The hidden impact of magnesium deficiency in cattle
Magnesium is essential for neuromuscular function, enzyme activity, and metabolic regulation in cattle. Lactating cows need around 60g of magnesium per day. Unlike calcium, blood magnesium levels cannot be tightly regulated hormonally and therefore rely heavily on daily intake and absorption efficiency.
When blood magnesium (Mg) levels fall below 1.2Mg/dL, animals may suffer from hypomagnesemia, leading to muscle tremors, convulsions, and, in severe cases, sudden death. This condition is especially critical in older and lactating cows. Research indicates that up to 30% of cattle affected by severe hypomagnesemia die before treatment can be administered, underscoring the urgent need for proactive prevention strategies.
Magnesium’s roles and deficiency consequences
Grass tetany is a specific form of hypomagnesemia, typically seen in the spring when lush pasture is high in potassium (K), which inhibits both passive and active magnesium absorption in the rumen. Excess nitrogen further reduces availability by forming insoluble complexes. Under these conditions, magnesium absorption may decrease by 20-50%, even when dietary supply appears adequate, leading to acute symptoms that are often fatal.
Hypocalcemia (low blood calcium), seen at calving and during lactation, is another metabolic disorder frequently seen when cows experience hypomagnesemia. Milk fever at calving is associated with a sudden calcium demand for colostrum with a drop in blood calcium. During lactation, gradual calcium depletion may occur when intake or mobilisation is insufficient. Low magnesium levels can impair calcium metabolism by reducing parathyroid hormone (PTH) responsiveness and vitamin D activation, increasing the risk of hypocalcemia.
Although distinctly different, grass tetany and hypocalcemia are closely connected. Low magnesium status not only increases the risk of tetany but also undermines calcium regulation and neuromuscular function. Research indicates that increasing magnesium intake from 0.3 to 0.4% of dietary dry matter can reduce the incidence of milk fever by up to 64%, highlighting that magnesium management is key, not only to preventing tetany but also to reducing calcium-related disorders.
Key prevention strategies
Prevention is by far the most effective approach. The most reliable strategy involves ensuring a consistent supply of magnesium, particularly during high-risk periods such as early spring grazing. Magnesium oxide (MgO) supplementation is widely recognised as one of the most effective ways to prevent hypomagnesemia, as magnesium oxide brings the highest ratio between magnesium content and magnesium bioavailability. To optimise magnesium supply, producers can include highly bioavailable magnesium-based solutions in the rations, such as the specific blend of magnesium oxide developed by Terresis Agriculture.
In addition to supplementation, modifying pasture composition may help reduce the risk of hypomagnesemia. Incorporating legumes such as clover or alfalfa into grazing areas has been shown to increase dietary magnesium intake, as these forages typically contain higher magnesium concentrations than grasses.
Conclusion
The combination of high potassium, high nitrogen, and low magnesium availability creates a perfect storm for hypomagnesemia, particularly in lactating cows with high metabolic demands. By incorporating the right magnesium-based solution into mineral programmes, adjusting pasture composition to include magnesium-rich forages, and managing potassium levels in the diet, producers can significantly reduce the incidence magnesium deficiency.
