Put Out to Pasture: the Critical Time for Grass Tetany
Making the change from winter feeding of grain and hay to the fresh pastures that are springing to life can be a stressful time on an animal’s digestive system. There is also a concern about the oft times necessity of supplement feeding as the pastures start to grow in. In early spring as both animal and farmer are anxious to resume their activities outside, a very real danger lurks around the corner: tetany. The technologies that have made our lives easier and have changed fallow fields to productive ones have also favored the development of a disorder called grass tetany. Grass tetany has two main causes; either an imbalance in the mineral content of the forage or under-feeding.
Grass tetany is a metabolic condition of ruminants (cows, sheep…) that has been recognized for the last hundred years or so and in the last thirty years has been associated with a decrease in the amount of magnesium (hypomagnesaemia) available in the blood. This condition develops soon after cows, or other ruminant animals, are moved from hay and concentrates to rapidly growing, lush, green pastures. The change in diet frequently contributes to a decrease in magnesium intake. Animals cannot absorb magnesium as efficiently when fed a lush grass diet as opposed to hay and feed concentrates. As the animal ages, they are unable to absorb magnesium as effectively. While tetany may not affect a high percentage of cows, the effect of herds that are struck in this manner can be devastating and overpowering. Animals that are most susceptible to the effects of tetany are the very young, older, and lactating females.
Potassium-based fertilizers such as potash or liquid manure have tremendous effects on the mineral balance of the grasses and fields they are spread on. When plants come into contact with potassium, their cells absorb and accumulate it more rapidly than they do with either calcium or magnesium. An animal that grazes on those plants is ingesting a high concentration of potassium. This mineral has a special effect on the neuro-muscular system. Potassium acts like a stimulant and certain organic compounds can accentuate the effects of potassium on neuro-muscular sensitivity and are factors in triggering attacks of tetany.
Under-feeding can result when the animal has little or poor quantities of food made available to it; pastures with poor crops of grass on which to graze or from digestive upsets and rumination disturbances. Both of these effects can occur simultaneously. Along with the effects to the neuro-muscular system, changes in the way food is digested are seen as well. Rapidly growing and young grass causes spiked ammonia production in the rumen and decreases the amount of absorbent magnesium available to the animal’s body. As the seasons and pasturelands mature, we see a variation in the available magnesium in the plants; up to 40% in August than in May.
Tetany is not a disease, but rather a collection of symptoms radiating from a direct cause; a severe, irrecoverable drop in magnesium and/or calcium concentration in the blood. Magnesium metabolism and calcium metabolism have direct effects on the other; when one is affected so it the other. Farmers often have difficulty distinguishing tetany from a multitude of other diseases and the animals aren’t often discovered until it is too late.
Affected animals will have lost flexibility in their hindquarters, have no appetite, and the extreme neuro-muscular sensitivity evidenced can have the animal falling into convulsions from noise, excitement, low temp, an injection, or even the breeze. If the tetany is not as severe, the animal maybe subject to muscle tremors and sideways jaw movements and emit large quantities of saliva. Its back will arch and the head thrown back. The heartbeat will be fast and irregular and the animal can have spells of apnea lasting up to thirty seconds. Once on the ground the animal has lost its ability to rise since it cannot bend its legs and the struggle of trying to rise causes a rise in the temperature; whereas it was normal with a tendency to drop. If treatment, discussed below, is given too late or not at all, then the tetany is always fatal to the animal. The animal will refuse all food and be dead within a few days. Post-mortem reports report indications of hypertrophy of the adrenal glands, degeneration of the liver, and calcium deposits in the kidneys and other organs, while the brain will have suffered internal lesions.
The most widely agreed form of treatment is simultaneous injections of magnesium and calcium chloride with an additional shot of magnesium to follow if needed. The shock of the injection may actually send the animal into convulsions due to the hyper-sensitivity of the neuro-muscular system. The calcium chloride is used if the magnesium injection is too high or the respiratory muscles become paralyzed. Not all the animals affected will respond to this treatment if it is given too late.
Grass tetany has advanced hand in hand with the use of temporary pastures. You can cut the risk to your herd by gradual putting them out to grass. At the beginning of the grazing season bringing the animals in at night helps to lower occurrences as well as limiting the time they are to pasture. Waiting for the pastures to mature allows magnesium and calcium concentration to rise naturally. Spring time is especially dangerous for tetany; by taking some precautions in the early season you can save money and the health of your animals.