Grass Tetany
The rain of the past few weeks have caused a flush of green grass everywhere. It has coincided with the arrival of a crop of new calves throughout the district.
Though wonderful to see, this grazing is not without its risks. Cows with autumn calves can develop grass tetany on new plant growth of grass and cereal crops.
Grass tetany is an often fatal metabolic disorder of cattle. It can occur anywhere, but is prevalent on the northern, central and southern tablelands and slopes of NSW.
Although it is primarily a deficiency of magnesium, it is a complex condition. Magnesium is an important component of the cerebrospinal fluid which surrounds the brain and spinal cord and it is essential for the transmission of nerve impulses. Magnesium cannot be stored in the body and is constantly being secreted in faeces, urine and milk. Cows in milk are especially at risk. Calcium levels also play a role in the expression of symptoms.
Symptoms:
Often farmers will be unaware of a problem until they find dead cows, with froth from the nose and mouth, and signs of a struggle before death.
In mild forms of hypomagnesemia, cows may show no symptoms until they are stressed by cold, oestrus or mustering for example, when symptoms may be seen. Early symptoms include twitching of the face and ears and face, a nervous, jumpy appearance and a stiff gait. In the next phase the cow is wild, her front legs are lifted too high in a goose stepping motion, the tail is held higher than normal and she may appear blind. This progresses to excitement, bellowing, galloping and staggering. The cow goes down on her side with stiff, outstretched legs. She paddles violently, her head arches back slightly and she froths at the mouth. The cow may die within minutes.
Low magnesium levels are also implicated in lazy calving syndrome.
Treatment:
There is often not enough time to call a veterinarian, but if one arrives in time, he/she will inject a calcium and magnesium compound intravenously, followed by subcutaneous magnesium. Intravenous injections require caution as giving it too fast, or giving too much, will kill the cow.
If you treat the cow yourself, do not disturb the cow until you are ready to start. Inject one or two plastic bags of commercially available calcium and magnesium solution under the skin, followed by 60 to 100 ml of 50% magnesium sulfate under the skin. Use sterile equipment. Inject each bag into a different site. If possible warm the bags to body heat in a bucket of hot water before use.
After recovery, feed the cow sixty grams of MgO (Causmag) daily.
Prevention:
The most effective supplement is hay treated with magnesium oxide. (Causmag) Daily Causmag requirement is 60 gram per cow per day. It takes two to three days before cows are protected. If supplementation is stopped, the protection ceases too. Excess Causmag can cause scouring.
To treat hay, a slurry is made of Causmag.
Causmag 6kg
Water 2L
Molasses 3kg
This will keep for 10 days.
The prepared slurry is poured onto biscuits or rolled out hay in sufficient quantities to ensure that each adult animal will consume 60g of Causmag daily.
(For other feeding methods, see DPI Primefact 421)
Causes:
Young grass and cereals have lower magnesium levels than older grass and cereals.
Grasses and cereal have lower magnesium levels than clovers and lucerne.
More advanced pastures normally have the three critical minerals, namely magnesium )Mg), calcium (Ca) and potassium (K) in balance. However when cold conditions below 7 degrees Celsius persist for four or more consecutive days, plant roots hibernate and cease to absorb nutrients from soil. As potassium is more chemically reactive than Ca of Mg, a build-up of potassium occurs around the roots of the plant. When temperature warms up again to night temperatures above 8 degrees and 16 degree days for four days, plants switch back to active root absorption and a rapid uptake of potassium (K) relative to Mg and Ca occurs. K suppresses Mg uptake in the rumen. This can trigger hypomagnesemia (lack of magnesium in blood) and grass tetany.
After about 5 days the plant mineral balance will stabilize again.
During a drought, plants will also absorb less minerals from soil, leading to buildup of available potassium in the soil.
After germinating rain, the excess available potassium is rapidly absorbed by new plant growth. This can trigger grass tetany. Ideally stock should be kept off pasture until it has grown to at least 12 cm in length.
Yarded animals that are being hand fed are also at risk after rain. Their manure which is rich in potassium, enriches the soil with potassium. In certain circumstances where rain has germinated short pick grass, animals may consume the grass as it appears. This is not always noticed by managers until losses occur.
Heavy nitrogen or potassium fertiliser application reduces magnesium availability to plants.
Anything that reduces the animals food intake can trigger grass tetany under the right conditions.
Prevention:
Grass tetany occurs where there are acid soils and high potassium levels. Prevention of soil acidification is a long term goal.
Spring calving cows will be less susceptible than autumn/winter calving cows.
Minimise stress in last 6 weeks of pregnancy and in cows with calves.
Avoid sudden changes in food. Leave the gate open to a fresh paddock so animals can move back and forth quietly until they get used to the new pasture.
Keep hay for cows and calves and give the green crop to steers. The best hay is legume hay. It contains the right nutrients to help prevent grass tetany.
Supplement susceptible livestock with Causmag or another MgO containing supplement.
Summary:
Grass tetany is a complex condition, with magnesium, calcium and potassium ratio playing a role.
The risk of a specific pasture causing problems can be evaluated by carrying out soil tests. Plant analysis is less meaningful as levels are constantly changing in the plant and can change within days.
This article provides general information only. Consult your vet to discuss how it applies to you.
Keywords: grass, tetany, magnesium, calcium, potassium
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