Over the last two weeks at least a dozen cattle producers on the Central Tablelands have reported either to me or to veterinary colleagues that they have found mature cows dead in the paddock.
In one case a stockowner found six of 50 Angus cows from one mob dead and the owner of a small herd has lost half their cows.
Others have only lost one to two cows.
Almost all cases have occurred in mature cows with calves at foot running on lush grass-based pastures.
In some cases, the deaths have occurred in cows with spring calves of weanable age. On one property I saw a cow just before she died.
She was on her side, non-responsive with extended trembling legs and whole-body tremors.
As a district veterinarian, treatment is not my role but the farmer had a flow pack on hand. Our intravenous and subcutaneous treatment was not successful.
However, I collected blood from the cow first.
I also necropsied and collected eye fluid from another cow that died overnight.
Both had very low magnesium levels consistent with hypomagnesaemia or grass tetany.
Two producers that suffered losses described seeing jittery, agitated cows that died before they could treat them.
Another farmer told me that he noticed a couple of cows that started to shake and appear nervous as he held the mob in the yards to wean the calves.
He successfully treated both these cows with calcium and magnesium under the skin.
On the Central Tablelands, grass tetany is the most common cause of death in mature cows with calves at foot in the winter and spring.
We expect it in cows with two to eight-week-old calves on short grassy pastures, during or after cold miserable weather.
But we don't expect grass tetany in the autumn when the weather is sunny and you can hear the grass growing.
We also don't expect it when cows have big, seven-to-eight-month-old calves at foot.
However, I was speaking to a colleague from central Victoria recently.
He told me that they regularly see grass tetany in the autumn in cows with calves at foot.
I recall that we had similar reports in the autumn of 2016.
My take is that we should be prepared for it when we enjoy exceptional autumns with abundant grass pastures.
These pastures may have depleted magnesium but may also be rich in potassium which blocks magnesium absorption across the rumen wall.
Cattle producers on the Central Tablelands are well versed in preventing grass tetany once a diagnosis is made.
Some will wean those big spring calves and this should solve the problem.
But we have seen cows dying immediately after weaning presumably because they are agitated and not eating as they call for their calves.
Based on these experiences I have advised cattle producers about to wean calves from at-risk cows to have magnesium flow packs on hand and to feed hay, preferably with Causmag added or to use a Causmag, salt or commercial mineral mix before and after weaning.
- Bruce Watt is the LLS Central Tablelands senior district vet.