Volts is important.
you have your water heaters yet? are they 120 or 240v?
I'd at least run 240v as much as I can and put subpanel in the second pasture, then run the 120v back to the first pasture.
1500w + 1500w + 200w = 3200w
3200w at 120v is about 26.7a at 280' will need to be about 2gage ($$$!!!). 3200w at 240v run at half of amp, at about 13.3a and only need 4ga, maybe 6ga, to save money on wire. Wire size is determined by amp and distance to maintian acceptable voltage drop.
higher volt require less amp for the same amount of work, so might want to run 240v out there to a subpanel in the pasture then run 120v to your heaters (or run 240v heaters, even).
but i am no VAC expert, though. my $0.02