Supplementing cows with extra magnesium (Mg) in late pregnancy and early lactation has become routine on most farms since the 1970’s. Mg supplementation helps prevent animal health problems, such as milk fever and grass staggers (tetany), and potentially increases milk production.

Magnesium and grass staggers (grass tetany)

Mg plays an important role in nerve and muscle function and functioning of the immune system. Although cows have significant stores of Mg in the bones, little of these stores are available to maintain levels in the blood. Therefore, the cow is dependent on the Mg supplied in the diet and from supplements to maintain blood levels.  Blood and urine tests can confirm Mg deficiency. Consult your vet. 

The initial symptoms of Mg deficiency are nervousness, ears pricked, nostrils flaring, eyes alert and head held high. Movement is stiff, like a cow is walking on stilts, and she will stagger when forced to move quickly. Cows suffer loss of appetite and reduced milk production.  Death results from a “tetany”, where the muscles contract uncontrollably, including the heart.

Preventing Magnesium Deficiency

It is recommended that dry cows receive a diet containing 0.35 percent Mg, and lactating cows 0.28 percent Mg.

There are a number of different sources of magnesium, and methods of adding these into a cow’s diet. Common methods include drenching, pasture dusting, hay slurries, through water, and as magnesium bolus. Each method has its own limitations and advantages, so it is up to each farm how they choose to supplement their magnesium.

Factors that increase Mg requirements of cows during the winter/spring period are:

• Diets naturally low in Mg and/or high in potassium (K) e.g. pastures (low Mg or high K), maize silage or fodder beet (low Mg), paddocks with high potash or effluent (high K).
• Cold wet weather in spring, depressing grass growth and cow intakes.
• High cow demand for Mg over calving and early lactation, e.g. due to high milk production.

For more information refer to Magnesium supplementation (Farmfact 3-1)

Table 1. Dietary magnesium concentrations and quantity of supplementary Mg required (grams/cow/day) for different types of dairy cattle.

Table 2. Quantities of magnesium sources to supply the required amounts of pure magnesium (down the cow's throat). 

Note: If dusting magnesium oxide on pasture, the quantities above need to be at least doubled, possibly tripled, to allow for field losses. When mixing with feed, double the rates above.

Table 3. Amount of magnesium oxide dusted on pasture (g/cow/day).