What is it about springtime that seems to increase the incidence of colic in horses? No-one knows for sure, but various studies indicate that changes in management and pasture conditions are the prime contenders. The factors that I feel most often contribute to spring colic are lush, spring pasture; internal parasites; commencement of the show season; and foaling.

Spring Pasture

Grass is high in readily digestible carbohydrates when it is young and rapidly growing (i.e. lush). These carbohydrates can be fermented by the micro-organisms in the horse's digestive tract, which results in rapid gas buildup and altered bowel motility. This is why spasmodic or "gas" colic (mild cramping, often with gas buildup) is relatively common when horses are suddenly given unrestricted access to lush pasture, especially when the pasture contains a significant amount of clover. In a study in England, a recent change in management was blamed for over 40% of the cases of spasmodic or mild, undiagnosed colic. The most common management change? Turnout onto lush pasture in the spring.

To prevent spasmodic/gas colic as a result of spring pasture, gradually increase the grazing time over a 2‚3 week period, rather than suddenly turning out confined horses at the end of winter. Another strategy that can help prevent colic in this situation, and even in horses that have been turned out all winter, is to provide horses on spring pasture with some hay until the grass begins to mature. The dry matter content of spring pasture can be as low as 20%, which means the grass may be as much as 80% water. For optimal digestive health and function, the basic dry matter requirement for horses is 1‚2% of body weight per day (which, in a 1,000-lb horse with no pasture access, is 10‚20 lbs of hay per day). But when spring pasture is the horse's only roughage source, the horse may not be able to eat enough grass in a day to meet its daily dry matter requirement. I've found that horses on spring pasture often eat a surprising amount of hay when it is provided, which indicates that they probably need it as a supplemental dry matter source.

Internal Parasites

Internal parasites, especially the large and small strongyles, may contribute to spring colic for two reasons:

1. The numbers of infective larvae on the pasture increase as the days warm up. Even though larval numbers on the pasture generally peak in the summer and early fall in temperate climates, strongyle eggs hatch at temperatures as low as 45 F. So, internal parasites are a potential cause of colic in horses grazing poorly managed spring pastures.

2. Encysted small strongyle larvae often emerge from the lining of the bowel en masse at the start of the grazing season. Unlike the large strongyles (redworms or bloodworms), the small strongyles (or cyathostomes) do not migrate through the bowel wall. Instead, the larvae burrow into the lining of the bowel and become encysted or dormant (hypobiotic) for part of their lifecycle. They can remain encysted for months before "hatching" or emerging into the interior of the bowel and maturing into adults. In temperate climates, the larvae tend to emerge in late winter/early spring. What triggers the encysted larvae to resume their development and emerge into the bowel interior all at once is not yet known, but it is probably some type of environmental factor. Emergence of large numbers of larvae can cause colic and/or diarrhea.

Ivermectin and several other dewormers are effective against adult strongyles and the later-stage larvae. But only two currently available dewormers claim to be effective against encysted small strongyle larvae: moxidectin (Quest), at the recommended dose and schedule; and fenbendazole (Panacur), when twice the standard dose is given each day for 5 days (i.e. double-dose daily for 5 days).

When to use these larvicidal treatments depends on your location, pasture management, and deworming program. As infective larvae are picked up while the horse is grazing, a common recommendation for horses in temperate climates is to use a larvicidal treatment in the fall, after the grazing season has ended. However, it can be given at any time, including early spring.

Avoiding Colic at Shows & Competitions

The most common types of colic in horses at shows or other competitions away from home are spasmodic/gas colic and large colon impaction (blockage of the large colon with feed material). The most likely factors are the stress of transport and strange surroundings; a change of water supply and routine (feeding, exercise, and turnout schedules); and, in some cases, a change of bedding. Therefore, the most effective preventive strategies involve minimizing change:

1. Feed the same ration as fed at home, including hay, and keep to the horse's regular feeding schedule as much as possible. (Although it is often a good idea to decrease the amount of grain fed just before and during a long trip.)

2. Make sure the horse has good quality hay available at all times. In addition to its dietary benefits, feeding plenty of hay reduces stress, gives the confined horse something to do throughout the day, and provides a good alternative to eating the bedding. Poor quality roughage (e.g. over-mature, stemmy hay) is a key factor in most impactions, especially when water intake is decreased for any reason, so hay quality is also important.

