Mineral supplementation for grazing cattle can be a complex topic. This is largely because it is always a moving target as forage mineral supply changes throughout the year. Mineral availability in pasture and hay forages fluctuates according to the season, fertilizer application, weather conditions, forage species, and other factors. Cattle mineral requirements also fluctuate with growth and stage of production. These factors converge in the spring of the year and can be consequential to herd health and productivity. The following information details what needs are priority during spring when forage is actively growing.
Minerals play vital roles to every day physiological processes in beef cattle. Minerals required for these processes are separated into two categories: macromiberals and microminerals.
Macrominerals are required in percentage amounts and include the following:
- calcium (Ca)
- phosphorus (P)
- magnesium (Mg)
- sodium (Na)
- potassium (K)
- chlorine (Cl)
- sulfur (S)
Microminerals are required in parts per million amounts and include the following:
- zinc (Zn)
- copper (Cu)
- iron (Fe)
- manganese (Mn)
- selenium (Se)
- molybdenum (Mo)
- Iodine (I)
- cobalt (Co)
In Alabama, cattle grazing spring forages are especially at risk for becoming deficient in Mg (and/or Ca), Zn, Cu and/or Se.
Mg deficiency causes a metabolic disorder called Grass Tetany or hypomagnesemia. Hypomagnesemia refers to low serum levels of Mg. This disorder results from a deficiency of Mg because of inadequate intake from the forage and mineral supplement and/or a higher requirement of Mg caused by negative interactions with other minerals or organic acids. A cow’s dietary requirement for Mg increases two-fold as she transitions from gestation to lactation. Additionally, older cows have less ability to mobilize Mg from bone stores than younger cows do, making this disorder more prone in older cows with good milking ability.
Spring calving cows grazing lush growing spring forage are highly susceptible to this disorder. Lush growing spring forages, in particular small grain annuals, can contain high quantities of K which has been shown to increase the requirement for Mg by decreasing Mg absorption across the gut. Elevated nitrogen (N) concentrations in forages also contributes to a decrease in Mg absorption. It has been shown that elevated K consumption along with elevated N consumption is more dangerous than either one alone.
Hypocalcemia (low serum Ca) is also a concern and symptoms are similar to grass tetany. In many cases, when serum inorganic P rises above the level of serum Ca, tetany symptoms from a deficiency of Ca and/or Mg occur. Pastures and hay meadows receiving heavy poultry litter fertilization can be problematic because of an imbalance of P with Ca and/or Mg.
Symptoms and Treatment
Cattle experiencing grass tetany will exhibit nervousness, muscle twitching, and staggers. Affected animals will likely be standing by themselves and will be easily agitated. Increased stress when moving the cow to a working facility may exacerbate the symptoms because of the role Mg (and Ca) plays in the nervous system. Eventually, the cow will go down on one side with muscle spasms and convulsions. If not treated the animal will die.
Once an animal is down, it is difficult for them to recover without veterinary intervention. Veterinary intervention results in an IV infusion of a magnesium-calcium-glucose solution. Physical symptoms are not usually observed until the animal is already down. For an emergency treatment (and short-term solution), a rectal enema of Mg using Mg sulfate (Epson salt) dissolved in water may allow enough Mg to be absorbed across the large intestine to overcome the symptoms. Regardless of treatment outcome, dietary changes must be made to the herd to prevent further occurrence of this deadly malady.
Providing a mineral supplement containing Mg is an easy and reliable method of preventing grass tetany. A common practice is to provide a free choice mineral supplement. It is recommended that mineral mixes contain between 10 and 14 percent Mg for cows grazing spring forages. Cattle need to consume 3 to 4 ounces of mineral mix per head daily. Ideal Mg mineral mixes are often referred to as high-mag because they contain an increased concentration of Mg. High-mag mixes have a bitter, often “metallic” taste which may cause palatability issues.
Transitioning cattle to a high-mag mineral one to two months (January-February) before lush forage growth begins will allow them an opportunity to adjust to the new mineral mix. Another option is to mix trace mineral and Mg oxide at a one-to-one ratio and feed it directly to the cattle without any other source of salt. If cattle refuse to consume a high-mag mineral mix, consider mixing (in equal amounts) trace mineral, Mg oxide, and a high energy feed such as ground corn or dried molasses. However, do not mix Mg oxide with protein supplements.
Micromineral Considerations and Mineral Sources
Microminerals that may be problematic during the spring grazing season will likely be Zn and/or Cu. Unlike Mg deficiency, where grass tetany results in direct physical symptoms, Zn and Cu deficiencies can result in hidden health and production losses. In Alabama, forage Zn and Cu levels are often lower than animal requirements. Deficiencies of these minerals can result in reduced immune responsiveness, lowered weaning weights and reproductive failure. Mineral supplements should contain a source of these minerals that are biologically available for absorption across the digestive tract. It is generally thought that organic sources of these minerals (the mineral attached to an organic constituent to include but not limited to amino acids and/or short protein molecules) are more biologically available than their inorganic counterpart. However, the organic source of these minerals is more expensive than their inorganic counterpart. The most biologically active inorganic source of Zn and Cu is the sulfate form. The oxide form of Zn and Cu is recognized as being low in biological availability and is not recommended. Therefore, when looking at mineral supplement feed tags, pay particular attention to source of Zn and Cu that is provided in the supplement. However, this is not true for the macromineral Mg. Mg oxide is the recognized standard for supplementing Mg in mineral supplements.
For in-depth information on beef cattle mineral nutrition concerns, contact the Alabama Extension animal sciences and forages regional agent in your area. For additional information regarding grass tetany prevention, see the Extension publication Management Practices to Reduce Grass Tetany, ANR – 0495.