Many producers will be feeding straw and grain or greenfeed-based rations to their cattle this winter. When feeding these types of feedstuffs, winter tetany may occur.
High levels of potassium and phosphorus, coupled with low levels of magnesium and calcium, can result in a magnesium deficiency. This deficiency causes winter tetany. Feeding either magnesium oxide or limestone can prevent the condition.
“Winter tetany results in similar symptoms as milk fever or grass tetany – nervousness, lack of co-ordination, anorexia, muscle tremors or twitching. Cattle may walk with a stiff gait,” said Juanita Kopp, a forage beef specialist with Alberta Agriculture in Stettler. “As the condition progresses, cows can go down, start to convulse and die.”
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The magnesium requirement of a pregnant cow is 0.12 percent of its dry matter intake. For a lactating cow it’s 0.2 percent DMI, and 0.1 percent DMI for growing and finishing cattle. The appropriate calcium:phosphorus ratio for all classes of cattle ranges from 1.5:1 to 7:1.
Forages are good sources of potassium and most will usually contain between one and four percent. Cereal straws, in particular barley, oat and wheat straw, contain on average 1.40, 1.55 and 1.44 percent potassium respectively.
They also have a low calcium and high phosphorus concentration, which influences the mineral imbalance. Because grains are low in calcium and straw is high in phosphorus, feeding this type of ration has a high tetany potential.
“To determine the risk of winter tetany, a tetany ratio can be calculated using the percentage of (potassium, magnesium and calcium) in the diet.”
Using the symbols in the periodic tables in which potassium is K, phosphorus is P, magnesium is Mg and calcium is Ca, Kopp expressed the tetany ratio as (K/ Mg + Ca).
“If this ratio is above 2.2, winter tetany may occur,” he said.
By adding limestone, which is a good source of calcium, the tetany ratio can be reduced to below 2.2. Magnesium oxide can also be fed, but in lesser amounts because it is not as palatable as limestone.
Mixing these magnesium or calcium supplements with loose salt will encourage intake. Supplementing magnesium or calcium will help prevent the onset of winter tetany.
“If a cow goes down and you suspect tetany, contact your veterinarian for immediate treatment,” advised Kopp.
