"My cows aren"t taking off after calving," said a recent caller to my Michigan State University Extension office. Maybe your dairy farm has experienced the same problem and you are wondering how to your herd get off to a better start after calving. What if there is something holding them back that you don"t see? Maybe there is.
Low blood serum calcium, called hypocalcemia, may be prevalent in your herd, even if you rarely see milk fever cases. It has been reported that in herds fed typical dry cow rations, approximately 50 percent of cows that are second lactation and older will be hypocalcemic in the first 24 hours after calving; even when the rate of clinical milk fever is less than 5 percent. Hypocalcemia may result in lower dry matter intake, lower milk production, higher incidence of metabolic disease and poorer reproductive performance.
Defining the disease
At the 2013 Tri-State Dairy Nutrition Conference, Garrett Oetzel, University of Wisconsin Department of Medical Sciences, defined subclinical hypocalcemia as a blood serum calcium level of less than 8.5 mg./dl. Clinical hypocalcemia, also known as milk fever, is severe hypocalcemia and can be categorized by the severity of the symptoms. Cows with Stage I hypocalcemia have early signs of milk fever without going down. Symptoms include nervousness, weakness, excitability, and frequent shifting of their weight frequently while standing. Stage II is defined as cows that are down but not flat on their side. They typically lie with their head turned into their flank and exhibit moderate to severe depression. Stage III cows are lying flat out and severely depressed. If not treated immediately, death is imminent.
However, many cows that are hypocalcemic may be in the subclinical stage and suffer the effects of hypocalcemia but go undetected. In fact, the economic cost of these subclinical cases to the herd is far greater than the cost of clinical milk fever because the incidence is so much higher and the impacts are seen in secondary health effects. Jesse Goff of Iowa State University calls subclinical or clinical milk fever a gateway disorder. For example, low serum calcium levels lead to reduced smooth muscle contraction. As a consequence, displacement of the abomasum (DA) may occur. Reduced contraction of the sphincter muscle at the teat end may allow bacteria to enter and cause mastitis.
We have been defining subclinical hypocalcemia by the blood serum level of cows from samples taken 12 to 24 hours after calving. While it is possible to take those samples and have them analyzed to monitor the extent of subclinical hypocalcemia, the fact is that we cannot control the problem at this time by monitoring and responding. Therefore, when it comes to subclinical hypocalcemia producers can either emphasize prevention or treatment. In this article we"ll discuss prevention and in a separate article, treatment.
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.