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Fungus factor bull

Much of the research on fescue toxicity has focused on females, but Clemson research shows fungus-infected pastures can affect bull fertility.


CLEMSON — Bulls and men seeking to be fathers are susceptible to the same stress. Heat down under may affect the outcome.

Bulls can’t swap briefs for boxers, but they do need to stop grazing on grass infected with a fungus that alters their body chemistry, triggering abnormal body temperatures and other symptoms. Clemson University researchers are searching for ways to neutralize the toxic effects.

Tall fescue is a robust perennial, providing millions of acres of pasture for livestock. Its hardiness in part comes from its relationship with the fungus. It thrives on popular varieties of the grass and in return helps the fescue resist insects, heat and drought. But what is good for the grass is bad for the grazer.

An endophyte, the fungus causes fescue toxicity, a condition that diminishes growth, health and reproduction in cattle as well as horses and sheep. About 8.5 million head of cattle graze on it. Infected fescue may lead to losses of as much as $1 billion yearly in lost body weight, illness and fewer pregnancies, according to beef industry estimates. Symptoms occur typically during the hot months — “summer slump” is the cattlemen’s diagnosis.

Much of the research on fescue toxicity has focused on females, which can fail to become pregnant or spontaneously abort their offspring. The failure rate for cows can run as high as 35 percent. Some researchers have turned their attention to males. It made sense to study both sexes, of course, but there was another realization. One bull can have more impact on the problem than one cow. A bull can cover as many as 25 cows via natural reproduction.

“In a race that requires sprinters, these sperm were barely walkers,” said a researcher from Southern Illinois University, referring to her work with infected bulls.

At Clemson University, Scott L. Pratt looks at the bulls from another angle. A molecular reproduction physiologist, Pratt focuses on the chemical molecules that deal with animals’ reactions to the toxin. The consumed infected grass and hot summer temperatures hamper the bulls’ ability to maintain normal body temperatures. Particularly heat sensitive are the testicles, where body temperature can affect sperm.

Pratt is working on finding genes and gene pathways that are affected by the toxin.

“The work we are doing to identify biochemical markers that are indicators of bull fertility may help with inconsistent breeding soundness exams, which is big problem,” Pratt said. “All labs have seen mild to no effects on the exams. Bulls on toxic tall fescue will pass a breeding soundness exam, but still be subfertile.”

His work could lead to better bull management strategies to maintain bull fertility while grazing toxic tall fescue.

It would add another way to deal with fescue toxicity. Currently, a cattle producer must take his animals off an infected pasture months before breeding. There are nontoxic fescues available, but planting them requires completely replacing a pasture, which can be costly and time-consuming.


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