American beef plants are under threat as the number of cattle destined for their facilities stays far below historical levels, reflecting the smallest herd in more than half a century.Cattle placed in US feedlots, where animals are fed until ready to be slaughtered, are expected to have dropped in November to the lowest levels for the month since 2015, according to a Bloomberg survey of analysts. The data, to be announced later Friday by the US Department of Agriculture, comes after cattle placements for Oc ...