Revisiting grass tetany and magnesium deficiency

Clifton Martin, Extension Educator, Agriculture and Natural Resources, Muskingum County

When the spring flush arrives lactating animals will be the most susceptible to grass tetany.

Managing agricultural field fertility through lime applications is a well-known practice with the goal of maximizing plant growth and productivity. One choice in the process is the decision to use calcitic lime or dolomitic lime as a source. Competing variables in the decision might be economics of short-term cost versus managing for a magnesium deficiency in the pasture. Generally, calcitic lime is cheaper to acquire and dolomitic lime is more expensive, but as we seek to manage a magnesium deficiency it may be advantageous to use dolomitic lime to deliver the needed nutrient to plants. This may be a consideration in a hedge to prevent grass tetany in a pasture. With spring just around the corner, it is a wonderful time to revisit the grass tetany challenge in forages.

What does the problem look like?

To get straight to the point, animal death is the outcome of a grass tetany problem if not properly treated. Grass tetany, also called hypomagnesemia, refers to blood magnesium concentration that is too low in the body and manifests muscle stiffness from a lack of muscle stimulation. This is a risk we manage because cool season grasses are known to be low in magnesium mineral, so we actively consider adequate magnesium supply in livestock management. Magnesium content in forage should be above 0.2 percent to avoid grass tetany problems in lactating animals. This is a disease that requires proactive decisions to stay ahead of the problem before it happens.

What are the contributing factors?

We must be thoughtful about factors impacting magnesium uptake.  Too much potassium is known to interfere with magnesium uptake and should be a consideration in pasture fertility management. The risk of grass tetany increases when the ratio of magnesium to potassium is less than 2:1. Too much dietary calcium and dietary fat will also reduce efficiency.  Sodium intake can also carry magnesium out of the body through urination. Briefly, over fertilization of pastures can also reduce magnesium content.  Nitrogen, potassium, calcium, and sulfates all interfere with magnesium concentration.  One decision elsewhere on the farm may turn out to be too much of a good thing when it comes to magnesium content and grass tetany.

What are some management choices?

Legume plants aid grass tetany management by delivering elevated levels of magnesium in the forage and is part of the reason for including a small amount of legume in a seeding. An alternate risk in the case of legumes becomes the problem of bloat if legume content is too high. Choosing a lime source has an impact from two sides: excessive calcium from calcitic lime can interfere with magnesium uptake while dolomitic lime delivers needed magnesium. Many decisions we make on our way toward sound agronomic management will themselves aid us in ensuring adequate magnesium. Standard tools like soil testing and forage testing will provide needed information to make decisions. Forage testing by wet chemistry is preferred for mineral analysis.

The choices that make the greatest impact will be forage species selection, fertilization practices, and providing additional dietary magnesium. As many of these go, the choices will be a balancing act that require adjustments over time. Strategic and limited applications of nitrogen can be applied in March to help jump start growth but should be light (20-40 lbx N/A) and only limited to a third of total acreage.

The spring flush will be here soon. Lactating animals will be the most susceptible to the risks associated with too much access to new lush green pastures. Consider less sensitive animal stock to utilize the spring growth. I find the magnesium challenge to be well documented in resources available to producers. As it goes with forage testing and soil testing, it may take a little studying to sort through all the factors. Should you find yourself seeking more information it can be found through your extension resources and I’m sure the retailer of your choice will be delighted to help you on your way. With some care and attention to the details we will all be on our way to setting up for a great spring.