Selenium is a micronutrient that horses require in trace amounts to support cellular function. Selenium levels in equine diets vary widely depending on where their forage is grown; levels of this trace mineral in the soil determine its final concentration in plants.

Selenium imbalances are common in horses. While it is more common for horses to be deficient in selenium than to get too much, consuming excess amounts of selenium is potentially life-threatening.

Acute selenium poisoning is usually caused by accidental over-supplementation with inorganic forms of this mineral. The majority of horses suffering from acute selenium toxicity die suddenly before symptoms are noted.

Horses can also develop chronic selenium toxicity, also referred to as alkali disease, due to prolonged consumption of plants high in selenium. Symptoms include hair loss and hoof abnormalities.

Treatment depends on whether the case is chronic or acute. Options for acute poisoning are limited and generally ineffective. Most horses die within two days of acute poisoning, even with medical intervention.

The treatment for chronic alkali disease is supportive and involves pain management, hoof care, removal of the selenium source, and dietary modification.

Selenium in Equine Diets

Selenium (Se) is a non-metallic element occurring naturally in sedimentary rocks, and is found in trace amounts in all feeds and soil. [1][2]

All natural elements exist in multiple chemical forms. Selenium is usually found in one of the following forms: [1][2]

  • Elemental Selenium: the pure form of selenium is relatively non-toxic and is an essential trace element
  • Selenite and Selenate: are inorganic forms that plants can use to make organic forms such as selenomethionine
  • Selenide: both selenite and selenate are converted into selenide in animals, which can be toxic to cells at high levels
  • Selenomethionine and Selenocysteine: amino acids that contain selenium and are incorporated into plant and animal proteins

Selenium in Soil

The geographical distribution of selenium in soil is uneven, and regions worldwide vary from being deficient to having toxic levels. The plants growing in these regions have varying selenium content; consumption of both selenium-rich and selenium-poor plant products can have harmful effects on humans and animals.

In the United States, the Northern Great Plains (eastern Wyoming, North Dakota, Montana, and Nebraska) and the Intermountain Region (Nevada, Utah, Idaho, western Montana, northern Arizona, western New Mexico, and far west Texas ) are selenium-rich.

The Great Lake region and large sections of the Northwest, Northeast, and Atlantic Coast are deficient in selenium. [3][4][5]

Selenium Map of Canada & the USA | Mad Barn Canada

Plant Uptake

Selenium intake in the equine diet is primarily determined by the amount of this mineral in forages and grains consumed by the horse.

Equine ingestion of selenium starts with selenium uptake from the soil by plants, the specifics of which vary depending on the plant species, environmental conditions, and selenium concentration in the soil.

Plants can be categorized according to their selenium uptake as follows: [6]

  • Hyperaccumulators: plants that accumulate more than 1,000 milligrams of selenium per kilogram of dry weight (> 1000 mg/kg dry matter) are hyperaccumulators. These include plants belonging to the Astragalus species, the Prince’s plume (Stanleya), and the Xylorhiza family. While hyperaccumulation can be beneficial to the plants, it can pose a threat to grazing animals. However, these plants are frequently very unpalatable, so grazing animals are unlikely to eat them unless they have no other options available
  • Secondary-accumulators: secondary accumulators are plants that can accumulate selenium in the range of 100–1000 mg/kg dry matter. These include False Flax (Camelina sativa), Sunflower (Helianthus), Broccoli (Brassic