Don’t Gamble With Green Grass

For some horses, overingesting certain grasses can lead to laminitis. Learn more about managing these horses and their pastures.

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To determine how to manage horses on pasture, you must first identify the ones most at risk for laminitis. | Photo: iStock

How to manage horses and their pastures to prevent laminitis

Grazing is a natural exercise for equids, but it isn’t without risk. For some horses, overingesting certain grasses can lead to laminitis, a painful, life-altering hoof disease that can be fatal in severe cases. It occurs when the laminae—the tissues that suspend the coffin bone within the hoof capsule—become damaged and inflamed.

So what is it about grass that creates such an issue for these animals? It boils down to the density of the nonstructural carbohydrates (NSCs), which are the sugars (glucose, fructose, sucrose), starches, and fructans that grasses create during photosynthesis. When a horse digests NSCs, his body breaks them down into glucose and fructose, which the small intestine absorbs. This can result in an increase in blood glucose concentration that can trigger an insulin surge.

“In the horses that are predisposed to metabolic disorders, when they ingest a high amount of NSCs, they secrete too much insulin,” explains Kathryn Watts, Colorado-based researcher and founder of “Abnormally high insulin can be a feature of pituitary pars intermedia dysfunction (PPID, formerly equine Cushing’s disease), or they can have high insulin as a standalone condition.”

Shannon Pratt-Phillips, PhD, a professor of equine nutrition and physiology at North Carolina State University, in Raleigh, studies obesity, metabolic issues, and glucose disturbances in horses. “Horses that tend to eat regular diets that are rich in starch and sugar can develop insulin dysregulation (ID), and for horses that are already at risk, consuming something that’s extra high in starch and sugar can send them over the edge where they might have a laminitic crisis,” she says.

Grasses and other plants photosynthesize to create sugar on sunny days. The plants generate more and more sugar throughout the day, so each plant’s sugar content is at its peak by the time the sun sets. If the temperatures are above 40 degrees F overnight, plants use those sugars to grow. So, by the time the sun rises in the morning, each plant’s overall sugar content is low again.

“In the spring and fall the ambient temperatures at night are too cool for the plant to use its sugar stores for growth, resulting in a higher concentration of sugars the next day, and the concentrations just get bigger and bigger every day,” Pratt-Phillips says.

“That can happen at any time of year depending on where you live,” adds Watts. “It is possible to founder (cause the coffin bone to rotate or sink in) a healthy horse in just a couple of hours if they’re predisposed and allowed to overeat grass that’s high in NSCs. Even if the concentration of NSCs in the grass is not maxed out, that same horse might still run into problems if they are allowed to overeat.”

When Is Grass a Risk?

A common misconception is that the only time of year grazing poses a risk to horses is in the spring. But, again, the issue depends more on where you live than what time of year it is. “In marine climates, like the Pacific Northwest, the U.K., and the coastal regions of Australia, there is no seasonality for laminitis,” says Watts. “There isn’t a lot of snow, and the grass stays green all winter long.”  

Is Your Horse at Risk?

To determine how to manage horses on pasture, you must first identify the ones most at risk for laminitis. “Most veterinarians are suggesting to proactive owners that all horses over the age of 15 get tested annually for ACTH (adrenocorticotropic hormone, excessive levels of which can lead to PPID) and insulin concentration,” says Watts. “Most of the epidemiological studies are saying that it’s the undetected metabolic disease that is implicated in most of the new cases of laminitis. So, risk management is all about testing horses to find out which ones are at high risk and then managing that risk.”

Everything You Need to Know About Laminitis in Horses
Everything You Need to Know About Laminitis in Horses

Thanks to modern veterinary medicine and better nutrition, horses are living longer than they used to and have more time to develop diseases such as PPID. Horses over age 15, therefore, are at higher risk of developing pasture-related laminitis than younger animals.

“Not all PPID horses have high insulin,” says Watts. “Those that do not may be able to graze with less risk, but PPID is a progressive disease, and insulin dysregulation may develop later. ID as a standalone condition is not age-­dependent, although symptoms generally do not display until after the horse or pony is done growing.”

Tips for Low-Risk Horses

If your horse is getting plenty of exercise, has been tested for ACTH and insulin levels, and is already in a setting where he has access to pasture 24/7, managing his health boils down to monitoring his body condition so you can make sure he isn’t consuming too much grass. Having constant pasture access means he’ll be naturally adapting to changes in grass throughout the year—the surges in growth in spring and summer and the dormancy of fall and winter. Even with that ongoing adaptation, however, be mindful of intake.

“Do they ever come up for air?” says Pratt-Phillips. “Or is their head buried in grass the whole time? Most horses will walk around and take breaks, but if you have a horse that never picks his head up, that might be a horse that’s more prone to overeating.” That healthy horse can soon become an unhealthy horse by consuming too many nutrients.

