GRASS TETANY: A Hidden Risk as Pastures Take Off

Nathalia Pamplona Barboza
3 hours ago
2 min read

By Nathalia Pamplona Barboza, TPM Sales & TRAC Productivity Consultant

Winter pastures are finally moving with warmer days and more sunshine. Ryegrass and cereals are jumping away, and while it’s good to see feed in front of stock, this rapid flush of growth also brings a familiar challenge: Grass tetany.

What is grass tetany?

Grass tetany (hypomagnesaemia) occurs when a cow’s magnesium intake or absorption drops below her daily needs. Magnesium can’t be mobilised quickly from the skeleton, so animals rely on a constant supply from the diet, absorbed through the rumen wall.

Lush, fast-growing feed is the classic risk because it is:

Low in fibre → feed races through the rumen.
High in potassium and nitrogen → reduces magnesium absorption.
High in water → dilutes nutrients further.

Why 2025 is a high-risk year

Because the autumn break came late, stock are now hitting fresh feed harder and faster than usual. With high fertiliser use and rapid pasture growth, conditions are primed for magnesium deficiency. Older cows (over 5 years) are especially vulnerable.

What to look for in the paddock

Restless or aggressive behaviour.
Muscle tremors, staggering, difficulty standing.
Sudden deaths with no warning.

Grass tetany can easily be confused with perennial ryegrass toxicity (PRGT), but the cause is very different. PRGT comes from plant toxins, while grass tetany is purely low blood magnesium.

Why prevention pays

Grass tetany strikes quickly — often the first sign is a dead cow. Losing one breeder at calving can wipe out the profit from several others. Prevention is always cheaper than cure.

How to manage the risk

Graze later: Aim for at least 2.5 leaves on ryegrass before grazing.
Transition gradually: If moving cows from confinement to pasture, take 10–14 days.
Avoid stress: Limit long walks and yarding during risk periods.
Provide magnesium every day: The most reliable option is a high-magnesium loose lick like our TPM Boost HMA.

How TRAC can help

TRAC Performance Minerals provides high-magnesium loose licks formulated for local conditions. Backed by our ruminant consultants, they’re simple to use and proven to help farmers manage grass tetany risk in seasons like this one.

If you have any questions, please do not hesitate to talk with one of our TRAC Ruminant Experts on 08 8733 1888 or email us at info@totalresult.com.au.