Senators from across the agricultural heartland pressed President Donald Trump’s nominee for agriculture secretary, Brooke Rollins, on Thursday, seeking firm commitments to support struggling farmers and ranchers over the next four years.

WASHINGTON — President Donald Trump’s pick to lead the U.S. Department of Agriculture said during her confirmation hearing Thursday that she has “a lot to learn” about highly pathogenic avian influenza or bird flu, the virus that’s wreaking havoc on the country’s poultry industry and dairy farms.

Brooke Rollins, a policy adviser in the first Trump administration, has been on a smoother course than other Cabinet nominees. President-elect Donald Trump’s pick for Agriculture secretary ...