The Japan Racing Association said Saturday it would exclude 156 horses from racing this weekend, an unprecedented number, after finding a prohibited substance in their feed additives.

The JRA held its races as scheduled on Saturday, and will do likewise on Sunday with the remaining horses. A total of 983 were originally supposed to compete this weekend.

The association said an imported additive sold by Japanese company Nosan Corp. was found to contain a performance-enhancing substance called theobromine, a stimulant that also strengthens cardiac activities.

“We weren’t able to test the 156 horses on Friday, so we decided to scratch the horses from competition to ensure the fairness of the competition,” said Shinobu Ito, a JRA official.

“We have been telling feed vendors to only ship products that have been tested in the past, but the products that were delivered this time contained additives that were not tested yet,” Ito said.

At Tokyo Racecourse, another JRA official said the association learned early Friday evening that feed vendors had been contacting stables, asking if they could retrieve the products that tested positive for the banned substance.

“The product has been used for a long time, but basically it must not be sold until after test results come back negative. These happened to be sold to stables before the results came out,” Yukinobu Shomura said.

While stables have long used the additives, the JRA calls for all feed products to be tested at the Laboratory of Racing Chemistry in units they were manufactured in. He said the JRA is still investigating how the product contained theobromine.

The additive, which tested positive for the banned substance, was manufactured between December and May, and was used in a total of 28 stables in Ibaraki and Shiga prefectures.

Nosan Corp. apologized in a statement, saying, “We will investigate the causes and take preventive steps.” The product, the statement said, does not use theobromine and it is not yet known how the substance got into the product.

The use of such drugs that temporarily enhance or reduce the performance of race horses are banned from races sanctioned by the JRA, which is holding 26 Grade-1 races this year.

The association took until Saturday morning to announce the matter because most of its staff were on their way to their assigned racecourses, and it required time to contact stable officials and come up with possible measures, the JRA said.

A total of 72 JRA races are scheduled at Tokyo Racecourse, Hanshin Racecourse and Hakodate Racecourse this weekend.