It is that time of year when clients start asking me if they should start feeding extra magnesium to avoid grass tetany.  Over the years, my advice has been that most people should just supplement on a year-round basis.  I know most people are familiar with grass tetany, but I thought this to be a good time for a refresher course on the cause, treatment and prevention of this condition.
Magnesium deficiency is known by many terms – grass tetany, winter tetany and grass staggers.  It is a metabolic disturbance in cattle and sheep associated most frequently with animals grazing young, rapidly growing forages.  The highest incidence of grass tetany is seen in early spring when cool season forages are taking off and spring calving cows are producing peak amounts of milk.  Mature cows are most at risk, but young cows and even calves can be affected.
Rapidly growing forage often will have low magnesium content that is a result of many factors.  Naturally low magnesium levels in some soils can cause low magnesium levels in plants.  External factors, such as heavy nitrogen or potassium fertilization, can serve to make magnesium unavailable for plant absorption.  Water logged soils, which are low in oxygen, prevent plants from taking up sufficient magnesium regardless of soil magnesium levels.  Forages heavily fertilized with manure also have a higher probability of causing grass tetany.
Symptoms of grass tetany include nervousness, staring eyes, uncoordinated gait, teeth grinding, erect ears, and increased heart and respiratory rates.  As the deficiency progresses, animals become extremely sensitive to sounds.  This is followed by muscle tremors, inability to stand, convulsions, and death.  Often the first finding by a producer is a dead cow that was completely normal looking the previous day.
Treatment of grass tetany involves intravenous administration of magnesium and calcium supplements, usually by a veterinarian.  Blood samples can be tested to confirm the diagnosis.  Cows usually respond to treatment rapidly and are able to stand anywhere from a few minutes to a couple of hours after.  If IV treatment is not able to be initiated when the cow is found, treatment with oral gels can temporarily raise magnesium levels long enough for IV magnesium to be administered.
Prevention strategies include moving herds to pastures with more mature grasses or legumes, feeding hay with a high percentage of legumes, and feeding mineral mixes with elevated magnesium levels.  This is the most common way to manage a herd over the course of time.  While most basic mineral mixes will have anywhere from 1 to 2 percent magnesium, high magnesium mineral mixes contain between 12 to 16 percent magnesium.  I made some calls to some of the local feed dealers, and the average difference in cost for the high magnesium mineral averaged $3/100 lbs.  With an average intake of 3 to 4 ounces per cow per day, the cost per cow is less than one penny per cow per day.  So an ounce of prevention is really worth a pound of cure.  Another way to increase magnesium levels is to apply dolomitic limestone to pastures.  Call your veterinarian if you have specific questions about magnesium supplementation in your herd.
Mike Bloss, DVM, owns Countryside Animal Clinic with his wife, Kristin Bloss, DVM, in Aurora, Mo.

AdministratorOn-Call / VetMissouri,On CallIt is that time of year when clients start asking me if they should start feeding extra magnesium to avoid grass tetany.  Over the years, my advice has been that most people should just supplement on a year-round basis.  I know most people are familiar with grass tetany, but...The Ozarks' most read farm newspaper, reaching more than 58,000 readers in Missouri, Arkansas and Oklahoma