The high-risk period for tetany and lactating ewes

ONE of the biggest killers of ewes post-turnout is grass tetany. So if turning sheep out to grass, be aware, understand the factors that influence grass tetany’s occurrence and what steps can be taken to prevent it.

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, which leads sheep to have convulsions followed by a coma and death. The main factors influencing the onset of grass tetany are adverse weather conditions, especially very wet spells which affect the utilisation and intake of grass and the ability of the ewe to absorb nutrients from grass. Farms located on more exposed sites, therefore have higher risk.

Rapid grass growth coupled with high soil potassium levels affects the grasses ability to take up magnesium from the soil. Ewe stress is another factor; ewes that are under stress for any particular reason will be at a higher risk if conditions either climatic or nutritional favour grass tetany.

Knowing the factors which influence the occurrence of grass tetany needs 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 sheep are detected early, treating them with 100 ml of magnesium sulphate solution in five/six different locations around the body is often successful. There are numerous methods that can be used to supplement ewes with additional magnesium during the months of March, April and May.

Ewes need a daily supply of magnesium; it is not something they can store in their body except in the bone, where it’s poorly mobilised. Generally ewes need one to two grams of magnesium per head per day. Under adverse weather conditions or when ewes are stressed, utilisation of grass will be poor, and the ability of the ewe to take up correct levels of magnesium will be halved.

So it is recommended during periods of high stress and poor utilisation of grass that ewes intake three to five grams of magnesium daily. Popular options for supplementation and controlling grass tetany are meal feeding and incorporating Cal-Mag into the meal. If you know the weather is going to get bad, feed your ewes some meal a few days in advance. This option can be expensive if you have adequate grass available and want to reduce meal feeding. Pastures being dusted with Cal-Mag need at least 10 cm of grass for the magnesium dust to stick to. High magnesium buckets or blocks are generally successful with three to four buckets per grazing group. But during dry weather the blocks may need to be wet in order to get satisfactory intakes. Magnesium bullets are convenient but can be regurgitated. Drinking water can be treated with magnesium, however in bad weather sheep obtain a lot of their water requirement from grazing grass.

Losing an otherwise healthy, prolific ewe to grass tetany can be prevented by supplementing magnesium during high-risk