Question
We are the lucky owners of a high-performance mare that carries one copy of the polysaccharide storage myopathy type 1 (PSSM1) gene. She’s a four-year-old Quarter Horse in professional reining training. She is always stalled except when she is being ridden, and she is in heavy work five or six days a week. She’s fed free-choice alfalfa hay, 6 lb (2.7 kg) of a commercial pelleted feed intended for performance horses, and a slew of supplements: an all-in-one “muscle recovery” product, fish oil, electrolytes, and free-choice Himalayan salt. She’s had two known incidents of tying-up, one last month and one five months ago. The latest was much more severe than the first. Am I doing everything I can for her from a nutritional standpoint?
Answer
The overarching dietary goal for PSSM1 horses is to provide a low-glycemic, low-insulinemic ration with the appropriate caloric density.
The mare’s current concentrate is high in nonstructural carbohydrates (NSC, such as sugar and starch) and low in fat, especially for a horse with a diagnosis of PSSM1. Changing this aspect of her diet may play a significant role in keeping further episodes of tying-up at bay.
A suitable concentrate would contain less than 15% NSC, 10-12% fat, and 11-13% protein. When choosing a new feed for her, contact the feed manufacturer for this information if it’s not readily available on the feed bag or tag. In addition to changing her feed, you may consider testing the hay for NSC levels, though alfalfa is typically below the 12% threshold suggested for PSSM1 horses. However, alfalfa may not be appropriate because of its high digestible energy content and the possible insulinemic potential of its high protein content.
You didn’t mention the mare’s body condition in your question. Horses with PSSM1 are often easy keepers and prone to weight gain. If your mare’s body condition has crept into the 6+, 7, or higher range, based on the familiar body condition scale of 1 to 9 developed by Henneke, then other dietary recommendations may be appropriate.
After reviewing the supplements fed to the mare, you may consider adding vitamin E, as this antioxidant supports muscle health. Natural-source vitamin E is more bioavailable than any synthetic source, and nanodispersed sources rapidly restore serum status. Nano-E fits this description perfectly.
Another management factor that may help your mare is increased access to turnout. Training and showing schedules often make turnout a difficult option for show horses, but many horses respond positively to this change. Regular exercise is important to horses with PSSM, as it helps manage the muscle glycogen and abnormal polysaccharide in the muscle fibers.
Want a deeper dive? Review Feeding Performance Horses with Myopathies, a paper presented at the American Association of Equine Practitioners Conference by Stephanie Valberg, D.V.M., Ph.D., an expert on equine muscle disorders, and Joe Pagan, Ph.D., founder and president of Kentucky Equine Research.
