I am sure some ploughmen sang to their teams, though I doubt anyone sang continously. If they did, then their throats must have been lined with cast iron, for ploughing often went on nearly continously throughout the daylight hours. "gee" and "haw" are commands I have heard were used - and in one demonstration with an ox team I witnessed the ploughman repeated the command in what appeared to be a set rythme, whenever he used it. I suppose some commands might have been more or less "sung"