We all need calcium. From dairy cows to humans, animals need calcium to survive. Calcium plays important roles in a range of biological processes including bone formation, muscle contraction, blood clotting and nervous system signaling. It is easy to see how not having enough calcium can be a bad thing. Generally, the way we tell if an animal has enough calcium is by measuring blood calcium concentrations. When an animal has a low concentration of calcium in their blood, they develop hypocalcemia, and in dairy cows, this can be a major concern.
If you’ve been working in the dairy industry, you have likely already heard of hypocalcemia. That’s because clinical hypocalcemia, also known as “milk fever,” is one of the most well-known diseases of dairy cows. Typically, cows suffer from milk fever in the early days after calving. Milk has a high calcium content, so when the udder pumps milk out of the cow, calcium leaves too. As a result, blood calcium concentrations usually drop. For some cows, this drop can become very severe, and these are the cows that develop clinical hypocalcemia. What makes this version of hypocalcemia “clinical” is that blood calcium concentrations have become so low that cows do not have enough calcium for regular bodily functions – they no longer stand or ruminate. When a cow goes down with milk fever, she needs intravenous calcium treatment or she might die, which tells us how serious clinical hypocalcemia can be.
Fortunately, despite the seriousness of clinical hypocalcemia, we worry less about this disease than we used to. Nowadays, most herds feed specialized dry-cow diets designed to help cows adjust to the change in calcium demand that happens in the first few days after calving. Because of these diets, clinical hypocalcemia only occurs in about 3% of dairy cows, which is great for the cows, producers and the industry at large.
So, if only 3% of cows develop clinical hypocalcemia, why have an article devoted to it? The answer is that clinical hypocalcemia isn’t the only kind of hypocalcemia that matters. Hypocalcemia can also be subclinical, which means that some cows experience low blood calcium concentrations without showing any outward signs. Even though they might be walking around, eating and milking in a way that seems totally normal, some cows might still have hypocalcemia. Outside of the difference in intensity (clinical hypocalcemia cases have much lower blood calcium concentrations than subclinical cases), a key feature of subclinical hypocalcemia that differentiates it from milk fever is the timing. Any time a cow develops clinical hypocalcemia, it is a bad thing; however, sometimes when a cow develops subclinical hypocalcemia it’s just a normal biological response – but sometimes it isn’t.
As noted above, most cows experience a drop in blood calcium concentrations at the start of lactation. For many cows, this is an example of normal subclinical hypocalcemia. However, most cows overcome this calcium imbalance quickly by activating calcium regulation pathways in the body. We consider this to be a normal biological response to the sudden start of milk production because, as it turns out, cows that experience this temporary subclinical hypocalcemia perform as well as cows that don’t experience a drop in calcium concentration at all, from both a health and milk production perspective.
But this is where the timing comes into play. If cows fail to recover from subclinical hypocalcemia by four days in milk (DIM) or develop subclinical hypocalcemia at this time, this is a bad sign. Cows with subclinical hypocalcemia at four DIM (defined as a blood calcium concentration ≤ 8.8 milligrams per deciliter) have an increased risk of disease, decreased reproductive success and decreased production compared to cows without hypocalcemia. For these reasons, we consider cows with subclinical hypocalcemia at four DIM to be dyscalcemic. Unfortunately, around 30% of multiparous olsteins on any given dairy experience dyscalcemia, which means this form of hypocalcemia has a big impact on cow welfare and herd productivity across the country.
It is important to understand that dyscalcemia may not be a problem in and of itself – it may just be a good marker of an underlying problem we haven’t yet recognized. Researchers are trying to understand why dyscalcemic cows experience the negative outcomes mentioned above. It may be directly related to calcium imbalance, but there could be other biological factors at play. We do know that dyscalcemic cows experience changes in milk constituents, elevated inflammation and impaired immunity, as well as depressed feed intake, rumination time and activity. These findings, combined with the knowledge that these cows also experience an increased risk of clinical diseases downstream, suggest these cows are not adapting well to lactation. Figure 1 is a summary of what we know about dyscalcemia.
As it is not yet clear exactly what causes dyscalcemia, it is also not clear what we can do to prevent it. Current research is investigating the impact of dry-cow dietary strategies and postpartum therapies on the incidence of dyscalcemia within a herd. For now, we recommend monitoring dyscalcemia status in a herd by measuring blood calcium concentrations at four DIM, which is a good indicator of how well cows are transitioning into lactation. We plan on continuing to learn about dyscalcemia, the underlying biology, prevention and treatment, so stay tuned.
