2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
By Sarah Probst
University of Illinois
College of Veterinary Medicine
Toxins produced by molds, poisonous plants, fungi, and poisonous insects can hide in hay. Specific management efforts when baling, buying, and feeding hay can reduce the likelihood that you will accidentally poison your horse.
Eliminating toxins from hay begins in the field. "First of all, walk the pasture you intend to cut and bale," says Dr. Val Beasley, veterinary toxicologist at the University of Illinois College of Veterinary Medicine in Urbana. "It is best not to cut areas with significant amounts of toxic plants. Early in the year, you might be able to use a selective herbicide. If you cannot get rid of the areas with the toxic plants, cut around them, or cut the hay but don't bale it and feed it to horses or other animals."
The following plants are among those that retain toxicity in the hay: nightshade, snakeroot, poison hemlock, red and silver maple leaves, wild onion, milkweed, dogbane, alsike clover, Senecio, Crotalaria, St. John's wort, smartweed, pokeweed, bouncing bet, corn cockle, buckthorn, damaged or moldy sweet clover, bracken fern, and jimsonweed.
Mold in hay also causes problems from inhalation the spores
in the hay. These mold metabolites and mycotoxins can poison a
horse. The mold
Stachybachrys-particularly a problem in Europe-can produce neurologic problems, severe irritation of the mouth and lips, damage to the mucosa of the stomach and the intestine, and immunosuppression. It can also affect people who handle the hay.
"To prevent mold from developing in your hay, cut it when it is not going to get wet; pick it up before it gets wet and after it is dry; and store it where it is going to stay dry. Be sure to have adequate ventilation, and not let it get excessively hot," advises Dr. Beasley. "If you use round bales, get rid of the hay on the outside." A non-toxic preservative, such as proprionic acid, can be a good way to prevent mold. Make sure whatever preservative you buy is safe for horses.
Some molds can present toxic problems even before the hay is cut. The mold Rhizoctonia leguminicola is noted as a black patch, especially on second-cutting red clover and sometimes other types of legume forages. When eaten, it releases a toxin that stimulates the salivary glands, and to a lesser extent, the lacrimal glands. Along with drooling, increased urination and increased defecation to the point of diarrhea may also occur. "Owners should avoid second-cutting red clover hay, if it has black patches," says Dr. Beasley.
It doesn't take very many blister beetles to poison a horse. These insects are especially a problem in alfalfa hay that is crimped when is cut. Crimping-bending and crushing the hay when you cut it-was popular several years ago. As you crush the hay you kill any toxic beetles in the hay. If you do not crimp, the beetle is more likely to survive and can crawl away before the baler comes along.
"Larval stages of blister beetles fall into two groups. One group feeds on the eggs of grasshoppers and the other on stores of solitary bees. The adult beetles subsequently feed on leaves and flowers. Thus alfalfa fields where there have been grasshoppers or bees should be monitored especially closely for blister beetles. As you might expect, first-cutting hay generally is least risky, and alfalfa hay should never be allowed to go into full flowering before it is harvested. Late in the season, blister beetles will move away from alfalfa plants to feed on flowering goldenrod when it is nearby," adds Dr. Beasley. Therefore, a field with goldenrod might have an increased likelihood for a blister beetle infestation.
When you buy hay, you should ask the seller what kind of grass and legumes make up the hay. Check for weeds. Ask where it was grown. While feeding the hay, keep a look out for plants that aren't suppose to be there. "If you have any doubt as to whether the plants are toxic, take them out. Feed the horses enough hay so that there is some left over and the horses can sort out and hopefully avoid what is toxic," says Dr. Beasley. "Remember that when horses evolved, they were able to move about freely and pick and choose what to eat. When we take away the horse's ability to choose, we must provide nutritious and essentially non-toxic feed and forage."
Endophytic fungi, which grow inside the shafts of fescue grass, should also be avoided. "Hay with endophyte-infested fescue may look good because toxins produced by the fungus deter insects. Fescue infected with fungi cause mares to go past term and have weak, oversized foals. You can buy endophyte-free fescue seed, but most fescue pastures have the endophyte," Dr. Beasley says.
Also, dusty hay often indicates the presence of mold. It might be darker, discolored, or even white. "Green, low-dust, no-dust hay with no evidence of mold or unusual weeds and a great predominance of desired grasses and/or legume forages usually means you are in good shape," says Dr. Beasley.
If you suspect your horse is suffering from any toxic substance,
call your local veterinarian.