CHICAGO, May 15 (Reuters) - Tyson Foods ( TSN ) cannot

predict when U.S. ranchers will start rebuilding the nation's

cattle herd in a meaningful way, CEO Donnie King said at a BMO

Global Farm to Market Conference on Wednesday.

Producers slashed the herd in recent years due to high feed

costs and a drought in the western United States, raising costs

for the livestock and squeezing margins in Tyson's beef

business.