With water deficits come some unexpected consequences. Hay could be at a premium and you might be inclined to feed less of it and/or use hay substitutes (see "Equine Hay Purchases and Substitutes During Drought"). Horses like to chew small amounts frequently, so a lack of appropriate fiber sources to chew on might stimulate horses to seek other fiber sources. This can take the form of the wooden boards of your barn, posts, trees or fences. Or, a horse may resort to eating dirt and sand. In some cases, the best thing found to chew is a weed, which could be a toxic plant.
Eating the wood on your farm turns into a time and financial hassle because it means you have to replace those parts to keep the place safe and cosmetically appealing. A horse’s habit of consuming dirt and sand is notorious for creating impaction colic, which is not only scary for you as a horse owner, but can also be expensive and even life threatening. Consumption of poisonous plants can also have disastrous consequences.
The trick is to balance hay conservation with the amount of forage necessary to keep a horse happy with what he has to consume and chew.
When hay growth is compromised by drought, there is sometimes a tendency to want to speed up the recovery through fertilization. When this occurs near water sources, another unexpected consequence can develop in the form of blue-green algae. High levels of nitrogen and phosphorus coupled with hot weather (and water) encourage growth of these cyanobacteria. Even without fertilizer contamination, water sources contaminated by manure and urine are also prone to developing blue-green algae blooms when climatic conditions favor their growth. While there are a couple thousand strains of blue-green algae, less than 100 are toxic to livestock. But toxicity can be life threatening, with signs of neurologic disease, diarrhea, respiratory distress, seizures or liver effects (including photosensitization).
The best advice is to fence horses away from stagnant ponds, minimize water contamination by manure, urine or fertilizer, and to always provide clean, fresh water.
Other toxicity issues exist due to drought such as the uptake by pasture plants of toxic materials, such as nitrates. Another more common toxic principle to be aware of in plants is mycotoxin, a by-product of fungal metabolism that occurs in moldy grains--those that include corn, milo, wheat, barley, rice and cottonseed are most susceptible. When extreme conditions, such as drought, favor fungal growth, the fungus can persist in the field, at harvest, in storage, or during processing of feed grains. Not all fungi produce mycotoxins, but there is always that possibility. Make a smell test every time you feed grain to your horse. Look at the grain also to see if there is a moldy appearance. When in doubt, don’t feed it.
Blister beetles in alfalfa hay are another serious concern that can happen in any climatic conditions, including drought. Beetles residing in an alfalfa field may be inadvertently baled with no one the wiser. Blooming alfalfa plants are most attractive to blister beetles and they are also likely to be present with large populations of grasshoppers since the beetles consume grasshopper larvae. Even a small amount of blister beetle (cantharidin) consumption--25 to 300 beetles--can be fatal to a horse. Large swarms of beetles may occur consisting of many tens of thousands in a field if conditions are right. Discuss with your alfalfa hay supplier about precautions taken to avoid blister beetles in the hay.
As if drought worries aren’t enough, there are the other relatively invisible toxic entities to fret over. But with this knowledge at hand, “to be forewarned is to be forearmed.” Be diligent in scrutiny of your horse’s environment, his feed and in your management.