Horses suffering from Equine Metabolic Syndrome (EMS) cannot regulate their blood insulin levels properly. After consuming meals high in sugars and starches, or what we call non-structural carbohydrates (NSC), horses with EMS experience above normal spikes in insulin that they have difficulty managing. This can lead to many dangerous health complications for EMS horses, with the most serious being laminitis. Thus, horse owners are forced to monitor their horse’s daily NSC intake and often ask our team if pastures are safe for their horses.

Is It Safe for Metabolic Horses to Be on Pasture?

Forage is the foundation of every horse’s diet and should be fed at a rate of 1.5 to 2.5% of a horse’s body weight per day. For example, a 1000-pound horse should receive 15 to 25 pounds of forage per day. This can either be in the form of hay or pasture.

Because the bulk of a horse’s daily feedstuff intake is forage, this is where most horses receive the majority of the NSC in their diets. Thus, strategies to reduce NSC intake in EMS horses has led to horse owners feeding expensive, bagged hay with a guaranteed low NSC level, soaking hay to reduce NSC levels, or seeking other low NSC alternatives in their horse’s diets. However, many horse owners wonder if pastures could be a safe option for their EMS horses.

The quick answer to whether or not pasture is safe for EMS horses is maybe, and only under certain conditions and with careful management. Even when following all of the precautions to limit NSC intake from pastures, pasture access may still provide more NSC than is appropriate for many horses with EMS. Things to consider and plan for when utilizing pastures for EMS horses are:

  • The time-of-day EMS horses are allowed to graze on pasture.
  • Environmental temperatures during the night impacts pasture NSC levels.
  • Stress on plants can lead to high NSC levels.
  • Species of grasses and their associated NSC levels.
  • Limit intake or time on pasture to reduce NSC intake.

When to Allow Metabolic Horses to Graze
 
Since the levels of NSC in the plants of a horse pasture are dynamic and constantly changing, understanding when to allow grazing is critical. With careful management and observation, EMS horses can be allowed to graze on pastures with decreased risk, but it does not eliminate the risk.

The basic plant cycle is to accumulate NSC during the day through photosynthesis, with levels peaking in the late afternoon. Then, during the hours of darkness, the plant utilizes the stored NSC to continue its growth. Additionally, it is important to note that most of the NSC is stored at the base of the plant. Thus, the first factor in deciding to allow EMS horses to graze on pasture is the time of day. The optimal time to allow an EMS horse on a pasture would be early morning. It is advisable to take them off the pasture in the mid- to late morning before the plants have time to accumulate higher levels of NSC. On bright sunny days, with horses that are particularly sensitive to NSC, the allowable time on pasture should be severely limited or reduced all together.

The second factor of when to allow EMS horses to graze on pasture would be the time of year. This is because the growing season will impact the species of forages growing in horse pastures. Additionally, environmental temperatures can also influence NSC levels.  Spring and fall are the most dangerous times of year to allow EMS horses to graze unmanaged. This is when both warm- and cool-season forages are in their prime growing season. Thus, NSC levels tend to be higher during these times of year.

Another consideration is when nights are cooler and under 40°F (4°C). At these low temperatures, most plants do not utilize the NSC stored in the day and halt their growth. However, the plants will keep accumulating NSC, making them dangerous for an EMS horse to consume. Alternatively, times when plants suffer stress from drought or even a freeze (under 32°F or 0°C) will also force the plant to accumulate higher levels of NSC and is a time when grazing should be avoided.

Following these guidelines assures that NSC will be at it’s lowest during grazing; however, that does not guarantee that it will low enough to be appropriate for any given individual horse.

The Best Types of Pasture Grasses for Metabolic Horses

For EMS horses, there are certain forages that are preferable to others. In general, warm-season grasses are preferred because they are lower in NSC. When compared to cool-season grasses, they are without fructans, which are sugars that contribute to overall NSC levels of plants. This was recently confirmed in a study published by equine scientists out of Rutgers University. They were investigating NSC levels in forages fed to horses and found:

  • Warm-season grasses (Bermudagrass, Crabgrass) had significantly lower NSC than cool-season grasses (Kentucky bluegrass, Orchardgrass, Fescue).
  • For all species of grass, NSC levels were lower in the morning compared to late afternoon and early evening. However, NSC was significantly higher in the late afternoon/early evening in the cool-season grasses.

The scientists concluded that the warm season grasses they studied would be better options for horses when needing to limit NSC intake. They also advised to allow NSC-sensitive horses and those prone to laminitis to only graze in the early mornings.

Pasture Management Tips for Metabolic Horses

The final consideration for EMS horses and whether to allow them to graze on pastures or not is to manage pastures to help keep NSC levels lower. Not only does general pasture management help, but here are some other key tips.

  • Keep pastures mowed at or near 6 inches. This is especially useful in the spring and fall. Doing this will help keep the plants in a constant stage of regrowth and they will not accumulate as much NSC as a more mature plant.
  • Avoid overgrazing. This is when pasture grasses average 3 inches or lower. In response, stressed plants produce more NSC and are best avoided.  

A final tip is to consider using a grazing muzzle. Briefly, grazing muzzles can reduce intake of pasture 30 to 83%. They also help restrict where on the plant a horse can graze, usually the tips of the plant. Thus, they help horses to avoid the denser NSC base of a plant.

Take Home Message

There are many factors and variables in deciding when to allow an EMS horse to graze on a pasture. By utilizing careful management strategies, some horses can be allowed some access to pasture. If providing pasture access to horses with EMS, they should be adapted to pasture slowly and strictly monitored for signs of laminitis. If you have any questions or concerns, especially if you have an EMS horse, please feel free to contact us for a free, personalized feeding plan.

References

Weinert-Nelson, J.R. et al. 2022. Diurnal variation in forage nutrient composition of mixed cool-season grass, crabgrass, and Bermudagrass pastures. J Equine Vet Sci 110:103836.

Williams, C.A. et al. 2019. Effects of grazing system, season, and forage carbohydrates on glucose and insulin dynamics on the grazing horse. J. Anim. Sci. 97:2541-54.
 

Chris Mortensen, Ph.D.