United Parcel Service said weak global trade and a disappointing holiday season slowed it down in the fourth quarter. UPS also forecast a ‘‘relatively flat’’ first quarter. ‘‘Overall we still see 2013 as a slow-growth economy,’’ said CEO D. Scott Davis. It lost $1.75 billion in the fourth quarter because of a $3 billion charge for pension liabilities, compared with a profit of $725 million a year ago. Without the charge, it would have earned $1.32 per share. Analysts expected $1.38.