Points to ponder about your horse pastures’ sugars

If you follow veterinarians’ and equine nutritionists’ advice, then the bulk of your horse’s diet is forage, either fresh pasture or hay. Some horses, however, might have health issues, such as insulin dysregulation (ID), that preclude free, ad libitum access to pastures. In this article we will review ID, plant anatomy and sugar content, and learn how to safely manage horses with ID on pasture.

What Is Insulin Dysregulation?

Simply explained, horses with ID produce excess amounts of insulin or respond to insulin abnormally. The pancreas produces insulin and releases it into the bloodstream after the horse consumes meals high in sugars/starch. Insulin prompts the body’s tissues, such as skeletal, muscle, or the liver, to take up glucose from the bloodstream for important metabolic processes such as energy production.

In cases of ID, surplus insulin produced results in high circulating levels of the hormone. This hyperinsulinemia can directly affect tissues of the foot (lamellae), resulting in the extremely painful disease laminitis. Currently, veterinarians report hyperinsulinemia-associated laminitis (HAL) as the leading cause of laminitis in horses.

The most important management strategy for ID, which is the hallmark feature of equine metabolic syndrome (EMS), is reducing horses’ insulin levels. This can be achieved, at least in part, by reducing the amount of sugar in their diets.

“Horses with ID must be closely managed if considering turnout onto pasture. It is not impossible, but careful monitoring of their insulin status and managing turnout to the times of day or days where nonstructural carbohydrate (NSC) content would be at its lowest is critical,” says Carey Williams, PhD, professor in the Department of Animal Sciences and Equine Science Center at Rutgers University, in New Brunswick, New Jersey.  

“Many ID horses might be too severe to have pasture turnout and should have an all-forage diet with NSC containing no more than 10-12%,” she adds.

To truly appreciate why pastures are not suitable for some horses, we need to understand the sugars, fructans, and NSC components of grass plants.

Plant Anatomy

Whereas animals have a musculoskeletal system to support their tissues, plants have cell walls rich in the sugars (carbohydrates) cellulose, hemicellulose, and lignin, which provide the cells strength and rigidity. This explains why nutritionists refer to them as structural carbohydrates. And while cellulose, hemicellulose, and lignin are made of sugar molecules, these molecules are in long chains (polymers) the small intestine cannot digest. Fibers containing structural carbohydrates are instead fermented in the large intestine to produce short-chain fatty acids horses use as their primary energy source.

Inside the plant cells are soluble sugars. These are either individual sugar molecules such as glucose and fructose or short chains of sugar molecules linked together such as sucrose, which is made of one glucose plus one fructose. These sugars are the primary drivers of insulin secretion from the pancreas when they are absorbed from the horse’s small intestine into the bloodstream.

Here are some useful definitions of the carbohydrate types relevant to horses’ diets:

Sugars are molecules containing carbon, hydrogen, and oxygen. These include simple sugars such as glucose and fructose, which again are made of one sugar molecule, or sucrose and lactose that are made up of a small number of sugar molecules bonded together.

“Sometimes you will also see these called ethanol-soluble carbohydrates (ESC) on a forage analysis,” says Williams.

Fructans are a type of short-chain carbohydrate that escapes digestion in the small intestine and is instead fermented in the hindgut. Fructans are primarily composed of long chains of fructose (not glucose as in starch, see below) and are found in higher amounts in forages than in grains.

Water-soluble carbohydrates are listed as WSC on a forage analysis and include both the sugars and fructans.

Starch, also a carbohydrate, is a long chain of glucose molecules bonded together lightly, making them easily digested by enzymes in the small intestine.

Nonstructural carbohydrates include all the sugars, fructans, and starches in a horse’s diet.

Fiber includes the structural carbohydrates (cellulose and hemicellulose) measured in forage analysis on a feed tag with tests such as neutral detergent fiber (NDF) and acid detergent fiber (ADF).

“These values indicate the percentages of different forms of fiber and lignin in horse feeds,” says Gordon Jones, PhD, extension agronomist and associate professor in the Department of Crop and Soil Science at Oregon State University, in Corvallis.

Again, while these fibers are carbohydrates (polysaccharides), the type of bonds between the individual sugar molecules are not broken down in the small intestine like those in the starch molecules. Instead, the hindgut microbial population ferments these carbohydrates.

Safely Allowing Pasture Access for ID Horses

Williams says fresh pasture offers a variety of nutritional and non-nutritional benefits over dried forages (hay), including the following:

• Fresh pasture provides higher levels of vitamins than dried forages or grains. This includes not only vitamin E but also other fat-soluble vitamins such as A and D₂, as well as C and many of the B vitamins;
• It reduces likelihood of colic;
• Lowers incidence of gastric ulcers;
• Decreases exacerbations of severe equine asthma;
• Increases bone mineral content in young horses;
• Allows horses to behave naturally, roaming and eating for most of the day and interacting with herdmates, which improves equine welfare;
• Can significantly reduce hay costs; and
• Reduces the need to deal with manure and bedding materials from stalls and dry lots, which saves labor costs and time, bedding materials, and spreading/removing manure and bedding from the farm.

The best way to allow horses to graze safely is to understand how plants produce and utilize sugars.

How and When Grass Plants Manufacture Sugars

Plants use photosynthesis to produce energy from the sun, converting that energy into sugars and other nonstructural carbohydrates they use as building blocks for growth. These sugars and nonstructural carbohydrates are the ones easily digested in the horse’s small intestine that stimulate insulin secretion.

“Sugar levels are highest during the late afternoon or early evening because plants accumulate sugars as they photosynthesize throughout the day,” Jones explains. “These sugars are used as an energy source by the plant at night for maintenance and growth.”

