Re: One Can...
"Always waits until we are back at the ranch and then promptly drains the water trough."
We have a three-day eventer here at the Ranch who does that when we take him out on one-day training rides off-ranch. When at a venue for an actual 3-day, he starts drinking "normally" late in the evening of the first day out. My Wife (a horse trainer these last 50+ years) says it's a mental issue. If it only happens occasionally, it's because the horse isn't feeling thirsty at that moment ... even if you think he ought to be! Regardless, the horse will drink if you wait long enough.
I've seen similar with peeing ... We have a horse here who refuses to pee when he's turned out. He'll hold it all day long, but as soon as we put him in his stall for the night he unloads (the Wife calls it "stage fright"). If we leave him out for a week, he re-learns to pee outside ... until we get back into the routine of bringing him in at night again. Within a couple days, he starts holding it until he's indoors.
All horses have their quirks. Some are quirkier than others.
::shrugs::