Equine nutrition specialist thinks of horses as ‘carbovores’

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Hay is an important forage source for horses. Photo: University of Kentucky

A specialist in equine nutrition says she considers horses to be “cabovores” because of their reliance on carbohydrates in their diet.

“Yes, horses are herbivores because they eat plants. But I think of them as ‘carbovores’ because carbohydrates are the primary component of a horse’s diet,” Laurie Lawrence told those attending a recent pasture management workshop in Kentucky.

The Pastures Please!! virtual workshop included three expert talks covering the management of carbohydrates in the equine diet, current herbicide effectiveness on weed control, and investments for pasture management.

Professor Lawrence, an equine nutrition researcher in the University of Kentucky’s Department of Animal and Food Sciences, shared information about managing carbohydrates in horse diets.

Horses consume mainly carbohydrates and not all carbohydrates are equal, she said.

There are different digestion processes for different types of carbohydrates. Horses have individualized needs; some need a more highly digestible, high-calorie diet, while others may be better off with a low digestible, lower-calorie diet. How best to meet individual dietary needs can be helped by an understanding of the composition of forage.

The main method of carbohydrate consumption for horses is by eating plants. Plants use photosynthesis to create carbohydrates out of water and carbon dioxide. Along with the synthesis of glucose during this process, plants also release oxygen. Understanding complex carbohydrates allows horse owners to be aware of how their horse uses carbohydrates for energy, she said.

The digestion of carbohydrates occurs in multiple parts of the digestive tract. Starches and sugars are absorbed as glucose in the small intestine, whereas other complex carbohydrates pass through the small intestine to be broken down more effectively in the large intestine.

Cellulose, hemicellulose, pectin and fructan have chemical bond structures that can only be broken down in the large intestine. The microbes in the cecum and colon break down and ferment the compounds, which creates volatile fatty acids that the horse absorbs to use as caloric energy.

As an example, Lawrence gave the nutritional breakdown of a common hay, mature orchardgrass. On an as-fed basis, its carbohydrate content is about 64%. Of the specific carbohydrate components in orchardgrass hay, low percentage digestible compounds such as lignin and cellulose are of minimal use to the horse in terms of nutrients. High percentage digestible compounds such as fructan and starch are of high use to horses.

Hay and pasture grazing are also different.

“Typically, we make hay when the plant is somewhat more mature. It has accumulated some mass and we would see the seed heads and more well-defined stems,” Lawrence said.

“On the other hand, when our horses are grazing, they are grazing material that is much more vegetative. It’s in an earlier stage of growth, so the plant leaves are much finer and much softer without the seed heads.”

Lawrence explained that while both hay and pasture are grasses, forage that is mature, such as hay, is much lower in calories because there are fewer digestible nutrients.

Forage that is immature and vegetative is much higher in calories because it has more digestible nutrients.

The need for digestible, high nutrient forages is influenced by each particular horse’s needs.

For example, a lactating mare, weanling or performance horse needs a high nutrient diet. Conversely, a sedentary “pasture ornament” would be better off maintaining a diet consisting of fewer digestible nutrients.

High sugar forages are sometimes preferable because they cause weight gain and encourage lactation, but they can also raise the blood glucose and insulin levels in horses, which may be a concern for horses with metabolic disorders. Too much fructan in the diet can also cause a cascade of internal issues that lead from disturbed microbial community to diarrhea, colic or laminitis.

With that in mind, some horse owners may decide to feed high digestible nutrient-rich forage but limit the intake. It is a strategy Lawrence cautioned might lead to other problems such as chewing wood on fences, run-in sheds or barns.

“Horses are driven to chew on things, and in normal environments they would probably be spending 60 to 70% of their time grazing and chewing. When we restrict forage intake, then we can have some side effects from that we may not want,” she said.

Her talk is below:

Article courtesy University of Kentucky College of Agriculture, Food and Environment.

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