Question
Analysis of my Montana-grown orchardgrass hay revealed a selenium level of 1.6 ppm on a dry matter basis. I am feeding a 1,100-lb (500-kg) Warmblood mare 20 lb (9 kg) per day divided into three meals. She’s in perfect weight for my use. Based on my calculations, she’s consuming 14 mg of selenium per day, which is high, given the value from the report. Is this calculation correct for her current intake? What are the toxic levels for selenium, and am I approaching this with what I am feeding my mare? She is also fed a low-starch feed, 1 lb (0.45 kg) a day.
Answer
Montana is known as a selenium-rich region, so it is not uncommon for hay grown there to contain higher than average selenium levels.
Your calculation for total selenium intake is correct: concentration of selenium (1.6 ppm is the same as 1.6 mg/kg) multiplied by hay intake (kg). For example: 1.6 ppm selenium x 9 kg hay = 14.4 mg of selenium per day provided from the hay. The feed adds a small amount of selenium, about 0.3 mg. Does your horse have access to pasture grazing as well? If so, pasture could be another source of selenium.
The maximum tolerable levels for selenium is 2 mg/kg of diet, or about 20 mg for an 1,100-lb (500-kg) horse consuming 2% of its body weight in feed. The mare is currently consuming about 1.4 mg of selenium per kilogram of diet. A general recommendation is to keep the total amount of selenium in the diet between 1-10 mg per day and stay below 20 mg per day, as toxicity may develop at this point. The average selenium recommendation for performance horses and larger breeds is between 2.5-5 mg of selenium per day.
Your horse’s selenium status can be measured using serum, plasma, and whole-blood selenium levels. Consult with your veterinarian for additional information on testing and to evaluate your horse’s health status and risk of developing chronic selenium overload.
Finding a low-selenium hay, one grown in an area known to have deficient to low soil selenium levels, might be best. Selenium content can be confirmed with testing, as you’ve done previously. These hays typically have a selenium level of 0.06-0.2 ppm. Using a hay lower in selenium can dilute the diet and reduce the total amount of selenium consumed daily, if it is not possible to switch entirely from the local grass hay you have.
One final note: you are not feeding the low-starch feed as recommended. If your horse has low energy needs, using a ration balancer or micronutrient supplement are the best ways to supply proper nutrition in a low-calorie, low-intake form. However, these products also have added selenium and will supply about 2-3 mg of selenium per day when fed at the correct amount, as they are designed to balance forages with low to adequate selenium levels. Some commercial companies will develop custom mineral and vitamin mixes for clients, and a custom mix may allow you to provide the other essential nutrients without the added selenium.
