Ground conditions have improved on farms, with drier land increasing the number of cows with calves going out to grass.

Settled weather has also seen farmers spreading fertiliser on silage and grazing swards to boost growth rates.

Both factors will increase the risk of lactating cows developing grass tetany, so make sure to keep on top of magnesium supplementation.


Tetany risks increase when cows graze lush, fast-growing swards which are low in fibre.

Cows under physical stress are particularly vulnerable, such as first-calved heifers and cows suckling twins.

Grazing swards that recently received slurry or compound fertilisers high in potash (K) will also increase that risk, as will changeable weather conditions.

Potash blocks magnesium uptake by grass, thereby leaving animals deficient and prone to taking staggers.


There are various options for supplementing magnesium, all of which have merits and drawbacks. Ultimately, it comes down to whatever is most practical and cost effective for your herd.

If using lick buckets, use one bucket per 10 cows in a grazing group. Not all cows will use buckets, so keep a close eye on animals when herding every day.

When using magnesium in water troughs, on wet days, cows at grass will drink less and can be under-supplied. Therefore, sometimes a second form of supplementation works well.

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