October 24, 2008

Weather change means change for producers grazing livestock

Fortunately, these can be prevented with proper management, said Ron Lemenager. Removing livestock from pastures for several days after a frost is the best way to reduce prussic acid poisoning from sudangrass and sorghum sudangrass pastures and bloat from alfalfa and legume based pastures, he said.

After the first frost, pasture bloat should be a concern to cow-calf operators and stocker operators, Lemenager said. Frothy bloat or legume bloat, caused by consuming forages with high levels of soluble protein such as alfalfa, winter wheat and white clover, minimizes the animal's ability to expel rumen gases.

Cattle suffering from bloat can die within an hour, he said. Symptoms include rapid swelling on the left side of the animal and signs of discomfort such as kicking at their side or stomping their hooves.

If animals ingest forages with high levels of prussic acid, the acid absorbs into the bloodstream and interferes with oxygen transfer, causing death within minutes, Lemenager explained. Symptoms may include excessive salivation, difficult breathing, staggering, convulsions and collapse. Ruminant animals are more susceptible than other species because cud chewing and rumen bacteria increase the release of cyanide, he said.

"While southwest and south central Indiana await their first autumn freeze, east central Indiana woke up to a killing frost Wednesday morning with temperatures as low as 23 degrees," said Ken Scheeringa, associate state climatologist for Indiana.  "Elsewhere in the state, patchy frost has occurred as air temperatures dipped just below freezing the past few days."

When summer-annual grasses are damaged by a frost, the material in plants known as cyanogenic glucosides is converted to prussic acid.  The concentration varies by plant species and is highest in sorghum, moderate in sorghum-sudangrass crosses and lowest in sudangrass.  Pearl millet is virtually free of cyanogenic glucosides.

Soils with high nitrogen levels or a deficiency in phosphorus or potassium produce plants with increased cyanide potential, Lemenager said. Immature plants and plant leaves also have a higher concentration. After frost damage, cyanide levels will likely be higher in fresh forage compared to hay or silage, because cyanide dissipates as the forage dries or is ensiled.

To prevent prussic acid poisoning, Lemenager recommends that producers avoid grazing their animals on nights when frost is likely, because high levels of the acid are produced within hours after a frost on susceptible species. Once a frost occurs, producers should not graze animals until five to seven days after a killing frost. They should also avoid grazing wilted plants or plants with young tillers after a frost, he said.

"Cattle that must be grazed on sorghum pastures that have not been killed by frost should have access to another type of hay," Lemenager said. "It's important for cattle to have full rumens before being turned out to pasture. Producers should watch them closely for the first few hours, and if symptoms are observed, call a vet immediately."

Producers can green-chop frost damaged plants, which lowers the risk compared to grazing directly because animals have less ability to selectively graze damaged tissue, Lemenager said.  But he warns to proceed with caution because the forage can still be toxic.

Hay also can be made from frost-damaged sorghum grasses with little or no risk of cyanide toxicity.  If it's dry enough to make hay, most of the cyanide gas will have dissipated.

"Normal silage-making allows the majority of cyanide to dissipate from frost-damaged annual sorghum grasses, but feeding should be delayed six to eight weeks after ensiling," Lemenager said.  "But producers need to be careful when working in the feed room, around silos or in silos because cyanide is heaver than air and will migrate downward as it's released from silage.

"Make sure ventilation is adequate, and send suspect silages to a lab to be analyzed for cyanide content before feeding."

To reduce the risk of bloat, Lemenager recommends:

* Using bloat control measures if pastures contain more than 50 percent legumes.

* Supplementing significant amounts of grass hay to cattle grazing bloat-provoking pasture.

* Allowing cattle to graze bloat-provoking pastures continuously instead of removing them during the day or at night.

* Waiting until the dew is off alfalfa before placing cattle into a new pasture.

* Avoid grazing cattle on young plants, where bloat potency is highest.

For questions and more information about preventative strategies, contact Lemenager at (765) 494-4819 or rpl@purdue.edu.

Writer:  Julie Douglas, (765) 496-1050, douglajk@purdue.edu.

Sources: Ron Lemenager, (765) 494-4819, rpl@purdue.edu.

Ken Scheeringa, (765) 494-8105, kens@purdue.edu.

Purdue News Service: (765) 494-2096; purduenews@purdue.edu

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