Grass tetany is caused by low magnesium (Mg) in livestock and other ruminants. Sometimes it is manifest as a cow that is acting out of character, seems oversensitive to touch or may even act blind. These may be early signs of grass tetany, caused by a lack of available Mg. Other common symptoms include hyperactivity or aggressiveness, stiff gait or goose-stepping, excessive pawing or chomping of the jaw, and severe cases will yield seizures and death. It is often an issue during periods of high Mg requirement, such as during lactation when Mg is lost through milk production and available Mg is limited in the food source. Grass tetany can be avoided if precautions are taken early on before any symptoms are manifested in the animals in the field.
In the plant
From the plant perspective, grass tetany happens because the plant grows faster than it can accumulate Mg. In the spring, the plant has a need to build leaves and stems, so growth takes precedence over mineral uptake. The minerals come in later with time. Mg is somewhat mobile throughout the plant and usually moves from older growth to new growth in the spring. However, if plants have been overgrazed, the amount of stored Mg is limited, and the plant will have to rely on available Mg in the soil. Cool spring soil temperatures limit Mg availability in the soil. This is exacerbated by a soil pH lower than 6.5 or higher than 8.5, where Mg is less available to the plant. Sandy soils also tend to be lower in Mg and may contribute to the problem. Pastures that had too much canopy removed the previous fall or winter will not likely have much residual, which is an essential storage bank for Mg in the spring. Grasses are often the biggest offenders, so including other species such as legumes or other forbs can offset the risk of Mg deficiency and grass tetany. Rangelands are typically low in Mg, and caution should be taken in the early spring months.
Another good practice to reduce grass tetany is to make sure you have a diverse mix of grasses, forbs and legumes in your pasture. Monocultures of grasses are often the source of Mg deficiencies during times of rapid growth. While spring is the most common time for grass tetany, it may also occur during fall growth or after a fertilizer application. Pastures that only offer grass give livestock no other option to help meet Mg requirements. Fertilizer applications can also influence tetany. Overapplications of fertilizers can cause grass tetany for two reasons: Rapid growth following nitrogen applications can lead to the same problems caused by growth in the spring, and nitrogen and potassium can impede the uptake of Mg if applied at excessive rates. Having the soil tested and applying only the necessary rates will not only save money, but it may also save your cow.
Cereal grains and brome grasses can be especially problematic for grass tetany on irrigated pastures. In a rangeland setting, crested wheatgrass can cause tetany in some conditions.
In the animal
Mg is essential for nerve impulses. When concentrations are below a certain level, it causes these impulses to fail, which results in hypomagnesemic tetany (commonly called grass tetany). Mild symptoms may include the animal acting differently than normal, grazing away from the herd or erratic behavior. In more severe situations, the animal may exhibit muscle twitching, stiff gait, lying down and pawing at the ground, and death. Quite often after a dead cow is found, there will be signs that it aggressively pawed at the ground around it. Grass tetany can often be mistaken for nitrate poisoning or “oat hay poisoning.” It is important to have the feed assessed by a diagnostics lab and the cause of death determined by an experienced veterinarian. Providing a mineral supplement with ample amounts of Mg before they go out on lush green pasture is an easy way to ensure you don’t have Mg deficiencies. Allow continuous access to that mineral until your grasses have started to elongate and the risk of grass tetany has passed.
Pregnant and lactating cows require 0.1% to 0.2% of their diet to be Mg. This is usually a problem because the lowest point of available Mg is right as spring calves are being born and Mg requirements are high. Livestock depend on their daily dietary intake for Mg, and if they do not have a regular source of Mg in their feed, they will get tetany.
Treatment
Animals suspected of having grass tetany should be worked very slowly and gently, so as not to elevate their heart rate. Mg and calcium solutions can be administered intravenously to combat the effects of grass tetany. After a cow has shown signs of tetany, it should be given Mg in its diet through mineral supplements, slow-release capsules or adding powder such as magnesium oxide, magnesium sulphate or magnesium chloride to their feed.
Cattle that have experienced grass tetany are more likely to get it again if Mg-deficient conditions continue to exist. To prevent future relapses of tetany, affected cows should be moved into a pasture that has legumes or other forbs rather than grass only. Shelter should be provided, and stress should be mitigated. Stress from being overworked or bad weather can cause symptoms to worsen.
Conclusion
There are a few practices that can be used to prevent grass tetany. Encourage Mg intake through additional sources such as mineral blocks and salt licks. This can be difficult because magnesium oxide is not very palatable. It is best used in a mix with other necessary nutrients. Don’t allow livestock access to pastures that have been overgrazed the year before. It is best to allow pastures to get approximately 8 to 10 inches tall before putting cows out. Mature plants have more Mg than new growth. Encourage legumes by applying phosphorus to pastures. Legumes have a higher concentration of Mg than grass. Lactating cows are the most susceptible to grass tetany because of their higher Mg requirements, so avoid placing them in known problem areas during fast growth in the spring or following a fertilizer application.
Grass tetany can be a problem, but the solutions are simple. Make sure livestock have access to a source of Mg and avoid overgrazing pastures and rangelands to ensure enough Mg is carried over for the following spring as the plant starts to grow again. Following these recommendations will help ensure that even high-Mg-requiring livestock such as lactating cows can avoid grass tetany.
