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Preventing grass tetany in cows

published: May 1st 2023
by: Karla H. Wilke and Mary Drewnosky, Nebra
source: Southern Livestock Standard

As grass begins to turn green, producers are anxious to get cows out to graze. However, cool season predominate areas tend to have lush spring growth which can lead to grass tetany in cows. While there are treatments for cows caught quick enough, prevention is always the best policy.

Why are cows at risk of grass tetany in the spring?

      Grass tetany occurs when circulating Magnesium (Mg) is low in the beef animal. Symptoms include staggering, convulsions, excitability, twitching, and can result in death. While it can affect growing cattle, it generally affects older lactating cows. The Mg requirement in the pregnant cow is 0.12% of the diet on a dry matter basis and jumps to 0.20% with lactation. Moreover, the Mg in colostrum is three times what it is in the milk the rest of the lactation. Thus, risk is highest early in lactation.

      Additionally, unlike some other minerals, Mg is not stored and mobilized in the tissues for times when it is deficient in the diet. Magnesium is absorbed across the rumen wall and how much Mg is circulating in the blood is highly dependent upon how much was consumed.

      In addition to the fact that the Mg requirement increases with lactation, feed that is high in potassium (K) or nitrogen (N), like lush, rapidly growing cool season forages, can decrease Mg absorption. So, more magnesium needs to be consumed when cows are grazing lush cool season pastures to get the same amount of magnesium into the blood.  Furthermore, cool, cloudy days associated with wet springs often increase the risk of grass tetany.

Using free choice minerals to prevent grass tetany

      To help prevent issues with grass tetany, producers should start providing a high Mg mineral to cows about a month before turning out on lush pasture to get them used to consuming it and continue to provide high Mg supplement until grass starts to elongate and mature and the risk of grass tetany is low.

      Mineral mixes are typically formulated for two oz. or four oz. intake. At two oz. intake, 100 cows should consume 87.5 lbs. of mineral mix per week and 175 lbs./week for a four oz. mineral mix. If they are eating more, then adding more salt will often decrease intake. However, the salt will dilute the mix so overall intake (salt plus mineral mix) should be greater than the initial target. Keep track of the actual mineral mix intake to ensure target intakes are reached. When using a free choice mineral mix to provide magnesium, do not provide separate salt as providing separate salt will reduce intake of the mineral mix.  

      To prevent grass tetany, a good target for a four oz. target intake free choice mineral is 10 to 13% Mg.  If adding magnesium to an existing mineral, then nine lbs. of magnesium oxide per 50 lbs. of mineral mix would be needed.  However, magnesium oxide is bitter and may reduce intake. If intake is low, then adding one lb. of dried distillers or soybean meal may help increase intake.  

Using a hand fed supplement to prevent grass tetany

      If using a hand-fed energy or protein supplement, then five lbs. of Magnesium oxide per 100 cows would need to be added. Sodium deficiency can increase the risk of magnesium deficiency, so if using a mineral fortified supplement that is provided daily, such as a cake (and not a free-choice mineral), make sure that free choice salt is provided.

      While providing high Mg mineral helps reduce the incidence of grass tetany, producers should talk to their local veterinarian and have a treatment plan in place for cows who do succumb.


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