Magnesium is a critical part of proper nutrition in livestock

Magnesium is a major mineral, meaning it is required in greater amounts and dietary requirement is typically reported as a percentage of the diet.

Cattle need a continuous supply of magnesium to facilitate proper enzyme and nervous system function. Magnesium is also an important factor in carbohydrate metabolism and skeletal integrity.

Cattle rely on absorption of magnesium from the rumen to meet most of their needs. According to the University of Missouri Extension, cattle cannot modify magnesium absorption and they only absorb less than 30 percent of the magnesium they consume. However, several things can affect the uptake of magnesium from forages, including current magnesium level, rumen pH, the passage rate of the diet through the digestive tract, and interaction with other minerals.

“The maximum tolerable level for magnesium is 0.4 percent of diet (dry matter basis),” explained Dr. Shane Gadberry, professor with the University of Arkansas.

Compared to other major minerals, including calcium, phosphorus, potassium, and sulfur, the bioavailability for magnesium from forages is low.

“The average amount of magnesium in Bermudagrass and fescue hay tests in Arkansas is 0.22 and 0.25 percent,” Gadberry said. “Legumes contain more magnesium than grasses and our alfalfa and clover tests average 0.28 percent magnesium.”

A magnesium deficiency is uncommon in livestock. However, if cattle are grazing lush-growth fescue or small grain pastures, the risk for deficiency is increased.

“The most likely time to experience a magnesium deficiency is with mature cows grazing spring pasture,” Gadberry explained. “Especially those recently fertilized with potash.”

A high rate of nitrogen and potassium fertilizer can contribute to grass tetany. Grass tetany is a metabolic disorder and is a serious condition. When livestock consume too much potassium, magnesium absorption is inhibited both in the forage and the livestock.

“Other contributing factors include the rapid digestion and passage rate with lush spring pasture, very low sodium content of forages, and cows generally having greater magnesium requirement during the spring,” said Gadberry.

Risk for grass tetany usually occurs following an extended period of cold weather combined with fertilization in cows that have been fed high-quality cool-season grass hays.

When trying to determine if your animal is susceptible to grass tetany, the answer is possibly; however, there are some livestock that are more susceptible than others.

“We usually consider suspect animals as older cows just prior to four to six weeks after calving,” Eldon Cole, livestock specialist with the University of Missouri Extension in Lawrence County, Mo., said.

Very thin, overweight and lactating livestock may also be more at risk for developing grass tetany. Some research indicates that breed is a factor and Angus livestock and their crosses may be more susceptible in general, but there is limited research to indicate this notion.

Cows that are affected may show excitability with muscle twitching, an exaggerated awareness and a stiff gait. Some animals may even appear aggressive and show additional signs like galloping, bellowing, and then staggering.

However, not all animals may act as noticeable. In less severe cases or early on, the only symptoms may be a change in character of the animal and difficulty in handling. More often than not, grass tetany is caught too late and the livestock are found dead.

If grass tetany is suspected, the first correction is to increase the magnesium content of supplemental minerals.

“Increasing to 10-13 percent is very common,” stated Gadberry. “Magnesium oxide is typically used to provide the supplemental magnesium.”

It’s also important to note that magnesium supplements may have an off flavor so it is important to start supplementation well before periods of risk increase and keep an eye on the consumption.

“We supplement magnesium in the winter period from January 1 to mid-April,” said Cole.

Some producers opt to feed a magnesium supplement year-round. Ideally, producers have the freedom to make that decision based on the livestock feedstuffs; however, it is extremely important to keep your eye out for magnesium if your agriculture practices align with any of the above.


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