Protect Against Forage Toxicity

Producers should be aware of toxicity concerns caused by two forages, kochia and sweet clover, commonly grazed or hayed for use in winter feeding, says Larry Hollis, Kansas State University Extension beef veterinarian.

September 24, 2010

5 Min Read
Protect Against Forage Toxicity

Producers should be aware of toxicity concerns caused by two forages, kochia and sweet clover, commonly grazed or hayed for use in winter feeding, says Larry Hollis, Kansas State University Extension beef veterinarian.

Kochia (Kochia scoparia), also known as fireweed or summer cypress, is an escaped ornamental plant that grows throughout much of the U.S. and is well-suited to semi-arid climates due to its drought, disease and insect resistance, and alkaline soil pH tolerance. It usually has good forage value, often containing from 11% to 22% crude protein, depending upon soil nitrogen content and stage of maturity at the time of grazing initiation or harvesting as hay, and has been nicknamed “poor man’s alfalfa.”

Well-managed kochia grazing or feeding of timely harvested kochia hay usually results in good livestock performance. However, if kochia plants are drought-stressed shortly before grazing or haying, allowed to grow over 18- to 24-in. tall, and begin to develop seedheads, or if insufficient cattle are placed in the field to keep the plants grazed below recommended heights, various toxicity problems may occur, Hollis says.

Nitrate toxicity may occur after periods of drought stress, but the most common problem arises as the plants approach maturity. Oxalate accumulations as high as 6% to 9% (100% dry matter basis) aren’t uncommon in almost-mature green plants. Oxalates absorbed into the circulation bind with calcium to form insoluble calcium oxalate. If consumed rapidly and in high quantities, it can lead to hypocalcemia. A more common problem, however, is the accumulation of calcium oxalate crystals in the kidneys, which causes kidney failure. A currently unidentified toxin has been found to cause liver failure in cattle, sheep and horses.

These problems typically occur when animals have been moved off overgrazed or drought-limited native pasture and into a post-harvest wheat field, an old corral or a drylot where kochia plants have grown and appear to be a ready source of forage for hungry livestock. Toxicity especially becomes problematic if the diet is made up almost exclusively of kochia, and animals have been consuming it for more than 30-60 days.

One of the first signs commonly reported with oxalate-induced kochia toxicity is increased water consumption as kidney function is altered by the toxin. Another common sign is photosensitization as a result of liver failure. This will appear as sunburning, particularly on lighter-skinned animals or white patches of skin on multi-colored animals. If either of these are noted, animals should be removed immediately from kochia fields or feeding of kochia hay should be stopped and animals given access to high-quality feedstuffs. Many of the animals not visibly affected will recover, but those more severely affected may die as a result of kochia toxicity.

Sweet clover toxicity. A casual glance would indicate a bumper crop of yellow and/or white sweet clover growing in ditches, pastures and Conservation Reserve Program fields across much of Kansas. The delayed arrival of spring, coupled with adequate moisture in most areas, created a favorable growing environment for this commonly found plant species. Because of its overabundance this year in areas suitable for haying, cattle producers should be reminded this plant may have potential side effects that should be taken into consideration when haying, storing or feeding.

Grazing fresh, undamaged yellow or white sweet clover normally is a safe management practice. Problems with sweet clover typically occur after it’s damaged or spoiled, either shortly before or as it’s made into hay. A naturally occurring substance in the plants called coumarin, which is responsible for the characteristic smell of sweet clover, is converted to a toxic substance called dicoumarin when the plant is damaged by hail, drought or frost prior to harvest, or by improper curing or harvest/storage conditions that allow the hay to become moldy.

Dicoumarin (the active ingredient in some rodent poisons) interferes with the synthesis and metabolism of vitamin K. Vitamin K is essential for proper blood clotting to occur, so dicoumarin toxicity problems are manifested primarily as bruising or bleeding disorders. Bleeding from one or more body orifices, a wound, or excessive bleeding following calving are usually the first signs. Young animals are more sensitive to the toxin than older animals. The toxin also can be passed though the milk to calves nursing cows fed affected hay.

Because the toxin has to accumulate in the animal’s body before symptoms appear, the damaged or moldy hay usually has to be fed a minimum of 2-3 weeks before any signs begin to appear. Toxicity usually is seen as a herd problem affecting many animals, and most commonly is seen during the winter after prolonged hay feeding. Affected hay will remain toxic for years.

To prevent sweet clover toxicity from occurring, it’s essential that recently damaged plants not be harvested for hay. Also, the stems of sweet clover plants should be examined to make sure they’re cured properly (thoroughly dry) before being baled. If conditions are right for hay to become moldy, they’re right for coumarin to convert to dicoumarin.

Questionable or obviously moldy hay should be tested for dicoumarin levels prior to feeding. Check with your county agent or veterinarian to see about having hay samples tested. Test the most heavily damaged or moldiest spots in the bale. If hay is found to contain toxic levels of dicoumarin and it is your primary hay supply, alternating between feeding 1-2 weeks of hay containing sweet clover and 1-2 weeks of good-quality alfalfa hay has been found to reduce the likelihood of toxicity signs. However, hay containing significant amounts of yellow sweet clover should not be fed as cows approach calving.

For more information about this form of toxicity, visit
-- KLA News and Market Report

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