February 2016
Farm & Field

Magnesium and the Rumen

What You Need to Know to Prevent Grass Tetany

As we enter into winter and spring, many will start high magnesium supplementation to help prevent winter tetany and grass tetany. I think we’ve all heard over and over that cattle need extra magnesium to prevent tetany. But have you ever wondered why?

Why are cattle so susceptible?

Cattle are more susceptible to tetany than any other domesticated species. Ruminants in general are less efficient in absorbing magnesium than non-ruminants. Of domesticated ruminants, cattle are three times worse at absorbing magnesium than sheep or goats. The reason cattle are more likely to experience magnesium deficiency is that the primary site of absorption occurs in the rumen.

The body relies more on daily intake of magnesium than on body reserves. Circulating levels of magnesium in the system are strongly affected by daily feed intake. Fasting causes a rapid decline in serum magnesium levels. This is why transport and bad weather can be triggers for onset of tetany symptoms.

Rumen environment

The chemical environment of the rumen is crucial for magnesium uptake in cattle. Magnesium is absorbed by active transport across the cell wall against an electrochemical gradient. Think of it like trying to push a heavy box up an incline. Extra potassium in the rumen increases the potential difference (making that incline steeper), thus reducing magnesium absorption. In contrast, the presence of sodium in the rumen decreases the potential difference (making the incline less steep), making magnesium absorption easier. So, in essence, sodium enhances magnesium absorption while potassium blocks it.

Rumen pH is also important because the solubility of magnesium is highly dependent on pH. Magnesium must be dissolved in the rumen contents in order to be absorbed. Magnesium is most soluble at a slightly acidic pH. Excess nitrogen in the diet can negatively influence magnesium absorption by creation of excess ammonia that in turn raises rumen pH and allows less magnesium to dissolve in the rumen contents.


Colostrum contains up to three times the magnesium of "regular" milk, thus increasing magnesium needs drastically at calving. However, even though "regular" milk isn’t a rich source of magnesium, the concentration is not influenced by the dam’s diet. So lactation will continue to drain maternal reserves in situations of a magnesium-deficient diet or interference in absorption. On the flip side, enhanced magnesium intake doesn’t increase the amount of magnesium found in milk.

Onset of tetany

The rate of onset is dependent on the degree of deficiency. Lactating cows can experience rapid decline while undernourished, non-lactating cattle (of both sexes) can experience a slower development of symptoms. Symptoms include extreme excitability, muscle twitching, frequent urination, grinding of teeth, staggering, uncoordinated and stiff gait, and eventually collapse with convulsions with classic paddling. Death usually follows soon afterwards. Progression may be a matter of hours or may stretch out for several days.

Treatment and prevention

Animals exhibiting symptoms require immediate treatment. Because calcium deficiency often accompanies magnesium deficiency, IV administration of both magnesium and calcium is a common treatment. Consult your veterinarian for more detailed treatment information.

Prevention is best achieved with a combination of daily intake of a supplement containing sufficient magnesium and proper management. SWEETLIX high magnesium supplements will be properly balanced and will be sufficiently palatable to overcome the bitterness of the added magnesium oxide. Management wise, make sure cattle don’t lose too much weight or become malnourished. Don’t allow them to run out of hay or supplement. Try to plan calving to avoid the most inclement weather if possible.


Winter tetany or grass tetany can be costly to cattle producers and can be caused either by a physical deficiency of magnesium or a metabolic imbalance such as excessive potassium or nitrogen in the diet. Prevention is best achieved by a combination of smart management practices and daily intake of a magnesium-fortified supplement. SWEETLIX offers many high magnesium options to best meet your production needs. Contact your local Quality Co-op location or visit us on the web at www.sweetlix.com to find out which SWEETLIX Livestock Supplement will best suit your needs.

Jackie Nix is an animal nutritionist with Ridley Block Operations (www.sweetlix.com). You can contact her at This email address is being protected from spambots. You need JavaScript enabled to view it. or 1-800-325-1486 for questions or to learn more about SWEETLIX mineral and protein supplements for cattle, goats, horses, sheep and wildlife. References available upon request.