J. Dudley Butler will end his tenure next week as head of the Grain Inspection Packers and Stockyards Administration, the agency said Thursday.

Butler oversaw key parts of the Obama administration's effort to pass sweeping antitrust reforms covering the meat packing industry.

A heated fight over the regulations pitted small farmers and ranchers against some of the nation's biggest meat companies. The effort ended late last year after Congress killed funding for the proposed regulations.

Agriculture Secretary Tom Vilsack said Butler's departure won't weaken antitrust enforcement at the Department of Agriculture.

"President Obama and I believe fair and competitive markets are critical to the success of American agriculture, and Dudley has worked tirelessly to advance this cause. USDA looks forward to continuing this work on behalf of our nation's producers," Vilsack said in an e-mail to the AP.

Butler said in a brief statement that he is confident enforcement efforts will continue.

The American Meat Institute, a lobbying group that opposed Butler's reforms, declined to comment on his resignation.

Farmer advocates saw Butler's departure as a loss.

"We're deeply disappointed, but not at all surprised," said Bill Bullard, chief executive of R-CALF USA, a group that represents small to mid-sized cattle producers. "The problem occurred when the meatpackers intensified their political pressure, and the administration buckled and would no longer support Dudley's efforts."

The Packers and Stockyards Administration has special antitrust authority over the nation's biggest meatpackers and poultry companies. During Butler's tenure, the PSA sought to expand its authority by giving greater protection to chicken farmers and cattle ranchers; in part; the reforms would have made it easier for farmers to sue packers over such issues as the prices packers pay or which production costs they cover.

Big meat companies opposed Butler's appointment in May, 2009. An attorney for more than 30 years, he had specialized in suing big companies like Tyson Foods Inc. on behalf of farmers.

After Butler took over the PSA, the Obama administration started trying to overhaul the agency. In June 2010, Butler and his team proposed rules the Department of Agriculture said would benefit consumers by stoking competition among the handful of companies that produce most of the nation's meat. Just four companies process about 80 percent of all beef in the United States.

The rule would have prevented meatpackers from showing preference to big feedlots by offering them special incentives, and it would have limited the control chicken processors have over farmers they contract with by barring them from requiring farmers to bear the full cost of improving their facilities.

Meat companies said the new regulations would have added extra costs and further cut already-thin profit margins. And, in November, Congress barred the USDA from funding the new rules.

In December, the USDA adopted a radically stripped down version. The final measure requires meat companies to let farmers opt out of arbitration clauses in their contracts. The other measures were abandoned or made into optional guidelines for the agriculture secretary to consider when judging if a meat company violates the Packers and Stockyards Act.

The USDA said Alan Christian, deputy administrator for the Packers and Stockyards Program, will serve as GIPSA's acting administrator until a replacement for Butler is named.