Aniaml Health Problems: Life-Threatening
Excerpt from Publication 19, Pasture Production, Order
Table of Contents
- Grass Tetany
- Nitrate Poisoning
- Prussic Acid Poisoning
- Other Animal Health Problem Recommendations
- Related Links
Grass tetany or hypomagnesemic tetany is a condition that occurs when
an animal's daily diet does not provide enough magnesium to maintain a
normal level of magnesium in the blood serum. It is seen in adult animals
sooner than young animals, and most often in cows and ewes shortly after
giving birth. Cows milking heavily or sheep with twin lambs or triplets
are more susceptible than lower-producing animals.
Symptoms of grass tetany are usually observed 5-10 days after animals
are turned out to pasture. Lush spring grass pastures can be low in magnesium,
with their high moisture content reducing dry matter intake - thereby
reducing the intake of available magnesium to a critical level. Typical
symptoms are: nervousness, tremors and twitching, staggering, convulsions
and death. Death occurs quickly, within 6-10 hours after the onset of
the first symptoms. An animal dead from grass tetany is usually found
with the ground about it churned up, foam on its mouth, and a pile of
loose manure behind it. Blood serum magnesium levels can drop so quickly
it is possible for tetany to occur on the first day out to pasture. Chronic
hypomagnesemia can also occur, with the plasma magnesium level dropping
slowly over a relatively long time. The clinical symptoms of the disorder
occur when the animal is under additional stress.
The magnesium supply from pastures is affected by the botanical composition
of the pasture, the stage of maturity of the plants, soil conditions and
fertilizer treatments. Legumes and herbaceous weeds tend to have a higher
magnesium content than grasses. White clover is one of the best accumulators.
Most of the commonly used grasses tend to have similar abilities to accumulate
magnesium, with the exception of tall fescue and orchardgrass. Tall fescue
is a better accumulator than most grasses, while orchardgrass is much
worse. With both legumes and grasses there is a decline in magnesium content
as the plants mature. Wet, cold soils increase the chances of tetany occurring.
Keep your animals off poorly drained pastures, particularly during periods
of rainfall in spring and fall.
Fertilization affects the magnesium content in pastures in 2 ways:
- it changes the botanical composition of the pasture. Use of nitrogen
on a legume-grass stand encourages grasses and can lead to a decline
in the legume content.
- it directly affects the magnesium concentration in the plants.
Nitrogen can increase the magnesium concentration in grasses when the soil
magnesium level is not limited. However, if the soil magnesium concentration
is low and there is a high potassium concentration, high nitrogen fertility
may result in an excessive uptake of nitrogen and potassium and a reduced
uptake of magnesium. Heavy potassium fertilization, alone, may also decrease
magnesium uptake by plants growing in soil low in magnesium. Soil testing
to determine the potassium and magnesium soil levels before fertilizing
is an important step in preventing grass tetany.
Other options to prevent grass tetany include:
- feeding supplemental magnesium
- offering hay before turning the animals out onto lush grass pastures
- avoiding high rates of nitrogen by applying split applications
- applying potassium in the fall, not spring.
If you suspect grass tetany, contact your veterinarian
as soon possible.
Nitrate is a normal plant metabolite. In a ruminant's digestive tract
nitrates are broken down into nitrites and ammonia. Toxicity may occur
when the rate of production of nitrites exceeds the rate of conversion
to ammonia. The formation of large amounts of nitrites is dangerous to
the animal. Nitrite is absorbed into the bloodstream where it reacts with
haemoglobin to form methaemoglobin. This compound is incapable of releasing
oxygen and, in lethal cases, the animal is killed by oxygen starvation.
Cattle are more susceptible to nitrate poisoning on pasture than sheep,
goats, or horses. Stressed animals are more vulnerable to nitrate poisoning.
The symptoms of acute poisoning are trembling, staggers, rapid breathing
and dark-looking mucous membranes. Death can occur in as little as 3-4
hrs. Nonfatal poisonings can cause poor growth, poor milk production and
abortions. Vitamin A storage is also affected in cattle. Contact a veterinarian
as soon as nitrate poisoning is suspected.
Normally nitrate levels are low in the grasses and legumes commonly
used in pastures. However, occasionally there can be sufficient nitrate
accumulation to cause poisoning. Several factors influence nitrate accumulation
- Environmental conditions such as drought, uneven rainfall distribution,
long periods of cloudy weather, high temperatures and frost increase
nitrate accumulation by slowing down the plant's normal growth rate.
If conditions improve and plants start actively growing, some of the
accumulated nitrates will dissipate and the danger of poisoning decreases.
- Some plants are naturally good accumulators of nitrates. These include
species commonly used for annual pastures (small cereals, sorghum, sudangrass,
forage rape, and kale) and such weeds as lamb's quarters, thistles,
pigweeds and witchgrass.
- Nitrate levels are highest in vegetative plants, with nitrate accumulation
peaking just before the plants start to bloom.