Thus, daily fluctuations in pasture sugar levels occur. Sugars are typically lowest in the morning, just before daybreak, when the sugars have been depleted overnight and photosynthesis has not yet ramped up for the day. Then, the sugars accumulate in the grasses during a nice sunny day and peak in the late afternoon or early evening before sunset.

“Because the timing of sunrise and sunset shifts through the course of the year, and because plants don’t abide by daylight saving time, it’s best not to follow specific timings on a clock but, rather, understand that sugars will typically be lowest just before the sun rises in the morning,” says Jones.

However, in addition to the time of day, other factors also affect sugar levels, including the following:

1. Season (time of year).

When nighttime temperatures dip below freezing, such as in the very early spring and late fall, the plant’s sugar use can decrease dramatically. This means sugar levels remain high at night and are further elevated by continued photosynthesis the next day.

“Pastures can therefore be higher in sugars in both the spring and fall, so we should limit grazing these pastures in horses with ID or with a history of laminitis at these times,” Jones explains. “If sugar intake is a concern, limiting the duration of grazing and using a grazing muzzle can help reduce intake of sugars.”

2. Plant species.

Cool-season grasses typically found in the northern half to two-thirds of the U.S. tend to produce more NSCs than warm-season grasses because they use different photosynthetic pathways. Cool-season grasses include tall fescue, orchardgrass, ryegrass, bluegrass, timothy, and bromegrass. These cool-season pastures are often unsafe for horses at risk of laminitis, and these hays should be tested for sugar content before they are fed to horses prone to laminitis. Warm-season grasses do not accumulate sugars like the cool-season grasses do. Examples of warm-season grasses, typically found in the southern U.S., include Bermuda grass, crabgrass, sorghum, switchgrass, millet, and teff.

Jones adds, “Teff is particularly popular for its low NSC content, but even teff, under certain growing conditions, can have high enough sugar concentrations (>10%) to cause problems for sensitive horses.”

Still, plant species alone is a poor indicator of the NSC content of grass because of the effects of weather, climate, stage of growth, and management (fertilization, mowing, etc.). Further, even warm-season grasses could contain NSCs beyond what’s recommended for metabolic horses or those at risk for laminitis.

“Legumes like alfalfa and clover are similar to warm-season grasses because they do not accumulate sugars,” Williams says. “Instead, they typically store starch. However, starch has an upper limit and will typically not go near the 10% maximum for NSC intake in EMS horses. Therefore, feeding a smaller portion of alfalfa can be a good thing for EMS horses given they are not overweight as alfalfa is typically higher in calories than grass hays.”

3. Species maturity.

Stage of maturity has a complex interaction with sugar content of pastures grasses, says Jones.

“In early spring, short leafy pasture can have high sugar content caused by low temperatures,” he says. “As plants begin growing quickly in spring, the sugars produced by photosynthesis are promptly used up by plants to produce new biomass, resulting in lower forage sugar content. When the grasses mature in early summer, fiber from the stems dilutes down sugars which are present. But as flowering occurs and seeds form, starch accumulates in the seed and that starch can cause problems of its own.”

4. Plant stress.

During drought or, again, a cold snap (temperatures dipping below 40 F), plants store sugars.

“This is because the plants continue to photosynthesize and create sugars, but the environmental stress prevents the plants from using those sugars to power growth,” Jones explains. “Therefore, the sugars accumulate in the forage rather than being converted to other compounds by the plant.”

5. Areas of the pasture.

Even in certain parts of the pasture there could be more and less sun. Parts of the pasture typically in the shade most of the day will be lower in NSC than pasture in full sun.

“Therefore, if dividing up your pasture with temporary fencing, consider fencing off a shady portion for your EMS buddy,” says Williams.

Grazing Prohibited for Some but Not All

In Williams’ opinion, most horses are fine grazing pasture 24/7.

“It is only those horses with EMS/ID that I would watch for during peak conditions,” she says. “Pasture is not evil for most horses. It is only a select few that could have problems.”

Jones agreed with Williams, adding, “Pasture can be an excellent source of high-quality feed for many horses that require high-quality feed. Some horses, including growing animals and those receiving moderate to intense exercise, have a high need for energy, and pasture is a good source. However, that high-energy, high-sugar feed is a problem for horses without a great demand for energy, particularly those predisposed to laminitis or EMS.”

Jones draws a parallel between the dietary needs of grazing horses and those of humans to illuminate his point. “A high-school athlete who is still growing and is highly active can consume large portions of carbohydrates such as pasta and sugar without a problem. These athletes need that energy to grow and perform in their sports. However, if you feed that same energy-rich diet to an adult working a desk job or someone in a retirement home, we’d intuitively expect weight gain followed by metabolic issues,” he says.

Based on current recommendations, metabolic horses should be fed diets low in NSCs, at least less than 10%. While NSC content can be measured via forage analysis for hay, this is not so simple with pasture.

“The sugar content of pastures can change quickly (within a day, as described above) and also on a day-to-day basis,” Jones explains. “Because it generally takes a few days to get test results back from the lab, your pasture may have a totally different sugar content by the time the lab results get back to you. The sugar content of dry hay, however, changes very little, so testing hay before it’s purchased or fed is the best and only reliable way to determine its sugar content.”

Take-Home Message

Horses must consume forage and ferment fiber in the large colon to produce energy in the form of short-chain fatty acids. Fresh pasture, however, might contain high sugar levels that could be unsuitable for some horses. Choosing safer times and grasses for your horses to graze allows them the luxury of turnout while safeguarding their feet from laminitis. There is, however, still no guarantee pastures are safe for all grazing horses, and horse owners looking to optimize their horse’s health and welfare must weigh the benefits of pasture turnout against the possible contraindications.