NEW YORK >> Butterball apparently has big fat mystery on its hands: The company says it doesn’t know why some of its turkeys wouldn’t plump up in time for Thanksgiving this year.

CEO Rod Brenneman says in an interview with the AP that it’s the first time it happened and that the company is investigating what went wrong. Butterball had announced last week that it will have a limited supply of large, fresh turkeys that are 16 pounds or heavier for the holidays.

“It’s a really good question. We don’t have an answer yet,” Brenneman said when asked about the cause. But he noted that turkeys are “biological creatures” subject to a variety of factors.

“For whatever reason, they just didn’t gain quite as well this year,” he said.

Like many other turkey producers, Butterball feeds its birds antibiotics to prevent and treat illnesses, which can occur from living in cramped quarters. The use of antibiotics, which also promote growth in livestock, has been the subject of concern that it could lead to antibiotic-resistant germs.

Butterball, a privately held company based in Garner, N.C., declined to say whether it made any changes to its feed this year. But the problem seems to have come up rather recently.

For much of the year, Butterball produces turkeys that are frozen and stored until they’re ready to be sold for the holidays. But then in October and November, it shifts into production for fresh turkeys. And that’s when the company ran into problems with the turkeys not gaining enough weight, Brenneman said.

Butterball declined to say whether the issue has been resolved or provide details on the extent of its shortage. But Big Y, a supermarket chain based in Springfield, Mass., said in a statement that it had been notified by the company that orders across the country were cut by 50 percent.

Mark Kastel, founder of The Cornucopia Institute, a Wisconsin-based group that advocates for organic farming, noted that major poultry producers tightly control production factors, making Butterball’s shortage unusual.

“I thought that was very mysterious. I could not think of a rational explanation,” Kastel said, suggesting that a change in the feed formula may have been to blame.

Butterball’s shortage shouldn’t be a problem for most since fresh turkeys only account for about 15 percent of sales, with frozen turkeys accounting for the rest. Butterball also makes only about one out of every five turkeys eaten on Thanksgiving.

Over at Cargill, spokesman Michael Martin says in an email that the company has never had a problem where it’s birds didn’t put on enough weight to produce an adequate supply of large turkeys.