Grass tetany is a highly fatal disease associated with
low levels of magnesium in the blood. Grass
tetany can affect all classes of cattle but older cows with calves at side
during winter and spring are most at risk. Very thin and overly fat animals are
also more susceptible. Calves, yearlings and replacement heifers rarely develop
grass tetany because they can more easily access body stores of magnesium.
Grass
tetany – causes:
Cattle hold magnesium in the bones and muscles but
cannot readily access and utilize these stores when needed. The animal
constantly loses magnesium in urine, feces and milk, so it needs magnesium in
its diet to meet daily requirements. A
cow in peak lactation (6–8 weeks following calving) needs a constant source of
magnesium to replace the large amount lost from the body in milk. Even when
feed levels of magnesium are low, the loss of magnesium in the milk remains the
same. Low magnesium in the blood of an
animal can be caused by low magnesium levels in feed and/or reduced magnesium
absorption.
Contributing
causes are:
·
Magnesium
levels are lower in cool season grasses and small grain forages than in legumes
or weeds.
·
Levels
are low in grasses grown on leached acid sandy soils.
·
Levels
are low when potash and nitrogen fertilizers are used and growth is vigorous.
·
High
moisture content in grass causing rapid gut transit and low uptake.
·
Reduced
absorption of magnesium resulting from high rumen potassium and nitrogen and
low rumen sodium.
·
Low
energy intake, fasting or sudden changes in feed.
·
Bad
weather, especially winter storms.
·
Low
roughage intake (young grasses have low roughage and often poor palatability).
·
Low
intake of phosphorus and salt.
Signs:
Animals suffering from grass tetany are often found
dead. There may be marks on the ground beside the animal indicating they were
leg paddling before death (lying on their side with stiff outstretched legs
that thrash backwards and forwards). Early
signs include some excitability with muscle twitching, an exaggerated awareness
and a stiff gait. Animals may appear aggressive and may progress through
galloping, bellowing and then staggering.
In less severe cases the only signs may be a change in the character of
the animal and difficulty in handling.
Treatment
of affected cattle:
Blood magnesium levels must be restored. Veterinary administration of an intravenous
calcium and magnesium solution produces best results. However, in acute cases where time is
critical, producers can inject a calcium and magnesium solution under the skin. Producers should also provide oral sources of
magnesium to affected herds to prevent relapses. These include:
·
Magnesium
oxide in loose mineral supplements.
·
Molasses
blocks or tubs with elevate levels of Magnesium.
·
Slow-release
boluses.
·
Magnesium
sulphate or soluble magnesium chloride added to hay or silage.
·
Adding
magnesium to concentrates or pellets.
These products are available from your veterinarian,
feed supplier or rural supplies company.
Please note that trying to increase magnesium intake through mineral or
feed supplementation can be difficult and can actually be detrimental in some
situations. If magnesium levels are
elevated too high in the supplement it will actually limit or stop consumption
of these products.
Prevention
and control:
·
Provide
a magnesium supplement to cattle that provide elevated levels of magnesium at
least 2 weeks prior to grazing lush or suspect pastures and forages and
certainly prior to parturition and the start of lactation.
·
Eliminate
factors which reduce magnesium absorption through pasture and grazing
management.
·
Provide
supplemental roughage in advance of grazing lush pasture.
Immediate
actions:
·
Increase
energy and roughage intake. Good quality hay is suitable.
·
Pellets
or grain can be added if introduced carefully and cattle are accustomed to
these.
·
Provide
salt if a natural source is not available.
·
Move
lactating cows (especially older animals) to high dry matter pastures if
available.
·
Reduce
stress factors (if corralled do it in a very low stress manner, limit transportation
if at all possible).
·
Provide
magnesium supplements (see below).
Long
term actions:
·
Correct
soil acidity with lime or dolomite (dolomite contains some magnesium).
·
Plant
clovers if adaptable species will grow in your environment.
·
Apply
phosphate fertilizer.
·
Limit
potash and nitrogen applications until soil acidity is corrected.
·
Keep
good records to inform future management.
For problem pastures, consider forage analysis
(primarily the leaves) for magnesium and potassium. Consult your veterinarian for
further advice.
Diseases
similar to grass tetany:
Accurate diagnosis of grass tetany by a veterinarian
is important because a number of significant diseases have similar signs. These
include:
·
Staggers
caused by perennial rye and annual ryegrass toxicity.
·
Nitrate/nitrite
poisoning (also seen on young, rapidly growing heavily fertilized grasses and
cereals).
·
Lead
poisoning – usually from discarded batteries.
·
Locally
occurring viruses and bacteria.
How
a veterinarian can help:
If cases of grass tetany have been confirmed in the
area, work with your veterinarian and be prepared to provide immediate
intervention if cattle are found with symptoms of grass tetany. Work with the veterinarian to develop a
management plan to prevent the onset of grass tetany within the herd. A veterinarian will be able to investigate
whether grass tetany or another disease is occurring. Producers who detect
signs of grass tetany in their stock, or notice any other unusual signs, should
contact their private veterinarian.