Horses Grazing Summer Pastures

Horses Grazing Summer Pastures

Regulate grass intake to prevent excess weight gain when fields are at their greenest.

Photo: iStock

Regulate grass intake to prevent excess weight gain when fields are at their greenest

Succulent swathes of summer grazing pasture can provide temporary relief to a horse owner's wallet in the form of reduced hay and feed bills. But at the same time, many owners report unwanted weight gain in horses consuming this lush forage--particularly if the animals come out of winter months already overweight (British researchers recently determined more than a quarter of 127 study horses were obese heading into the spring grazing season).

"Horses love fresh pasture," says Brian D. Nielsen, PhD, PAS, Dipl. ACAN, professor of equine exercise physiology at Michigan State University's Department of Animal Science. "They've been on hay all winter long, and as soon as there's a little bit of green in the pasture they will ignore the hay and go for the green.

"It's very palatable ... and it's high in sugars so there are a lot of calories there," he continues. "It's a great situation because you (might not need) to feed hay anymore, and you can put some weight on hard keepers. A little extra condition on a horse may not be that big of a deal, but overweight horses with insulin resistance (a reduction in sensitivity to insulin that inhibits cells' ability to transport glucose out of the bloodstream and store it as glycogen) are more inclined to develop laminitis (inflammation of the interlocking leaflike tissues attaching the hoof to the coffin bone). On lush green pastures blood glucose can go up. If the insulin goes too high that might precipitate laminitis."

While there's no "one-size-fits-all" way to control a horse's potential weight gain and ensure he still gets the nutrients he needs, understanding what you're up against can help you make informed decisions.

Timing Pasture Consumption

It's likely many owners are familiar with this scene: You turn your horses out on summer pasture in the morning, they drop their heads, begin to eat, and stay that way for the rest of the day. But have you ever wondered how much grass your horse is eating and whether he's getting too much? Paul D. Siciliano, PhD, from North Carolina State University's Department of Animal Science, and his colleagues are working on ways to predict how much horses eat on an hourly basis and how owners can use that information to determine optimal turnout intervals for meeting nutritional needs. This knowledge can help preserve pasture, save money, and prevent health problems caused by overeating. (See reasons why weight gain can be so harmful to your horse's health at

"Horses can consume somewhere between 2 and 3% of their body weight in dry matter when turned out to pasture each day," says Siciliano. "If you do some calculations, in a well-managed pasture that amount of calories potentially consumed is more than what the horse requires. Of course that doesn't go on forever, and at some point the horses will eat all the pasture and owners have to purchase hay. In the end, they end up spending more money than they need to if they let their horses eat all the pasture they want."

Siciliano's current research focus is on regulating pasture intake to prevent excess weight gain and wasted nutritional resources. He has conducted these studies within a narrow set of parameters that might not apply to every horse; however, he's noticed some similar trends among his and other research groups' studies.

Siciliano says the study results revealed that in general, if you have a well-managed pasture with eight to 10 inches of forage and minimal bare ground and weeds, most horses on a maintenance-only diet (such as a lightly ridden horse) are going to fulfill their nutritional requirements in eight to 10 hours of grazing. "You also have to take into consideration that pasture is dynamic in that forage available for grazing changes with the seasons of the year," he explains. "Additionally, you aren't going to have that eight to 10 inches all the time, such as with a drought situation. You'll also have to supplement the mineral requirements."

He also discovered that restricting pasture for weight and health requirements wasn't as straightforward as previously thought. The researchers studied eight horses; each was confined to a 5-by-5-meter (about 270 square feet) grazing cell containing nontoxic endophyte-infected tall fescue (some varieties of this forage can cause pregnancy complications in mares) for four hours before they were moved to a new, same-sized grazing cell for an additional four hours. In the first four hours they ate nearly 60% of their eight-hour caloric intake. Then they slowed down for the next four. "This showed us that restricting pasture intake for three to four hours may not be as effective a means of reducing caloric intake as is commonly thought," he says. "We found that the rate of pasture intake per hour goes up when horses are restricted (in their turnout time). If you turn your horse out for four hours he may eat the same as if you turned him out for eight."

