A:  This is a great question because the majority of horses are fed hay as their main source of forage at least some time during the year if not all year. Therefore, when managing a horse with metabolic concerns, hay is an important consideration. It can be easy for owners to become consumed with concerns over the nonstructural carbohydrate (NSC) content of supplemental feeds that they are providing while forgetting to consider that by far the greatest quantity of feed being consumed is the horse’s forage, often in the form of hay.

In an ideal situation, a horse with metabolic problems should only be fed hay that has been tested and has a NSC content of less than 12 percent on a dry matter basis. The reality is that most owners do not have the luxury of tested hay, so it can be helpful to understand some basics of hay NSC content.

The plants that make up most hays can be divided into three different groups, the legumes such as alfalfa, the cold-season grass hays such as orchard, timothy, and brome, and the warm-season grass hays such as Bermuda and teff.

Alfalfa is the most predominant legume hay and is typically considered to be a low-glycemic-index hay with low NSC and, while this might make it appear to be the ideal solution for horses sensitive to NSC, legume hays also tend to be higher in calories per pound and are high in protein. Since many of these horses are easy keepers, feeding a high-calorie hay will reduce the total amount they can consume. There is also preliminary research that suggests that feeding very high-protein feeds can stimulate insulin release in horses with equine metabolic syndrome, which might explain why some owners report that their insulin-resistant horses do not do well when fed alfalfa.

Grass hays are generally lower in protein and calories than legumes, so horses can consume more than they can if fed alfalfa. However, cold-season grasses can be quite high in NSC because they store the energy they generate from photosynthesis differently than warm-season grasses. They are able to store much larger amounts of NSC because they can generate a form of complex sugar known as fructan which warm-season grasses do not. Warm-season varieties can generate more starch but, generally, cold-season varieties of grass cut under the same conditions, and at the same maturity level will generally have higher NSC contents than the warm-season varieties. This is one of the reasons why teff has become a very popular hay variety for horses with metabolic problems.

It is important to understand, though, that many things influence NSC levels and it is possible to get a cold-season hay such as timothy with a lower NSC than a warm season hay like teff. However, without testing, it is impossible to know the exact NSC level. I have certainly seen enough variation between tests of different varieties of grass hays that it is very hard to claim that a certain variety will always be low or always high in NSC. One exception to this would be ryegrass hay, which does tend to have much higher NSC levels than other cold-season varieties.

Other factors besides plant type affect NSC content of hay, such as maturity, growing conditions, and time of day the hay is cut. Typically, more mature, stemmier hays are lower in NSC because they have a higher structural carbohydrate content. Grass grown under less-than-ideal conditions, such as drought or poor fertilization, especially cold-season varieties, can generate more NSCs as a coping mechanism. As the day goes on, NSC stores increase as the plant has more time to photosynthesize. Therefore, hays cut in the late afternoon could have higher NSC than those cut first thing in the morning. Similarly, if cut on a cloudy day or after several cloudy days, these plants might contain less NSC than after a period of intense sun.

With so many variables it is impossible to say for certain that one hay is going to be better than another, but a stemmier, later-cut hay or a warm-season variety should be lower in NSC. If feeding a sensitive horse untested hay, soaking the hay for 30-60 minutes can remove some of the soluble sugars. Alternatively, there are a number of forage replacers on the market such as chopped hay or hay pellets that are tested and guaranteed to have lower levels of NSC. These can be incorporated into the diet to replace some or all the untested hay to lower the overall NSC intake for sensitive horses.