Beef packer margins rallied sharply last week and posted their second week of gains to end essentially at breakeven levels. Margins rose $142/head and hit an estimated -$0.89 as fed cattle prices collapsed $7/cwt and beef values rose nearly $4/cwt. Packer margins should continue to improve with the seasonal strength in beef demand and with much of the strength having come out of the cattle markets.  Feedlot profitability dipped last week due to ...