Muscle cramping (tied-up muscles) associated with exercise is a fairly common problem in athletic horses. It has been termed azoturia and Monday morning disease, but is now known as exertional rhabdomyolysis (ER). Whatever it’s called, however, tying up episodes can be prevented or minimized by changes in the horse’s management and diet in most instances.
In the 19th and early 20th centuries, ER was seen most commonly in draft horses kept on full grain rations during a short layoff, such as a weekend. Occasionally, these horses experienced muscle problems when they went back to work—hence the term Monday morning disease.
In recent years we’ve learned there are actually several different forms of this syndrome, with different causes. Some horses have sporadic, infrequent, or one-time episodes with no underlying abnormality in muscle function. Others continue to tie up because of a defect in which the muscles collect an abnormal amount of sugar. The latter problem is an inherited condition in some heavy-muscled horses such as Quarter Horses, draft horses and warmbloods. Another type of tying up occurs most frequently in young Thoroughbreds (especially fillies) in race training.
Dr. Stephanie Valberg at University of Minnesota, and Dr. Beth Valentine at Cornell, and now at Oregon State University, did the research that revealed the details. “Valberg calls the condition that occurs in heavy muscled horses PSSM (polysaccharide storage myopathy), while Valentine calls it EPSM (equine polysaccharide storage myopathy),” says equine nutritionist Stephen Duren. It amounts to the same thing: too much sugar in the system.
ER is now broken down into two categories: sporadic and chronic. Chronic conditions are due to specific inherited abnormalities and can be of two distinct types—polysaccharide storage myopathy, seen in heavy muscled horses; and recurrent exertional rhabdomyolysis (RER), which shows up in Thoroughbreds, Standardbreds and Arabians.
“This type of tying up only happens occasionally, and there’s no underlying functional defect in the muscles,” says Valberg. “There’s just an imbalance that must be corrected.” The horse may have done something out of the ordinary, such as a soft, unconditioned horse being overworked, working too long and hard in hot weather, or straining a muscle during exertion. Or he may have a mild injury that caused him to work the muscles on one side of his body harder than usual.
“If a horse has an underlying respiratory infection such as influenza and is trying to exercise, this can also cause muscle soreness,” explains Valberg. “If the diet isn’t balanced (not enough vitamin E and selenium, or enough salts during hot weather when the horse is working and sweating) this may predispose a horse to problems. If you make sure the horse has a regular training regime in which the amount of exercise is not excessive for his conditioning, and the diet is well balanced for vitamins, minerals and electrolytes (and not overdo the grain), the horse will do fine if rested and put back into work or training,” she says.
PSSM/EPSM (Sugar Imbalance)
Horses that tie up repeatedly are a different story. “These horses tend to store too much glycogen (sugar) in the muscles because they are sensitive to insulin and don’t seem to be regulating energy metabolism properly in their muscles,” says Valberg. “They tie up when they start to exercise. They develop this problem early in life and are often just walking or trotting when they tie up—showing signs within the first 20 minutes of exercise. Once we recognized that, we devised some training and nutritional approaches to try to eliminate the tying up episodes.
“We found that you have to get these horses very gradually fit, and start very slowly with just a few minutes of exercise a day. You keep increasing it and use interval training to improve their exercise tolerance. This enhances a lot of the enzymes in the muscles to allow them to use energy and get things back into better balance. This helps keep them using the glycogen they keep storing in the muscles,” she says.
“We also change their feed so they get as little starch and sugar as possible. We don’t use grain or sweet feed, molasses or rich hay. We give them another form of energy they can utilize better, such as fat. Most of these horses are very easy keepers so they don’t need grain, and it’s important to keep them from becoming fat. The feeds specifically designed for horses with this problem are high in fat and fiber and low in starch,” explains Valberg. Her research group developed Re-Leve for this type of horse.
These horses also need to be outside and exercising, rather than confined, but are better off in a large drylot or a not very green pasture, since lush grass contains a lot of sugar.
Thoroughbreds (and Arabians and Standardbreds) face a different issue. “You usually don’t see them tying up until they are in training,” says Valberg. The Thoroughbreds generally don’t start tying up until they are in race training and quite fit.
“They tend to be two- and three-year-olds. Young ones have more problems than mature horses, and it’s usually the more nervous ones that are affected, such as nervous two-year-old fillies,” she says. The tying up episodes are usually associated with excitement and happen when they are being held back by an exercise rider, even if they are walking.
Valberg discovered that these horses tended to have more tying up episodes if they were standing in a stall for two days before the exercise. “It’s better to get them out every day and not give them days off,” she says. Most horses in race training would do better if kept in small paddocks where they can move around and self exercise.
Why do these horses tie up? “We think they inherit an abnormality that triggers tying up when they get excited,” Valberg says. “There are certain lines of Thoroughbreds, for instance, that are more predisposed to this. We think part of the reason mares are more predisposed to this problem is because they are more easily stressed and upset, particularly when they are cycling.”
Duren says that many of these horses, whether they suffer from PSSM or RER, respond well to a high-fiber, high-fat, low-starch, low-sugar diet. “Many of these horses have an increased ability to store muscle glycogen, so you need to keep their blood sugar low, so they don’t keep packing those cells full of sugar,” explains Duren.
With some horses, all you need to do is eliminate or greatly reduce the grain portion of the diet to halt the tying-up episodes. “Horses with mild symptoms respond to just taking away the grain and replacing it with a vitamin/mineral supplement (the nutrients that were being supplied in the grain). But horses with severe symptoms need their hay changed, as well,” he says.
