Few nutritionists will dispute that grass is the most natural feedstuff for horses. The horse’s digestive tract is well designed for continuous or “trickle” delivery of pasture forage. As well, it seems intuitive that the opportunity for socialization and exercise provided when horses have access to pasture is important for their health and well-being. If given free access to pasture (all day), horses will spend 60-70% of their time grazing, with considerable walking activity, especially in large fields. Unfortunately, for many horses free access to pasture is but a dream; only a restricted period of grazing, if any, is available to a large proportion of the domesticated horse population.

From a nutritional standpoint, common questions regarding pasture grass and grazing activity include: “What is the benefit of pasture for my horse?” “Can my horse get all the required nutrition from grass alone?” And, related to the previous question, “If my horse is at pasture all day long, do I need to feed him some kind of supplement?”

The answer to all of these questions is … it depends! Several factors have to be taken into consideration, including the age and use of the horse, season, species of pasture grass, pasture management practices, and the time available to the horse for grazing activity. You need some understanding of these factors to best utilize pasture as a part of your horse’s nutritional program.

Grazing Behavior

Horses like to graze and, in general, spend time in wide-open spaces. However, to estimate the potential for pasture forages to meet nutritional needs, we need knowledge of the horse’s typical grazing behavior. From the available data it is clear that extended grazing periods are needed for pasture to meet (or nearly meet) a horse’s nutritional requirements. This makes sense when you consider the horse’s overall energy needs and the fact that pasture grasses can be 50-80% water.

As mentioned, on average horses graze approximately 70% of the time spent at pasture. So, if a horse has 24-hour access to pasture, he’ll spend up to 17 hours engaged in grazing activity. However, a number of factors will influence grazing time: The time devoted to grazing decreases with severe weather (cold or hot), increased pasture forage availability (e.g., lush ryegrass), being alone, and when bothered by flies. In general, grazing time is inversely proportional to the quality and amount of pasture forage (i.e., a horse will spend less time grazing a lush, spring pasture when compared to a dry, mid-to-late summer pasture with lower forage availability).

Interestingly, when horses are kept in stalls and fed hay free choice, they spend a similar amount of time eating. Of course, it is more typical for stalled horses to be fed twice during a 24-hour period—under these circumstances, consumption time represents only about 15% of a 24-hour period. And we sometimes wonder why stalled horses often develop undesirable vices such as wood chewing!

Horses spend about 10% of their time walking when at pasture. However, more time will be spent walking on poorer quality pasture as the horse searches for palatable forage. Similarly, they will walk more if alone, presumably because they are in search of companionship. Only about one to two hours is spent lying down, mostly in the period three to four hours before dawn.

Horses are classified as selective grazers; they do not simply eat forage available in greatest abundance, but seem to base their selection on palatability. So, in a pasture containing several grass species, a patchy grazing pattern can develop as the preferred forages are depleted first. This leads to very inefficient utilization of pasture forage, a situation that can be minimized by proper pasture management, including the practice of rotational grazing.

Benefits of Pasture

The opportunity to graze for extended periods has two very important benefits for the horse. First, forage (or fiber of some type) is a basic necessity for normal functioning of the equine gastrointestinal tract, and a well-managed pasture will be an economical source of high-quality feed. Indeed, as will be discussed shortly, under some circumstances pasture can meet most of your horse’s nutritional needs. Second, pasture is a great place for horses to exercise, and this exercise also can be important in maintaining healthy gastrointestinal function.

The amount of voluntary exercise will be proportional to the time at pasture. In one study of Thoroughbred yearlings, Japanese researchers investigated the effects of grazing time on feed intake and energy expenditure, as indicated by the distance covered while at pasture¹. One group of horses was allowed to graze for 17 hours per day, while the other was restricted to seven hours grazing in a 10-acre pasture. The horses at pasture for 17 hours per day traveled between eight and 9.5 miles, whereas horses restricted to seven hours grazing per day covered 2.5 to three miles.

Most of this distance will be covered at the walk. Even so, energy expenditure per day (calories burned) will be considerably higher for a horse given access to pasture when compared to a horse kept in a stall for much of the day. This voluntary exercise also has a conditioning effect. In a recent study, there was a 20-25% increase in the aerobic capacity (VO₂max) of young Thoroughbreds which were otherwise untrained but turned out to pasture for between seven and 20 hours per day.

Now to the important question: How much nutrition can be derived from pasture? First, a few nutritional basics. Mature horses generally consume 2-2.5% of their body weight in feed each day (on a dry matter, DM, basis). For example, a 1,000-pound (454-kg) horse fed hay plus grain concentrate (feeds that are about 90% DM) should consume about 20-25 pounds (9.1-11.3 kg) of feed daily. For gastrointestinal health, horses need to eat about 1% of their body weight in hay or pasture grasses and legumes daily (10 pounds, or 4.5 kg, of DM intake for a 1,000-pound, or 454-kg, horse).

So, how much grass can be eaten per hour of grazing activity? This will vary with pasture forage quantity, quality, and palatability, and also with the amount of time horses are on pasture. However, about 1-1.4 pounds (0.5-0.6 kg) per hour (DM basis) is a reasonable range assuming quantity is not a limiting factor. This means that a horse with 24-hour access to good-quality pasture grazing 17 hours each day can consume up to 25 pounds (11.3 kg) as forage, which is plenty to satisfy his daily DM needs. A minimum of eight to 10 hours at pasture would be needed to achieve a DM intake of at least 1% of body weight. Anything less than this duration of grazing and the horse will need supplemental forage (such as hay) to satisfy his forage needs.

Pasture can be an excellent source of digestible nutrients for the horse, high in both digestible energy and crude protein. The “Pasture Grass Nutrient Content” graph below shows the nutrient composition of some pasture grasses at two stages of growth—vegetative (spring growth) and mature (mid-to-late summer). This table also relates the nutrients available in these grasses to some of the nutritional needs of different classes of horses. While the data in this table are representative of the nutrient content of these pasture species, many factors (including soil type, annual rainfall patterns, and fertilization) will influence the nutrient values of a given pasture.

In general, forage nutrient content is highest during the spring and fall when there is “flush” growth from increasing ambient temperature (spring) or rainfall (fall). Mature or dormant grass has much lower energy and protein content and overall digestibility when compared to a lush, growing spring pasture. The concentration of some minerals also decreases with plant maturity. Given this seasonal (and, potentially, year-to-year) change in pasture quality, some large breeding farms will regularly analyze the nutrient content of pasture to fine-tune feeding programs.

For the same reason, it is unrealistic for me to make precise recommendations for balancing rations in all situations where pasture is an important component of the diet. Extension specialists and nutritionists in your area are best able to make these recommendations. For the most part, though, it is fair to assume that a dormant winter pasture or dry summer pasture is more useful for exercise and socialization than for delivery of nutrition. However, when growing conditions are good and pasture supply is plentiful (such as in spring and fall), pasture can replace hay and reduce the quantity of concentrates required.

For mature, idle (i.e., pasture ornaments), or laid-up horses, or mares in early pregnancy, spring or fall pasture can deliver the most required nutrients providing the horse has adequate time for grazing and there is plenty of land per horse. Only a mineral supplement might be necessary in these situations.

For working (athletic) and growing horses and for late pregnant (last trimester) and lactating mares, even the best-quality spring or early summer pasture grazed all day long might not get the job done in terms of exercise performance or growth and development. In most situations, these horses will need some type of concentrate or supplement to meet their nutritional needs, and these needs will be greater when pasture is mature or dormant.

The issue of land space is important. All bets are off at high stocking densities because, regardless of the time available for grazing, pasture supply will be inadequate to meet the horse’s needs. Horses with a mature weight of 1,000 to 1,200 pounds need the following amounts of pasture at a— minimum: Mare and foal, 1.75 to two acres (0.7-0.8 hectares); yearlings and mature horses, 1.5 to two acres (0.6-0.8 hectares); and weanlings, 0.5 to one acre (0.2-0.4 hectares). If acreage is limited (e.g., less than an acre per horse), exercise will be the main pasture use as only a minimal amount of feed will be supplied.

Can Pasture Be Harmful?

In some circumstances, pasture can be a double-edged sword—a good source of nutrients on one hand, but a potential hazard to your horse’s health on the other. One example is the risk of heavy parasite burdens in horses grazing poorly managed pasture. Another example is the increased risk of colic and laminitis when horses graze fresh spring (or lush fall) pasture. We often associate these conditions with an abrupt increase in grain (starch) feeding, with fermentation of excess starch in the hindgut and an overall disturbance to gastrointestinal function that can lead to the development of colic, diarrhea, and/or laminitis. However, these same events can take place in association with an abrupt increase in pasture intake.

Modern pasture grasses are high in sugars, including sucrose, glucose, fructose, and fructan (particularly during rapid growth phases). The simple sugars (sucrose, glucose, and fructose) can be digested by the horse in the small intestine. On the other hand, other sugars such as fructan cannot be digested by enzymes present in the horse’s small intestine and, as a result, reach the large intestine undigested. There, the sugars are rapidly fermented by the action of microbial enzymes with the production of lactic acid and a decrease in cecal pH, all of which can spell trouble (intestinal upset and/or laminitis).

One strategy to prevent these problems is to gradually introduce horses to spring pasture. For the first day or two after the onset of flush growth, try to limit grazing to no more than 30 minutes to an hour. Then gradually increase grazing time over the next week to 10 days. For laminitis-prone horses and ponies, however, the grazing of spring or lush pasture is best avoided. Even a limited amount of grazing can be enough to trigger laminitis in some of these susceptible animals.

It also warrants mention that free access to pasture is problematic in overweight horses and ponies. The exercise associated with turnout is important in the weight management of these animals, but this should be done in a dry lot.

Pasture can be an excellent source of feed, exercise, and socialization for most horses if managed properly. Work with your local extension agent to develop the best horse and pasture management plan for your farm.