I don't know - I'll try it when I go out. I just asked my husband a few minutes ago about his water trough and he said he's drinking as much as everyone else is. Beats me, but I do know he'll stay in his stall instead of going out in the paddock to eat his hay if the weather is really miserable.
I have one that will not drink in the trailer. He has gone up to 8 hours. He will eat but will not drink different water. Next road trip, I'm going to bring my own I guess. I don't know if I'm going to have trouble with the mare. We have only went on "around the block" trips but she won't eat. Makes the gelding happy though caz he'll take it:') Funny critters.