Grass tetany is a nutrition-related health condition that generally occurs when cows are grazing cool-season grasses in early spring or wheat pasture in the fall. In all actuality, tetany can occur in some form at any time of the year. Fall or "Winter tetany" can occur in wintertime when cows are fed harvested forages. Grass hays, including cereal grain hays, tend to be low in magnesium and need to be properly supplemented. Feeding legume hay may alleviate the problem, but it will not fix an immediate problem.
Non-legume hays (Mixed-Grass), may average 0.18% magnesium, but some hays may be as low as 0.03 to 0.05% magnesium on a dry matter basis. Forage levels below 0.18% magnesium are marginal, while levels less than 0.12% will cause your cattle to suffer from winter tetany. Cereal-grain hays and grass hays may be high in potassium, but calcium and magnesium levels may be low. A plant magnesium level of 2-2.5% is considered a safe level. Normal blood levels are 1.7-3.2 mg./dl in mature cows.
Mature animals are far more susceptible than younger animals because of their inability to mobilize magnesium from their bones to meet their requirements.
This is especially critical during lactation.
Cows with young calves are more at risk than steers, heifers, dry cows and cows with calves more than 4 months of age, but heavy milking cows are the most susceptible to tetany. Tetany-afflicted cows may show signs of nervousness, reduced forage intake, reduced milk production and muscular twitching along the face, shoulder and flank. It progresses to staggering, when cattle fall on their sides with the head thrown back, excessive salivation and grinding of the teeth. This is no laughing matter folk’s! The time between the first signs and death may be as short as four to eight hours.
Obviously, treatment must begin as soon as possible. It's important to quickly get some form of magnesium into the animal and relocate the cattle until preventive measures can be taken. The treatment of choice is an intravenous (IV) injection with calcium-magnesium gluconate because it gives the most rapid response. Another way to treat your cattle if your not comfortable with starting a bovine I.V., is by drenching the animal with a source of magnesium, such as magnesium sulfate, or using a rectally-infused enema of magnesium sulfate are other options.
With an IV treatment, the blood levels rise rapidly but fall back to the previous level within three to six hours, so additional measures must be taken. To prevent winter tetany on tetany-prone grass or harvested feeds (grasses, cereal grain hays), feeding alfalfa or other legume hay may reduce the risk. Cows at this time of year (off green graze-able pastures), should always have a mineral source available to them that includes a source of magnesium and calcium. We always (year-round), have a free-choice "Mol-Mag" block in a mineral feeder to prevent this from occurring.
We, as BEEF Producers, are the best defense against tetany. As part of management, a complete forage analysis should have been run on hay & pastures at some time during the grazing/haying season. If you have a known problem with tetany or suspect a problem, the forages in question should be analyzed specifically for magnesium, calcium, nitrate and potassium.
When your receive your forage analysis information, a "tetany ratio" can be calculated by your Vet or nutritionist to see if the forage is tetany-prone. The formula is: tetany ratio = % potassium % calcium + % magnesium. If the ratio is greater than 2.2, then the forage in question is tetany prone and preventive measures should be taken immediately.
If grazing wheat pasture, crested wheat or tall fescue or feeding straw, cornstalks or other low-quality, tetany-prone roughage, you should be prepared ahead of time to plan preventive measures and reduce losses.