Magnesium supplementation is used to prevent two conditions in cattle: grass staggers (hypomagnesaemia) and milk fever (hypocalcaemia).
While dairy animals are more susceptible to classic milk fever due to their high milk production, beef cattle can also be affected, showing signs of lethargy, agitation, muscle tremors and then going down. Older cattle (over 8 years) are at higher risk of milk fever.
While milk fever is primarily a calcium deficiency, we don’t give calcium to cattle directly as large amounts are needed daily, and we want cattle to mobilise calcium from their own bone stores, or increase absorption from the gut. Magnesium is needed to produce hormones that are important for the absorption of calcium from the gut and the mobilisation of calcium from bones.
Magnesium isn’t stored like other minerals, so daily intake is important.
How can we supplement magnesium?
- Pasture dusting with magnesium oxide (causmag) or supplying causmag to fed hay or silage pre- and post-calving, are methods that can work well, but pasture dusting is really restricted to calving cattle behind a wire or in small paddocks, and is challenging in winter. This is the preferred method by many dairy farmers.
- Magnesium added to trough water as sulphate or chloride may be an option in some situations where water is mixed and standing water is removed. The taste puts some cows off and when combined with a wet period may reduce water intake dramatically.
- Another option is to supplement with a magnesium bolus. Rumetrace magnesium capsules provide nine to twelve weeks of protection. They require one week to begin dissolving so they are typically given two to three weeks pre-calving. A second capsule can be given in older animals, as often the end of the 9 weeks may coincide with peak milk production and demand. Capsules give great peace of mind but can be challenging to administer.
As magnesium cannot be stored by the cow, magnesium supplementation must begin at least two to three weeks before calving and continue daily until peak milk, realistically six to eight weeks after calving. The length of supplementation will depend on your calving spread and the season, as wet weather increases magnesium demand.
Other resources you might find helpful:
Common Metabolic Down Cow conditions and Treatment plans.pdf
