By Christine Skelly, Ph.D., Equine Extension Specialist, Department of Animal Science, Michigan State University
Pastures provide a valuable feed source for horses
Pastures are vitamin-rich and provide substantial energy and protein for many classes of horses. The psychological benefit for a horse as well as the health benefit of not being confined to a stall is of high value. If a horse has access to a good quality pasture (a pasture that can provide nourishment, unlike a turnout lot, 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.
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 energy requirements for milk production.
A pony or donkey, being smaller in size and requiring less energy, can easily become obese 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 done frequently to ensure they are not getting too fat or too thin. It may be important to limit grazing during times of lush growth such as early spring in order to prevent obesity or to supplement grazing with hay or grain during low pasture productivity, that is, in times of drought or towards the end of the grazing season.
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 horses have been confined, fed a hay/grain ration, and then abruptly turned out to pasture in the spring. The lush spring growth of pasture foliage, either grasses or legumes, is high in moisture content (75-85%) but also relatively higher in protein, vitamins and minerals on a dry matter (DM) basis than during other seasons of the year. 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.
Horses kept in stalls through the winter and abruptly allowed free access to pasture might overeat because of the palatability of the lush green foliage. This over-consumption can put a horse at risk for certain 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 change in the ration if done rapidly can cause excessive gas or an intestinal upset in certain horses, leading to colic.
4. Founder (Laminitis) - 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 pasturing, horses are also at risk.
Hooves of a horse with laminitis.
Horse pasture and fructan concentrations
Much attention is now being paid to the type of carbohydrates present in forage since more horses are being diagnosed as insulin-resistant. In particular, easy keeping light horse breeds like Morgans, Paso Finos and Tennessee Walker, mustangs, ponies, and miniature donkeys, are targets for this condition, especially if they are allowed to be obese. 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 in the hind gut (similar to a high-starch diet). While fructan content in hay and pastures is currently receiving the most attention since it is a potential trigger for laminitis in susceptible horses, all sugars and starches should be considered potential triggers.
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. 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 reaction, sugars begin to accumulate in grasses during the early morning and reach their peak by the afternoon. They steadily decline as the sunlight intensity decreases and reach their lowest values during the night. Fructans increase in grasses during drought conditions as the plant tries to resist the stress of low water.
For horses that have chronic laminitis, it is important to limit their grazing during peak periods of sugar content in pastures. Hay should also be selected based on sugar content for these horses.
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 a glucose tolerance test 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 fructan 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. There are management practices that can prevent or minimize problems when introducing horses to pasture in the spring.
1. Restrict the grazing time. Allow horses on the pasture for 20 minutes the first day and increase the grazing time by five minutes per day over a two-week period until they have adapted to the new feed source.
2. 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.
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 founder, 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 founder must be turned out on pasture, use a grazing muzzle to limit pasture consumption.
This mare’s pasture consumption is carefully managed with a grazing muzzle. Kim Nylander photo.
Additional Resources Related to the Topic
My Horse University and eXtension Free Archived Webcasts:
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
Pasture Management for Horse Acreages, Dr. Dave Freeman, Recorded February 2009
eXtension/horse Learning Lessons: