Grass staggers (hypomagnesaemia) in dairy cows

Hypomagnesaemia (low blood magnesium), often known as grass staggers or grass tetany, is a condition caused by insufficient magnesium intake. It’s also a significant risk for the development of clinical or subclinical hypocalcaemia (milk fever).

Magnesium is a vital component of metabolic events. It’s essential for nerve and muscle function, as well as bone formation.

The estimated mean annual incidence of grass staggers is less than 1%.

Cases often occur in spring-calving beef suckler cows, but they can also occur in dairy cows, especially if they’re not supplemented during the dry period.

Subclinical disease often goes unrecognised, but investigations have revealed an annual rate of 3–4% in lactating dairy cows.

Cows are almost entirely dependent on a continual daily dietary intake of magnesium as there’s only a small amount available in the body.

When the diet contains less than 0.4% magnesium (DM basis), or when dry matter intake is low, cows can develop grass staggers.

• Low magnesium in the diet
• Low dry matter intake
• Higher rumen pH, which reduces the solubility of magnesium, making it less absorbable
• Forages with high potassium content and those heavily fertilised with potash lead to rumen pH increases
• High ammonia levels from nitrogen fertilisers can reduce magnesium absorption
• High dietary potassium can reduce magnesium absorption in the rumen
• Very low dietary sodium may reduce magnesium uptake in the rumen
• Lush, wet pastures increase the rate of passage through the rumen, reducing magnesium absorption
• Some unsaturated fatty acids (linoleic, linolenic acids) can form insoluble salts of magnesium and reduce its absorption
• Older cows absorb less magnesium

Normal magnesium levels should be 0.8–1.0 mmol/L. Blood levels less than 0.8 mmol/L are associated with clinical signs of grass staggers.

Due to the high risk of culling in affected individuals and the possibility for large groups to be affected, grass staggers can be costly.

The most common signs are acute disease or sudden death, making this a veterinary emergency and must be treated immediately.

In many instances, magnesium levels can decline slowly over weeks without clinical disease.

Grass staggers does not correlate well with clinical signs. The signs can often be seen as either a single affected individual or outbreaks of disease in large groups.

Signs may include:

• Reduced dry matter intake
• Nervousness
• Reluctance to be milked or herded
• Lower milk fat and a drop in yield
• Uncoordinated movement
• Hyperexcitability
• Muscle spasms and seizures (chomping of jaws and frothy salivation)

In the final stages, cattle are unable to stand before death.

Magnesium plays a major role in the control of blood calcium. More than 75% of milk fever cases seen in mid-lactation are due to low magnesium intake.

Prevention depends on appropriate supplementation of all groups by reviewing dietary levels. Consider seasonal risk periods, i.e. post turnout, especially during periods of rapid grass growth.

Check mineral levels in diets and forages every six months. Soil test results can identify any deficiencies in the grazing platform that may affect nutrient uptake.

Recommended dietary levels:

• Close-up dry cows: 0.39–0.5% DM. Use the higher end when dietary potassium is above 1.5%
• Lactating cows: 0.25–0.3% DM

Choose the best way that works for your system:

• Free minerals and licks
• Compound feeds
• Boluses
• Water supplementation using magnesium chloride flakes (250 g per 100 L for lactating cows; do not exceed this or water may become unpalatable)
• Pasture dressing every 10–14 days using calcined magnesite

Different sources contain different levels of magnesium.

Magnesium sulphate (MgSO4) and magnesium chloride (MgCl2) contain around 9% and 12–18% magnesium, respectively.

In some cases, magnesium salts might need to be combined with molasses, concentrates or hay to make them more palatable.

Magnesium oxide (MgO), known as calcined magnesite or cal mag, contains around 55% magnesium and can be fed in smaller amounts. It’s a palatable and effective rumen buffer.

Sample a representative group of cows:

• Within 12 hours of calving to check magnesium status
• In early lactation to confirm the diet is adequate

Aim for 0.8 mmol/L or above. Low levels could indicate a lack of magnesium in the diet or poor absorption.

Review the magnesium content in the diet on a regular basis, especially during high-risk periods.

• This is an emergency and requires immediate treatment by a vet
• Do not encourage rising for at least 30 minutes after treatment to avoid the stimulation of tetany (muscular spasms) and convulsions
• Recovery will often be achieved in 1 hour if successful, but relapses are common within 12 hours
• When the cow can reach a stable sternal recumbency (lying on her breast bone), give oral magnesium to support longer-lasting magnesium levels
• Offer feed to encourage intake and prevent further relapse

• One case often suggests a wider issue in the group. Consider wider testing and supplementation
• Work with your vet to adopt preventative strategies and assess risk periods during the year