The most important thing your horse should consume, other than water, is forage. Forage is the long stems and leaves of plants, most often offered in the form of pasture, hay or haylage. Hay is made from pasture plants that are cut and left to dry out, to about 15% moisture. Haylage is baled at a higher moisture content – about 40-60%.
The nutritional composition of forage is influenced by the form it takes, but is mostly affected by plant type and, to some degree, maturity and cutting. Plants that are typically used for horse feeds are grasses and legumes.
Grasses are monocotyledons, which means they are flowering plants that germinate with one leaf. When grass is growing, each new leaf is attached at a 180-degree angle from the older, previous leaf. Each grass leaf has an upper part (the blade) and the lower part (the sheath). Most species of grasses available in Canada are considered “cool season grasses” that grow at temperatures of about 15-25°C. Examples include ryegrass, fescue, orchardgrass, bluegrass and timothy. These cool season plants are “C3” plants that convert carbon dioxide into three-carbon molecules.
Legumes are also flowering plants, but have compound leaves and several (three or more) leaflets. They also tend to have deeper roots than grasses, and seeds are formed in pods. Legumes are especially unique because they have a symbiotic relationship with the bacteria Rhizobia sp that lives in the roots of these plants. The bacteria has the ability to convert atmospheric nitrogen from air within the soil into organic nitrogen that can be used by the plant. This allows legumes to have access to additional nitrogen, which is used by the plant to make protein. Therefore, legumes tend to be higher in protein than grasses. In fact, if legumes are planted alongside grasses, the nitrogen that is “fixed” by the bacteria is also available to neighbouring plants. Species of legumes include alfalfa, clover (white, red, alsike, sweet), vetch (crown and tufted) and birdsfoot trefoil. Alfalfa is the legume most commonly used for hay production.
Ultimately, in terms of nutrition for the horse, legume hay or pasture tends to be more nutrient dense than grasses, because of the additional protein. Legumes also tend to have higher calcium concentration and slightly higher energy (calorie) content than grasses (see Table 1). Grasses tend to be higher in non-structural carbohydrates (NSC, starch and simple sugars) than legumes, though the range varies widely (8-18% in grasses, vs 9-13% in legumes).
Hay producers tend to seed a variety of plants in their hay fields in an effort to optimize nutritional quality as well as tonnage harvested over several cuttings. Therefore, hay can be classified further as “mixed mostly legume” or “mixed mostly grass.” The only way to truly know the nutrient density of your hay is to get it analyzed.
In general, 100% legume hay is too nutrient dense for most classes of horses. The excess protein would have to be metabolized in the body to produce urine (a process that requires water, energy and works the kidneys) to get rid of the excess nitrogen. Excess protein can also be detrimental to athletic performance. Because legume hay tends to be more expensive to purchase, and much of the added protein is unused by most horses, the result is very expensive urine and a smelly stall! Some classes of horses do have higher nutrient requirements, such as broodmares and growing horses, and may do well with a mixed hay that does contain some legume for added protein and calcium. Athletic and idle horses can easily meet their protein requirements through good quality grass hay and performance horses also get protein from the concentrates (grain) they are generally fed.
Table 2 shows the protein requirements for several classes of horses (assuming 500 kg mature weight) and how much grass or legume hay would be required to meet those protein needs. The amounts of hay needed to meet protein requirements may not be practical to feed in terms of amounts fed or calories required by the horse, in which case, concentrates must be added. Most horses will consume about 1.5-2.5% of their body weight (7.5 kg-12.5 kg) per day.
The best way to determine which type of hay is ideal for your horses is twofold. One, you should get a hay analysis to accurately determine the nutrient content of your hay, rather than using averages as shown in Table 1. Two, work with a nutritionist to compare what your hay provides and what your horse needs, to help determine what other nutrients are needed and what other feeds/supplements may be required by your horse.