Grass tetany can be a serious cattle grazing problem. It is often associated with dramatic diet changes caused when cattle are allowed access to extremely lush, high quality forage. Early spring when cattle are turned out on wheat pasture, ryegrass pastures, and other lush, green forages is the usual initiation of disease onset. However, anytime cattle diets are suddenly switched from an extremely low quality forage (like long grazed crop aftermath) to a very high quality forage (like dairy quality alfalfa hay) grass tetany should become a concern. It is sometimes called "grass staggers", wheat pasture poisoning, or hypomagnesemia. It is associated with an imbalance in the mineral components of blood serum with an especially reduced magnesium level. Stress seems to increase the likelihood of its occurrence, so things like frost, diet changes, and weather can complicate management for tetany.
It primarily affects older, lactating cows, particularly those 30 days either side of their peak milk production (which generally occurs two to four months postcalving). It may also affect dry cows and growing cattle, so managers should always be aware of the risk and preventive measures. Often, the highest producing cows in a herd are most affected.
Typical signs of grass tetany are uncoordinated gait (cattle walk like they are drunk) and terminate with convulsions, coma, and death. Many times the symptoms go unnoticed and animals are found dead. Usually evidence of thrashing is apparent around the cow if grass tetany is the cause of death. Grass tetany can be confused with milk fever. While clinical signs can be quite similar, the time of appearance usually does vary. Milk fever is within hours after calving, grass tetany later when the cows are really milking hard. Milk fever is associated with low blood calcium level. Also, another difference in symptoms is milk fever cows become paralyzed rather than show violent muscular response. Nitrate toxicity can occur during similar grazing situations as grass tetany. In cases of nitrate poisoning the blood usually turns chocolate brown in color and there is grayish to brownish discoloration of non pigmented mucous membranes.
It actually is caused by cows grazing or being fed a forage low in magnesium content or availability. The availability of magnesium is reduced if the forage was fertilized with high levels of nitrogen or potassium. This can happen with either commercial fertilizer or heavy manure applications.
Several methods of prevention are recommended. Control grazing or diet modification are normal preventive management decisions. Preventing early grazing of young and lush grass; turning cattle out on more mature pastures will eliminate tetany occurrence. Fields and pastures can be top dressed with magnesium as a preventative. Other producers have fed higher quality forage for 10 to 14 days prior to turn out, which seems to reduce the dramatic change in diet. But the most prevalent management decision is to supplement magnesium in a freechoice mineral. If supplementation is the route, take care to ensure that each animal gets the proper amount of supplement daily. Magnesium is not stored in the body; therefore, feeding additional magnesium oxide months ahead of grazing lush forage will not be beneficial. However, beginning high magnesium supplementation one to two weeks prior to the grazing season assures all cows are consuming the mineral in adequate amounts. Depending on the fortification levels, cows will generally need between .5 and 2 ounces of mineral per head per day. A rapid decline in blood magnesium levels may occur as soon as 48 hours after supplementation stops. So supplementation should continue until the forage matures past the lush stage of development.
A cow requires 13 to 15 grams of magnesium daily for maintenance and lactation. Some situations may require at least 36 grams to prevent development of tetany in the herd. Commercial mineral mixes containing 10 to 15 percent magnesium are the norm for prevention.
Control of grass tetany should be centered on prevention. Cows should be observed closely for several days after being turned out, and a veterinarian should be consulted at the first sign of trouble.