“If you have a horse that was on a drylot over the winter, and suddenly the horse has access to a perfectly manicured pasture, then that’s a very different type of situation,” she adds. “You have to introduce them to pasture very slowly, perhaps starting with 30 minutes each day for a week and slowly increasing from there.”

Tips for High-Risk Horses

Horses with signs of high circulating insulin levels (e.g., ridges on hooves, abnormal fat deposits) are particularly at risk of developing pasture-associated laminitis, says Watts. Horses with body scores above 6 (on the 1-to-9 scale, with 9 being obese) or that have PPID or a history of laminitis also need to be watched carefully. For these horses the best time for pasture turnout is before dawn (again, after nights when it hasn’t dropped below 40 F).

“When you rise at dawn, graze your pony on your lawn,” recites Watts. “Sugars rise in the afternoon; for foundered ponies this spells doom.”

Watts says using a grazing muzzle can be a good solution for some horses. “The goal is to minimize intake, but we want them out there (on pasture) for as long as possible, because we want them moving and eating,” she says.

Consider any pony to be at risk for laminitis. Pasture is like an all-you-can-eat buffet, and for ponies, which evolved in harsh environments and often have higher insulin levels and metabolic rates, this can be disastrous.

“Ponies can really pack in the pasture,” says Pratt-Phillips. “They can eat twice as much as a horse can per unit of body weight. So even grass with a relatively low sugar concentration can potentially be problematic.” Grazing muzzles and timed grazing can be helpful when managing your pony on pasture.

The horse with a history of laminitis should probably never have full access to grass, says Pratt-Phillips. “If you don’t have a facility that allows you to keep your horse off grass, you should have that horse wear a grazing muzzle all the time so that they can only eat sparse amounts of grass,” she explains. “You can be more relaxed with these horses if you know your pasture is composed of low-sugar or warm-season grasses (e.g., Bermuda grass, crabgrass, bahiagrass), but I probably wouldn’t risk it.”

Watts recommends every owner pay close attention to their pastured horses: “Every horse with ID seems to have a preferred ‘fat depot,’” she says. “Feel those fat pads on a regular basis. If you have a horse that has a tendency to have a cresty neck, after a couple of hours of grazing, you can sometimes feel their neck harden.”

Any dramatic changes to those fat pads can indicate a reaction to sugar. “A journal is very useful for detecting small changes that occur over time,” she adds. “Measure their neck and write it down, take pictures of their fat depots once a month, have your farrier take pictures of their feet, and compare the changes over time.”

It’s also very important to watch at-risk horses move—every day. Watts suggests owners teach their horses to trot on command, either at liberty or in-hand, so they can monitor changes in their movement, which can be red flags. “For any horse that is high risk for laminitis and is out on grass, I want to see them trot,” she says. “It takes an expert to see changes in the walk, but at trot most people can see it.”

Pasture management--such as fertilization, weed control, and not letting grasses go to seed--is key to reducing grazing horses' risk of developing laminitis. | Photo: iStock

Pasture Management

Managing your horses’ pasture is another key way to reduce their risk of developing laminitis or to allow at-risk horses access to grass. Consider these steps:

  1. Keep your pasture fertilized. “There are a lot of data in plant physiology journals that show that grass that’s deficient in nitrogen tends to be higher in sugar,” says Watts. “If there isn’t enough nitrogen to create protein, the plant can’t grow, and it tends to accumulate sugar.”
  2. Manage your weeds—they can also be high in sugar. Clover, dandelions, plantain, chicory, thistles, and other broad-leaf weeds can be problematic.
  3. Don’t let grass go to seed. “The growth points where the seeds develop is where the sugar accumulates,” says Watts. “As the grass starts to head out, the portion of the plant that’s highest in sugars is now elevated and easier to graze.”
  4. Plant warm-season grasses if you have mild winters. Bermuda grass, teff, and bahiagrass don’t tend to accumulate as much sugar as cool-season grasses such as Kentucky bluegrass, timothy, orchardgrass, brome, and fescue.
  5. Strip graze, or divide your pasture into smaller areas or “strips” that reduce horses’ risk of overeating. As you move horses from one strip to the next, you can maximize the pasture usage without impacting its nutritional value.

Take-Home Message

If you’ve determined your horse is at low risk for developing laminitis, and you can manage pasture quality and quantity, then by all means give him access to it. “Grazing is one of the best things a horse can do,” says Pratt-Phillips. “Grass fulfills almost all the protein requirements, calories, and vitamins that a horse needs.”

With high-risk horses, remember that grazing risks don’t disappear at the end of spring, and every horse must be managed as an individual. “If the insulin stays high,” Watts says, “there are some horses that just can’t ever have grass again.”


Written by:

Corie Traylor is a full-time writer living in Portland, Oregon. She grew up riding horses on her family’s small farm and is currently retraining her OTTB, Bess, to be a dressage horse.

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