The Complicated Relationship Between Horses and Grass
Every horse owner’s idea of paradise is to see their horse grazing on a lush green field; however, there are a number of serious issues that should be considered prior to allowing grazing on rich grass.
Horses are foragers and ideally should consume 1.5-2% of their body weight in forage/hay per day. When choosing the type of forage, horse owners must factor in the horse’s overall health, age, weight, and activity level/energy needs. The same pasture will comprise vastly different component proportions at different times of the year. While grass contains water, vitamins, minerals, protein, sugar, starch, and structural fibers, spring grass contains a very large percentage of water and a much lower concentration of indigestible fiber than grass found later in the year.
The high water content found in lush spring grasses may lead to loose manure in many horses. As well, spring grass has increased concentrations of highly fermentable carbohydrates which can wreak havoc on the equine digestive tract. In particular, horses and ponies that are overweight with insulin resistance (Equine Metabolic Syndrome) are at risk for complications from consuming grasses high in sugar content.
Non-structural carbohydrates (NSCs) include starches, fructans and sugars (glucose, fructose and sucrose). Horses susceptible to NSC-related diseases such as Equine Metabolic Syndrome (Cushings), laminitis and Polysaccharide Storage Myopathy (PSSM) should be turned out on dry lots and fed hay that is lower in NSCs. Young grass tends to be higher in sugars while mid and late-bloom grass is generally lower in sugar.
Cool season grasses store their sugars in the form of fructans. When temperatures remain below 40 degrees fahrenheit at night, certain grasses do not grow. As a result, fructans are stored at higher concentrations in the stems and leaves, making the grass more desirable to grazing horses. Unfortunately, the equine digestive tract is unable to break down fructans in the stomach and small intestine. Instead the easily fermented fructans are passed into the hindgut, resulting in the rapid production and build-up of lactic acid. This accumulation of lactic acid has been shown to be a direct cause of colic and laminitis in horses maintained on pasture.
It is essential that horses be introduced to grass (especially new spring grass) gradually so that their digestive system be allowed to adapt. For overweight or otherwise susceptible horses, unlimited grazing should be avoided year-round. Horses that are allowed to graze on lush green grass must be fed hay as well. The hay will provide much-needed fiber (lacking in the grass) which will help to maintain the health of the digestive tract.
While allowing free-choice grazing on lush pasture seems like the ideal situation for owners to provide their horses, the consequences can be deadly. Limiting intake by allowing short durations of grazing or by using a grazing muzzle are unfortunately necessary measures which must be taken for at-risk horses. Ultimately significant weight gain, colic, and laminitis are potential outcomes of all-you-can-eat grazing. It is best to speak with a veterinarian about the overall body-condition, energy needs, and metabolic status of your horse and what type of grazing is indicated when developing a turn-out program.
