Animal Health Update, July 2004
SELENIUM DEFICIENCY IN ARIZONA RANGE CATTLE

Selenium is one of 15 essential trace minerals or micronutrients required by animals and humans. It is naturally present in varying amounts in soils and is taken up by range forage plants at levels that usually reflect soil content or availability of the element.  It is required nutritionally only in "trace" amounts (i.e. less than 0.1 % of feed dry matter) and either deficient or excessive intakes can be detrimental to livestock.  Intakes of trace elements that are considered "adequate" to meet nutritional requirements of livestock fall within a fairly narrow range between deficiency and toxicity.  To prevent signs of selenium deficiency, the National Research Council recommends that beef cattle rations contain 0.1ppm (parts-per-million) selenium. Because of wide variations in soil levels, cattle consuming native forages are at risk of selenium deficiency in many regions of the United States.  In Arizona, analysis of soils, range forages, and blood samples from range cattle suggest that deficiencies of selenium are common in east-central Arizona and in the southern border regions.

Functions of selenium: Selenium is an essential component of antioxidant enzyme systems in the body that protect cells and their internal components from oxidative damage from oxidizing substances formed during normal cellular metabolic processes. Oxidative damage to cells and their components, if not controlled, can lead to tissue damage and impaired function that vary in severity depending on a number of factors. Vitamin E and several other antioxidant enzyme systems are involved as well but we have not found vitamin E deficiency to be common in Arizona cattle grazing natural range forages.  Vitamin E, however, has been found to be deficient in some cattle in feedlots.

Effects of selenium deficiency:

In calves, severe selenium deficiency can cause muscle degeneration (nutritional myopathy or white muscle disease) that can lead to death.  In less severely or "sub clinically" deficient calves, impairment of the immune system has been reported resulting in poor response to vaccination and greater susceptibility to infectious causes of pneumonia and scours. 

In cows, some effects of chronic, sub-optimal selenium intakes that have been reported are: decreased conception rates, retained placentas, abortions, mastitis, weak, stillborn, or lethargic calves, and occasional downer cows following calving. Keep in mind that some reproductive effects can be subtle (i.e. "sub clinical") and may not be readily observed unless cattle are observed routinely and good records are kept. At the UA V-V Ranch in east-central Arizona, provision of a loose salt-mineral mix containing 90 ppm selenium along with recommended levels of other trace minerals and calcium and phosphorus was shown to improve early breeding season (January-April) conception rates in treated cows compared to un-supplemented control cows. 

Diagnosis of selenium deficiency: 

Direct assessment of selenium status: Selenium status of cattle can be evaluated most conveniently by analyzing whole blood samples (in a purple-top EDTA tube) from ~10 cows in a herd.  Whole blood is best because it reflects long-term intakes and selenium that is active in antioxidant enzyme systems as well as current dietary intake. Serum samples are less desirable because selenium levels in serum reflect only current status and are more difficult to interpret, particularly in cows in mid to late pregnancy.

Indirect assessment of selenium status:

This can be done by analysis of forage samples and is an acceptable option but multiple composite samples of the moderately palatable, more abundant forage species from several areas of the range should be tested.  Forage analysis is also a good option when determining the status of copper, another nutritionally important trace element.  Blood or serum levels of copper are not diagnostically useful. We normally analyze forage for a number of other nutritionally relevant trace minerals including selenium and copper.

Options for managing selenium-deficient range cattle:

1. The first option is the routine and continuous feeding of salt-mineral supplements containing customized levels of selenium usually as sodium selenite. This type of supplement is the most commonly used option in range cattle determined to have sub-optimal selenium status.  Salt-mineral mixes can be prepared in either loose or blocked form containing up to a maximum of 120 ppm selenium and are usually the most practical and cost-effective. An advantage of this method is that the requirements of cattle for salt, calcium, phosphorus, and other trace minerals along with selenium can be met all at once. A disadvantage of a loose mix is that the provision of simple mineral feeders on the range is required. Cattle offered new, customized supplements for the first time may seem to refuse them initially and may have to be "trained" to consume them by removing other salt sources.

Caution: loose salt-mineral mixes should not be offered suddenly to water or salt-deprived cattle. Over consumption may occur and will lead to salt poisoning and death of the animals. Salt-deprived animals should be given plain white hard salt blocks prior to the provision of loose salt mixes so that they become salt-sufficient. Adequate water in reasonably close proximity to the supplement should be available at all times.

Cattle in remote, difficult-to-reach areas of the range should have plain white salt blocks available as well so that they do not become salt deprived if the loose salt-mineral mix is depleted and not re-supplied for some time. So-called mineralized salt blocks, also known as "brown blocks" or "camouflage blocks" that contain added trace minerals normally do not contain sufficient selenium to meet the needs of selenium-deficient cattle.

Supplements containing "organic," "complexed," or "chelated" selenium are also available and are claimed to be more effective but are often more expensive. All supplements should be formulated to provide adult cows a maximum daily intake of about 3 mg (milligrams) selenium.

To our knowledge, there is currently no Arizona feed company capable of manufacturing blocks of custom mineral supplements to a satisfactory hardness.

2. A second, more expensive, but effective option that has recently become available is sustained-release ruminal boluses (CoSecureâ boluses).  These have been evaluated in cattle on the UA V-V Ranch and overall were effective in raising blood selenium levels in cows and their calves, with some variation by year and breed.  The boluses that were evaluated last about 6 months in the cow's rumen as they slowly dissolve and release selenium, copper, and cobalt and so need to be administered twice a year.  An obvious disadvantage of boluses is the necessity for handling cattle in a squeeze chute individually twice a year in order to administer the boluses orally.  In some types of operations, however, this option is well worth considering.  Another disadvantage is the salt requirement of cattle is not met when this method is used unless salt is provided separately.

3. The third, least desirable option for long-term supplementation is administration of selenium by injection.  Again, this is effective but must be repeated periodically, as elevation of blood selenium levels will only last 30-60 days.  Obvious disadvantages besides the cost of these products are the labor and facilities required for repeated injections and the possibility of injection-site blemishes in cattle that are marketed.

Recommendations:

1.   Selenium supplementation should be customized to meet, but not exceed, the selenium requirements of cattle. 

2.   We recommend that any salt-mineral supplements containing customized mineral levels be routinely batch-tested to determine their actual selenium content.

3.   Selenium status of cattle should be monitored by follow-up blood testing in a minimum of about 60-90 days after routine supplement consumption by cattle has begun.

4.   The intake of mineral supplements by cattle should be monitored closely. Records should be kept of consumption.

5.   Laboratory testing is available through the AzVDL Toxicology and Nutritional Testing Section (520-621-2356, extension 17). AzVDL or Veterinary Extension personnel (520-621-2356, extension 19) can assist producers in monitoring a supplementation program.  Feel free to call for information.

Ted H. Noon, DVM
AzVDL Toxicology & Nutritional Testing Section

Howard Frederick, PhD
Adjunct Professor, Department of Veterinary Science and Microbiology

S. Peder Cuneo, DVM, MS
UA Extension Veterinarian

â Telsol, Ltd.. P.O. Box HH7, Leeds, United Kingdom LS82YE. Manufacturer of CoSecure. Mention of a proprietary product does not constitute a guarantee or warranty of the product by the University of Arizona Agricultural Experiment Station or the authors and does not imply its approval to the exclusion of other products that may also be suitable.