Understanding how grass grows and how horses use sugars in grass and hay can help you better manage your equine charges.

Grass is grass, right? Wrong! That lovely green pasture you’ve diligently watered and kept weed-free can be like Jekyll and Hyde. If your horse is at risk for grass founder or has a low tolerance for high levels of sugar, a pasture that might be perfect feed in the morning can be his biggest enemy in the afternoon.

Sugars are building blocks for plant growth. Grasses create sugar during daylight hours by using carbon dioxide, water, and energy from the sun via photosynthesis. The sugar made by day is then turned into fiber for cell walls and energy for other necessary life processes. During the night sugar sources are generally depleted. Thus, the safest time of day for horses at risk for grass founder to graze is early in the morning.

Sarah Ralston, VMD, PhD, associate professor of Animal Science at Rutgers University, says 15 years ago very few people—not even equine nutritionists—paid much attention to the sugar content in pasture grasses and hay. “In the past 10 years, however, thanks in large measure to the efforts of Katy Watts (whose studies/articles are found at www.safergrass.org), people have become more aware of the need to pay attention to the cycle of sugar production in grass, for horses that are extremely prone to laminitis,” says Ralston.

Timed Grazing

Tania Cubitt, PhD, equine nutritionist in Middleburg, Va., with Performance Horse Nutrition, says grass sugar content is highest in the spring, when grass is growing most vigorously. “We also need to be concerned in the fall, for at-risk horses, if there’s been fall rain and regrowth of cool-season grasses, though this is usually not as drastic as in the spring,” says Cubitt.

“If you have a laminitic horse, he shouldn’t be eating lush green grass; he should be kept in a drylot and fed hay,” she says. “If a horse is just sensitive to sugars and hasn’t shown any signs of laminitis—and can still tolerate some grass—then early morning turnout is best.”

“We did some research at Virginia Tech (a PhD project by Bridgett McIntosh) in which we used 15 horses and monitored them for 36 hours,” says Cubitt. “We kept five of them in stalls and fed hay, and 10 roamed a big pasture. Every hour (for 36 hours) we took blood samples, pasture samples, and fecal samples. We did this twice in the spring, once in summer, once in the fall, and once in winter, to get the seasonal effects as well as the daily effects.”

There are some variations in sugar levels—not only daily and seasonally, but also regionally around the world. “In different regions people are getting conflicting results in pasture carbohydrate research, and some people try to claim that their results are correct and others are wrong,” says Cubitt. “I think there are regional differences. Here in Virginia we found by 4 to 5 p.m., we had peak sugar accumulation. We recommend that at-risk horses be turned out early in the morning, such as between 5 a.m. and 10 a.m.”

This can change, however, from day to day. “On a cloudy day there’s less photosynthesis and sugars do not accumulate as fast,” Cubitt adds. “When the sun is out, there’s more produced. Then in the evening the plant uses sugars and grows during the night. By early morning the sugar level has usually dropped because the plant has used its energy store and it’s safer.”

Ralston points out that in some situations the sugar content might still be high in the morning if the grass has undergone stress during the night. If it’s cool at night, the grass might not grow much. After a frost, especially, the sugar level can still be high.

“In a stress situation such as drought, excessive heat, or an overnight freeze, grasses will not have converted the sugar they synthesize during the day into fiber for growth overnight, and remain relatively high in sugar in the morning,” she explains.

“If horse owners understand this, they can make management decisions based on the weather,” says Cubitt. “Grasses may not grow in a drought, yet the sun still beats down on them and they still accumulate sugar. The short grass in a pasture that didn’t get watered (and didn’t grow much) will be full of sugar. Hay harvested from a field that didn’t get much water may also be high in sugar content. The sugar keeps accumulating because the grass can’t use it up.”

Testing Grass

Some grasses are more problematic than others, and this can vary greatly. “There are many species that have a wide variance in sugar levels (under the same weather conditions and time of day),” says Ralston.

“For instance, there are hundreds of subtypes of fescue, and some are very high in sugar and some are not,” she notes. “Kentucky bluegrass tends to be lower in sugar than the fescues, but this is a very vague generalization. Testing the grass won’t help you determine if a certain grass is high or low in sugar because it will all depend on the time of day you take the sample.

“If you want to see how high the sugar level might go—and scare yourself to death—you could take a sample at 4 p.m. in the afternoon, and you might never want to turn horses out there again!” Ralston adds. “By the same token, if your horses are grazing this pasture quite happily with no problems, there’s no need to worry. The most important thing is to know whether a particular horse is at risk or not.”

Cubitt recommends testing hay samples if you have at-risk horses. “What it has at point of harvest will be what you’ve got,” she says. “It won’t fluctuate on a daily basis like the pasture. Whatever happens to the grass before it is cut as hay will be the telling factors.”

Owners need to realize the vast majority of horses are not at risk from sugars in pastures. Ralston says that only about 10-20% of horses might be at risk. Ask your veterinarian about your horse’s risk.

Weight Watchers

“On the flip side, this isn’t all bad, because people have become more conscious about carbohydrates,” says Cubitt. “Generally, the way we feed horses has become more low-calorie, which on the whole has been a good thing for many horses. Our horse population was getting a little like the human population—overweight. Many horses are pasture ornaments and pleasure animals, rather than working in a strenuous career, and we need to feed them accordingly."

“This may mean using low-calorie balancer feeds or some of the low-carb feeds that you also feed at a fairly low rate because they’re meant for the inactive or obese horse,” adds Cubitt. “So indirectly, even if the horse wasn’t at risk, some calorie control (such as limiting his time at pasture) may still help him.”

Unnatural Pastures

Even though a horse’s natural habitat is free-choice, full-time grazing, that natural environment was native prairie grasses and slow-growing bunchgrasses, not the lush, irrigated fields of improved “tame” pasture/hay of a modern horse farm.

“The horse evolved eating grasses that contained more fiber and much less sugar,” says Cubitt.

Irrigated pastures or pastures in humid climates contain “improved” plants that produce more pounds of forage per acre. “Pasture and hay grasses today were developed for the dairy and beef industry,” she notes. “The ability to accumulate sugars was desired because the more sugar, the more milk the dairy cow would produce or the faster a beef animal would grow. The native bunch grasses are not as thick, nor as tolerant to trampling, as many tame grasses.”

It takes many more acres of bunch grass (than it would of cultivated tame grasses) to feed a horse. Rangelands in the West, for instance, provide good grazing for cattle and horses, but it might take 10 acres per animal, or even 100 acres per animal on arid ranges of Nevada where grass is sparse.

“I grew up in Australia, where some areas are very arid and it takes many acres to feed an animal,” says Cubitt. “By contrast, my grandparents live in New Zealand, where it’s greener and you can have several animals per acre. “With domestication, horses often live in stalls or small pastures and can’t move around, whereas in the wild they walk almost continually,” she explains.

If horse owners stop to think, they realize their pastures are not a natural environment for the horse. This can make a difference in the decisions they make on how to manage their horses most appropriately.

Take-Home Message

Understanding how grass grows, and how sugars are used and available in grass and hay, can help you better manage your overweight and at-risk horses. High sugars in grass and hay can be dangerous for horses prone to certain problems, so keep an eye on the weather, grass growth, and horse weight.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.

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