Calcium and magnesium are critical after calving

Clinical hypocalcemia, also called milk fever, occurs when a cow’s blood calcium levels drop too low. This most often affects mature dairy cows during the first three days after calving.

Leading up to calving, calcium is steadily drawn from the cow’s body to support fetal bone development. After calving, calcium demand increases even more as milk production begins. As a result, blood calcium levels can fall by 40% to 80%. Magnesium and phosphorus often decline at the same time.

When blood calcium is around 5 milligrams per deciliter (mg./dL.) or less, milk fever can present as excitability and restlessness. Cows may walk unsteadily, bellow and have muscle and ear twitches. Their eyes may appear to protrude. With levels below 3 mg./dL., cows feel cold to the touch, stop eating, and cannot urinate, defecate or stand. In extreme cases, cows can become unresponsive and die.

Milk fever can be treated, but speed is critical. Affected cows need intravenous calcium as quickly as possible. Because calcium can be toxic to the heart if administered improperly, IV calcium should ideally be given by a veterinarian. Some cows require more than one treatment, and additional magnesium or phosphorus solutions may be helpful.

While roughly 5% of dairy cows experience clinical milk fever, far more common is subclinical hypocalcemia, defined as blood calcium levels of 8 mg./dL. or less two days after calving. Studies show that about 25% of first-lactation cows and up to 54% of fifth-lactation cows are affected. Because these cows don’t collapse or show dramatic signs, the condition often goes unnoticed, but it can predispose them to costly problems. It decreases appetite, alters metabolism and impairs immunity, leading to higher infection rates. The disorder also results in higher plasma cortisol levels, which indicates stress. 

Cows with subclinical milk fever have lower first-service conception rates and are at greater risk for metritis, displaced abomasum, ketosis and retained placenta. Poor reproductive performance is a major reason cows are culled.

Cows are at greatest risk for low calcium immediately after calving. Routine monitoring, along with early treatment if needed, can prevent milk fever and reduce the long-term effects. It is worth testing cows to find out if they have subclinical hypocalcemia.

Low blood magnesium often accompanies milk fever. Magnesium is critical in calcium metabolism, and unlike calcium, a cow’s body stores very little magnesium. They must consume it daily. Blood magnesium below 1.2 mg./dL. can result in grass tetany. Even levels around 1.7 mg./dL. can contribute to secondary hypocalcemia. 

Ensuring adequate magnesium intake is essential. Lactating cows are often supplemented with magnesium oxide (MgO), which is also a rumen alkalizer. However, not all MgO products are equal. Pure mag ox (about 60% magnesium) is not ideal, as its tight crystal structure limits absorption. Feed-grade MgO with 54%-56% magnesium is more bioavailable.

Magnesium sulfate is commonly used in dry cow diets, especially when milk fever is a concern. This anionic salt is used to induce metabolic acidosis during the final three weeks before calving. Mag sulfate is highly water-soluble, though it may clump in humid conditions. It’s a more expensive product but very effective, despite its somewhat unpalatable taste.

Dolomite (dolomitic limestone) is another useful source, containing about 11% magnesium and 22% calcium. It’s generally palatable to cattle and has a finer particle size to improve magnesium availability.

Even when sufficient magnesium is fed, cows must be able to use it effectively. Potassium (K) interferes with magnesium absorption in the rumen, and even normal dietary K levels can reduce absorption by 40% or more. As K increases, magnesium supplementation should increase as well. One way to improve magnesium absorption is to include monensin (Rumensin) in the diet. While this ionophore does not completely reverse potassium’s effects, it does increase magnesium uptake, reduce the need for supplemental magnesium and raise the overall energy efficiency of the diet.

Visit with your MFA livestock consultants for the supplements and feeding practices you need to keep your dairy herd healthy and productive. 

***