Feeding Flaxseed

Is this plant safe to feed your horse?

A horse owner recently sent The American Quarter Horse Journal a column from a horse magazine pointing out potential toxic effects of feeding flaxseed to horses. Because flaxseed is commonly fed as a supplement, I thought it would be a good idea to look into the issue. Here is what I have learned.

The Plant

Flax is a small, blue-flowered plant that grows throughout the United States. The plant contains cyanogenic glycosides that can produce highly toxic hydrogen cyanide (prussic acid) if the plant cells are damaged. This occurs most frequently when a cold snap freezes the plans.

Cyanide contained in the damaged leaves affects the animal's respiratory system and can cause sudden death. Other plants that can produce cyanide after sudden damage to their leaves include sweet peas, sudan grass, Johnson grass, sorghum and white clover.

Get the latest horse health updates in the award-winning American Quarter Horse Journal. The Journal is delivered monthly and is packed with valuable information for any horse enthusiast.


Flax produces a tiny seed that is commonly fed to livestock. It has also become popular in human diets because it is thought to lower serum cholesterol, prevent heart disease, lower blood pressure, improve skin quality and stimulate the body's immune response. Some studies in laboratory animals suggest that it can also aid in the treatment and/or prevention of some forms of cancer.

Because the tiny flaxseed is hard and difficult to chew, most of it goes undigested if it is not processed prior to feeding. Traditionally, flaxseed has been crushed to produce linseed oil and the resultant flaxseed (also called linseed) meal has been used as a protein supplement for livestock.

Similar to most grains, the composition of flax can vary based on variety, environmental factors and methods of analysis. But generally, flaxseed contains 42 percent oil, 20 percent protein and 28 percent dietary fiber. Flaxseed also contains high concentrations of Omega 3 fatty acids, which horses need to maintain good health. Omega 3 fatty acids are not produced by the body and must be obtained through the horse's diet.

Research conducted at the University of Guelph demonstrated that horses suffering from sweet itch, a common skin disease caused each summer by Culicoides insects (midges), improved dramatically following daily supplementation of their diet with one pound of milled flaxseed. Other benefits of flaxseed supplementation include stimulation of the immune system, relief of arthritis and reduction of pain due to inflammation, an increase in the ability of cells to take up oxygen, improved skin and hair coat and scavenging of free radicals. Because it is high in dietary fiber, it can also help prevent impaction and sand colics.

Whole flaxseed, as opposed to milled flaxseed that has been ground, should be soaked in cold water for two to six hours and then boiled for 10 to 30 minutes to soften it and destroy any prussic acid that might be present. It is then fed, about ¼ cup (measured dry before soaking), as part of a bran mash once or twice a week per horse per feeding. The whole seed keeps well in storage for a long time, but ground flax will deteriorate fairly quickly. Many of the newer horse feeds contain milled flaxseed or linseed meal that will be listed on the label.

Get trusted horse health advice from the widely read American Quarter Horse Journal. The Journal also brings you event coverage, horse training tips, showing advice, stories of ranch life, event calendars and much more. Subscribe today!


There has been some concern that flaxseed causes hypothyroidism in horses, but I could not find a reliable source that confirmed this fear. Hypothyroidism is poorly understood in the horse. AQHA is funding research to learn more about the disease.

It also appears that although the flax plant can become toxic if severely damaged by an early frost, the seed and oil are relatively safe if fed in small amounts. As discussed earlier, the seed can always be boiled to destroy any toxic prussic acid if you're concerned about feeding flaxseed.

If you have any questions on flaxseed supplementation, talk to your local university extension equine nutritionist.

For more information on keeping your horse healthy, consult an American Association of Equine Practitioners-member veterinarian in your area. For a list of members, go to or call (800) GETADVM.