Feeding Horses that 'Tie Up' Due to RER

Q. I have an upper-level show jumper, and she’s had a few bouts of “tying-up” over the past few years. My veterinarian recently did a muscle biopsy and diagnosed my mare with recurrent exertional rhabdomyolysis (RER). As I understand it, RER isn’t the same as polysaccharide storage myopathy (PSSM, which also causes affected horses to tie up and requires nutritional management), but I’m wondering if diet changes might help?

A. I applaud you getting a definitive diagnosis by having the biopsy done so you know exactly which disease is afflicting your mare. As you state, RER and PSSM are not the same thing, even though both can result in a horse tying up. Researchers believe RER is due to an inherited abnormality in the regulation of muscle contraction and relaxation. There are different forms of PSSM: Horses with type 1 have genetic mutations of the glycogen synthase gene, whereas those with type 2 lack this mutation but show abnormal staining of glycogen when biopsy tissue undergoes microscopic examination.

NSCs and RER

Despite the different causes of tying-up between RER and PSSM, both conditions benefit from lower dietary nonstructural carbohydrate (NSC, the sum of the starch and water-soluble carbohydrates [WSC] in the diet) intake. Veterinarians recommend feeding PSSM horses diets low in NSC, because these carbohydrates have a direct impact on muscle glycogen; however, RER doesn’t affect muscle glycogen stores, so the reduced NSC is for a different reason. Horses with RER might tend toward excitability and tension, and high-NSC diets are more likely to cause these behaviors, which in turn may lead to muscle damage.

RER: Starch Calories vs. Fat Calories

RER is often seen in horses with Thoroughbred ancestry, so research in managing the condition has focused on this breed. Interestingly, mares tend to have higher incidences of RER than colts and geldings. Research using Thoroughbreds with RER showed that, when these horses were fed a moderate level of calories that included a high-starch concentrate at 2.5 kilograms per day, they showed few signs of tying-up when exercised. However, when researchers increased calorie and high-starch concentrate intake, the horses became more likely to tie up. This did not occur if fat provided the additional calories. For this reason, when horses with RER need extra calories, they should derive no more than 20% of their daily calories from NSC, and 20-25% of their calories should come from fat.

While you could control the intake of a regular higher starch feed and add oil for more calories, you might struggle to get your horse to consume adequate fat this way due to palatability. It’s also difficult to know exactly how many calories are coming from NSC versus fat. Often, a better approach is to find a feed specifically formulated as low-NSC and high-fat. Look for feeds that are ideally less than 15% but no more than 20% NSC and at least 10% fat. If you’re unable to feed at the manufacturer’s recommended portion, you will also need to add a low-NSC ration balancer pellet to ensure the diet is properly balanced. Keep in mind that when dietary fat increases, the horse’s vitamin E needs also increase. Commercial feeds typically account for this, but if using oil you should add an additional 600 IU of natural vitamin E for every cup of oil fed.

When feeding hay, choose quality grass hay, as some horses with RER might be sensitive to alfalfa. However, a small amount of alfalfa might be beneficial for horses with a history of gastric ulcers. In Thoroughbreds, consuming hay with an NSC of 17% didn’t increase the likelihood of an RER episode. Therefore, feeding very-low-NSC hay might not be as important for horses with RER as it is with PSSM horses.

Final Thoughts

As with the other tying-up conditions, diet is only one part of the successful management of RER. Horses with RER need to be kept calm, which might mean moving to a quieter stall. In a busy barn where many horses get worked on a schedule, those with RER should be worked first and not left anxiously waiting for their turn. These horses might do better in smaller barns that are quieter and where they can be handled by the same person every day. Routine is important. Turnout for horses with RER is beneficial, because it helps keep them moving, and being turned out with another compatible horse may help keep them calm. Days off should be kept to a minimum and, when possible, shouldn’t be consecutive.