Magnesium Supplements for Cows

Phil Rogers MRCVS <>
Grange Research Centre, Dunsany, Co. Meath, Ireland
Friday February 20, 2004

Hypomagnesaemia | Mg supplements (Prepartum | Postpartum | Tetany-control) | Water medication | Mg excess | Free-access | Check Mg content of supplements

Hypomagnesaemia: Low blood Mg may decrease appetite, milk yield and possibly fertility (via reduced DM intake). It increases the risk of tetany and restricts Ca release from bone, increasing the risk of milk fever, dystocia, retained placenta, prolapsed uterus.


Mg supplements Details
Prepartum Mg supplements Feed 10-20 g Mg/cow/d (say 15 g) for 4-6 weeks before calving to and ensure high turnover of bone Ca and reduce the risk of milk fever and other problems at calving.
Postpartum Mg supplements Feed 10-20 (say 15) g Mg/cow/d to lactating cows on silage + concentrate to reduce the risk of decreased feed intake and milk yield.
Tetany-control supplements Feed 20-40 (say 30) g Mg/cow/d to lactating cows in the tetany season to reduce the risk of tetany. In very severe challenge (for example very lush grass with very high N, K and fatty acid levels), up to 60 g Mg (4 oz of calcined magnesite)/cow/d may be needed to ensure normal blood Mg levels and help to maintain appetite. For best control, the Mg should be included in the concentrate supplement or in a small amount of palatable carrier (say 0.75-1.50 kg). The better Co-Ops supply about 40 g Mg/cow/d in a specified amount (0.5, 1, 2, 3 or 4 kg) of their special Dairy Summer Feeds.
Water medication One may add 15-30 g Mg, as soluble Mg salts (especially 150-300 g/cow/d of hydrated Mg chloride (10% Mg) or 60-120 g/cow/d of dehydrated Mg chloride (25% Mg)) to the trough water. This is best done by simple plastic drums with a special nozzle.

Excessive intakes (usually >250 g/cow/d) of Epsom Salts (Mg sulphate, 10% Mg) via water can reduce water intake, and cause scouring and reduced milk yield.

Free-access (free-choice) systems DIY mix of 50:50 molasses:calcined magnesite. DIY mixes, or commercial high-Mg blocks or licks etc are less reliable than fixed-rate feeding of minerals (in feed, on feed, or in the water supply).
Mg excess High Mg intake in cows may cause scouring and drop in milk yield. Drystock, especially young castrated males, may develop urinary calculi and obstruction if high Mg levels are fed.
Check the Mg content of Mg supplements Commercial feeds and mineral supplements do not always provide the intended (or stated) amount of Mg. If there is any doubt, samples of the Mg supplement can be tested at Grange. In that case, we need to know the daily allowance of supplement also.