Weather Extremes

A record hurricane season and drought conditions throughout the United States last year hammered home the message that horse owners need to be prepared for the effects of extreme weather conditions. The immediate effects of these conditions are usually visible in the form of increased feed prices, but there are many other underlying effects that are not always readily recognized by horse owners.

Drought and wet-weather conditions can create environments that are accommodating to molds and fungi in hays and grains, and these conditions can damage grasses, allowing weeds to outcompete them in pastures. This can be especially dangerous when pastures are overgrazed and contain toxic weeds or nutrient accumulator plants.

Molds and Fungi

Ann Swinker, PhD, extension horse specialist at Pennsylvania State University, says molds can be a serious problem when horses are exposed to them, more so than most other types of livestock. She says horse owners are generally wary of molds present in their hay, but they aren't always able to recognize molds or fungi present in their grain or even in their pasture fields.

Aflatoxins (a type fungal toxin) can infect corn or other high-energy grains, and they are ordinarily found in crops that are stressed because of drought conditions. Aflatoxins are poisonous, carcinogenic by-products of several species of fungi and are produced as the fungi grow in grains and other feed products.

Aflatoxins can cause diarrhea, colic, anemia, immune suppression, oral inflammation, and liver damage. The most commonly known syndrome caused by these mycotoxins is equine leukoencephalomalacia or "blind staggers." This condition is caused by a gray (or cotton white) fungus called Fusarium moniliforme that often invades Midwest cornfields.

Blind staggers are usually associated with drought or very wet conditions that create an environment for the fungus to grow. Horses affected with this disorder will show signs of reduced response to external stimuli, followed by circling or aimless wandering, excitability, head pressing, blindness, partial paralysis, weight loss, and jaundice. After a period of time, affected horses will usually become laterally recumbent (on their side and unable to get up) and eventually die.

Aside from molds and fungi, Swinker says, "Another thing we really need to be concerned about is ergot fungal infected grains." Many of the feed companies routinely test for ergot and Fusarium, in grains before mixing commercial feeds.

Ergot toxicity in tall fescue, which is caused by Claviceps purpura, negatively affects blood flow to the uterus and is usually associated with fescue-related abortions in broodmares. The condition can be very serious because it takes only a small amount of contaminated fescue to cause toxicity in horses. It takes about 3% of forage contaminated with ergot, and about 0.3% of affected grains in the diet, to cause adverse effects in horses.

Other bacteria and molds can irritate the horse's respiratory system and cause a dry type of cough. If the irritation is allowed to continue over time, it can cause chronic obstructive pulmonary disease (also known as heaves).

Swinker says chronic obstructive pulmonary disease is one of the most common respiratory disorders in horses, especially those in wet environments. Horse owners should be vigilant while inspecting their hay and grain for mold.

"If you throw a flake of hay on the stall floor and a slight puff of white dust emerges, you may have a mold problem," she explains.

In addition, very dry weather and dusty corrals and paddocks can result in respiratory irritation in horses. Swinker notes, "You may need to apply water to dry lots during severely dry spells." Add just enough water to the dry lots to reduce the rising dust, but not so much that the base becomes slippery and dangerous for the horse.

Some of these conditions caused by molds and fungi can be serious or even life-threatening, although not all of them are. There are some instances of toxicity that mainly cause inconveniences to horse owners. Case in point is Rhizoctonia leguminicola-infected clover. When consumed while grazing or as hay, clover infected with this fungus will often cause horses to slobber excessively, which is known as slaframine poisoning. The fungus infects red clover, white clover, alsike clover, and alfalfa. Slaframine poisoning is not life-threatening, but it can cause excessive salivation, oral ulcers, colic, and possibly laminitis.

