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College of Food, Agricultural, and Environmental Sciences


Dry Weather Can Pose A Feeding Risk for Livestock

August 9, 2002

COLUMBUS, Ohio - Livestock grazing on plants or fed cereal grains during drought conditions may run the risk of being poisoned, causing illness and even death.

Ohio State University Extension livestock specialists said that certain plants like weeds and ornamentals are toxic, and during times of dry conditions when no other pasture feed is available, livestock may be inclined to consume them.

"Toxic weeds are typically in the pastures all the time and livestock normally leave them alone," said Steve Boyles, an Ohio State Extension beef cattle specialist. "The concern, however, is that during drought situations that's the only thing green still standing and that may make them more desirable to the animals." Boyles said livestock producers should get to know some of the more common toxic weeds and plants and keep livestock well fed to ensure they would not be tempted to eat them. Some toxic plants include holly, rhubarb, ivy, morning glory and brackenfern.

"One concern may be the risk of cyanide poisoning," said Boyles. "Symptoms include labored breathing, staggering, trembling muscles and convulsions." He said some sources of cyanide poisoning include twigs and leaves of wild and cultivated cherry trees and certain marsh grasses, such as arrowgrass. The grass contains a high salt content and lack of salt on the pasture may drive livestock to feed on the plant to meet salt requirements.

Farmers can minimize livestock illness from poisonous plants by following the suggested guidelines: * Learn the identification of poisonous plants. * Do not manage forage in high-density areas of poisonous plants. * Supplement feed with salt, minerals and other nutrients. * Avoid grazing animals in areas of abundant poisonous plants. * Provide adequate water to prevent nonselective grazing.

A more dangerous source of cyanide poisoning is found in forage grasses, such as sorghum, sudangrasses and sorghum-sudangrass crosses. Boyles said the grasses produce prussic acid that, under dry conditions, can accumulate in high levels. The active component of the poison is hydrocyanic acid, which is extremely toxic to livestock, especially cattle and sheep.

Boyles recommends producers follow the suggested guidelines when grazing sorghum and sudangrass varieties: * Plant sudangrass and sudangrass hybrids instead of sorghum varieties, since sudangrass hybrids have lower prussic acid potential. * Do not graze sheep on sudangrass or hybrids until the plants are 12 inches to 15 inches tall; 18 inches to 24 inches tall for cattle. * Do not graze sorghum-sudangrass hybrids until the plants are at least 24 inches tall. * Regrowth sorghum should not be grazed until after the plant is completely killed by frost or dried. Young, regrowth forage can be very toxic. * Do not graze forage varieties after a drought, or if the plants show visible signs of moisture stress. Test the plants for toxicity levels before grazing. * Do not graze hungry livestock on forage varieties. The more they consume the greater the risk of being poisoned. It is much safer to offer feed such as hay or silage. In the case of silage, it should be allowed to ferment for at least two weeks before it is fed.

Information on prussic acid poisoning can be obtained through an Ohio State Extension fact sheet titled, "Livestock and Prussic Acid Poisoning" by logging on to the Ohio State Extension 2002 Drought Web site at

Producers should not only be aware of potentially poisonous plants, but also of plant toxins produced by other organisms.

Maurice Eastridge, an Ohio State Extension dairy specialist, said drought conditions increase the risk for the development of mycotoxins - toxins produced by molds during the growth or storage of cereal grains, especially corn. Some of the more common mycotoxins include aflatoxin, vomitoxin, citirin, ochratoxin, fumonisin, and zeralenone.

"Molds thrive in high temperatures and, during drought conditions plants that are stressed are more conducive to mold growth," said Eastridge. "The organisms can proliferate if the kernels are damaged during harvest or under storage. Feeding moldy feed to livestock is unhealthy and also puts the animals at an increased risk of being affected by mycotoxins." Eastridge said the best management practice to keep mold development under control is to store grain under optimal drying conditions before feeding it to livestock. Mold growth takes place in most feeds when moisture content is above 15 percent.

Many health-related problems associated with moldy feed can go unnoticed in livestock, but moldy feed is generally unpalatable and will result in less feed intake, which leads to weight loss and reduced milk production "Cattle are more resistant to mycotoxins than other livestock, like swine and poultry," said Eastridge. "But it's important to monitor feed intake and conduct standard feed tests. Not all mycotoxins are regulated by the FDA." Aflatoxins, produced by strains of the fungus Aspergillus, are the only mycotoxins regulated by the Food and Drug Administration. Corn grain containing over 20 parts per billion of aflatoxin cannot be marketed commercially, and milk with .5 parts per billion is also prohibited for sale.

Additional information on molds, mycotoxins and poisonous plants is available through an Ohio State Extension fact sheet titled, "What Do I Do If Mycotoxins Are Present?" by logging on to the Ohio State Extension 2002 Drought Web site at

Candace Pollock
Maurice Eastridge, Steve Boyles