Fast-growing spring forages have always been a blessing and a curse. The blessing is that cattle can switch to getting their primary nutrition from pasture rather than stored feeds and their associated labor requirements. The curse is that early spring grass forage is high in moisture and has some unique nutritional qualities that, in turn, can cause some unique nutritional problems. One such problem is grass tetany.
Although cows are subject to grass tetany on any type of grass pasture, animals are especially susceptible when grazing winter annual cereals such as wheat. Oklahoma State University’s Paul Beck explains that spring calving often coincides with the rapid growth of lush wheat pasture.
The extension beef specialist says these pastures are ideal for meeting the nutritional requirements of a lactating cow, but there are serious concerns about grass tetany, which can appear suddenly and be fatal if not prevented.
Grass tetany typically occurs as the result of inadequate magnesium absorption in the digestive tract. This is caused by a low forage magnesium concentration and a high potassium concentration in the lush, spring forage and results in mineral imbalances that interfere with magnesium absorption in the cow.
“Mature cows early in lactation are most susceptible to grass tetany because their magnesium requirement increases at calving through milk production,” Beck notes. “Older cows are at even greater risk because their ability to mobilize magnesium from bone is reduced.”
What to watch for
Cows afflicted with grass tetany will be subject to nervousness, unusual behavior, muscle twitching, an unsteady gait, and have difficulty standing up. If the condition persists, cows may become excitable, develop labored breathing, go down, or die. “Because stressed cattle worsen rapidly, care must be taken when handling affected animals,” Beck advises.
Cereal pastures that have been well fertilized offer the greatest risk for grass tetany. Beck says farmers can assess the risk of a pasture by doing a forage mineral analysis, which can be used to calculate a tetany ratio.
“The tetany ratio examines the quantity of potassium to the quantity of magnesium and calcium, and ratios greater than 2.2 are likely to result in grass tetany,” he explains. The problem with relying on a forage mineral analysis is that it takes at least a week to get the results, which may be too late.
Be proactive
“Prevention is far more effective than treatment,” Beck asserts. “The most common and reliable approach is to provide supplemental magnesium, beginning at least one month prior to turning cows onto wheat pasture and continuing through the high-risk period.”
High-magnesium mineral supplements typically contain 10% to 12% magnesium, usually in the form of magnesium oxide. Beck notes that an intake of 3 to 4 ounces per day will provide 40% to 50% of a cow’s daily magnesium requirement; however, he cautions that because magnesium oxide is unpalatable, intake needs to be monitored closely.
