In the past two decades, says forage researcher, Kathryn Watts, pastures in the United States and Europe have been undergoing a transformation. Increasingly, grass varieties that concentrate energy and are hardy even under extreme weather conditions are taking hold, according to Watts, who owns a company called Rocky Mountain Research and Consulting in Center, Colo.
"Plant breeders have been focused on increasing carbohydrate levels in new grass varieties," she explains. "High sugar levels in forage increase palatability and intake. This increases weight gain and milk production in cattle. In addition, high carbohydrate levels allow plants to grow earlier in the spring, and stand up to drought and harsh winters."
All of which may make beef and dairy cattle more productive but can spell big trouble for horses. That's because these grasses contain concentrated sources of energy, which can overwhelm the equine digestive system, leading to colic and/or the metabolic train wreck that ultimately produces laminitis.
Of course, starch, which is found in grains as well as growing plants, is the most familiar culprit in carbohydrate overload in horses. But recently researchers have been focusing on the effects of another type of plant sugar called fructan, which is found primarily in cool season grasses.
Fructan is similar to starch but horse keepers will want to make note of it's peculiar characteristics: Fructan, which has been implicated in precipitating laminitis in susceptible horses, is found not only in cool season grasses but in hay made from those grasses as well.
Known as nonstructural carbohydrates (NSC), both starch and fructan are produced through photosynthesis, a process in which plants use sunlight and carbon dioxide to make sugar. Starch, comprised of a string of glucose molecules, is the primary energy source in most species of grasses, but some popular pasture plants also produce fructan, which consists of fructose and glucose.
"About 15 percent of the world's plants produce fructan," says N. Jerry Chatterton, research leader for the USDA Agricultural Research Service's Forage and Range Research Laboratory in Logan, Utah. "Most of those plants also produce starch so they are not mutually exclusive. It's an ancillary form of carbohydrate storage."
In fact, a key difference between fructan and starch is how they are stored within plants. Because starch is stored in the same space where photosynthesis takes place--the chloroplast found primarily in the plant's leaves--photosynthesis tends to slow when large amounts of starch accumulate. Fructan, on the other hand, accumulates in the plant's vacuoles, with the highest concentration in the stem, so large stockpiles of the sugar won't necessarily put the brakes on photosynthesis.
"On a cool morning, when temperatures are low but the sun is bright and shiny, photosynthesis goes on its merry way but the plant isn't growing because it's cold, so fructan accumulates," says Chatterton. "Cool season grasses keep making more sugar whereas the warm season plant shuts down."
No one is certain why plants produce fructan, rather than the more common carbohydrate, starch. Some plant geneticists speculate that the substance may be a drought-adaptation or a means of protecting the plant against frosty temperatures and other environmental extremes.
"Is the plant accumulating fructan as a carbohydrate balance or is it accumulating it to protect itself as an antifreeze?" says Andrew Cairns, PhD, principal research scientist in the plant genetics and breeding department at the Institute of Grassland and Environmental Research (IGER) in Wales. "It's feasible that through evolution, the chloroplast in certain grasses have not been as effective so the plant has found another way to store carbohydrate. We don't know for sure."
What is known is that fructan levels depend on temperature, light intensity and moisture content. "It also depends on the molecular weight--that's the length of the chain [of fructose] in the plant--as well as the genetic potential of the plant to produce fructan," explains Annette Longland, PhD, senior research scientist in the plant genetics and breeding department, at the IGER.
In addition, NSC levels within a given plant can also vary widely throughout a day. Levels of this sugar tend to be lower in the morning, says Longland, but climb steadily as the hours of sunlight increase the rate of photosynthesis. In one sampling of two perennial ryegrass species, she found individual plants generally contained 15 percent fructan at 6 a.m. but the percentage would jump to 26 percent by 2 p.m., under normal weather conditions.
Plants can also be loaded with NSC at what would seem like the least likely times of year. To illustrate this point, Watts describes some forage samples taken from her home property in Colorado last October when the days were sunny but the nights were frigid. "These grasses were in no way lush," she says. "The grass was dark green, half-dead, and freezing down for the winter with about 40 percent dry matter. When tested, the grass contained 27 percent sugar and fructans. If a horse is eating 20 pounds of dry matter a day, that's 5.4 pounds of sugar and fructan."
What's more, hay--generally considered a safer forage for laminitis-prone horses---can retain high levels of fructan. Why? Because cut grass can continue storing fructans between cutting and baling, says Cairns. "I've studied individual leaves and they remain alive for about 50 hours and can photosynthesize carbohydrate," he says. "When you cut your field in the sunshine, that material is lying there, disconnected from the rest of the plant. So it's photosynthesizing but has no where to send the carbohydrates."
The weather conditions when grasses are cut and hay is baled are also important because fructan, unlike starch, is water soluble. "When it rains on cut grass, some of the fructan leaches out. If it stays wet and doesn't dry fast, respiration [utilization] continues and you lose more carbs opposed to grass that is cut and dried quickly," says Chatterton.
Protecting At-Risk Horses
While most horses can graze on grasses high in NSC without consequence, owners of cresty, easy-keeping ponies, insulin-resistant horses, those with hormone imbalances and others at-risk for laminitis are wise to try reducing NSC in the forage they feed. Admittedly this can be a tough goal to achieve given that sugar levels within plants can fluctuate from hour to hour. In one load of hay, a bale cut in the morning may have much less NSC than a neighboring bale cut late in the day.