If you are planning to feed any of these types of hays, Swinker says it's best to allow them to cure for a lengthy period of time (about 10 months, according to Swinker) because the fungi tends to break down over time. (For more on fungal toxins see

The major problem with these types of relatively benign conditions, Swinker says, is they can often resemble other, more serious conditions (such as rabies). The reverse of this is an owner who thinks a horse with rabies has slobbers, which can sometimes delay necessary medical attention. "Any time you have a concern about your horse, you should contact your veterinarian," Swinker explains.

Some of the other toxic plants to watch for during extreme wet or drought conditions include pig weed, which is a selenium accumulator, and ginseng, which is an interesting plant because it is more toxic during the day and less so at night.

Changing climatic conditions (such as a change in temperature) can possibly increase and/or decrease the toxic condition of these plants.

Nutrient Accumulators

Drought conditions can cause weeds and grasses to accumulate elements such as nitrate or selenium in higher than normal concentrations. When Swinker was out west at Colorado State University, during very dry weather she noted an increased number of horse owners telling her that their horses were losing their manes and tails and showing signs of laminitis (these are clinical signs associated with selenium toxicity).

"When we would hit severe or even slight drought conditions, we would see a big problem with selenium plant accumulators," explains Swinker.

Some plants have what are known as "tap root" systems. During normal conditions, these roots grow up to a depth of three feet, where they can extract micronutrients from the topsoil. However, during drought conditions, their roots are forced to grow deeper into the "mother soil" where nutrients are more concentrated. These plants then store the concentrated nutrients (selenium) that are later transferred to horses when they graze. Plants that are normally slight accumulators become major ones during severe drought, and some other plants that are not normally accumulators will store concentrated levels of these nutrients. Horses avoid these types of plants under normal circumstances, but when grass is scarce or pastures are overgrazed, they will sometimes turn to them for nourishment.

Now that Swinker is in the East, during dry bouts of weather, she receives calls about nitrate accumulators in forages, similar to their selenium counterparts. Slightly high nitrate concentrations will result in broodmares aborting. By the time the source of the problem is determined, the horses will have suffered severe effects. Swinker says the best solution for horses ingesting these types of plants is to remove them from the affected forage. (For more on selenium toxicity and selenium accumulators see

The effects of drought are not limited to the drought period. Affected plants and grasses are often cut and made into hay. When fed to horse months (or even a year) later, the effects are the same.

Horses on overgrazed or drought-stricken pastures will need to be fed hay or other alternative forage sources to keep them from eating infected weeds or accumulator plants.

Your local extension service will be able to test your hay or pasture to determine if there are accumulators present.

Adequate Water Sources

Horses require more water during a drought because they are forced to eat more fibrous, less palatable grasses and weeds. In addition to hydration, horses require extra water to maintain the movement of the coarser material through their guts.

Drought conditions can also severely affect water source quality. Because of evaporation, natural water sources can contain higher concentrations of minerals and/or contaminants that would be diluted under normal circumstances.

Stagnant water pools are also ideal conditions for blooms of blue-green algae, which is very toxic to horses and can even cause death. If you are concerned about water contaminants, consider having your water tested, or fence horses away from the stagnant water.


In less-than-ideal pasture situations, a foal can place a great physical demand on its dam. Lori K. Warren, PhD, assistant professor of equine nutrition at the University of Florida, says, "Creep-fed foals can be weaned earlier and with less stress, which is particularly important if your mares are doing poorly and not producing much milk."

Warren cautions that owners should be aware that foals are generally very active after weaning, and excessive running on drought-hardened or slippery ground created by rains can lead to bone or muscle injuries.

Warren says, "Lack of grass in a dry field may force your horse to graze nearer to manure piles than normal. Ensure your de-worming program is adequate. Also, dusty, dry conditions may increase the risk for your horse to develop heaves. Foal pneumonia (caused by Rhodococcus equi) has also been observed to be more common in drought years."

Additionally, when the dry spell ends there might be severe storms. Swinker says owners should be aware of tree branches that could be knocked down during a storm.Some leaves, such as wild cherry, are toxic to horses. Less than a quarter of a pound of these leaves can be fatal to a horse.

