CATTLE are more susceptible to tetany than any other domesticated animal, partly because ruminants 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 that 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.
The chemical environment of the rumen environment is crucial for magnesium uptake in cattle. Magnesium is absorbed by active transport across the cell wall against an electrochemical gradient.
Extra potassium in the rumen increases the potential difference, thus reducing magnesium absorption. In contrast, the presence of sodium in the rumen decreases the potential difference, 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 which 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 cows diet.
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 soon follows. Progression may be a matter of hours or may stretch out for several days.
Treatment and prevention
Animals exhibiting symptoms are required to be addressed immediately. Because calcium deficiency often accompanies magnesium deficiency, administration of both magnesium and calcium is a common treatment.
Prevention is best achieved with a combination of daily intake of a palatable supplement containing sufficient magnesium and proper management. Management wise, make sure that cattle don't lose too much weight or become malnourished. Also don't allow them to run out of hay or supplement.
- Toby Doak is a ruminant nutrition specialist with Alltech Lienert Australia.
Want daily news highlights delivered to your inbox? Sign up to the Queensland Country Life newsletter below.
Sign up for our newsletter to stay up to date.