The Role of Magnesium and Sodium in Grass Tetany

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Salt and Trace Minerals for Livestock, Poultry and Other Animals

THE ROLE OF MAGNESIUM AND SODIUM IN GRASS TETANY

Ruminant animals are susceptible to a disease condition characterized by low blood magnesium known variously as grass tetany, hypomagnesemic tetany, grass staggers, lactation tetany, wheat pasture poisoning, and winter tetany. Animal losses vary from year to year but are economically important. For example, in the United Kingdom clinical cases occur in an estimated 1% of the cattle and a third of these cases result in death. In the United States the most susceptible animals are mature beef cows in early lactation grazing lush, rapidly growing grass. Although difficult to measure, dollar losses to cattle producers are estimated to be several million dollars every year.

Magnesium is a mineral that is required by all animals and functions as an enzyme cofactor. It is very important to the central nervous system because it competes with calcium in the excitation-secretion coupling process. This role is directly related to the most common symptom of grass tetany, tetanic contraction of the muscles. Initial symptoms are usually characterized by excessive alertness, wary appearance, and fine twitching of muscles of the face and ears. The affected animal will be uncoordinated and walk with a stiff gait. After a few minutes to 3 to 4 hours, the animal may suddenly drop with convulsive spasms, lying on its side with rapid paddling movements of the limbs. Death often occurs in convulsions, or the animal may become comatose before dying. It is not uncommon for all symptoms to occur in a span of 2 to 3 hours so that the producer does not notice any problems until the cow is already dead. Usually levels of blood magnesium will be approximately 0.5 mg/100 ml of blood compared to normal values of about 1.7 mg/100 ml.

In reviewing the scientific literature it is obvious that the cause of grass tetany is very complex. Although it is characterized by a deficiency of available dietary Mg, many other factors interact to determine whether an individual animal will exhibit the symptoms. In spite of its complexity, it is imperative to begin to put the pieces of the puzzle together if effective prevention programs are going to be implemented. This is even more important because treatment is often unsuccessful.

The most consistent trait of grass tetany is its occurrence in older, lactating animals consuming lush, cool-season grasses that have received some degree of fertilization. The problem is seasonal and often occurs 5 to 10 days after the onset of cold, wet weather. The forage is low in magnesium, sodium, and soluble carbohydrates, but high in nitrogen, potassium, and higher fatty acids. Incidence of grass tetany is higher in herds composed of British beef or dairy breeds than in Brahman breeds (175).

Feeding high levels of potassium generally has depressed blood serum magnesium in ruminants (172) as a result of reduced magnesium absorption (178). The main effect of potassium is on preintestinal magnesium absorption. Tomas and Potter (180) reported that magnesium infused into the omasum or abomasum was completely recovered at the duodenum, but 36% to 61% of magnesium infused into the rumen was not recovered at the duodenum. Correspondingly, it has been shown that ruminal infusion of potassium in sheep resulted in a large decrease in magnesium absorption, but infusing potassium into the abomasum or ileum had no effect.

Magnesium is transported across the ruminal mucosa by an active sodium-linked process (172). In fact, Marten and Rayssiguier (177) suggested that dietary or salivary sodium deficiency decreases the sodium:potassium ratio in rumen fluid, which results in depressed magnesium absorption. They reported a fourfold increase in magnesium absorption when the sodium:potassium ratio in rumen fluid increased from 0.5 to 5.0. These studies utilized sheep where the rumens were emptied and then filled with buffer solutions containing different sodium:potassium ratios.

Marten et al. (176) reported that magnesium absorption in sheep increased from 22.3% to 34.5% when 2.3 grams sodium were added to a low-sodium, dried grass diet. The ruminal sodium:potassium ratio in this study increased from 0.9 to 5.5. These data suggest that inadequate salt supplementation may increase the susceptibility of animals to grass tetany. Swerczek (322) also observed that cattle with access to loose salt rarely have grass tetany. Certainly when there is a marginal magnesium deficiency, increasing the sodium:potassium ratio in the rumen fluid may be critical to maximizing the absorption of the magnesium that is available.

