Grass tetany myths debunked
Ruminant Extension Veterinarian, University of Kentucky
Although the Internet provides a wealth of good information, all too often it contains false or misleading advice because no one directly oversees the content nor questions its validity. Often it is difficult to separate the good from the bad, especially when the author’s credentials seem credible and the journal or magazine seems reputable. With the prefix “Dr.” and a long list of possible suffixes (DVM, PhD, MD, Board-Certified), the reader is willing to believe and accept “expert” conclusions, even if drawn only on observation and never tested critically or reviewed by other experts in the same field. Blanket statements such as “wrestling is real”, “Elvis is alive”, “the IRS is here to help you” and “salt prevents grass tetany” all contain some grain of truth yet should also raise a flag of doubt. While the debate over wrestling, Elvis and the IRS may never be solved, the prevention of grass tetany has been thoroughly studied and high magnesium mineral supplementation is the clear winner.
What is “Grass Tetany” and when are cattle susceptible: Grass tetany, also known as spring tetany, grass staggers, wheat pasture poisoning, winter tetany or lactation tetany, is due to a low level of magnesium (Mg) in the blood. Absorption is completely dependent on the amount obtained from the diet. Deficiencies occur most often in beef and dairy cows in early lactation grazing lush pastures high in potassium (K+) and nitrogen (N+) and low in magnesium (Mg++) and sodium (Na+). Affected cattle may also have low blood calcium concurrently. Typically grass tetany occurs when grazing ryegrass, small grains (i.e. wheat, rye) and cool season perennial grasses in late winter and early spring (Feb-April) although it can occur in fall-calving cows. Fast-growing spring grass is usually high in potassium and crude protein, and low in sodium and magnesium.
Myth 1- Feeding plain white salt to cows will prevent grass tetany:
This claim is shared every spring and, indeed, there are producers who do not have grass tetany that only feed salt. How can that be? Simply put, for those lucky producers, the minerals available in their soils and forages are enough to meet the needs of their cows for their age and amount of milk they are producing. A number of complex factors contribute to the ability of magnesium to be absorbed through the rumen (stomach) wall. Primarily there is a “pump” mechanism that actively transports the dissolved Mg across the rumen wall to the bloodstream. This pump doesn’t work when potassium is high and sodium is low because this changes the electrical potential necessary to drive it. Adding salt to the ration will improve Mg transport when forage sodium is low but too much salt will also increase urination. This will ultimately result in loss of magnesium in the urine. Too much salt, as with any substance, can be dangerous and even fatal.
Research has shown that the negative effects of high potassium that is often present in early spring grass cannot be overcome by the addition of large quantities of salt. However, a high rumen magnesium level, achieved by feeding high magnesium mineral mixes, will allow magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump.
Myth 2-Limited salt in mineral mixes has led to an overconsumption of minerals:
Regional soil types, soil fertility and different forage species can result in different mineral intakes of grazing livestock on every farm. A blanket statement disregarding these factors is oversimplifying a very complex situation. Trace minerals such as copper, selenium, and zinc are all essential nutrients vital for proper growth, production, and immune system function. Trace mineral deficiencies are extremely common in Kentucky and can predispose animals to serious and sometimes fatal disease conditions. Interactions occur between all of the various metals, minerals, and other elements in the diet, and optimal amounts of all elements are essential for proper nutrition. Trace mineral mixes are formulated to meet the needs of cattle, including the need for salt. The keys to using a free-choice product are to ensure cattle have access to mineral 100 percent of the time and to use a palatable product.
Hypomagnesemia is often referred to as an “iceberg” disease because only a few clinical cases occur but there are many unobserved or subclinical cases that may become problems after a stressful event such as a weather change.
Myth 3-Grass tetany only occurs in the spring:
“Winter tetany” in beef cattle is an underlying form of hypomagnesemia caused by a chronic energy shortage and insufficient intake of magnesium. It may be observed when feeding forage silage from cereal grains such as wheat and rye during the winter since it is often high in potassium and nitrogen but low in magnesium. These cattle are borderline low in blood magnesium concentration and clinical signs of grass tetany are triggered by a stressor such as cold weather.
The Truth about Prevention: Prevention is based on providing a high concentration of soluble magnesium in the rumen during times when conditions for grass tetany exist. As long as the active transport pump for magnesium is working well and driving magnesium across the rumen wall, problems should not develop. However, when factors prevent this from working (such as high potassium level in the forage in lush spring grass), the second or “backup” pathway is to increase the amount of magnesium in the diet with a high magnesium mineral mix. A high rumen magnesium level will allow magnesium to passively flow into the bloodstream of the cow without the need for the active transport pump.
Supplementation with high magnesium mineral should begin at least 30 days prior to calving. Cows require 20 grams of magnesium daily or 4 ounces per day of a 15 percent magnesium mineral mix during the late winter and early spring. Mineral feeders should not be allowed to be empty because consistent intake is important for clinical disease prevention. UK Beef IRM mineral recommendations for free choice supplements for grazing beef cattle include 14 percent magnesium in the complete mineral mix and all from magnesium oxide (no dolomitic limestone or magnesium mica). These complete mineral mixtures also supply additional sodium in the form of salt to aid in combatting high potassium intakes. Consumption should be monitored because mineral intake is generally inadequate if using poor quality mineral products. Feeding ionophores (monensin, lasalocid) has been shown to improve magnesium absorption efficiency. High magnesium mineral may be discontinued in late spring once the grass is more mature, the water content of the forage is decreased, and daily temperatures reach at or above 60?F.
In addition to supplying supplemental magnesium, several management factors may decrease the risk of grass tetany. These include: 1) Soil test and apply fertilizer based on soil test results and use no more potassium than recommended since grasses are luxury consumers of potassium; 2) Legumes are high in magnesium and will help offset the problem although their growth is often limited in late winter; 3) Feed small amounts of hay and/or grain to cattle on lush pasture during susceptible periods or limit grazing to 2-3 hours per day; 4) Graze the less susceptible or non-lactating animals (heifers, dry cows, stocker cattle) on the higher risk pastures.
In summary, increasing magnesium intake by supplementing with magnesium oxide, offering adequate salt to prevent sodium deficiency, and increasing total energy intake are all effective tools in preventing grass tetany. These are exceptionally important when moving from winter rations to young spring grass pasture, especially in heavily milking cows. Grass tetany is considered a true veterinary emergency requiring prompt treatment with magnesium to prevent death.