Arthur Dobbs, in 1744, uses "American Oxen" to refer to Beeves (Cow).
Arthur Dobbs, An Account of the Countries Adjoining to the Hudson Bay (London: J. Robinson, 1744), 41
The American Oxen, or Beeves, have a large Bunch upon their Backs, which is by far the most delicious Part of them for Food, it being all as sweet as Marrow, juicy and rich, and weights several Pounds.