THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Selenium Biofortification of Pasture Forageby Juliana Ranches and John Arthington
Selenium (Se) is a trace element that is essential in small amount for humans and for animals; however the element is not essential for plants. Humans and animals require Se for the function of a large number of Se-dependent enzymes, called selenoproteins. For humans and cattle, there are 25 selenoproteins, but only half of them have their metabolic functions identified. The most widely understood selenoprotein is the antioxidant enzyme glutathione peroxidase (GPX), which is well known for catalyzing the reduction of hydrogen peroxide and organic hydroperoxides, thus protecting cells from oxidative damage. The lack of Se can lead a large number of negative impacts with the most widely recognized being white muscle disease in calves. Other problems associated with Se deficiency include muscular weakness, reduced weight gain, diarrhea, stillbirths, abortions, retained placenta and diminished fertility.
The benefits promoted by Se supplementation have been shown in different fields of research. From human nutrition, researchers proposed that dietary Se is involved in cancer prevention, immune function, aging, and male reproduction. Studies with dairy calves have shown that feeding dams a supranutritional Se-yeast supplement or adding pharmacological dosages of Na selenite to colostrum both increase serum-IgG concentrations and total serum-IgG content in Se-supplemented calves. Another study with dairy cows, showed that Se supplementation 1 mo before calving increased blood GPx activity, slightly reduced the prevalence of intramammary infections at calving, and lowered SCC at the time of calving, regardless of Se source. Researchers from Oregon State University, working with beef cattle, revealed that short-term exposure of cattle to Se-fertilized forage elevated whole blood Se concentrations and levels were sufficient to maintain adequate concentrations throughout grazing periods when there would be limited access to Se supplements.
In some regions of the World, such as parts of the western United States, Se toxicity can be a problem. There are two general types of toxicity, acute and chronic. Acute Se toxicity is caused by the consumption, usually in a single feeding, of a sufficient quantity of highly seleniferous plants. The indicator plants include certain species of Astragalus, prince's plume, and some woody asters. This kind of poisoning, produces severe symptoms and death occurs within a few hours. A second form of Se poisoning is the chronic toxicity, there are two different types of chronic poisoning dependent on the chemical form of the ingested selenium. "Blind staggers" occurs when animals ingest water-soluble Se compounds naturally found in accumulator plants. Toxicity from eating plants or grain with protein-bound, insoluble selenium is called "alkali disease." Selenium is the only trace mineral that is sometimes found in toxic concentrations in forages grown in specific regions of the US.
The range between adequate and toxic concentrations is narrower for Se compared to other essential trace minerals; however, in most regions of the country, Se-deficient forage is much more common than cases of Se excess. In a survey of 253 cow/calf operations in 18 US states, over 18% were classified as marginally or severely Se deficient by blood Se parameters. Among the states analyzed, those located in the southeast region had the greatest percentage of operations classified as marginally or severely Se deficient (42.4%). Soils containing less than 0.5 mg/kg total are classified as Se deficient. According to National Research Council, Se-deficient regions in the U.S. include New England, New York, New Jersey, Delaware, Pennsylvania, Maryland, West Virginia, Florida, Ohio, Indiana, Illinois, Michigan, Wisconsin, Washington State, Oregon, Montana, Arizona, and coastal regions of Virginia, the Carolinas and Georgia. Although Se is not an essential element in plant nutrition, the consumption of plants and plant products is the primary route by which animals and humans receive their dietary Se in the absence of any special supplementation. While higher plants do not require Se, they readily take it up from their environment and incorporate it into organic compounds using Se assimilation enzymes. The plants are responsible for the conversion of selenate-Se into organic Se compounds, this conversion is believed to occur in the chloroplasts.
One potential method for addressing Se nutrition in grazing cattle is the implementation of pasture Se applications with the intent of increasing plant Se content and thus the Se status of cattle grazing these forages. This strategy is is called “biofortification”, and has been utilized in Findland since 1985. By definition biofortification is a strategy to increase the nutrient content of food.
Selenium from selenate sources appears to be more available for plant uptake compared to selenite sources. In Florida, spraying bermudagrass with Na selenate at Se application ranges of 49 to 196 g Se/acre (via Na selenate) resulted in substantial increases in forage Se content by 2 wk after application, decreasing rapidly by 12 wk post-aplication. Feeding forages grown on Se-fertilized hay fields impacts both Se status and performance of grazing cattle. In a study conducted at Oregon State University, weaned Angus-type calves were fed Se-fertilized alfalfa hay over a 7-week period. Alfalfa hay was grown on fields receiving applications of Na selenate in amounts providing 0, 9, 18, or 37 g Se/acre. These application rates resulted in a linear increase in Se content of hay (harvested 40 days after application) as Se application rate increased (Figure 1; Inset A). In addition, calves consuming these hay treatments (approximately 2.5% BW daily) experienced a linear increase in whole blood Se concentrations as Se application rate (and Se content of hay) increased (Figure 1; Inset B).
In a recent study at the UF/IFAS, Range Cattle REC, we produced a high-Se hay crop by spraying a Jiggs bermudagrass hayfield with Na selenate at a rate of 105 g Se/acre. Selenium content of hay, harvested 8 wk after Na selenate application, was greater for Se-treated vs. control pastures (7.73 ± 1.81 vs. 0.07 ± 0.04 ppm DM; P <0.001). In a subsequent study, this hay crop was fed to weaned calves and Se status was evaluated over a 42-d study. Calves were stratified by initial BW and randomly assigned to treatments including high-Se hay, low-Se hay + supplemental Na selenite, or No supplemental Se (n = 14, 14, and 4 calves, respectively). Calves were housed in drylot pens (2 calves/pen; 7, 7, and 2 pens per treatment). A pair-feeding design was utilized, whereas each pen of high-Se hay calves was paired to a pen of Na selenite - supplemented calves. Calves assigned to the high-Se hay treatment were provided ground, high-Se hay for a 4 h period each morning. Pen dry matter intake was calculated and total daily Se intake/pen was estimated. Each Na selenite paired pen was then provided the same daily amount of Se via Na selenite hand-mixed into a limit-fed grain supplement. Therefore, each pen of calves receiving high-Se hay had a paired partner pen of calves receiving the same amount of Se via Na selenite. Liver Se concentrations remained unchanged for the negative control calves receiving no supplemental Se over the 42-d feeding period, but they were increased (P < 0.001) in calves receiving both high-Se hay and Na selenite treatments. Calves receiving high-Se hay had greater (P < 0.05) liver Se concentrations on d 21 and 42 than calves receiving Na selenite. Of notable interest, this difference was attributed only to the paired pens consuming < 3 mg Se daily. From these initial data, we hypothesize that there is a differential availability of Se in forage vs. inorganic sources dependent upon the total daily intake with a critical point of approximately 3 mg/d in beef calves. We are currently examining these data further in both periparturient cows and calves.
In summary, Se is the essential trace mineral most commonly found to be deficient among beef cows and calves in Florida. Biofortification of pasture forage with Se appears to be effective in increasing calf Se status. Further investigation is warrented to understand the impacts of this management system on cow and calf productivity and overall system economics.
Figure 1. Effects of Na selenate application to alfalfa hay fields on subsequent forage Se content (A) and Se status (B) of calves consuming the hay. Conversion; g/ha ÷ 2.45 = g/acre. Data adapted from Hall et al. 2013. PLOS ONE. 8:E58188