Grass tetany and lactating ewes
MOST mid-season flocks are lambing or at the point of lambing, which puts pressure on farm labour and housing facilities.
Farmers at this stage will have already identified fields for early nitrogen, and will spread a light amount of nitrogen.
As lambing progresses and housing is stretched, it will be necessary to turn out ewes that have lambed to grass.
One of the biggest killers of ewes post-turnout is grass tetany or hypomagnesaemia.
So, if turning sheep out to grass, think of grass tetany, understand the factors that influence its occurrence and what steps can be taken to prevent this condition.
Grass tetany is an acute and often-fatal condition that affects lactating sheep at grass.
It is caused by a deficiency in blood magnesium levels that leads the animals to have convulsions followed by a coma and death. The main factors influencing the onset of grass tetany are bad weather conditions, especially very wet spells, which affect the utilization and intake of grass; and the ability of the ewe to absorb nutrients from that grass.
Rapid grass-growth coupled with high-soil potassium levels (applications in excess of 70kg/ha in spring time) affects the grasses ability to take up the magnesium. Ewe stress is another factor. Ewes that are under stress because of poor weather, disease, malnutrition, weaning, other causes will be at a higher risk if conditions, either climatic or nutritional, allow grass tetany to flourish.
Knowing the main factors which influence the onset of grass tetany requires vigilance in identifying affected animals.
Some of the more common symptoms include nervousness, staggering, twitching and, in the later stages, ewes may be found lying on the ground and kicking due to muscle tremors.
Where affected animals are detected early, treating them with magnesium sulphate solution is often successful.
How far the condition has progressed will largely determine the outcome of treatment.
In cases where treatment is delayed, the outcome is often not good and can lead to permanent brain damage and death.
Magnesium is not stored in the body, except in the bone where it is poorly mobilised.
That’s why it is important that the ewe gets adequate magnesium on a daily basis before and during risk periods.
There are numerous methods that can be used to supplement ewes with additional magnesium. On grass tetany prone farms, supplementation will be required during the months of March, April and May.
The most popular options for controlling grass tetany are meal-feeding and incorporating Cal-Mag into the meal at the rate of 10 grams per head per day.
This option can be expensive if you have adequate grass available and want to reduce meal feeding.
Pastures being dusted need to have at least 10cm of grass for the magnesium dust to stick to.
High magnesium buckets or blocks are generally successful, but during dry weather the blocks may need to be wetted in order to get satisfactory intakes.
Magnesium bullets are convenient, but can be regurgitated. Treating drinking water with magnesium is not recommended as sheep drink little or no water when they are most at risk. Losing a ewe to grass tetany that was otherwise perfectly healthy and prolific is a financial loss, and you are also left with one or two lambs with no mother. So understand grass tetany, and make sure to know when your flock is at risk.