Oftentimes there are no outwardly visible signs that a cow is suffering from subclinical milk fever.

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In order for a dairy farm to remain productive and profitable it must have lots of fresh cows. So the primary goal of every dairy farmer should be getting all the cows in the herd pregnant and calving them out during the course of the year. Yet there are a host of potential challenges that beset a cow at the time of freshening because so many things are happening to her body all at once. One of the biggest challenges on a dairy farm is preventing a metabolic disease called hypocalcemia, also known as milk fever.

A cow is diagnosed as having clinical milk fever when her blood calcium level is so low that she’s in a state of physical paralysis, with muscles that are unable to function. She often starts off being wobbly on her feet and will later collapse and be unable to stand or even sit up. In many cases she’s already afflicted with hypocalcemia before she delivers her calf. If left untreated, she will be unable to give birth, leading to the death of the unborn calf. Eventually her lungs and heart will shut down and she will die. Clinical milk fever is always treatable, although not always successfully, but at least the dairy farmer knows that something is wrong.

Subclinical milk fever (SCMF), on the other hand, affects a cow much less severely, and oftentimes there are no outwardly visible signs that she’s afflicted. However, many organs, such as the rumen and uterus, are not working at their full capacity. Cows with SCMF may already be experiencing poor rumination, resulting in poor nutrient metabolism, and may need assistance in giving birth. Oftentimes these cows will continue with their SCMF and twist a stomach or retain fetal membranes.

Studies have shown that 47 percent of all cows that are freshening in a second or greater lactation will have some level of SCMF. Moreover, as many as 25 percent of first-lactation cows are likely to be experiencing some level of SCMF, as well. Even though many cases of SCMF will often resolve on their own with no treatments with intravenous (IV) or oral calcium products, any level of SCMF will still have a negative impact on rumen function and adequate feed intakes at the onset of lactation. Decreased feed intakes will lead directly to negative energy balance and contribute to the cow’s inability to reach higher milk production peaks early in the lactation.

Virtually all fresh cows are subject to a negative energy balance due to a decrease in feed intake during the last week of pregnancy as well as a compromised immune system at the onset of lactation. Subclinical milk fever will exacerbate these two conditions. Once feed intakes and energy metabolism are decreased, a cow will start using her own body fat reserves to meet her energy needs. This can result in ketosis, a condition associated with excessive body fat mobilization. Ketosis often causes a cow to lose interest in eating, leaving her with increased odds of a twisted stomach, as well as losing nearly a gallon of milk per day at first test day.

Calcium also plays a critical role in the activation of immune cells at the time of calving. White blood cells, known as neutrophils, are responsible for eliminating infection-causing bacteria in the uterus. Cows with SCMF are significantly more prone to uterine infections as well as reduced involution of the uterus after calving. Studies have shown that there can be as much as a 30 percent reduction in conception in cows that have experienced SCMF compared to herd mates that have remained healthy. Cows suffering from SCMF ultimately have delayed pregnancies that result in longer and less profitable lactations.

The graphic on page 15 depicts the complex interrelationships of metabolic diseases and challenges confronting the fresh cow. Nearly all dairy cows will be challenged with some degree of immunosuppression and negative energy balance at the time of freshening. Mastitis, retained placentas and metritis are the result of suppressed immunity. All three can result in reproductive disorders and may delay or prevent pregnancy. The added presence of SCMF will only make matters worse. Displaced abomasums, ketosis and ovarian dysfunction are associated with nutritional limitations and can also have an effect on reproduction. Again, the added presence of SCMF will only make matters worse. If not addressed or treated properly, these metabolic disorders will result in the death or the culling of your milk cows.

The increased expectations for cows to produce more milk at the onset of lactation have undoubtedly contributed to greater incidences of SCMF. Genetics and nature have not provided the ability for today’s high-producing cow’s metabolism to respond quickly enough in providing the necessary calcium required at that time of transition to lactation. Calcium requirements double from the dry period and the final growth of the fetus to when a cow must start producing milk.

Diagnosing SCMF can only be done through blood testing, since these cows do not exhibit clinical signs of milk fever. The traditionally recognized minimum calcium level in blood is 8.0 milligrams per deciliter (mg/dl) of blood. Healthy dairy cows should always have blood calcium levels above 8.0 mg/dl.

In recent years, close-up dry cow diets that slightly acidify the blood have had good success in minimizing milk fever challenges in fresh cows. This anionic diet enables a more rapid mobilization of calcium from the cow’s bones and gets it into the system, where it will both enable adequate muscle function and immune response, as well as supply enough calcium for milk production. Close-up dry cow diets must also be adequately balanced for energy and protein and formulated to allow rumen function to accelerate as the cow’s appetite increases after freshening.

Milk fever, both clinical and subclinical, can be troublesome and frustrating for dairy farmers. Milk fever robs cows of milk production and health as well as reducing her profitability for the dairy farmer. Proactive management by the dairy farmer, veterinarian and nutritional professional will keep milk fever and the host of potential problems it causes to a minimum.

John Hibma is a dairy nutritional consultant and works for Central Connecticut Cooperative Farmers Association in Manchester, Connecticut.