Early spring is the period when cattle are at most risk for grass tetany in Iowa. Grass tetany is actually a magnesium deficiency (hypomagnesemia). The problem occurs at different times of the year worldwide, but the timing seems to have a few things in common. Lush spring grass can grow very fast if the growing conditions and fertility are right. Usually this involves plenty of moisture and fertility. Grass that is susceptible to producing grass tetany often is low in magnesium (Mg), calcium (Ca) and sodium (Na), but higher in potassium (K). Grass tetany can occur in legume-grass mixtures when the temperature is below about 60 degrees since much of the growth at that stage will be grass. Grass that is conducive to producing tetany in cattle usually is high in protein, but high enough in water to limit the intake of energy by the cattle.

Susceptible cattle usually are cows nursing calves in peak or near peak lactation. This is when the requirements of cows for both Mg and Ca are the highest. Cows in the early stages of tetany will become nervous, and you may notice muscle twitching. As the condition progresses the cows will lose coordination and go down. Timely treatment with fluids that contain Mg and Ca by your veterinarian is often effective. A blood Mg test can confirm that grass tetany was the cause. 

Of course the best course is always prevention. A high magnesium mineral mix that is at least 12-15% Mg is recommended during periods of grass tetany risk. Because it takes some time to build up the stores of Mg, it is best to start the higher level of supplementation two to four weeks prior to pasture turnout. Some sources of Mg can be somewhat unpalatable so it is important to monitor the mineral intake. Some adjustment of the salt content, additions of palatable feeds, or force feeding the mineral through a grain supplement may be needed if free choice consumption is insufficient.

To paraphrase Ben Franklin, “an ounce of prevention (in this case three to four ounces) can be worth more than “the cure."

Source: Dan Loy, IBC interim director, and Mary Drewnoski, animal science postdoctoral research associate