So high-grain diets are out. Most of the starch calories can be replaced by fat. A horse can usually tolerate a pound of fat in the daily ration better than 15 pounds of grain. “In general, these horses should be fed good quality hay, and if they need extra calories for energy (for a strenuous performance career), the fat can be added by feeding it over alfalfa pellet or beet pulp rather than grain,” says Duren. Plus, both alfalfa and beet pulp are high in fiber. But isn’t beet pulp high in sugar? That’s easily dealt with. “When you soak beet pulp and dump the water off, this takes away the sugar, even if it was beet pulp with molasses added,” Duren says. Another good energy feed: Re-Leve, which Valberg’s group formulated, in part, to provide enough calories for hard-working horses.
The feed also has to be very palatable, because athletic horses become very finicky when they have to eat that much.
Cutting sugar intake is not easy. “Many people think they should put the horse on a grass hay rather than alfalfa, since alfalfa is richer and may have more calories. But most grass hays actually have more sugar, here in the West, than an alfalfa hay. You can’t look at hay and determine sugar content; you have to analyze it. So if a horse is very sensitive to this problem, modifications have to be made in the hay portion of the diet, as well,” explains Duren.
Some people, knowing that a tying-up horse needs a high fiber, high fat diet, add too much fat. “If the horse’s level of work is not strenuous, this may be too many calories and the horse ends up being overweight,” says Duren. Even if it was a hardworking horse, if he’s laid off for awhile or not back to peak performance yet after a tying up episode, don’t overdo the fat. A hardworking horse may be able to utilize one to two cups of vegetable oil per day, but if he’s only doing moderate or light work, this is too many calories.
Before you change a horse’s diet, it’s important to have the problem diagnosed. Some horses tie up once, and this is generally not an issue. But if a horse ties up again, it might pay to have some testing done, to find out what is going on. Management of the horse is also important, taking care in how he is exercised and paying attention to the exercise routine. “Consistency of exercise and managing the horse’s environment can be just as important as diet,” says Duren.
COMMERCIAL FEED OPTIONS
There are now a lot of low-carbohydrate feeds on the market. “They contain more energy than hay, but don’t have near the calories of grain, and don’t have the sugar content. Some of these feeds can be used as a base, and you can add more fat to them if needed,” says Stephen Duren, equine nutritionist.“The problem with many feed labels is that they don’t tell you how much starch is in the feed,” says Dr. Stephanie Valberg. For that information, “you may have to ask the dealer’s nutritionist. There are many high-fat feeds available that are also high in starch, and these won’t work for horses that tie up,” she says. Look for a product that’s at least 10 or 12 percent fat, but low in non-structural carbohydrates (NSC), which is how the starch is usually measured. “For these horses we want the NSC level in feed no higher than 15 percent,” says Valberg. Always check labels for NSC and fat content. Amy Gill, PhD (equine nutritionist in Lexington, Kentucky) says that up to 20 percent of a horse’s total calories can be fat. “This includes the fat in forages and concentrates. Now that we are using rations that are high in fat and high soluble fiber, most diets are about 10 to 15 percent (of calories) from fat. The fat can also be supplied as vegetable oils (corn, soybean, wheat germ oil, etc.), rice bran or rice bran oil.” She cautions against use of animal fats. “Most horses won’t eat them and they’re not as digestible, for herbivores. They do better with vegetable sources of fat,” she says. Some of the specially designed feeds for horses that tie up include Re-Leve (Hallway), Respond (Agway), and XTM (Nutrena). “The XTM is a high fat, high soluble fiber feed with just a little bit of oats in it. Even though it’s about 25 percent starch, this supplement is only part of the diet, and your goal is to get the total diet below 15 percent starch,” says Gill. Soybean hulls—the material that’s left after the oil and meal are removed—are used in a lot of horse feeds. “This is a very soluble, highly digestible fiber. Microbes in the hindgut can utilize almost all of it, and it picks up water and helps keep the gut healthier,” she says. And that’s important. Grain is digested in the stomach and small intestine, whereas forage (the natural diet of horses) is mainly digested in the hindgut (cecum and colon). “The goal of many of these feeds is to maximize use of the hindgut, which is the horse’s natural way to digest food, and still supply enough calories for the level of work they do—in a way that won’t upset the hindgut or cause acidosis like grain can do,” says Gill. —HST
The Problem With Domestication
Equine nutritionist Stephen Duren feels that horsemen have inflicted a number of problems on today’s horses by using a lot of unnatural feeds—giving horses grain and hay instead of pasture, for instance. Horses that are allowed to live in a natural environment, on native grass rather than being kept in stalls and fed artificial diets (which includes lush green “tame” pastures), rarely suffer from Cushings, insulin resistance or tying up. “The horse is a foraging animal. If we feed as much forage as we can, and the minimum amount of grain and extra calories needed for a hard working horse to maintain body condition, we rarely have problems,” he says.
“In horses that tie up, often the owners are pouring a lot of grain into them. Some horses require that many calories, but in these instances we need to be more creative in how we provide them, using fat calories and fiber calories like beet pulp,” he explains.
Dr. Amy Gill agrees and says, “We’ve been selectively linebreeding and inbreeding for hundreds of years, and the way they live is also unnatural. We’ve created metabolic problems like tying up,” she says.
Selenium, vitamin E and other antioxidants are often given to help prevent damage to muscles in horses that tie up. “These are important, but if your horse has PSSM, they won’t prevent the problem. Tying up is a symptom, in many cases, rather than a disease. You have to figure out the cause and try to deal with it,” says Duren.