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OSU Extension

College of Food, Agricultural, and Environmental Sciences

March 10, 2016 - 3:11pm -- Anonymous

When the calendar turns to March, spring won’t be far behind and for beef cow/calf enterprises that means looking ahead to spring pasture management.  One aspect that must be considered is taking steps to prevent grass tetany when grazing lush spring grass.  Recently Michelle Arnold, DVM, University of Kentucky Ruminant Veterinarian wrote a very good article for the OSU Extension beef cattle letter.  I’ll excerpt some of the major points of that article here, but for the complete article go to:

Grass tetany, grass staggers, is due to a low level of magnesium (Mg) in the blood. The amount of magnesium in the blood is completely dependent on the amount obtained from the daily diet. Deficiencies occur most often in beef cows when they are nursing a calf and grazing young, green grass in early spring. Fast-growing spring pastures are high in potassium (K+) and nitrogen (N+) and low in magnesium (Mg++) and sodium (Na+).  Affected cattle often have low blood calcium concurrently.  A number of complex factors contribute to the ability of magnesium to be absorbed through the rumen (stomach) wall.  Primarily there is a “pump” mechanism that actively transports the dissolved form of Mg across the rumen wall to the bloodstream. This pump doesn’t work when potassium in the rumen is high and sodium is low because this changes the electrical potential necessary to drive it.

Prevention is based on providing soluble magnesium in the rumen during times when conditions are right for grass tetany. As long as the active transport pump for magnesium is working well and driving magnesium across the rumen wall, problems should not develop. However, when factors prevent this pump from working (such as high potassium level in lush spring grass), the second or “backup” pathway is to increase the amount of magnesium in the diet with a high magnesium mineral mix.  A high rumen magnesium level will allow magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump. Supplementation with high magnesium mineral should begin at least 30 days prior to calving.  Cows require 20 grams of magnesium daily or 4 ounces per day of a 15% magnesium mineral mix during the late winter and early spring.  Mineral feeders should not be allowed to be empty because consistent intake is important for clinical disease prevention.  Mineral recommendations for free choice supplements for grazing beef cattle include 15% salt and 14% magnesium in the complete mineral mix and all magnesium from magnesium oxide (no dolomitic limestone or magnesium mica). These complete mineral mixtures supply the necessary sodium in the form of salt to aid in combatting high potassium intakes. Consumption should be monitored because cattle seldom eat enough trace mineral if using poor quality products.  Feeding ionophores (monensin, lasalocid) has been shown to improve magnesium absorption efficiency. High magnesium mineral may be discontinued in late spring once the grass is more mature, the water content of the forage is decreased, and daily temperatures reach at or above 60°F.

Other management options that can decrease the risk of grass tetany include: Feed small amounts of hay and/or grain to cattle on lush pasture during susceptible periods or limit grazing to 2-3 hours per day and graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the higher risk pastures.