By Hubert J. Karreman, V.M.D.
What about grass tetany?
Grass tetany is low blood levels of magnesium. Grass tetany occurs when there is lush growth of pasture; however, it can be any kind of pasture (not just bloat-causing legumes); grass tetany is actually more likely on grass stands. It will more quickly affect a fresh cow that has more metabolic needs than a later lactation cow. Its symptoms are somewhat like milk fever, and in actuality they easily could occur together.
The main symptoms include stiffness in a leg or body side (unlike milk fever where muscles are weak and lax). The cow will almost fall over but usually immediately right herself, but after this goes on a while she will lie down. However, cows are very uncomfortable when lying down because the muscles in certain areas will become stiff. They can also then all of a sudden get up with normal strength (unlike a milk fever), and this back-and-forth can go on for a while. They will act a bit more agitated than usual (similar to early milk fever) and stay that way (whereas with milk fever they will become depressed and dull). A recent history of eating early pasture growth will aid in the diagnosis and its subsequent proper treatment.
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Treatment consists of correcting the metabolic disturbance by giving magnesium, either orally in the form of large capsules of Epsom salts (magnesium sulfate) or six to ten large “pink pills” (magnesium oxide), or by giving intravenous CMPK (calcium, magnesium, phosphorus and potassium).
However, it can be darn difficult to hit a vein properly in a cow that wants to keep shifting around when it is standing while it also partially attempts to lay down. Just wait until it is down, then put a halter on her and tie the halter to a non-mobile post. Don’t tie it back to her leg like in milk fever because of the strength she can use to try to get free (and mess up your IV needle position). Give the bottle at no higher than her backbone (since it has calcium in it). If it is an older cow, continue into a second bottle, and if you have a stethoscope handy listen to the heart to make sure it is beating regularly (it may be at a quicker rate, but it should be beating totally regularly) while you give the entire second bottle. By the end, the cow should be noticeably calmer than prior to treatment. Regular 23 percent calcium IV will not work; the treatment must have magnesium in it. Follow-up treatment includes administering the “pink pills” or Epsom salt.
Cows left untreated and found down and unable to rise will often show more severe neurologic symptoms, such as paddling the ground area near them, and will have a staring expression. They then may continue into convulsions, coma, or death (which can happen in a few hours). In a severe situation, give the IV treatment fairly slowly since the solution contains calcium and potassium as well, and the animal’s system at this point will be acutely sensitive to the effects of added potassium and calcium.
Grass tetany is caused by pastures with low magnesium and/or high potassium. Magnesium absorption is reduced when the concentration of ammonia in the rumen is high (as with lush pasture of any sort). The combination of low-magnesium pastures and rapid growth leads to this condition. Although rare, it may be seen during spring.
Source: Four-Seasons Cattle Care