Nonetheless, says Watts, there are things you can do to adjust a horse's feeding and turnout program to reduce the risk of fructan-induced grass laminitis: "You really have to be aware of what you're doing and only make one change at a time and see if that helps your horse." With that in mind, Watts offers the following suggestions:
Have your pasture grasses and dry forage tested. The level of NSC intake required to trigger laminitis is unknown, but the general rule of thumb, says Watts, is that the risk drops with the level of the sugar.
Lab tests cannot isolate fructans, but they do measure it along with sugar and starch percentage as NSC. "With a horse who has been laminitic or has acute metabolic problems associated with glucose intolerance, I try keeping NSC below 10 percent until the horse is stabilized," says Watts.
Watts recommends sending pasture and hay samples to Dairy One Cooperative Inc., an Ithaca, New York-based forage laboratory that can measure NSC, among other things, through forage analysis. Fees range from $12 to $77 depending on the testing protocol. (Dairy One maintains an online feed composition library, which provides NSC averages for some forage species. To view this chart or for information on individual forage analysis, go to www.dairyone.com or call 800-496-3344).
Identify forage alternatives. Depending on your climate, you may consider reseeding your pasture with warm-season grass species that are lower in fructan, such as Bermuda grass, bluestem or switchgrass. Of course, you'll need to stick to grasses that will thrive in your climate and will tolerate grazing. If you plan to seed or reseed a field, contact your local extension agent or farm/ranch service company for information on horse-friendly forages available in your region.
If reseeding your pasture is not feasible, consider acquiring "low-carb" hay made from warm-season grass species. Of these, Bermuda Grass hay is one of the most popular, and according to Dairy One, it ranges from nine to 17 percent NSC with an average of 13 percent. Grass hay also averages around 13 percent and legume hay, around 11 percent. In contrast, oat hay measures an average of 23 percent, with peaks up to 30 percent. (Though low in fructan, alfalfa is high in starch; evidence suggests that the extra protein in alfalfa can stress the digestive system when fed in large quantities.)
Another fiber alternative is beet pulp, a by-product of sugar beets, usually soaked in water--so that it puffs-up like rice--before it is fed. Beet pulp is often mixed with a grain ration but it is digested like forage. To reduce sugar intake, says Watts, purchase "non-molasses" beet pulp and soak the serving for at least 30 minutes, then rinse it to remove any sugar residue.
Soak hay before serving. Because sugars and fructan are water soluble, you can significantly reduce its levels in hay by soaking the forage for 30 to 60 minutes in a large bucket. In 2003, Watts conducted a study in which she compared sugar levels in hay samples soaked for various amounts of time, using room temperature and hot tap water. She noted that soaking hay for 30 minutes reduced NSC levels by an average of 19 percent. After 60 minutes, NSC levels dropped an average of 31 percent.
Soaking hay in hot water for 30 minutes yielded the same results as using cold water for 60 minutes.
Practice sensible pasture management. Mowing and weeding your fields not only promotes the health of your pasture, it may be better for your horses. "Grasses grow from the bottom, pushing the leaf out above them," Cairns explains. "If you've cut the leaves off, it stimulates growth in the remaining tissue. That growth requires carbohydrates so the reserves are used to make more leaf material." To minimize the accumulation of carbohydrates in pasture grasses, keep your pasture between four and eight inches in height, mowing as often as necessary to maintain this range.
Some studies suggest that stress from overgrazing or nutrient deficiencies can increase fructan levels in grasses. To avoid taxing your pastures, rest each field every two months. If you're unsure whether your soil requires additional fertilization, contact your local extension agent or farmer's co-op and schedule a soil sample analysis.
Restrict grazing time. Sometimes the best thing you can do for horses who are susceptible to laminitis is reduce exposure to tempting but risky forage. A grazing muzzle can restrict the amount of grass a horse grasps in one bite and this may be enough to protect a horse even when fructan levels rise.
Another option is to limit the horse's time on pasture, allow him to graze in the morning when fructan levels are lower, rather than late afternoon or early evening. Says Watts, "The safest time to graze is early morning; after a night when the minimum temperature was above 40 degrees Fahrenheit; and on grass that is in a vegetative (leaf-growing) stage. The most dangerous time for laminitis-prone animals is late afternoon or evening on a sunny day; when grass is heading or flowering; on grass that is stressed; and anytime of the day when the previous night's temperatures were below 40 degrees."
Even as "super grasses" containing high NSC levels grow in popularity, there is hope that forage better suited for equine needs will also be developed. In fact, researchers at the U.S. Department of Agriculture's Forage and Range Research Laboratory are collaborating with Watts to study 24 forage species planted in Colorado and Utah last year. They hope to harvest plant tissue samples in 2004 and 2005 to measure and compare NSC content at different times of the day and year.
But this is just a start, Watts says. "The equine community needs to be more informed and politically active to fund and encourage further research into forage for horses. We need better communication between horse people and forage researchers."
In the meantime, careful management can protect laminitis-prone horses from the effects of high-fructan forage. We may even learn safer management practices from horse keepers of years past. "There's some folklore out there that supports what we're learning today," says Cairns. "My wife's grandmother bred ponies 50 years ago and she never turned them out during the day, only at night. Of course, there were no studies then, and horse owners didn't know the causes of colic or laminitis. But they were on to something."
This article originally appeared in the April 2004 issue of EQUUS magazine.