Pasture Forage Sugar Concentration

Another important aspect of grass consumption involves the plant's biology: Plant growth rate and sugar content (the raw material that the plant will eventually turn into fiber, protein, and energy for growth, or to storage forms of carbohydrate) fluctuate throughout the seasons and according to weather conditions. Thus, a pasture might be low in sugar one week and much higher the next. This sugar concentration is important because, as Nielsen earlier acknowledged, high sugar (which is a simple carbohydrate) can lead to weight and health problems.

Kathryn Watts, agronomist and consultant at Rocky Mountain Research and Consulting Inc., in Center, Colo., has researched grass sugar levels thoroughly. She says a plant's ability to make sugar relies on sunlight, which drives photosynthesis. "We don't have the epidemiology to back this up," says Watts, "but theoretically you're going to have more problems with (grasses having) high sugar concentration in sunnier areas versus the cloudier areas."

Plant stress--created by conditions such as drought--can limit growth and, thus, influence sugar concentration. "If there's a limiting factor for growth and the sun is shining, the sugar concentration is going to increase," she says. "Stress plus sun equals high sugar. When sugar concentration reaches set genetic trigger point, storage carbohydrates start forming for later use, after the growth limiting factor is removed. Warm-season grasses like Bermuda form starch from excess sugar, while cool-season grasses like orchard and timothy and broadleaf weeds like dandelion form fructan. Both (grass types) are nutritionally dense and contribute to obesity."

Another example of stress is cold. Although less common during summer, cool and sunny days cause many chemicals working inside plants to slow down and even stop. "Research done in a growth chamber has shown that in sunny and cool conditions, concentration of sugar can double or triple in a couple of weeks," she says. "So if you live in a cold and sunny place (you'll) have a lot of sugary grass."

Yet another example of stress is a lack of nutrients. If a plant lacks nitrogen, it won't grow, but it will continue making sugar. "Nitrogen, phosphorus, and sometimes potassium deficiencies may cause high sugar in grass," says Watts. "If you put on nitrogen fertilizer to correct the nutrient stress, you may end up with less sugar per mouthful but more sugar per acre because you'll have lots more grass. So it still matters how you manage your horse's intake."

Nielsen says horse owners often try to solve weight gain issues by putting horses in neglected or weedy pastures, but that plan can backfire. According to Watts, dry grass that looks almost dead might still be high in sugar because of the conditions limiting its growth. "A lot of the weeds in neglected pasture are much higher in sugar than the grass it replaced," she says. "Some of the high-sugar weeds are dandelion, plantain, clover, and thistles. These have higher potential for sugar production than grass under stressful conditions. You might think that your horse won't eat these but when the stress hits the plants, the sugars concentrate and the weeds become more palatable. And when most grass is dead, the sugary dandelions and plantains are still alive."

Weight Gain Prevention

There is no simple solution to preventing pasture weight gain and resulting health issues, as every horse is different. Some can eat grass continually and gain little weight; others gain weight quickly.

For horses with metabolic conditions or that are particularly susceptible to weight gain, consider limiting turnout time to two hours. You can also limit pasture time during periods of rapid plant growth, or during times of plant stress, such as the ones described: during a drought or when the weather is cool and sunny. "A grazing muzzle might be the practical option, especially if you want your horses to remain together," says Nielsen. "You can drylot your horse and feed hay, but you'll be paying for that, which is irritating when you have free pasture available. But sometimes for your horse's well-being you have to feed hay. You can also simply ride your horse more. Sometimes a horse left out at pasture isn't ridden as much as one that's in the barn, but exercise can compensate for the extra calories eaten at pasture."

Every pasture is also different, even those in the same region or even at the same farm. Watts recommends having an agronomist who understands horses' nutritional requirements evaluate your pasture.

Take-Home Message

Sugar, starch, and fructan concentration in pasture plants might vary significantly due to many circumstances. "We may be able to control some of these factors by better management of the pasture," says Watts. "Other environmental factors outside our control will require us to manage horses' intake to prevent overconsumption."

Currently researchers are determining how to better predict dry matter intake and how region, range conditions, seasons, horse type, and other factors impact grass consumption. "The important message is that a horse has a great ability to consume relatively large amounts of pasture and meet his requirements on pasture," says Siciliano. "The more we understand how to control that intake, the more efficient we're going to be in maintaining the pasture and the health of the horse. "

About the Author

Sharon Biggs Waller

Sharon Biggs Waller is a freelance writer for equine ­science and human interest publications. Her work has appeared in several publications and on several websites, and she is a classical dressage instructor.

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