Feeding for Polysaccharide Storage Myopathy (PSSM)

PSSM is a genetic flaw that prevents the use of glycogen as a source of fuel by the muscle cells. In the normal horse, glycogen is broken down into sugar to be used by muscle for energy. Horses with PSSM can’t complete this metabolic process, which leads to excess glycogen build-up in the muscle, resulting in cramping and weakness.


In order to manage PSSM from a dietary standpoint, the total diet must be evaluated in order to minimize the amounts of sugar and starch, while utilizing fat and fiber for calories.

Assessing the Value of Hay and/or Pasture

Analysis of the hay/pasture is essential for the proper management of diets for PSSM horses. Hays can vary in nutritional content, with some containing Non-Structural Carbohydrate (NSC) values over 15%. In addition, the amount of sugars and starch in the hay can vary immensely, so knowing the NSC value of your hay allows for a much more accurate total diet evaluation.

Estimating NSC Values for Various Forages
  • The NSC value in legumes forages such as alfalfa, clover and peanut hay have a range of 8%-13% NSC, with an average of around 11%.
  • The NSC values for various grass hays (Orchard, Timothy, Brome or Bermuda) range from 8%-20%, with an average NSC of around 12%.
Feed and/or Grain Assessment

The major source of glycogen in a typical horse’s diet is found in the feed or grain, so identifying the amount of sugars and starch (NSC) in feed for the PSSM horse is essential. Feeds and grains high in sugars and starch (NSC) should be avoided. Energy (calories) should be provided in the form of digestible fiber and fatty acids from fats or oils. Other nutrients, including vitamins, minerals and amino acids, can be provided in concentrate form such as diet balancers.

The key to proper dietary management of PSSM situations is knowing the nutritional values of the forages, feeds and supplements that are available. Progressive Nutrition® utilizes a ration-balancing program that incorporates these nutritional components into a “total diet nutrient assessment” to provide optimal feeding management for these horses. Progressive Nutrition® has incorporated the most recent and up-to-date nutritional research on PSSM into our ration-balancing program.

Current Nutrient Guidelines

Currently, we recommend feeding PSSM horses a diet containing less than 15% of the total calories coming from NSC and a minimum of 15% of the total calories coming from fatty acid sources. Progressive Nutrition® can assist you in a diet evaluation and selection that best fits your feeding situation.