GRASS TETANY – HYPOMAGNESAEMIA
Grass tetany is a condition in lactating cattle resulting from insufficient Magnesium. The usual finding is simply usually dead cattle usually on a cold frosty morning. . There are 3 phases to the symptoms. 1) an uncharacteristic ‘stirry’ or aggressive behavior making them difficult to handle, with foam from the mouth and nose 2) short but violent convulsive period before 3) death. The phases may be brought on or advanced by any excitement – handling, feeding out etc.
Grass tetany is a production disease – it occurs in high producing animals – lactating cows, usually in 5 to 6 year olds (maximum milk producing age.), and usually 6 weeks post calving (maximum stage of lactation)
Blood magnesium levels fluctuate throughout the day depending on feeding and milk production. The cows stop eating through the night, and levels fall – so most cases occur in the mornings. Many cow herds may have borderline blood levels of magnesium in early winter and all it takes is one cold morning to tip a few over to the too low stage.
Grasses contain lower levels of magnesium than broadleaf plants, especially when newly shooting. Beware on newly sewn pastures in early winter! Winter ’08 and ‘09 were particularly bad for this problem.
Grass tetany is not always simply due to low magnesium. Like other conditions, there is a complex interrelationship between different elements. These must also be considered. Sodium, or more particularly the sodium: potassium ratio, is of critical importance. Sodium is found in common salt. If there is insufficient sodium, magnesium cannot be absorbed into the bloodstream. Recent use of potassium (potash) or nitrogenous fertilizers will depress magnesium availability. “Red Gum” soils are naturally higher in potassium, so therefore present a greater risk.
Concurrent low calcium (sub-clinical Milk Fever) will exacerbate low blood levels of magnesium, but can also mask the ‘typical’ aggressive signs of the low magnesium. Low calcium results in poor muscle contraction and a depressed mental state and hence cows don’t always convulse and appear as aggressive.
- Hay feeding – this does not need to be high quality for the purpose of aiding magnesium levels (it does for overall nutrition or course!). Beware some brought in hay maybe high in potassium hence having an undesired affect on the sodium: potassium ratio which exacerbates the problem rather than solving it. Check the history of the farm if you are not sure, or get the hay tested.
- Causmag – magnesium oxide powder – at 50 to 60 grams per head per day – mixed with water and sprayed on hay. This is a very messy and tedious job. Causmag is not very palatable, so it needs to be applied to highly palatable fodder.
- Magnesium blocks –intake is very irregular between animals.
- Magnesium capsules – last 90 – 100 days @ about $18.00 per head (plus the applicator), to be administrated two weeks before the anticipated problem period.
- Salt (note – NOT the same as either the above-mentioned Magnesium blocks, or Epsom salt) – Sodium Chloride, just the same as table salt. Aims to improve the sodium: potassium ratio and increase the absorption of available Magnesium. Warning – this mechanism is not widely known by many resellers and some clients have reported they are talked into taking Magnesium Sulphate (Epsom Salts) instead – forget it!
- Magnesium Sulphate (espsom salts) or magnesium hydroxide have poor gut absorption and are therefore useless.
Hay – get the point?
+ salt with either
(Epsom salts) orally
On occasion, cattle will be found down, affected to varying degrees.
- Minimize stimulation to the affected cow – only go near her to treat her, and if at all practicable, move the remainder of the herd away – but don’t stir the up unless you want another case!
- First Aid – administer 1-2 packs each of
A) a “4 in 1” preparation such as Vetcal® or Calcigol® and
B) Magnesium Sulphate solution, both preparations given under the skin, then massaged out to aid absorption.
- Leave her alone again!
- If the cow is beyond just being down and jittery, and is convulsing, expect the worse. On rare occasions, such cattle will respond to intravenous therapy, but intravenous magnesium has a fairly high incidence causing acute heart failure.
- For a “pure” grass tetany case, expect recovery over one to several hours. Cases resulting from a combination of milk fever (low calcium) and grass tetany will often respond within the hour, as the hypocalcaemia component recovers quickly.
Don’t ignore the messenger!
Remember that the first affected cow is telling you the remainder is in trouble – commence prevention management. The ‘iceberg’ effect.
Differential Diagnoses – What else could it be?
Irritable, uncoordinated cattle
□ Ketosis/Acetonaemia – usually pre calving or immediately after calving.
□ Phalaris staggers – in phalaris present, are any “dry” cattle affected”
□ other toxins – access to toxic weeds, fermented grain, vineyard waste etc.
□ Acetonaemia – always pre calving
□ Milk Fever – usually are dopey rather that excited – look for neck bent around to flank, dry nose, no faeces passed.
□ Physical injury,
Cows found dead
□ Entertoxaemia/pulpy kidney – each year, several (different each year) properties lose 1-6 adult cows. Look for rapid decomposition, bloat after death, blood stained foam or discharge from orifices. Prevented by annual vaccination with 5 in 1.
□ Bloat – usually on pastures not associated with grass tetany e.g. high legumes. Bloat recedes after death.
Post Mortem Samples – grass tetany and milk fever may be diagnosed up to 24 hours post death by analysis of the aqueous humor – fluid from the eye. Discuss collection with us.