AT A GLANCE Massive herd expansion helps fuel surge in trading Analyst: Feedlots are in as good a position as they’ve been in years Beef producers are building the U.S. cattle herd at one of the fastest paces on record, as higher cattle prices and stronger exports fuel healthy profit margins. The expansion comes just a few years after the U.S. herd shrank to a six-decade low amid severe drought in the Southern U.S. Plains and near-record corn prices. The market resurgence is reflected in CME Group’s benchmark Live Cattle futures contract, for which the number of open positions, or open interest, has…