Beef packer margins pulled back last week but remained positive for just the sixth time this year. Margins fell about $64/head due to weaker beef prices (exemplified by a $10/cwt decline in the Choice cutout) that offset a modest $1.80/cwt decline in fed cattle prices. The drop credit for packers also worsened slightly and contributed to the weaker margins. Packer margins increasingly look threatened as the beef markets follow their seasonal pattern lower while cattle prices surge from the bullish Cattle on Feed report and the outlook for an extended suspension of feeder cattle imports from Mexico.  Feedlot profit margins were mixed last week ...