Magnesium

Why supplement cows with Mg?

Magnesium (Mg) deficiency in dairy cows was first recognised in New Zealand in the 1970’s. Since then, supplementing with Mg in late pregnancy and early lactation has become routine on most farms. Magnesium supplementation helps prevent animal health problems, such as milk fever and grass staggers (tetany), and potentially increases milk production.

Mg and milk fever

Magnesium plays an important role in milk fever prevention (calcium deficiency). Magnesium is required for the production of hormones that aid absorption of calcium (Ca) from the gut, and mobilisation of Ca from bones. Supplementing with Mg for two to three weeks pre-calving will reduce the risk of milk fever. However, it does not build up a store of Mg in the cow.

Mg and grass staggers

Magnesium also 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, or hypomagnesaemia, are nervousness, ears pricked, nostrils flaring, eyes alert and head held high. Movement is stiff, like they are walking on stilts, and cows 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.

Tips for preventing metabolic issues

• Supplementing with magnesium will reduce the risk of milk fever and grass staggers (grass tetany)
• Supplement with magnesium daily for two to three weeks pre-calving until after spring pasture growth rates have slowed (around December)
• It can be difficult to supply cows with enough magnesium just through the water supply, combine magnesium through the water supply with magnesium dusted daily onto pasture or silage
• Dietary requirements of magnesium will vary due to cow liveweight (dependent on breed) for dose rates of magnesium from different sources to meet daily requirements)
• After calving, provide 150g/cow/day ground limestone (lime flour) daily during the colostrum period- double this rate if dusting; can also consider calcium-enriched molasses.
• Springer cows should not be over-fed. Cows that are at or above BCS targets should be fed at 90% of requirements for 2 to 3 weeks pre-calving, while cows below BCS targets should be fed at 100% of requirements.
• Consider using starter drenches on high-risk cows at calving - 7+ year olds, or very fat, or with a history of metabolic problems.

Mg requirements

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

Cows that are at or above BCS targets should be fed at 90% of requirements for 2 to 3 weeks pre-calving, while cows that are below BCS targets should be fed at 100% of requirements.

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.

Magnesium supplementation and Salmonellosis

There have been reports from New Zealand and Australia of Salmonella infections occurring in dairy cows that had received Mg supplementation, delivered as granules, prills, pellets, powder, and via the drinking water.

Before starting Mg supplementation of your dairy herd, consult your veterinarian to discuss the potential risks and to determine if any health or environmental factors should be taken into consideration to reduce the risk of the development of Salmonellosis. If Salmonellosis does occur in a herd being supplemented with Mg, farmers should notify their veterinarian and their milk processor.