Dozens of horses on a breeding ranch in Oklahoma that for generations has provided livestock for rodeos were said to have died after eating feed that was contaminated with an additive given to cattle, a veterinarian was quoted as saying, prompting investigations by officials in Oklahoma and Kansas.

The Beutler and Son Rodeo Company has supplied horses, bulls and calves for roping and bucking events in rodeos across the United States since 1929. The ranch has passed through generations of Beutlers just north of Elk City, Okla., a community of about 11,000 people in the western part of the state.

Local news media this week reported that Bennie Beutler, who now owns and runs the ranch with his son Rhett Beutler, said that horses started to die last week in increasing numbers after being fed. “We put the feed out and we lost a couple,” Mr. Beutler said in an interview with News 9, a local CBS affiliate, on Wednesday.

It was not immediately clear how many horses had died by Thursday, although some reports said at least 70 horses had died. The deaths “heart-broke us all,” Mr. Beutler told News 9. Some of the horses were mares whose foals will now be bottle-fed, he said.

On Thursday, the Oklahoma Department of Agriculture, Food and Forestry said in an emailed statement that it sent an inspector to the ranch on Monday after learning on Friday about the loss of the horses from an unnamed veterinarian who said there was a “potential issue with horses relating to feed.”

The inspector took a sample of the feed, which originated in Kansas, for testing, and the results are expected to be completed within weeks. The Kansas Department of Agriculture said on Thursday that it had started an investigation of the records and of the labeling and operating procedures of the feed company.