Agriculture Secretary Mike Johanns will defend an administration plan to reopen the border to Canadian cattle at a Senate hearing on Thursday, where he will face questions about whether the action will expose the U.S. beef industry to more cases of mad cow disease.

The U.S. Agriculture Department plans to allow imports of younger cattle beginning March 7, ending a ban imposed 20 months ago with the first native case of mad cow disease in Canada. Two more cases were reported in January.

Besides arguments over beef safety and fears that imports will drive down U.S. cattle prices, U.S. treatment of Canada could set a precedent for other nations to resume purchases of American beef. The first U.S. case of mad cow disease, or bovine spongiform encephalopathy (BSE), was reported 13 months ago in a Washington state dairy cow.

Senate Agriculture Committee chairman Saxby Chambliss said Johanns will be expected “to give us reasons why” trade can safely be resumed.

“We need to trade with Canada but it has to be on the right basis,” the Georgia Republican said. “We can’t afford to trade with somebody who is going to continue to have BSE cases.”

The Bush administration sent a team of experts to Canada in late January to assess Canadian safeguards against spread of the fatal cattle disease. People can catch a human version of BSE by eating beef products carrying the infectious prions.

Johanns and other USDA officials at the hearing will be prepared to discuss the ongoing review, said a USDA spokeswoman. When USDA announced the review, it said the results “will be used to determine the appropriate next steps” for U.S.-Canada beef trade.

The Democratic leader on the Agriculture Committee, Tom Harkin of Iowa, says USDA should withdraw the Canada rule. Another committee member, Republican Craig Thomas of Wyoming, said there were “very strong feelings from most of us we need to change that rule” because of the two recent mad cow cases.

Democrat Blanche Lincoln of Arkansas said she wanted to hear from Johanns about USDA’s work to persuade traditional customers for U.S. beef to lift their bans on it.

Three U.S. meatpackers - Tyson Foods Inc., National Beef Packing Co. LLC and Swift & Co. - have cut beef production partly because Canadian cattle are not available for slaughter. In recent years, Canada shipped about 1 million head annually to the United States, two thirds for slaughter and the rest for fattening in U.S. feedlots.

While meatpackers have filed suit in federal court, seeking full resumption of cattle trade with Canada, the R-Calf United Stockgrowers of America group of U.S. ranchers and feedlot operators has gone to court to keep the border closed.