Teach Me Thursday: Sweat marks

I've been paying attention to Digby's sweat lately. He's not the best drinker in the world. I give him electrolytes year round to encourage him to drink more. Horses are actually supposed to get 1-2 oz of salt a day, when they're not in work. Typically grains do not provide even close to this amount and often horses can't lick enough on salt licks to get this daily. In the winter, if I don't give him an electrolyte, he doesn't drink a lot and then is prone to colic. In the summer, he doesn't sweat well. Every summer I just keep an eye to make sure he's sweating well. This summer, however, I've been keeping a closer eye. I've been trying to look and make sure the sweat marks, both under his saddle and otherwise, are symmetrical. I'm not 100% sure what I should be looking for. For example, he seems to sweat more in the middle of his neck and at the base near his withers. Does this mean this muscle is working harder? I suspect yes.

"Give me cookies NOWWW" face!

Let's hear it! What do sweat marks mean to you? Do you pay attention to them? Do you look at the marks on your horses body or just where the saddle goes? Any tips?