Danger in the Grass

Martin W. Adams, PhD, PAS – Equine Nutritionist for Southern States

When the spring season is here, it brings the opportunity for horses to enjoy fresh pasture. While pasture affords a nutritious and economical way to provide forage for your horses, there are some general and specific management concerns that need to be kept in mind to assure the health and safety of your pasture pals and avoid laminitis in horses.

A general concern for horses in the spring is the risk of founder or laminitis from excessive consumption of soluble carbohydrates (starch and sugar) found in the pasture. Laminitis is inflammation of the laminae in the hooves of the horse, and can result in temporary to permanent lameness. Care must be taken to introduce the horse to pasture slowly in the spring, gradually increasing the amount of grazing time to allow the intestinal microbes to adjust to the new diet and prevent a large intake of soluble carbohydrates.

The greatest danger of laminitis occurs with cool season pasture grasses, like fescue, orchard grass, timothy, etc., as they are the forages with the highest concentrations of fructans. Fructans are sugars that are not digested in the small intestine, but are fermented in the large intestine to produce lactic acid and increase the risk of colic and laminitis. Researchers have induced laminitis in the horse experimentally by overdosing the horse with fructans, so horse owners need to be especially cautious with cool season grass pastures. Research has also shown that cool season pasture grasses are less stressed and produce less sugar, including fructans, when they are well fertilized. So a good management practice is to have regular soil sampling and analysis to maintain proper fertilizer application which will provide a safer pasture for your horses.

Another general concern for the horse on pasture is excessive consumption of calories. While many horse owners don't have enough pasture for the number of horses they own, others may have too much. A mature horse on good pasture of two to three acres or more can easily consume more calories than it needs to meet and maintain daily energy requirements and can become obese. Obesity can result in a condition known as insulin resistance, which causes abnormally high levels of blood glucose and insulin. Equine nutrition research has shown that horses with a history of grass founder are insulin resistant and have higher blood levels of insulin. Insulin has been used to cause laminitis experimentally in horses and high blood levels of insulin are now considered a predictor of laminitis. So prevention of obesity with good pasture management can reduce the risk of laminitis in your horses.

A specific concern for pastured horses is management of horses that are known to have insulin resistance and have the problem we now call Equine Metabolic Syndrome (EMS). The health concern which older horses may get is Equine Cushing's Disease (ECD). Most pony breeds and certain horse breeds (Morgans and Paso Finos) are more likely to have EMS, but any horse of any breed can develop it. Older horses are more likely to have ECD (over 20 years of age) and horses with EMS tend to be intermediate in age (8 to 18 years of age). One of the biggest problems with these conditions is that they are associated with an increased occurrence of laminitis. Management of pasture grazing must be carefully monitored to reduce the risk of obesity and intake of soluble carbohydrates that might trigger laminitis in horses with these problems.

Following are some guidelines to use to properly manage pasture for your horses and keep them safe from colic and laminitis:

• Introduce horses gradually to pasture in the spring, only allowing 30 minutes to one hour grazing the first day for sensitive horses and slowly increasing the time by an hour per day until a full day of pasture grazing is allowed. A grazing muzzle to limit intake on a newly introduced pasture or to reduce intake continuously for an easy keeper on an abundant pasture is a good management tool to utilize to reduce intake.
• Perform regular soil sampling and analysis of your horse pastures and use it to determine proper rates of fertilizer application (limestone for pH control as well as nitrogen, phosphorus and potassium) for proper growth and persistence.
• Score the body condition of your horses and limit the time on pasture for any horse that has a body condition score of greater than 6.0 (on a scale of 1.0 to 9.0). For horses with an existing metabolic problem, such as EMS or ECD, you may want to maintain them at a lower body condition score of 5.0. Other methods to limit pasture intake would be to use a grazing muzzle on any obese or problem horse on the pasture, or to move an obese or problem horse to an exercise paddock or sacrifice lot with minimal pasture availability.
• Severely limit or abstain from grazing horses that are known to be insulin resistant or have EMS. This is a special concern in the spring or fall when pastures can be lush with new growth and high sugar content. Sugar content of grasses is higher in the afternoon than in the morning and sugar content is lowest at night, so grazing should be restricted to the safest times of the day to graze, early morning and night times.