Our summer has turned into some weekly rains and with that, the green grass growing season has been extended. Grass is the most natural food for horses, but fresh growing grass can be your worst enemy if you have an insulin resistant (IR) horse. Commonly known as Equine Metabolic Syndrome (EMS), insulin resistance presents a special challenge when it comes to grazing. Letting you IR horse gorge on grass unchecked, and you may have laminitis or a serious case of founder on your hands.
Horses love grass because it’s sweet and full of sugar. In grass, the main sugars that the plant uses for energy are called fructans. Fructans are produced during photosynthesis during daylight hours and stored for used during the night. Fructan concentration in the grass can vary depending on the type of grass, temperature, moisture and season. Cool season grasses in areas with cold winters are usually high fructan producers.
Fructan levels spike under the following conditions:
• When there are low nighttime temperatures and warm day temperatures.
• When there are bright, sunny, warm days where plants are photosynthesizing.
• When the plants are in the active growing stage.
• When there is hot, dry weather.
• Anytime the plant is stressed, such as overgrazing.
Horses are at risk of developing laminitis during these spikes and laminitis can appear immediately or more than a week after overgrazing. Other sweets such as bananas, apples and carrots can cause a similar reaction. Natural sugars can have a toxic effect in the body of an IR horse.
High intake of non-structural carbohydrates like sugars, starch, and fructans can result in extremely high blood insulin levels, which often triggers laminitis. It is important for owners of these horses to work with a veterinarian to decide whether the horse can graze fresh pasture or if it should only be fed low-NSC hay. Metabolically normal horses can graze pastures safely as long as they do not become overweight.
In the hot summer months, pasture grasses have greater stability in the sugar and starch levels. But in early spring and fall with cooler nights especially falling below 40 degrees, it can become risky to allow pasture grazing in the early morning for IR horses.
Without testing, being concerned about your pasture and your IR horse is only a guessing game. The only way to know for sure is to have your pasture analyzed. Testing for sugar and starch gives you a clear picture of how your pasture is behaving. Specifically, measuring ethanol soluble carbohydrate (ESC) will tell you the sugar content, the main factor influencing insulin levels.
Tips on Grazing Metabolic Horses
• Work closely with your veterinarian
Check bloodwork regularly to determine whether your horse should be allowed any pasture at all. Management and body weight changes can modify the risk level of grazing for some horses.
• Graze in the early morning
Fructans accumulate during the day when the sun is shining, then decrease overnight as the plant uses them for growth.
• Don’t graze when it’s cold overnight
When nighttime temperatures dip to 45 degrees and below, the plant process that uses up NSC slows down. As a result, NSC levels do not drop overnight and are still high in the morning. A series of bright, sunny days followed by cool nights can cause plants to accumulate very high levels of NSC.
• Don’t graze during or after environmental stressors
Drought and hard frosts can cause plants to increase NCS. Wait a week or more after a drought-ending rain or a killing frost (28 degress) before grazing again.
• Consider season and forage type
Cool-season grasses in temperate climates tend to have very high NSC in the spring and lower NSC in the summer with moderate NSC in the fall. Warm-season grass and legumes NSC do not usually fluctuate as much or get high levels.
