Beef packer margins reversed sharply lower last week, swinging back into negative territory after six straight weeks of positive returns. Margins fell $145/head to –$75 as fed cattle prices rebounded $7/cwt (live basis), while the Choice cutout slipped nearly $7/cwt. The rapid compression erased most of the leverage packers gained following late-November panic selling tied to plant-closure headlines. With breakeven values once again below the cash market, packer profitability appears increasingly vulnerable as boxed beef values soften seasonally heading into late December and early January.  Feedlot placement margins deteriorated further...