V1703
Grass
Tetany
Cows grazing grass-dominant
pastures account for most
cases of tetany.
Carl R. Dahlen
Beef Cattle Specialist
Charlie Stoltenow
Veterinarian and
Assistant Director of Agriculture
and Natural Resources
Grass tetany, or hypomagnesia,
is caused by low blood levels
of magnesium.
It is most prevalent when cows and
ewes that are in late gestation or
heavy lactation graze lush spring
growth, usually in April and May,
but cases can be seen in the fall.
Rapidly growing forages have
low levels of magnesium, and the
availability of magnesium is further
reduced by high levels of protein
and potassium in the forage.
Cows and ewes in early lactation or late gestation
are most susceptible to grass tetany.
April 2014
Grass tetany also is known as
grass staggers and wheat pasture
poisoning. Grass tetany is most
prevalent in spring pastures that
had very short grazing heights the
previous fall or contain little to no
carryover residue.
Species most likely to cause grass
tetany include cool-season grasses
(crested wheatgrass, bromegrass,
bluegrass and timothy), annual
cereal grasses (wheat, rye, oats)
and native range that lacks standing
litter from the previous year.
Cows grazing grass-dominant
pastures account for most cases of
tetany. Cloudy, windy, rainy weather
with the daytime temperature
between 40 and 60 F seems to be the
environmental conditions most likely
to induce grass tetany symptoms.
Leaving sufficient forage in the
fall will not only trap snow, retain
subsoil moisture and reduce runoff
potential, but it also will provide
sufficient magnesium in the old
growth to help balance the mineral
profile and reduce the potential
for grass tetany in the spring.
In addition, pastures containing
legumes or legume/grass mixes
will reduce the chances of
livestock developing tetany.
Tetany seldom occurs when legumes
or legume-grass mixtures are a
major portion of the animal’s diet.
Legumes usually contain at least
twice the magnesium as grasses
grown in the same soil.
Pastures fertilized with nitrogen (N)
and potassium (K) fertilizers also are
more likely to induce grass tetany.
The plants’ uptake of nitrogen and
potassium from the fertilizer further
decreases the amount of magnesium
available to the animal. Test the soils
to ensure that nutrient rates in the
fertilizer are not in excess of plant
requirements for growth.
❚ Susceptibility
to Tetany
Cows and ewes in heavy
lactation and late gestation are
most susceptible to the disease.
If conditions are favorable for
grass tetany, dry cows and heifers,
stocker cattle and cows with calves
older than 4 months can be grazed
on those pastures.
❚ Symptoms
Animals with grass tetany may
experience excitable and erratic
behavior. They can appear to be
blind. Clinical signs can include
hypersensitivity to touch, muscle
tremors, frequent urination,
staggered walking pattern,
separation from the herd,
convulsions, coma and,
ultimately, death.
The onset of the condition can be
very rapid, and the first clinical sign
producers may see is a dead animal.
The ground surrounding a dead
cow should be studied for marks
indicating whether a cow was lying
down and thrashing her feet while
convulsing prior to death.
A veterinarian should investigate
all unexplained sudden deaths
immediately. Anthrax also can
cause unexplained sudden deaths
(see NDSU Extension publication
“Anthrax,” V561).
❚ Magnesium
Intake Important
Animals have limited body
reserves of magnesium; therefore,
they need consistent intake from
feed to maintain blood magnesium
at required levels, especially during
late pregnancy and early lactation.
Reductions in feed intake associated
with weather, estrus activity, injury
or management may increase the
likelihood of grass tetany if the
forage growth conditions are
favorable for the disease.
However, not all cows with low
blood magnesium develop grass
tetany; why certain animals exhibit
symptoms of the disease while
others remain unaffected is unclear.
Brahman or Brahman-influenced
cattle are less susceptible to tetany,
compared with British breeds,
which are less susceptible than
dairy breeds.
In addition, animals that develop
tetany once and recover are likely
to develop tetany again if
low-magnesium forage conditions
are present. A normal level of
magnesium is about 2.25 milligrams
(mg) per 100 milliliters (ml) of serum.
The level of serum magnesium in
cows affected by grass tetany usually
is below 1 mg per 100 ml of serum.
In some cases, animals with low
blood levels of magnesium that
appeared normal developed
severe tetany after handling.
Low-stress handling of
susceptible cattle when
growing conditions are
favorable is another
important step in
preventing grass tetany.
Wait until grass
is 8 to 10 inches
tall before
turning cattle
out to problem
pastures.
Graze less
susceptible
classes of cattle
on potential
problem
pastures.
❚ Prevention
Techniques
Producers should consult their
veterinarian about a treatment
if they observe symptoms.
To be most effective, treatment
should be administered early
after symptoms appear.
Generally, veterinarians
recommend slowly giving
animals exhibiting symptoms
an intravenous treatment with
magnesium and calcium
(which also is low in rapidly
growing forage) in a dextrose
solution. In addition, if several
cattle develop grass tetany in a
pasture, consider the remaining
animals at risk.
Here are some techniques
to prevent the disease:
✔ Encourage daily intake of magnesium. Magnesium oxide
is the most common source of supplemental magnesium.
Keep mineral boxes filled and scattered at several locations
in the pasture, especially October through May.
✔ Do not start grazing pastures too early in the spring;
wait until new growth is 8 to 10 inches tall. Magnesium is more
available in mature grass plants than in new, lush, growing shoots.
✔ Make sure lactating cows receive 0.20 percent magnesium in
the diet on a dry-matter basis. This is equal to 18 to 21 grams
of magnesium intake daily.
✔ Use salt mixtures containing magnesium oxide as a magnesium
source. To be effective, the mineral mix should contain at least
10 percent magnesium.
✔ Mix magnesium oxide with other supplements because it is
unpalatable (tastes bad).
✔ Graze legume or mixed legume-grass pastures first because early,
lush grass growth is more problematic than legume mixed pastures.
Note: Grazing unlimited lush growth of legume pastures
can lead to cases of bloat. Take appropriate precautions
if managing cattle on legume or legume-mixed pastures.
✔ Graze less susceptible animals on problem pastures.
These animals include dry cows, heifers, stocker cattle and cows
nursing calves more than 4 months old. Cows and sheep in heavy
lactation are the most susceptible classes of livestock.
For more information on this and other topics, see www.ag.ndsu.edu
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