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Magnesium
is an essential nutrient for all animals, serving a variety of vital
functions within the body. The absence of adequate magnesium in the diets
of grazing animals can cause a serious metabolic disorder known as “Grass
Tetany.”
This
condition, which also is called Wheat Pasture Poisoning, Grass Staggers,
and Hypomagnesemic Tetany, is most common in cattle grazing succulent
spring grass pastures or fall cool-season grass pastures and winter
annuals that contain high levels of protein and potassium. A less common
form of the disorder known as Winter Tetany occurs with the consumption of
poor quality, magnesium–depleted forages such as dormant winter
pastures.
Grass tetany occurs when the level of magnesium in blood
falls below a critical threshold (below 1.2 mg per 100 ml). The incidence
of this disorder varies greatly among herds, but in extreme conditions can
be as high as 20%. Cattle diagnosed with grass tetany can have death
losses exceeding 30% if not treated promptly. The potential economic
impact of grass tetany is therefore large. Onset of the disease is rapid,
and death may occur in as little as six hours after the first appearance
of clinical signs. Marginal deficiencies of magnesium, though not
life-threatening, are less visible and also may have substantial economic
consequences due to poor animal performance. Providing a palatable
magnesium supplement with high biological availability is an effective
strategy for meeting magnesium requirements of grazing
cattle.
Lactating cows are most susceptible to grass tetany,
especially in the first few weeks immediately after calving. The disorder
also can affect non-lactating females, calves, and yearling cattle. The
predisposition of lactating cows to tetany is driven by the higher
magnesium requirements associated with milk production. Sensitivity to
magnesium deficiency also is a function of age. Incidence of the metabolic
disorder generally increases as animals become older, presumably as a
result of their inability to rapidly mobilize magnesium stored in the
bones. Younger animals can mobilize bone magnesium more readily, and are
therefore less prone to develop critical deficiencies that lead to grass
tetany.
Grass tetany is relatively rare among cattle grazing
pastures that contain a high percentage of legumes such as clover or
alfalfa, because these forages typically contain more magnesium than
grasses. Succulent, vegetative grasses found in early spring pastures,
fall regrowth pastures, and winter annuals (such as wheat, rye, and
ryegrass pastures) frequently do not contain adequate concentrations of
magnesium, and therefore must be supplemented to achieve optimal
performance and to reduce the incidence of grass tetany. Poor quality
hays, straws, and weathered forages also can be magnesium deficient, and
should be supplemented accordingly to avoid complications with winter
tetany.
The biological availability of magnesium sources is
influenced by a variety of factors. Magnesium oxide, though relatively
unpalatable, is an excellent source of magnesium for cattle. On the other
hand, dolomitic limestone is a poor source of magnesium due to its low
biological availability. Absorption of magnesium by ruminants is
influenced by the frequency of feeding. Compared to feeding once per day,
magnesium absorption is more than doubled with continuous feeding. Feeding
supplemental magnesium in conjunction with small levels of readily
available carbohydrates (sugars) increases absorption dramatically, and
has the additional advantage of helping to disguise the objectionable
flavor associated with many sources of supplemental
magnesium.
Midwest
Cooperatives has Smart Lic Barrels with Mag and also Mag Oxide bags to
keep grass tetany out of your herd:
Contact
your Midwest Cooperatives feed Dealer for more
information.
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