From Utah to Iowa to Maine and all points in between, it’s been a warm summer. Some weeks in some states have been downright hot.
Hydration for you AND your horse is key.
So is salt intake.
I like tortilla chips myself.
My horses like Redmond Rock.
It’s from salt mines in Utah, not from Pakistan (where they mine the popular Himalayan salt) or from some factory where it’s processed synthetically.
Dr. Rachel Flaherty of Maine Equine Associates told me horses in light work need 1-2 ounces of salt daily. When it’s hot and humid and they’re sweating a lot, they need twice as much or even more.
You can dose your horses by giving them an allotment every day, Flaherty said. “But it’s still good to offer a free choice salt source of some type. And always provide plenty of water when supplementing.”
I prefer free choice with no dosing. It lets the horse choose how much salt it wants and means they rely less on a human’s somewhat arbitrary measure.
Another note about salt:
Many companies make “trace mineral” salt blocks for horses. They may include calcium, zinc, phosphorus, copper, selenium, and 
magnesium. They appear in much smaller quantities than sodium, but can still be important, especially if horses aren’t getting them from grass or hay. (Intense farming and fertilizing has depleted many fields of these minerals.) Redmond tests its salt quarterly and posts the guaranteed analysis here.
Joe Camp gives his horses free choice alright, but doesn’t like salt blocks or rocks or anything they have to lick. They get granulated natural salt. Read more here.