Q&A: Feeding an Overweight PSSM Horse

A KER nutrition advisor recently helped a horse owner with the following question.

I own five-year-old Quarter Horse mare that weighs about 1,200 lb (545 kg), body condition score (BCS) of 7, so fleshy and overweight. Current ration includes low-quality pasture and a salt block. Exercise is on-and-off due to soundness issues, but she sweats heavily when she is worked, even minimally. Diagnosed with PSSM recently, the mare fatigues quickly and is easily irritated. Her management will change soon, and she will be required to be stalled for 10 hours each day. What can I do to help her nutritionally? Would a low-starch feed help her once she’s removed from pasture half the day?

A diet low in starch and high in fat and fermentable fiber is often recommended for horses that suffer from exertional rhabdomyolysis, such as polysaccharide storage myopathy (PSSM).

Because you have described your mare as overweight, feeding a low-starch, high-fat feed while simultaneously reducing exercise might compound the weight issue.

A ration balancer will provide all the vitamins and minerals she requires, plus some additional protein that may be not be available in the hay she will be fed when housed. Ration balancers are typically low in nonstructural carbohydrates and fat.

One way of encouraging your horse to use its internal fat stores for exercise is to withhold concentrate for six hours prior to exercise, as this causes an increase in available fatty acids to be used during work. Small amounts of hay can be fed prior to exercise, however.

Once you have initiated the housing and exercise changes, you may find that your mare requires more calories than the hay provides. At this point, consider adding fat to her diet. Recommendations for the absolute amount of fat in the diet for horses that suffer from chronic tying-up is specific to individual horses. Generally, between 0.5-2 cups of oil is added to fiber-based diets, or dietary fat is provided in the diet by feeding a high-fat (10-12%) commercial feed or by adding fat supplements (rice bran, flax, concentrated fat products). However, based on the response of your mare to these dietary changes, more or less fat may be needed to reduce the incidence of a tying-up episode. Again, adding fat to her diet would only be appropriate if she is unable to maintain her weight on a forage-only diet once she is stalled for part of the day.

Because you noted that your mare sweats heavily in response to light exercise, it is important to replace the lost electrolytes in sweat to avoid electrolyte imbalances. Offering free-choice salt and an additional electrolyte such as Restore SR will ensure electrolytes are replaced and available for normal muscle and nerve function.

Vitamin E is an important antioxidant involved in neuromuscular function, and research has shown horses with muscle disorders can respond positively to daily supplementation. Natural-source vitamin E (d-alpha-tocopherol) is the best form of vitamin E to offer, especially when compared to synthetic forms (dl-apha-tocopherol). Nano•E is a water-soluble, natural-source form of vitamin E that is rapidly available to the horse, providing protection to the muscle. Nano•E should be added to the diet in relation to the amount of dietary fat in her diet.

Nano-E allows targeted antioxidant support, so you can increase the amount given during stressful periods that may trigger a tying-up episode or after intense training sessions without affecting other nutrients in her diet.

These types of horses benefit most from maximizing turnout (drylot or pasture) and a daily exercise program. Reducing the number of days off and replacing some of these days with hand-walking or slow work under saddle can help reduce the incidence of tying-up, especially in the fit performance horse.

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