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Magnesium Supplementation For Cows

26 March 2019

It is that time of year again to remember about magnesium supplementation.  Magnesium is not stored and a daily dietary intake is required and if output exceeds intake then cows are at risk of staggers. Lactating spring calvers are most at risk due to the demands of milk production.  Lush, fast growing grass doesn’t have as much magnesium in it and has a high rate of passage so less time for absorption.  High potash levels in grazing grass can also disrupt magnesium absorption for example in fields that have had heavy slurry or dung applications.  Target intake is around 25-30g of magnesium/cow/day.

Below are methods of supplementation to supply the daily requirements:

  1. Hi Mag rolls, normally 1kg supplies daily Magnesium requirement (check with supplier) these are easy fed on the ground. 1kg will also supply around 10 MJ of energy which is important when grass supplies are short.
  2. Mineralising your own cereals, cheaper but need to account for wastage when feeding on the ground and possibility losing mineral on the ground – 100g/head of a 25% magnesium mineral required.
  3. Liquid molasses fortified with magnesium – harder to regulate intakes
  4. Hi Mag buckets or free access mineral – aim around 20% mag in buckets and 25% mag for powdered minerals. Downside is you are relying on all cows taking the mineral – ensure good access to minerals/enough buckets are put out for the number of cows.
  5. Slow release Magnesium bolus, lasts 4 weeks (1 application, 2 boluses/cow)
  6. Treating water supply – not as effective at grass, takes managing, shouldn’t be relied on.

Points to note:

  1. Lactating cows are most at risk and cows under nutritional stress i.e from lack of grass.
  2. The less stress the better – if grass is below 6cm cows must be supplemented for energy as well as for magnesium.
  3. Ensure good magnesium supplementation a week before cows are turned out.
  4. Turn cows out with a full belly and if possible on a good day.

Karen Stewart,

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