Stretching Your Hay Supply

There are times when hay supplies can run short; the 2005 hurricanes are a perfect example. Having an alternative plan can help ease the stress on you and your horses.

As mentioned before, horses require a minimum of 1% of their body weight in forage each day. When adequate forage is not available, horse owners must look to alternatives such as cubed or pelleted hays (alfalfa being the most common of these) or supplements such as beet pulp.

Swinker cautions, "When going from long-stem hay to pelleted hay, horses with behavioral problems (such as cribbing or weaving) may often increase these behavior vices."

When supplementing beet pulp for long-stem hay, some owners prefer to soak it in water to allow it to expand, though studies have shown that it's not necessary (see Swinker advises horse owners "to do your homework and study up on feedstuffs before using them." Beet pulp is an excellent source of fiber and energy.

It's also important to understand the effects weather has on harvested hay. It's not always the length of time the hay is cured after harvest; it's what environmental conditions are occurring during that time.

Quiz: Which hay would you expect to have the higher protein? Hay that was first cut in late July under overcast conditions and a lot of rain, or the same field cut under very dry conditions in June? If you picked June hay, Swinker says you are wrong.

These were the conditions over two separate years that Swinker and her colleagues examined from the same pasture, but under different weather conditions. The late-cut hay under cool wet growing conditions produced 14% protein, while the earlier cut hay under overly dry, hot conditions only produced 10% protein.

Take-Home Message

Extreme weather conditions (be it flood or drought) can affect all crops. It's important to be prepared for higher feed costs, and to know several trusted suppliers.

Trying to cut expenses by feeding lesser-quality feeds can come back to haunt you with even higher veterinary bills.

Warren says, "With just a few minor adjustments in your feeding and management practices, your horses and your pastures can be maintained in good health during a drought (or wet conditions)."


Alternative Feed Sources

Extreme weather conditions can place a heavy strain on pastures, hay fields, and feedstuff growth, which could mean you need to supplement your horse's diet with alternatives. One thing to remember when evaluating alternative feed sources, says Lori Warren, MS, PhD, extension horse specialist at the University of Florida, is that horses require a fiber intake of at least 1% of their body weight per day.

The following is a list of alternative feed sources:

  • Last year's hay It's possible that hay suppliers might still have leftover hay from the previous year. Older hay is fine if not dusty/musty. If stored properly, it should retain most of its original nutrients. Vitamin A might be limited in year old hay, so you might need to supplement.
  • Hay cubes Hay cubes can be used to supplement or replace baled hay in the horse's diet. If replacing alfalfa hay with alfalfa cubes, you should continue to feed the same quantity (10 pounds of alfalfa hay = 10 pounds of alfalfa cubes).
  • Alfalfa pellets These can also serve as a forage source or supplement your forage. However, feeding processed cubed or pelleted forage to a horse that "rushes" during eating could result in choking.
  • Beet Pulp Beet pulp is a highly digestible byproduct of sugar beet processing. It can be used to supplement up to half the normal hay ration (seven to 10 pounds), but it usually should be soaked before feeding.
  • Grain In general, grains are a poor source of fiber, but when hay is limited, increasing the horse's grain intake can replace a portion of the hay in the diet. As long as the horse consumes 1% of his body weight in forage, the rest can be supplemented with grain. (Note: to reduce the risk of colic and laminitis, gradually increase grain intake over a two-week period, and never feed more than five pounds per feeding.)--Chad Mendell

Feeding Tips

  • High-fiber roughage makes up the majority of a horse's diet.
  • Horses should ideally receive 1-2% of their body weight per day as roughage to maintain normal digestive function.
  • Grain rations should be divided into small meals throughout the day.
  • Dietary changes should be made gradually over a two-week period.
  • Be sure to provide free-choice salt in addition to fresh water in any weather conditions.

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for .

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