from reuters: The United States Department of Agriculture was holding a press conference on Tuesday as rumors that a case of mad cow disease was discovered in California sent live cattle futures tumbling. The spokesman declined to comment on the topic of the briefing. The contents of the briefing are embargoed until 3 p.m. ET. Live cattle futures on the Chicago Mercantile Exchange fell by as much as the 3-cent-per-lb daily limit on Tuesday on fund liquidation and selling stemming from the rumors. The first outbreak of mad cow in the United States occurred in late 2003.