WASHINGTON (Reuters) - The Senate Judiciary Committee’s antitrust subcommittee will hear from the top U.S. antitrust bosses next month to discuss a wave of big mergers over the past year, according to a staffer briefed on the hearing. The Justice Department’s Bill Baer, the assistant attorney general for the antitrust division, and Edith Ramirez, chairwoman of the Federal Trade Commission, will testify, the staffer said. The two divide the work of antitrust enforcement. The hearing will be March 9 at the Senate Judiciary Committee’s antitrust panel, which is headed by Senator Mike Lee, a Republican from Utah. One matter that will be taken up is the FTC’s use of Section Five, a portion of its authority that it uses to investigate companies which urge rivals to collude with them on price. The agencies are looking at a startling number of major, controversial mergers. The Justice Department is investigating two large insurance mergers - Aetna Inc (AET.N) buying Humana Inc (HUM.N) and Anthem Inc (ANTM.N) buying Cigna Corp (CI.N) - to insure they comply with antitrust law. It is also looking at Halliburton Co’s (HAL.N) purchase of Baker Hughes Inc BHI.N, among a long list of other deals. The FTC is suing Staples Inc SPLS.O in hopes of stopping it from buying Office Depot Inc (ODP.O). It also said in late 2015 that it aimed to stop three mergers of medical centers, one in West Virginia, another in Chicago and a third in Pennsylvania. 