3. Make sure the horse has fresh, clean water at all times. Most large colon impactions occur when water intake is decreased for any reason. In horses that are reluctant to drink "strange" water, take water from home or flavor the water with something the horse likes (e.g. molasses, peppermint, cider vinegar). If you're planning on flavoring the water, start doing it at home at least a week before you leave, to give the horse plenty of time to get used to it. Adding salt or electrolytes to the horse's ration (to the feeed, not the water) also encourages the horse to drink.

4. Exercise the horse every day. On days when you are not competing, longe, ride, or drive the horse. Also, hand-graze the horse for at least 30 minutes every day, provided there is a suitable clean area; otherwise, hand-walk the horse.

5. Use the same type of bedding as you use at home, if possible. Horses bedded on shavings at home will often eat straw bedding if there is insufficient hay available. So, to avoid impaction colic in horses that must be bedded on straw, feed plenty of good quality hay.

And don't forget about the horses left at home. Make sure that whoever is taking care of them is careful to keep their diets and routines consistent.

Colic in Broodmares

Broodmares are subject to the same types of colic found in other horses. Most common are spasmodic/gas colic, large colon impaction, and large colon displacement (in which the colon moves out of position, but not to the extent of twisting). The most important management strategies for preventing these conditions in any horse include the following:

1. Keep grain-based concentrates (grain in any form, grain-based sweet feed and pellets, etc.) to a minimum for that horse's needs. Feeding a high-grain ration is a common factor in horses with large colon displacements.

2. Feed plenty of high-quality roughage (good quality hay and/or pasture).

3. Ensure ample access to fresh, clean water at all times. Mares in peak lactation require much greater amounts of water than nonlactating horses, so make sure the mare has plenty of water, and preferably more than one water source.

4. Provide as much pasture turnout as possible, for both roughage intake and exercise. Large colon impactions seem to be more prevalent in horses that are confined.

5. Maintain an effective deworming program.

Large colon twists. Although very uncommon overall, one condition is more likely to occur in broodmares than in other horses: large colon volvulus (twisting of the large colon). This dramatic and life-threatening condition is more likely in the first few weeks after foaling. The key factors are probably the sudden increase in available space within the abdomen and abnormal bowel motility.

The mare's energy requirements in late pregnancy and early lactation are high, and are most often met by increasing the amount of grain-based concentrates in the diet. This is an appropriate nutritional strategy. However, whenever a large grain meal is fed, fermentation of the carbohydrate in the colon results in gas production. This can set up a cycle of rapid inflation and gradual deflation in the colons of horses fed large amounts of grain. This effect, together with altered bowel motility caused by the products of carbohydrate fermentation, could make colon displacement more likely. (It also explains how displacements and twists can occur before foaling and in nonpregnant mares and male horses.) There has been a suggestion that marginally low calcium levels could also be involved in large colon displacements and twists in mares in the weeks after foaling.

So, recommendations for preventing large colon twists in broodmares are as follows:

1. Feed plenty of good quality roughage, particularly in the first few weeks after foaling. In late-pregnant mares, roughage intake may be limited by the size of the fetus, so forage quality is very important during this time. Feed the best quality pasture and/or hay available, and increase the amount of roughage fed to the mare after foaling. (As mentioned, lush spring pasture contains large amounts of readily fermentable carbohydrate, so it can have the same effect as a large grain meal. It is often wise to provide good quality hay to recently foaled mares on lush pasture.)

2. Feed a high-fat balanced ration to meet the mare's increasing energy needs. A mare's dietary energy requirements increase in the last 2‚3 months of pregnancy and are highest during the first 2‚3 months of lactation. Adding fat to the mare's diet allows you to minimize the amount of traditional grain-based concentrates, while still meeting her energy needs. But when adding fat to the mare's diet, whether as vegetable oil, animal fat, or rice bran, be sure that the diet remains balanced for protein, vitamins, and minerals. The commercial high-fat balanced rations often are best because they are already balanced.

3. Make sure that the mare's diet contains adequate calcium, and is well balanced for other minerals.

Not every case of colic can be prevented. But by applying these simple management strategies, many cases of spring colic can be avoided. All the best for this spring!

Dr. Chris King is an Australian equine veterinarian currently living in North Carolina. She has written numerous articles for veterinary and equestrian publications and two books for horse owners and trainers: Equine Lameness and Preventing Colic in Horses.