By Christine Skelly, Ph.D., Equine Extension Specialist, Department of Animal Science, Michigan State University
(First published in 2017 and revised April 2021)
Pastures provide a valuable feed source for horses
Pastures are vitamin-rich and provide substantial energy and protein for many classes of horses. There is both a psychological and physical health benefit for horses to be turned out versus stalled. If a horse has access to a good quality pasture (a pasture that can provide nourishment, unlike a turnout area that is devoid of edible forage), the pasture should be considered as part of that horse’s daily ration. For the idle mature horse, adequate access to pasture, plenty of fresh water, and a trace mineral salt block may fulfill its dietary needs.
Grazing provides both psychological and physical health benefits for horses.
A horse requires a dry matter feed intake between 1.5 – 3.0% of its body weight (BW) based on its energy requirements. The estimates for voluntary dry matter intake (VDMI) of horses grazing pastures range from 1.5 – 3.1% of their BW. An adult horse will average VDMI on pasture at 2% BW per day. A lactating broodmare consumes more pasture with a VDMI of 2.8% BW per day to meet her high nutritional requirements for milk production.
A pony or donkey, being smaller in size and requiring less energy, can easily become overweight when given free access to grazing even a moderate-quality pasture. All equines utilizing pasture as their sole form of energy should have body condition scores accessed frequently to ensure they are not getting too fat or too thin. It may be necessary to limit grazing during times of lush growth such as early spring to prevent obesity or to supplement grazing with hay or grain during low pasture productivity to keep your horse at a healthy weight.
This pasture is overgrazed and provides very little nutrition.
Spring pastures put some horses at health risks
If horses are kept on pasture year-round, they usually adjust to the new foliage as it grows in the early spring. Most management problems occur when stalled horses fed a hay and grain diet are abruptly turned out to graze pasture in the spring. The lush spring growth of pasture foliage, either grasses or legumes, is higher in moisture content (75-85%). In addition the energy and protein content of foliage can be as much as 50% higher in early vegetative growth compared to that in vegetative growth twelve weeks later. Abruptly changing the horse’s forage source from hay to pasture can disrupt the microbial population in their hindgut and trigger laminitis or colic.
Horses kept in stalls through the winter and allowed early free access to pasture may overeat because of the palatability of the lush green foliage. This over-consumption can put a horse at risk for the following nutrition-related problems:
1. Overweight/obesity - Depending on individual metabolism, some horses can gain significant weight on pasture alone.
2. Diarrhea - Higher moisture content and the change in ration can trigger a “loose stool.” The feces usually become firmer as the horse adapts to the pasture, but in some cases a horse must be confined temporarily and put on a hay-based ration to restore normal bowel function. In a small number of cases, a horse might require medical treatment to clear up the diarrhea.
3. Colic - Although colic is not common in these situations, any rapid change in the ration can cause excessive gas or an intestinal upset in certain horses, leading to colic.
4. Laminitis (Founder) - Risk of this disease is always a concern when the ration is changed abruptly, especially when going from hay-based ration to pasture. Even though as a group, ponies are more susceptible to laminitis from early pastures, horses are also at risk.
Hooves of a horse with laminitis.
Horse pasture and fructan concentrations
Much attention is paid to the type of carbohydrates present in forage for horses diagnosed as insulin resistant. In particular, easy keeping light horse breeds like Morgans, Paso Finos, Tennessee Walkers, mustangs, ponies, and miniature donkeys, are targets for this condition, especially if they are overweight. Ingestion of water-soluble carbohydrates (glucose, fructose, sucrose, and fructan) found in rich spring grasses may trigger laminitis in some horses as a result of the rapid fermentation of the sugars and starches in the hind gut (similar to a high-starch diet). While fructan content in hay and pasture receives the most attention, all sugars and starches should be considered potential triggers in susceptible horses.
Fructan concentrations can vary greatly according to the growing season, weather conditions, and time of day. Fructans are found in the largest quantities in cool season grasses like Timothy, Orchardgrass and Tall fescue (endophyte free variety). Early spring pastures are highest in sugar content with mid-summer pastures being lowest and early fall/autumn pastures being intermediate in sugar content. In addition, as a result of photosynthesis, sugars begin to accumulate in grasses beginning in the morning sunlight, reaching their peak accumulation by late afternoon. The plants will utilize the stored sugars and starches overnight and the cycle begins again with the morning sunlight. Fructans also increase in stressed grasses during times of drought, frost, or overgrazing as the plant responds to the stressors by storing energy in the form of sugars and starch.
For horses that have chronic laminitis, it is important to limit their grazing during peak periods of sugar content in pastures. In cool season grasses, sugar and starch will accumulate on sunny days and when the pasture is stressed (drought, frost or overgrazing). Hay should be tested for non-structural carbohydrates (NSC) and carefully fed to horses susceptible to laminitis.
Insulin-resistant: When cells do not respond appropriately to insulin in the body. Insulin transports glucose from blood to muscle and tissues. An accurate diagnosis of insulin-resistance requires an examination and testing by a veterinarian.
Laminitis: A metabolic disorder commonly referred to as founder, can affect all four feet, but is most common in the front feet. Laminitis causes the sensitive and insensitive lamina of the hoof wall to become inflamed, leading to separation.
Lush spring pastures can be high in sugar and starch content.
Managing horses on spring pastures
Early spring pastures are lush in nutrients. A horse that has been stalled through the long winter may be at risk to laminitis if their forage source is abruptly switched from hay to lush spring pasture. The following management practices can prevent or minimize problems when introducing horses to pasture in the spring:
1. Feed hay immediately before horses are turned out on pasture during the adjustment period. They fill their stomachs somewhat, thus helping to prevent overeating. This practice curbs their appetite and allows them to discriminate between appropriate pasture foliage and weeds. There can be potentially toxic weeds in some pastures and, if horses are very hungry, they may not be selective about what they eat.
2. Restrict grazing time. Allow horses on the pasture for 15 minutes the first week and increase the grazing time by 15 minutes per week over a six-week period until they have adapted to the new feed source.
3. Supplement grazing with hay. If pasture foliage is sparse, supplementing with hay might be necessary to provide sufficient energy and other nutrients. This practice also helps prevent consuming weeds because under some pasture conditions the weeds outgrow other foliage.
4. Avoid grazing early spring pasture. If a horse has a history of laminitis, avoid grazing in the first four weeks of spring growth; then follow Rules 1 and 2 when introducing a horse to the pasture.
5. Use a grazing muzzle. If a horse that is susceptible to laminitis must be turned out on pasture, use a grazing muzzle to limit their pasture consumption.
This mare's pasture consumption is carefully managed with a grazing muzzle. Source: Kim Nylander
Additional Resources Related to the Topic My Horse University Free Archived Webcasts:
Pasture Associated Laminitis Prevention Strategies, Paul D. Sicilliano and Shannon Pratt Phillips, Recorded July 2015
Equine Metabolic Syndrome, Dr. Raymond Geor Recorded October 2010
Countermeasures for Equine Laminitis: Carbohydrate Profiles in Feeds and Forages, Dr. Bridgett McIntosh, Recorded April 2009