Published May, 2013

How important is the micromineral selenium to antioxidant activity in horses? A University of Kentucky (UK) research team recently set out to find out. The team evaluated selenium status' impact on antioxidant factors in mature horses.

Selenium (Se) plays an integral role in many of the equine body's functions, including production of the enzyme glutathione peroxidase, an antioxidant that's vital in preventing and repairing oxidative damage to cells. In horses residing and grazing in selenium-deficient geographic areas (including the Pacific Northwest, Great Lakes, and New England regions of the United States), this antioxidant capacity could be compromised if owners don't provide selenium supplementation. Impaired immunity, elevated blood levels of muscle enzymes, and tying up have all been reported in horses with selenium deficiencies.

In the recent UK study, which took place over 18 months, Mieke Brummer, PhD; Laurie Lawrence, PhD; and colleagues randomly assigned 28 pasture-kept horses (eight mature geldings and 20 nonpregnant, mature mares) to one of four treatment groups: low selenium (LS), adequate selenium (AS), high organic selenium (SP), or high inorganic selenium (SS). The AS horses served as controls, and the researchers maintained them on a diet formulated to provide 120% of National Research Council (NRC)-recommended dietary selenium intake throughout the study.

To deplete horses' selenium stores, the other three treatment groups consumed a diet consisting of 60% of the NRC's recommended selenium intake for the first 196 days. Blood samples taken during this period confirmed that horses’ selenium stores had been depleted.

A 189-day repletion phase followed, during which LS horses consumed the same diet as during the depletion phase. The SP and SS horses, however, received 0.3 mg/kg dry matter (DM) of either a commercially available organic selenium and yeast supplement (Sel-Plex, produced by Alltech Inc.) or an inorganic sodium selenite supplement, respectively, top-dressed onto their normal ration. The inorganic form of selenium (sodium selenite) is commonly used in today's horse feeds; however, researchers believe an organic form of the micromineral more closely mimics natural selenium in horses' diets, making it more digestible.

During each treatment phase, researchers took blood samples to analyze for indicators of selenium status, including glutathione peroxidase activity, whole blood Se (how much selenium was found in horses' bloodstreams), antioxidant parameters, and vitamin E concentrations.

Upon reviewing the results of their analysis, the team found that:

• During the depletion phase, whole blood Se concentrations and glutathione peroxidase activity both decreased significantly in the SP, SS, and LS groups. The team noted the whole blood Se levels and glutathione peroxidase activity also decreased in the AS horses during the first 84 days before leveling off, whereas the remaining groups continued to decrease throughout the period.

  Data from the repletion phase indicated that glutathione peroxidase activity continued to increase when horses consumed a daily selenium intake above the current NRC-recommended level of 0.1 mg Se/kg dry matter. This indicated that the NRC recommendation might not be adequate to support maximum glutathione peroxidase activity in the horse.

• At the beginning of the repletion phase, whole blood Se values were similar among the LS, SP, and SS treatment groups; however, within 28 days, the whole blood Se value for the LS group was significantly lower than the AS, SP, and SS groups. Within 154 days, the SP and SS groups had a greater amount of whole blood Se compared to the LS and AS groups. Glutathione peroxidase activity showed a similar but delayed response to that of whole blood Se, meaning the enzyme’s values rose at a slower rate. The researchers did not see differences in glutathione peroxidase activity until 56 days after treatments began, compared to 28 days for whole blood Se.
• There was no difference in antioxidant parameters between treatments during either phase, and there was no effect of treatment on vitamin E levels during either phase. Vitamin E is another vital component in regulating the horse's antioxidant status and acts concurrently with Se to prevent cellular damage. There is some evidence that suggests adequate dietary vitamin E can negate a selenium deficiency, and the team believes that the lack of effect of dietary selenium supplements on antioxidant status might be because of adequate vitamin E in the diet.

Based on the repletion data, the team ultimately concluded that horses might benefit from greater amounts of selenium in the diet compared to NRC recommendations to increase glutathione peroxidase activity.

So should horse owners add a selenium supplement to their horses' diet?

"Not necessarily," Brummer said. "The objective of this study was to evaluate the effect of a total dietary selenium intake (meaning that found in both forage and grain) of 0.3 mg Se/kg DM compared to the NRC recommendation of 0.1 mg Se/kg. In other species 0.3 mg/kg is frequently recommended; however, equine data is lacking to support the same amount in horses.

"Many of the commercial horse feeds are formulated to provide enough selenium for a total dietary intake of 0.3 mg/kg," she continued. "Therefore, in the light of (the risk of selenium toxicity), it is extremely important to ask an equine nutritionist (or veterinarian) to make a full assessment of total selenium provided by both forage and grain prior to adding a supplement."

The study, "Measures of antioxidant status of the horse in response to selenium depletion and repletion," will appear in an upcoming issue of the Journal of Animal Science. 

By Kristen M. Janicki, MS, PAS