text: Because pork production in Sonora is a relatively new development and because the United States has restricted the importation of swine and pork products for the last 20 years , APHIS bases its potential impact analysis on some assumptions regarding the level of pork production in Sonora in the future , the percentage of pork production that might be processed in Sonora for shipment to the United States , and the demand for pork in the United States .
  
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  For the last 20 years the United States has placed restrictions on pork products being imported .