Fertilizing Pastures After a Drought: Ensuring Adequate Mineral Intake and Preventing Grass Tetany

By Ted Wiseman

Spring is here, bringing lush pasture growth with April rains and warmer temperatures. Although many of our pastures may have gone into fall and winter in poor condition, do not be tempted to fertilize your pastures this spring. High levels of nitrogen and potassium in the soil can increase the risk of grass tetany. Cool season grasses during this time are often low in magnesium, an essential mineral for normal nerve and muscle function. Low magnesium in the blood can cause “tetany,” leading to muscle stiffness and potentially death if not treated quickly. Both feeding and management practices are needed to prevent this disease. Fertilizing in the spring will increase those chances grasses are luxury consumers of potassium.

Ruminants (like cattle, sheep, and goats) are not very efficient at absorbing magnesium from their diet. They absorb magnesium in their digestive tract in the rumen through an exchange between potassium and sodium. Early spring grass often has high potassium levels, which can interfere with magnesium absorption. Excess sodium intake can lead to more magnesium being excreted in urine. Unlike calcium, magnesium cannot be easily compensated from bones to correct a low level.

Feeding high magnesium mineral supplements is a preventative measure to reduce or prevent grass tetany. Cows require 20 grams of magnesium daily which can be accomplished through consuming 3 to 4 ounces per day of a mineral mix containing 12 to 15 percent magnesium. Just because you provide high magnesium mineral mix doesn’t mean your livestock are in the clear. You need to monitor mineral intake and make sure all animals are consuming desired amounts. Magnesium alone can result in decreased palatability and decrease consumption. If livestock do not consume adequate levels from a free choice mineral mix, the supplemental magnesium can be combined with more palatable feeds such as dried molasses or soybean meal. Commercial mixes are available, and a few University Specialists have provided home-made mineral mixes online. The high magnesium mineral supplements should be provided prior to turning livestock out onto lush pastures. Recommendations that I have found suggest provided these supplements 2 weeks to 30 days prior to turning out livestock onto pastures. High magnesium supplements do not need to be provided year-round but will not create any problems if you feed it too early or late into the season.

Females are more prone to the disease especially the older heavy lactating females. Steers, heifers, dry cows and growing calves are less susceptible, but can still contract tetany in the right conditions. Symptoms often include going off feed, nervousness, show muscle spasms, convulsions, irritability, lapse into a comma, aggressiveness and lastly death. Producers should keep a close eye on livestock in the early grazing season, since these symptoms often occur rapidly. Should you suspect grass tetany contact your veterinarian immediately.

Mineral supplements come in various forms. Inorganic Trace Minerals: Oxide, Sulfate, Chloride, Hydroxy. Or Organic Trace Minerals: Complexes, Chelates, Proteinates, Polysaccharides and Propionates. Organic minerals are absorbed better, followed by Hydroxy, Carbonate, Sulfate, and Oxide forms. Magnesium oxide is an exception and is bioavailable. Highly bioavailable mineral mixes are usually more expensive.

When choosing mineral products, read the labels carefully. Look for intended species, mineral levels, feeding method, targeted intake and source of each mineral.