases because of high background concentrations in forages. These are the herds described as feeding a typical dry cow diet and being found to have 50 percent subclinical hypocalcemia.
Anionic salts for close-up dry cows
In recent years, in order to ready the cows for this calcium demand at calving, the recommendation has been to feed anionic supplements. In essence, these lower the pH of body fluids, enabling greater mobilization of bone calcium. Dave Beede of Michigan State University has demonstrated that lowering the dietary cation-anion difference (DCAD) in the prefresh (close-up) diet with anionic supplementation reduces the incidence of hypocalcemia. We can monitor the effectiveness of the anionic salts by checking urine pH prefresh. Urine pH should decrease from around 8.0 to less than 7.0 when anions (sulfate and chloride) are provided during the close-up period.
However, there are limitations. Anionic supplements increase the cost of the ration. Additionally, while the pH only needs to be lowered for 4 - 5 days prior to calving, for practical purposes, producers will feed anions to their close-up group typically for 2 - 3 weeks. Negative impacts that anions may have on heifers are not known. Therefore, producers will have to have to decide whether to have a separate close-up heifer group.
In addition, some anionic salts are not thought to be palatable and may reduce dry matter intake, resulting in a negative impact. Therefore, supplements that reduce intake should not be used for close-up dry cows where other factors, such as inadequate feed bunk space per cow or crowding in the close-up pen will add to a reduction in dry matter intake. Even in herds in which the close-up dry cows were supplemented with anionic salts, the incidence of subclinical hypocalcemia was around 15 percent.
Oetzel also presented information at the conference about treatment options which is covered in “Treating hypocalcemia routinely”.
Watch your fresh cows. Look for signs that they aren’t getting off to a good start and check the rate of early lactation problems for your herd. It may be that you have been unaware of prevalent subclinical hypocalcemia impacting your cows.