Now is Time to Start Providing High Magnesium Mineral to Lactating Cows to Prevent Grass Tetany


Grass tetany occurs in cattle when low magnesium levels in the blood impact the animal’s nervous system. Although growing cattle can be affected, older lactating cows are at the highest risk, especially during early lactation.

Conditions align in spring to decrease magnesium levels in the blood of lactating cows. Lactation increases the cow’s magnesium requirement and vegetative cool season grass, especially on cool, cloudy days, decreases magnesium absorption. In addition, magnesium levels are dependent on consumption instead of being stored like other minerals.

To prevent grass tetany, the general rule is to provide free choice high magnesium mineral to lactating cows 30 days prior to spring grazing to accustom them to consuming it. In northwest Nebraska, this is typically around April 15. Continue to provide high mag mineral until the risk of grass tetany decreases as cool season grasses mature. This typically occurs in the middle to late part of June. 

Often pairs are fed in the pasture to prevent the spread of bacteria which can occur in confinement giving lactating cows immediate access to the lush green grass. If this is the case, high magnesium mineral may need to be provided starting in late March or early April before grazing becomes a significant part of the diet.

Mineral mixes are typically designed to be offered free choice and are formulated for an average intake of 2 to 4 ounces per head per day. For example, a 50-pound bag of mineral should last 100 cows 2 to 4 days (2 ounces per head per day multiplied by 100 head, divided by 16 ounces to convert to pounds equals 12.5 pounds. 50 pounds divided by 12.5 pounds equals 4 days). If mineral is fed from April 15 to June 15 (60 days), 15-30 50-pound bags of mineral will be required depending on desired intake. Always check the product label for feeding recommendations.

Salt can be mixed with mineral to reduce intake as long as intake does not drop below the desired amount. Providing additional salt other than what is mixed with the mineral could reduce mineral mix intake.

Providing high magnesium mineral only reduces the risk of grass tetany. Being able to identify the symptoms and having a treatment plan in place with a veterinarian are a must. If intervention does not occur quickly, grass tetany can be fatal. Symptoms of grass tetany include staggering, excitability, convulsions or seizures, and twitching.

For more information on mineral and vitamin needs, reference NebGuide G2340 “Formulation Considerations for Mineral and Vitamin Supplements for Beef Cows” available at