Understanding forage carbohydrates for horses


Horses are different. That’s all I have got to say. Horses are not just taller (and faster) cows. The differences are especially important when trying to understand how to find the right forage to feed them. 

For cattle, basically no forage is too good nutritionally, and forage certainly does not cause metabolic problems just because it is high quality. With cattle, we generalize and say that the higher the quality the better. 

Not so with horses. The purpose of this short article is not to do a deep dive into the complexity of carbohydrate metabolism in horses, but to understand why some horse owners want to purchase ‘low carb’ hay. 

To understand this, we can start with the difference in the digestive tract of cattle and horses. Cattle are foregut fermenters. By that we mean that forage is fermented (digested) in the front portion of the gut – rumen to be specific – followed by enzymatic action in the true stomach and small intestine. 

In horses, the digestion begins in the stomach followed by the small intestine. Starches and simple sugars are enzymatically digested in the small intestine, releasing glucose into the bloodstream.Microbial fermentation and digestion of fiber occurs in the hindgut (colon and cecum). So horses are hindgut fermenters. 

Some horses need to have a diet low in sugar and starch because they have a condition known as equine metabolic syndrome (EMS) or related metabolic disorders. These horses are characterized by obesity, insulin dysregulation and laminitis. They are genetically pre-disposed to gain weight and are commonly called ‘easy keepers.’ 

This metabolic disruption is more complicated than a tendency to gain weight. With EMS, cells become insensitive to insulin, leading to an overproduction of insulin by the pancreas especially after a meal. These elevated insulin levels may last all day. This condition is similar to Type 2 diabetes in humans. Elevated circulatory insulin resulting from EMS is associated with laminitis. 

Laminitis is inflammation of the lamina in the hoof.  Laminitis occurs in both cattle and horses, but horses with EMS can actually develop laminitis on high quality pasture. Euthanasia may be necessary with severe cases of laminitis in horses. 

Obesity and EMS in horses are partly due to genetics. Once a horse has EMS or is obese, forage composition becomes much more important.  Owners of overweight horses, horses with EMS or horses at risk for EMS often want hay low in non-structural carbohydrages. To determine NSC, add the water soluble carbohydrates and starch from the forage analysis. It has been suggested that when the sum of WSC and starch is less than 10 to 12%, the hay can be considered to be appropriate for horses with EMS or at risk for EMS

We need to take a closer look at the carbohydrate profile of hay and what parameters need to be considered to judge if a hay is suitable for horses prone to obesity or with EMS. First, forages contain a mix of structural and non-structural carbohydrates. The structural carbohydrates in a forage are contained in the cell wall material which is only partially digested in the hindgut. The release of energy from fiber fermentation does not cause the elevation of blood glucose which stimulates the release of insulin into the bloodstream. 

Non-structural carbohydrates are estimated by adding the water-soluble carbohydrates and starch reported on forage quality reports. Non-structural carbohydrates contain the simple (glucose, sucrose, fructose) as well as some more complex (sucrose, starch, fructans) carbohydrates. Simple sugars and the glucose produced from starch digestion are absorbed from the small intestine and their absorption significantly affects the release of insulin.  

Some hays are generally low in NSC. Forages such as the warm season grasses bermudagrass and teff are generally lower in NSC than cool season grasses like timothy, orchardgrass and perennial ryegrass . However, mature, first cutting orchardgrass might be low in NSC. The only way to know is get a hay test. 

Consider the two hays below. Both are first cutting orchardgrass but differ greatly in NSC. Hay number 1 would have an NSC of 20.3% (WSC plus starch). This forage was cut in early May and had few seedheads. Hay number 2 was still a leafy orchardgrass but cut at a later stage of maturity. The NSC for the second orchardgrass sample was 6.9%, well within the allowable range for metabolically problematic horses.    Ryegrass hay on the other hand typically has high levels of NSC and would therefore be a poor choice for horses with metabolic problems.

The only way to know the NSC level in hay is to get it tested. 

Confounding all of this to me is that lush green pasture can be extremely high in NSC. Most of the focus on carbohydrate management for metabolic horses centers on hay, but clearly limiting access to lush pasture would also be part of a sound management scheme to limit intake of readily digestible carbohydrates.

If this seems confusing to you, you are not alone. I call Dr. Laurie Lawrence, our pre-eminent horse nutritionist at UK at least once a year for my ‘carbohydrate lesson.’ She reminds me that it is easy to become too focused on low carbohydrate hay. Only a small percentage of horses actually have metabolic conditions like insulin resistance. Providing a well-balanced diet in the form of pasture and hay is the first step. Feeding a mature grass hay may be a good thing for older, less active horses because it is lower in energy and it keeps the horse busy chewing. As Dr. Lawrence is prone to say, a bored horse is up to no good. Always get a forage test for your hay, and insist on one from your hay suppliers. You cannot manage your feeding without it.

Happy foraging. 

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