THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Selenium Nutrition of Florida Beef Cows
For questions or comments regarding this publication contact: Dr. John Arthington, University of Florida, IFAS
Mineral supplementation in beef cattle can be divided into two broad categories, macro-minerals and micro-minerals. As a rule of thumb, micro-minerals are required in amounts less than 1 gram per day compared to macro-minerals, which are often required at levels greater than 1 gram daily. Multiple trace minerals are essential for basic physiological functions in beef cattle and among these selenium, copper, cobalt, and zinc are commonly found to be deficient in Florida forages. Each of these trace mineral nutrients are involved in numerous biological processes that impact immunity, reproduction, and growth. Although deficiencies are common for each, selenium is almost always found to be severely deficient in nearly all perennial grasses in Florida. Surveys from University of Florida researchers (Arthington and McDowell), evaluating perennial pastures in Florida, reported an average forage selenium concentration of 0.06 ppm (dry matter basis; average of 33 ranches/farms) resulting in a deficiency rate of 89%. Under these conditions, selenium deficiency is inevitable if efforts to address adequate supplementation are not implemented.
Selenium deficiency is widely recognized throughout much of the tropical and subtropical regions of the World, including Florida. Selenium is essential for the maintenance of tissue integrity. Widely recognized deficiency symptoms include the degeneration of tissue resulting in a clinical condition referred to as “white muscle disease”, which is characterized as white-colored striations in the muscle tissue. Sub-acute selenium deficiency leads to poor muscle function and coordination. This outcome may be a leading contributor to the condition referred to as “Weak Calf Syndrome”. Links to reproduction are largely characterized by an increased incidence of retained placentas. Research from Ohio State University, reported a 38% reduction in retained placentas in selenium-adequate versus inadequate cows. In an additional field study by the same authors, they observed a 42% reduction in the incidence of retained placentas in cows receiving a pre-partum injection of a blend of selenium and vitamin E compared to control cows. In addition, selenium supplementation also appears to be effective in lessening the recovery time of cows inflicted with metritis. In one study, metritis-inflicted cows receiving supplemental selenium had fewer uterine recovery days compared to metritis-inflicted cows not supplemented with selenium.
Current dietary recommendations for beef cattle suggest that a total dietary concentration of 0.10 ppm of selenium (dry matter basis) is sufficient to support selenium-dependent biological functions. Assuming average dietary intake of dry matter, this would equate to approximately 1 mg of Se daily. Considerable debate exists as to whether this amount is sufficient, particularly under conditions of stress, rapid growth, or in the presence of excess sulfur, an antagonist of selenium. Irrespective of the actual requirement, feed manufacturers are federally mandated to formulate supplements that may allow for, but not exceed, 3 mg of selenium daily. Most all researchers agree that this maximum amount is sufficient to support beef cattle in all stages of production. However, adequate intake of supplemental selenium, particularly among grazing cattle, can be unreliable.
Many grazing beef cattle in Florida are provided supplemental selenium through the provision of free-choice, salt-based mineral supplements. As an example of the regulated selenium inclusion, many supplements are formulated to provide about 2 to 3 mg of selenium daily when the free-choice supplement is consumed at the targeted rate. A product formulated for a targeted intake of 2 ounces daily will commonly contain 50 ppm selenium. If the cow consumes the targeted intake, she will consume a total daily intake of 2.8 mg of selenium, which is sufficient to fully support her selenium requirement.
The scenario described above illustrates a simple and successful strategy for addressing selenium nutrition of Florida beef cows through a free-choice supplement. The problem with this approach is that the cow’s selenium nutrition is entirely dependent upon her voluntary consumption of the free-choice supplement. As producers, we know this assumption can be flawed. In the summer months, when forage moisture content is high and air temperature and humidity is the greatest, cows display a craving for salt. During these times, free-choice intake of mineral supplements is high, often too high. Although over-consumption is unlikely to harm the cow, it is an inefficient method for nutrient supplementation. In contrast, during the dryer, cooler winter months, beef cows typically do not crave salt, and therefore, free-choice intake of minerals is reduced. This is the most troublesome time of the year because our Florida beef cows are typically calving during the winter and early spring. Selenium nutrition is particularly important during this critical time to support post-calving uterine recovery and resumption of cyclicity. It is likely that seasonal selenium deficiency of Florida beef cows is one of the factors impacting lengthened post-partum anestrus.
There are several methods to address selenium nutrition of grazing beef cows when free-choice intake of mineral is inadequate. The simplest method is to feed a selenium-fortified energy/protein supplement during the winter and early spring. If producers are purchasing commercially-formulated winter supplements (dry or liquid), selenium may already be included. It is important to consult with the manufacturer to determine the daily targeted selenium intake of these supplements to be certain selenium nutrition is addressed. If commodity supplements are fed, or no winter/spring supplement is offered, producers can feed a selenium-containing range cube 2 to 3 days weekly. This method of supplementation is called “controlled mineral feeding”. Controlled mineral feeding bypasses the variation associated with a cow’s voluntary intake of free-choice mineral supplements. Because the range cubes are highly-palatable, the cows readily consume them immediately upon offer. We often recommend that producers formulate these cubes with a complete mineral premix, including salt. Using this supplementation strategy, producers can discontinue free-choice mineral offering until the spring. These mineral-fortified range cubes can be fed on the ground, but be cautious to spread them over a large area to ensure all cows have access.
Pre-weaned calves also are likely to suffer from selenium deficiency. Milk is a poor source of selenium and the small amount of forage consumed by calves is almost void of selenium. Like cows, calves need to consume enough of the free-choice mineral to satisfy their selenium requirement. Our studies suggest that this is rarely the case. Born selenium adequate or marginally adequate, most Florida beef calves experience a slow and steady decline in selenium status up to the time of weaning. This doesn’t always present itself as a problem. Following weaning, when selenium-fortified feeds are offered, calves rapidly replete themselves and become selenium adequate in a short period of time. However, the immediate post-weaning period can be a troublesome, stressful time for beef calves, particularly in Florida. Marginal or deficient selenium status can lead to poor immune responsiveness at a time when high-health status is most needed. Current research at the UF/IFAS Range Cattle Research and Education Center is directed at management methods that seek to improve the selenium status of pre-weaned calves.
In summary, selenium is recognized as the most commonly deficient essential trace mineral in perennial pasture forages in Florida. Selenium participates as a cofactor in powerful antioxidant systems that protect and support tissue integrity. Research has shown that adequate selenium nutrition is important through the cow’s post-partum period, thus impacting the time of post-partum anestrus. Unfortunately, during the main Florida calving season (winter and early spring), selenium intake is limited by reduced free-choice intake of salt-based supplements. Florida cow/calf producers are likely to benefit from efforts to improve selenium intake during this critical time.
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