I have had a very busy month already and still 2 weeks left, grass tetany has really kicked in with the onset of cold weather. The following article is a good start to understanding magnesium shortfalls and particularly tetany. In the next edition I will add a follow up with strategies on how to avoid this insidious annual problem.
Grass Tetany is a disorder where the level of Magnesium in the cerebrospinal fluid, which surrounds the brain and spinal cord, drops below a critical level. In the development of grass tetany, the level of magnesium in the blood drops or decreases before the levels in the cerebrospinal fluid. Low levels of blood Calcium are also usually associated with low levels of blood magnesium, or hypomagnesaemia particularly in late pregnant stock and cows with calf at foot. These low levels of calcium and magnesium have an affect on muscle function to the point where the animal can die of suffocation because it cannot breathe.
Grass tetany is not always a simple deficiency of magnesium, the disorder can be quite complex and different circumstances can lead to reductions in blood and cerebrospinal magnesium levels.
In it’s simple form the problem is a lack of magnesium, in it’s more complicated form potassium can interfere with magnesium absorption in the rumen. Potassium concentrations in the rumen can increase when,
For most farmers the first sign of problems are usually death at the onset of grass tetany outbreaks, typically there is froth from the mouth and nose, and the ground around the animal is roughed up as though the animal has been trying to dig a hole. Excitement and muscular spasms are most common and indicative of this disorder.
Low magnesium intake can come from young grass and cereals, as they usually have less magnesium than more mature grasses and crops, grasses have lower magnesium than clovers and other legumes including Lucerne. This means that risk factors are high when grazing grass dominant paddocks or cereal crops especially when the plant is immature and quickly growing. Spraying for broadleaf weeds can also contribute to grass dominance as can heavy infestations of red legged earth mite on clover based pastures.
Heavy nitrogen or potassium based fertiliser application will reduce the amount of magnesium available to the plant from the soil and so reduce the plant magnesium levels. So, anything that reduces feed intake at this time, reduces magnesium intake ,bad weather, yarding, mustering, poor pasture management etc all contribute to grass tetany.
The timing of outbreak is usually confined to Autumn/Winter because,
Tetany prone pastures have low:
Magnesium concentrations, usually < 2g/kg dry matter
Calcium concentrations <3 g/kg dry matter
Sodium concentrations < 1.5 g/kg dry matter
And high levels of;
Potassium . >20g/kg
Clovers tend to contain higher amounts of magnesium and Calcium, so, if supplementing hay to these animals grazing grass dominant pastures or cereals, use a legume based hay as well as magnesium supplement.
Phosphorous is also vitally important, if animals are grazing low P pastures then magnesium absorption can also be hindered.
Compass Feeds has a product, Tetmag which has elevated levels of Calcium, Magnesium, Sodium, Phosphorous etc to compliment the nutrient shortfall in grass dominant pastures. Tetmag is very easy to feed because it is an adlib product, which is fed in troughs or other containers strategically placed near hay supply. Cattle usage or consumption is 60-100 grams/head/day and sheep require 10/20 grams/head/day. Remember that protection stops when supplementation stops.