A California waste management company has launched a legal challenge to a new U.S. labor board standard for "joint employment" that could make it easier for unions and regulators to hold companies accountable for the practices of staffing agencies, contractors and franchisees with which they partner. Browning-Ferris Industries, a subsidiary of Republic Services Inc., last week sought review of the National Labor Relations Board's decision on joint employmen t by a federal appeals court in Washington.