Taking a cue from this thread, I did an experiment on a dog I've been working.

He weighed 40.1 lbs before his work today. He was put on the jenny for 30 minutes today. He had two spells of where he walked for 2 minutes, each time, while on the jenny. At the end of that half hour, he had ran 4.89 miles at an average speed of 9.6mph.

His weight after he cooled down? 38.14 lbs. That is just a half hour work out for this particular dog, and he lost that much water weight. Hopefully that will give people an idea of just how much water a dog can potentially lose if he's a serious working dog. Obviously, lots of dogs don't work that hard so they won't lose that much water, but it is something that everyone should pay extremely close attention to if they want to have the dog running at it's optimum level come show time.

REPLENISH your fluids!