Springtime brings grass tetany

Written by Dr. Jim White on .

Cattle producers see the highest incidence of grass tetany in spring. And, over time, we have convinced ourselves that grass tetany is a shortage of magnesium in the blood (which it is by name and definition: “hypomagnesemia” is low magnesium in blood). So most producers address tetany problems by offering their cattle increased magnesium supplementation, and, once that remedy is in place, think all is well. 

The reason we associate grass tetany with spring is because of the low magnesium and/or high potassium content of rapidly growing grasses. Think lush, succulent spring fescue. In addition to the low magnesium, we have seen other factors that seem to be associated with grass tetany, many of them concurring with spring. There is the high soluble crude protein content of forages and weather stress—both for plants and animals. Lactation can play a role, too. 

Milk is a significant magnesium and calcium sink. And, we know that the nitrogen fertility status of pasture can play a role— with higher rates of nitrogen resulting in greater incidence of grass tetany. 

Meanwhile, the type of supplement offered cattle, particularly magnesium, calcium, salt and fermentable carbohydrates tend to reduce the incidence of tetany.

With so many factors involved, you can blame tetany on one or more of them. Grass tetany-hypomagnesemia may be due to low magnesium consumption. Or, it may occur due to low magnesium absorption/uptake. High potassium levels have been linked to reduced magnesium absorption. A German study showed that over 95 percent of the uptake of magnesium depends on the gradient of sodium, which implicates sodium deficiency as a potential cause for tetany. 

In other words, cows’ ability to regulate the concentration of intracellular free magnesium is directly related to sodium. The principal source of sodium for the animal will be the salt you feed.

Most of the time salt is offered to cattle free choice or forced in the diet. We know that we have to feed sodium because herbivores really like sodium, but plants do not. So without supplement, cattle will be deficient of sodium. 

Weather, fertility, species and growth stage will influence the amount of potassium in plants. The variation and imbalance of minerals in the rapidly growing lush grass triggers grass tetany. Another source of mineral concentration change might be due to frost damage, and young forage plants are particularly sensitive to frost damage. 

What are the effects of frost damage on tall fescue? March 2007 was warmer than usual, and there had been significant growth in pastures. But in April, there was a cold snap across the Midwest that resulted in substantial freeze damage to fescue stands. In the days following the event there were reported incidences of grass tetany in spring-calved beef cows. 

Researchers at the University of Missouri worked on the problem and concluded that the amount of sodium in fescue tissue was highly correlated to the freeze damage and the increased incidence of tetany. 

We might be past the final hard frost, but like 2007, there is always a chance lush grass will suffer frost damage. Now is the time to develop an appropriate strategy to reduce the risk of grass tetany this spring.

There are several factors which increase the likelihood of grass tetany. First off, magnesium is a dietary requirement for all cattle. Growing beef cattle have a magnesium requirement of about 0.1 percent of dry matter; for lactating beef cows, that requirement is doubled; triple that for wet dairy cows. For nursing cows, the requirement for calcium increases. So do the requirements for magnesium. The heaviest milking cows are at greatest risk of grass tetany. Storms, trucking, running the self-feeder empty or other stressors that cause cattle to stop eating can precipitate grass tetany. 

An obvious approach to defending against tetany is offering a salt mix with magnesium. This is generally the fastest and most certain method of addressing grass tetany. If modest protein and energy supplementation are needed where labor is an issue, 20-percent tubs with magnesium are a good choice. If the forage base is protein-and-energy adequate, products like MFA’s Hi-Mag Mineral, MFA Tasty Mag Mineral or Mag-Ade meal would be options. 

If you are hand-feeding cubes or a supplement at recommended rates, the likelihood of tetany is remote. Lactating cattle should consume 15 to 25 grams of magnesium per day. If a feed is on offer at all times, say a free-choice mineral, the supplement disappearance should be checked frequently. Magnesium is not well stored in the body, so frequent consumption is needed. Solubility (or bioavailability) is important. The more soluble a magnesium source is, the more bioavailable it is. 

Dr. Jim White is ruminant nutritionist for MFA Incorporated.

Originally published in the April Today's Farmer magazine ©2013 MFA Incorporated. All rights reserved. Here is a LINK to the original version of this story.

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