Another characteristic of diets causing grass tetany is that they are high in soluble protein and low in soluble carbohydrates. Marten and Rayssiguier (177) suggested that the imbalance between protein and carbohydrate in the rumen may lead to a deficiency of absorbable energy from the rumen. If volatile fatty acids and carbon dioxide production are depressed due to a lack of carbohydrate, blood flow to the rumen wall may be decreased resulting in low magnesium absorption. This may be a self-perpetuating phenomenon in that Ammerman et al. (171) showed that ruminal cellulose digestion and thus volatile fatty acid production was decreased by a magnesium deficiency.

Increased lipid or higher fatty acid concentrations in fertilized grasses may increase grass tetany. Magnesium and fatty acids may react to form magnesium soaps which are largely unavailable. Feeding peanut oil has been shown to depress plasma magnesium concentrations of grazing dairy cows (181). Although the level of lipids in most grasses is probably not high enough to cause grass tetany by itself, it is likely one of several nutritional factors in a cascade that, when summed, prompt the disease.

The ultimate goal of any prevention program should be to increase magnesium absorption so that blood levels of magnesium are maintained above 1.5 mg/100 ml. Because the quantity of magnesium that can be mobilized from body stores decreases with age, sufficient magnesium must be consumed on a regular basis. O’Kelley and Fontenot (179) showed that gestating beef cows required about 10 grams magnesium per day. Because magnesium is plentiful in milk, requirements more than doubled to approximately 22 grams during peak lactation.

Compounds used as supplemental magnesium sources include magnesium in the form of oxide, hydroxide, carbonate, sulfate, chloride, or as dolomite. Magnesium oxide contains the highest concentration of magnesium and has been used most commonly to prevent grass tetany.

Most magnesium compounds are unpalatable to ruminants and can not be fed successfully by themselves. Mixing the magnesium source with salt has some very important advantages. First, ruminants have an appetite for salt. Their desire for salt will increase intake of the magnesium source and insure that consumption will occur on a regular basis. Secondly, consumption of sodium and magnesium simultaneously may be critical to increasing magnesium absorption. As discussed previously, proper sodium:potassium ratio in the rumen may be the key to obtaining efficient utilization of supplemental magnesium. In addition, salt-magnesium mixtures can be self fed without the high-cost labor of hand-feeding a magnesium fortified supplement on a daily basis. Finally, grazing lush forages and lactation usually increases a ruminant’s appetite for salt which means that increased magnesium intake is likely to occur at the time of greatest need.

Adding a readily available carbohydrate source to the salt-magnesium supplement may also be beneficial. Frye et al. (173) found that adding dry molasses, ground corn, cottonseed meal or alfalfa meal to a salt-magnesium oxide supplement increased intake. The salt:magnesium oxide:carbohydrate sources were mixed in a 1:1:1 ratio. Palatability of the mixture is critical if lactating cows are going to consume the 22 grams of magnesium required per day. In addition, the availability in the mixture of readily fermentable carbohydrate sources such as molasses or corn may improve absorption of magnesium from the rumen by increasing volatile fatty acid production resulting in more blood flow to the rumen.

Salt-magnesium supplements should be placed where animals have easy access. In large pastures several mineral feeders should be located throughout the pasture to ensure ready availability. To minimize competition, one mineral feeder should be available for every 15 to 20 cows. If cows are going to be grazed on pasture that are tetany prone, feeding a salt-magnesium mixture a few days in advance will help animals adjust to the mixture and assure a more uniform intake.

Potential economic losses from grass tetany make prevention the cheapest insurance available. The salt-magnesium-carbohydrate supplements will only cost two to four dollars per head for the spring grazing season. In a 100 cow herd, preventing the loss of only one cow every three years would more than pay for the additional costs of supplementation.

Magnesium is important to all farm animals. Many horse scientists now recommend the addition of 5% magnesium oxide to salt for horses to protect against a possible magnesium deficiency. Dairy scientists are also investigating the use of magnesium oxide as one means of raising milk butterfat levels in cows fed high concentrate diets. High concentrate diets depress the fat level in milk. Therefore, magnesium supplementation, which at first was used only for beef cattle, is now used with dairy cattle, horses, sheep and other animals. An adequate and continuous supply of dietary magnesium is needed to prevent magnesium tetany, since animals have limited capacities to store and mobilize magnesium. More studies are needed on magnesium availability in feeds and the factors that affect it.