The National Rural Assembly last week urged the federal government to fully enforce anti-trust laws. The thinking among a lot of beef and pork producers is that markets — that is, prices! — are manipulated by the dwindling number of large companies that dominate the meat industry. That's why some farm and ranch groups have strongly opposed the move by JBS Swift, the Brazilian beef processor, to buy both the National Beef Packing Co. of Kansas City and Virginia's Smithfield Foods Inc. This week Wisconsin Sen. Herb Kohl, a Democrat, wrote the Department of Justice saying he believed the purchases would further reduce competition in the meat industry. "By reducing the number of major buyers for ranchers' cattle from five to three _ and in some regions even one or two _ this deal will give the remaining beef processors enormous buying power," wrote Kohl, chairman of the Senate subcommittee on antitrust, competition policy and consumer rights. If the sale goes through, three companies — JBS Swift, Tyson and Cargill — would control 80 percent of the beef market.