- There is a direct response in plant nitrate levels to increasing levels
of nitrogen fertilizer. High nitrogen rates on grass pastures can cause
some grass species (orchardgrass, tall fescue, meadow foxtail and reed
canarygrass) to accumulate unsafe levels of nitrates. Nitrate accumulation
is also greater with nitrate forms of fertilizer than with urea or ammonium
Due to the diverse factors influencing nitrate accumulation in plants,
it is not possible to easily predict when nitrate poisoning may occur.
To minimize the chance of poisoning, do not graze pastures containing
a high percentage of plants that are good nitrate accumulators, or
grass pastures treated with high amounts of nitrogen fertilizer, during
or after periods of poor growth. Wait 10-14 days for the pasture to
recover. A check on nitrate levels can be made by sending a sample
(made up of pasture clippings) to a feed-testing laboratory. Water
may also be a source of nitrates to grazing animals. If nitrate poisoning
is suspected, test both the plants and water to determine the source
of the problem.
Bloat results when an animal's rumen and reticulum become distended
with gas. With pasture bloat the gas is trapped in millions of bubbles,
giving it a foamy appearance.
The animal is unable to belch or burp up these small bubbles. The
visible result is that the animal's left flank starts to swell. The
animal becomes uncomfortable, may kick at its belly, or get up and
down more frequently than normal. Breathing becomes difficult and
the animal takes shallow, rapid breaths. The tongue may protrude and
there is some slobbering. Death can take place within 2-3 hrs after
the consumption of the bloating forage.
Bloating normally occurs on lush pastures with high alfalfa or clover
content, but can also happen when animals graze cereal crops, forage
rape, and young grass pastures with a high protein content. Alfalfa
and ladino clover are considered more hazardous than white Dutch,
red or alsike clovers. Cattle appear to be more susceptible to bloat
than other ruminants. In addition, some individual animals are more
likely to bloat than others.
The bloat hazard may be reduced in a number of ways.
- Plant pasture mixtures that do not have high percentages of bloat-causing
legumes. In cattle pastures limit the amount of bloat-causing legumes
to 30%, while in pastures for other ruminants do not exceed 50%.
- Use birdsfoot trefoil. It is the only commonly used legume that
does not cause bloat.
- Never turn hungry animals onto suspect pastures. Feed hay to animals
before they enter the pasture to prevent them from gorging on bloat-causing
- Introduce animals to pastures that have bloat-causing legumes
for only a short time. During the next few days, gradually lengthen
the time the animals may remain in the field.
- Introduce animals to suspect pastures when the plants are dry.
Heavy dew or moisture from rain increases the chances of bloating
- Wait until bloat-causing plants are flowering before grazing.
Vegetative plants and legumes in pre-bud to bud stage are more likely
to cause bloat.
- Use rotational grazing to maintain a steady rate of feed intake
and to reduce selective grazing.
- Wait a few days after a killing frost before grazing suspect pastures.
The risk of bloat occurring increases immediately after a killing
- Use an anti-foaming agent such as Poloxalene when the potential
for bloat is great.
Bloat can happen at any time during the grazing
season. A close watch is needed at all times, as the bloat-producing
potential of a pasture can change quickly.
Bloated animals must be treated quickly. With the first signs of
bloat, remove the animals from the pasture and consult a veterinarian.
Keep the animals on their feet and dose the animals with an anti-foaming
agent or surfactant such as mineral oil, raw linseed oil or liquid
dishwashing detergent. In severe cases, when the animal has gone down
and death is imminent, you must puncture the rumen and allow the gas
Prussic Acid Poisoning
Several plant species contain glycosides that can poison animals.
Cyanogenic glycosides can be found in wild cherries, marsh-arrow grass,
a few strains of New Zealand white clover and in plants in the sorghum
family. The amount of cyanogenic glycosides found in these plants
is influenced by plant genetics, stage of growth and environmental
When ingested, cyanogenic glycosides form prussic acid, which is
highly toxic. The prussic acid interferes with the exchange of oxygen
from the lungs to the body tissues.
The symptoms of acute poisoning are muscle tremours, difficult,
rapid respiration, and convulsions. Death can occur so quickly that
the other symptoms are not observed.
The highest levels of glycosides are usually found at early stages
of plant growth. Do not graze sorghum-sudan hybrids until it reaches
a minimum height of 75 cm and sudan grass a minimum of 45 cm
in height. Environmental stress of any kind can increase the amount
of cyanogenic glycoside to hazardous levels. After stresses such as
drought, long periods of cloudy weather or frost, do not graze sorghum
for at least a week. A check on cyanogenic glycoside levels can be
done at a feed-testing laboratory.
... on forages and pastures, visit Forages
and Pastures (OMAFRA)
... on weed control, order Publication
75 Guide to Weed Control: Forage Crops
... on agronomy for field crops, order Pub.
811 Agronomy Guide for Field Crops: Chapter 3 Forages
... on field crop protection, order Publication
812, Field Crop Protection Guide
... on livestock, visit Livestock
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