When it comes to livestock and toxins in hay, an ounce of prevention is worth a pound of cure.

That's the advice of Birgit Puschner, veterinary toxicologist at the University of California, Davis.

Most plant poisonings can't be treated, says Puschner. So, to avoid potential problems, hay growers should know what poisonous plants may be growing in their area — and what symptoms animals might exhibit if they have eaten toxic plants. If a poisoning is suspected, be ready to help determine what may have caused it.

“Consult with a veterinary toxicologist at the earliest stage of a case,” says Puschner. “Because many potentially poisonous plants or even insects are unique to a certain location, or to a specific time of year, talk to a toxicologist familiar with the geographic area where the hay was grown.”

Growers should keep and mark hay samples from each cutting and field they were taken from, in case they are accused of supplying toxic hay, she advises.

When Puschner's lab is notified of a suspected poisoning, animal tissue samples and samples of suspect hay are analyzed to see if there is a connection between the two.

Hay can contain toxins through the following:

  • Nitrate-accumulating plants, including weeds such as pigweed, lambsquarter and members of the nightshade family. These can be highly toxic to cattle. Oat hay, corn and sorghum are among the highest-risk crops, but alfalfa may also be high in nitrates.

    Heavy pasture fertilization, herbicide application, drought, persistent cloudy weather and below-normal temperatures may increase nitrate levels in plants. So to minimize poisoning risks, growers should carefully use nitrogen fertilizers, harvest under appropriate conditions, supplement with high-carbohydrate feeds and test forages for nitrate concentrations.

    Forage nitrate levels of 0.3%, or 3,000 ppm, and above are potentially dangerous. Acute poisoning is likely if nitrate levels exceed 1%, or 10,000 ppm.

  • Plants containing pyrrolizidine alkaloids (PA). Common groundsel and tansy ragwort are common PA-containing weeds in hayfields in California, along the East Coast and in Canada. Animals usually won't eat such plants in pasture settings, but often can't sort the plants out of hay. It can take two to eight months for animals to show PA-induced symptoms. While there is some degradation of PAs in silage, the PA content of hay remains constant over many months, Puschner says.

    “Careful inspection of feed is important and contaminated feed should be discarded or fed to less-susceptible species,” she suggests.

  • Oleander, an ornamental evergreen shrub cultivated widely in the South. As few as seven average-sized oleander leaves can be lethal to a horse, while 12 can cause death in cattle, says Puschner. She suggests keeping animals away from areas with oleander, especially if dried leaves or clippings are present.

  • Summer pheasant's eye. Introduced to North America as a horticultural plant, summer pheasant's eye, a.k.a. summer Adonis, escaped cultivation and is now all over the West. Horses, hogs and sheep have been poisoned and sometimes die from feed harvested from fields contaminated with this toxic weed. Horses poisoned from this plant have developed colic, diarrhea and other symptoms.

  • Bristlegrass. A horse develops blisters and lesions in the mouth and ulcerations on the tongue and lips from eating alfalfa hay tainted with bristlegrass, which has sharp and barbed bristles. In many diagnosed cases, alfalfa hay was contaminated with large amounts of yellow bristlegrass and green, yellow and bristly foxtail, all from the same genus.

  • Blister beetles. These insects produce cantharidin, which can reduce milk production in cattle and cause death in horses. Blister beetles are most common in the South.

    “Cantharidin is heat-stable and thus processed alfalfa, such as pellets or cubes, can also contain the toxic compound,” says Puschner. “Cantharidin can be present in hay after the beetles have been crushed.”

  • Botulism. A good rodent-control program helps prevent botulism poisoning. If hay or hay cubes are contaminated with a carcass, botulism toxin may be present.

“Prevention is the best way to deal with botulism,” Puschner points out. “Forage should be examined for evidence of decaying carcasses or other foreign matter.”