Q. Recently, a horse at the barn suffered an impaction colic, and the treating vet said the hay might be partially to blame. I board my horse at a facility that grows its own hay. Each year they take one cutting, typically in September, which seems late compared to other farmers in our area who cut hay in June. Does this have something to do with why the vet thought the hay might have contributed to the impaction?

A. I’m sorry to hear that a horse in your barn suffered a colic and hope the horse recovered without major incident. Impaction colic can be deadly and may be caused by a number of factors, such as dehydration, poorly chewed feed, and poor forage quality. I suspect the veterinarian was concerned about poor forage quality being a potentially contributing factor in this case given the maturity of your hay.

As grasses grow, they become increasingly stemmy, developing a strong stem structure that allows the plant to remain vertical. The components in the plant that provide this strength are known as structural carbohydrates and include such carbohydrates as pectin, fructan, hemicellulose, cellulose, and lignin. Structural carbohydrates exist in cell walls and are difficult for horses to digest. In fact, mammals don’t create the enzyme necessary to break down the bonds that hold some of these carbohydrates together. This requires microbial fermentation.

A portion of every herbivore’s digestive tract is dedicated to forage fermentation. It contains billions of bacteria that produce the enzymes needed to break apart most structural carbohydrates. However, even the microbes struggle with complex carbohydrates such as lignin, and not all cellulose will be fermented. So forages that are high in these structural carbohydrates will have reduced overall digestibility.

This leaves somewhat undigested material to negotiate the flexures (bends) of the large colon. Flexures are locations in the horse’s digestive tract particularly prone to becoming impaction colic sites. As the relative proportion of indigestible structural carbohydrate increases, overall digestibility decreases and impaction colic risk rises.

The more mature a plant is when harvested for hay, the greater the relative proportion of these indigestible carbohydrates. A hay cut a couple of months later in the growing season will be far stemmier and contain many more of these indigestible carbohydrates than one cut earlier in the season. Not all later-cut hays are bad, though. For some horses, a later-cut hay is actually ideal because it typically contains less starch and sugar and has fewer calories, making it a good choice for easy keepers and horses with insulin dysregulation. The trick is to cut the hay late enough to capture these benefits but not so late as to result in increased impaction colic risk.

This is why lab analysis of hay is beneficial. Lab tests can indicate the hay’s overall digestibility and the content of some of the structural carbohydrate fractions, such as cellulose and lignin. Most forage analyses will include a value for the acid detergent fiber (ADF) and neutral detergent fiber (NDF) content. ADF is a measure of cellulose and lignin, and NDF is a measure of hemicellulose, cellulose, and lignin. You can estimate hemicellulose, therefore, by subtracting the ADF value from the NDF value. Some results give a value for lignin, as well, which is the truly indigestible plant component. As lignin levels increase with plant maturity, cellulose digestibility decreases.

Typically, NDF values between 40-50% and ADF values between 30-35% are good for horses. ADF values above 45% have little nutritional value, and if the NDF value is greater than 56%, your horse might not even want to eat the hay. I recommend keeping ADF below 40% and below 35% if feeding broodmares, foals, or performance horses, especially harder keepers.

See if you can persuade your barn owners to cut their hay a little earlier. Perhaps get last year’s cutting tested and check the results. If the ADF and NDF are very high, try to educate