Make sure the cow pond and stream are don't have stagnant water. I would use water from a clean source in a barrel. You just don't know what was upstream and decided to pee or poo in that stream.
If you can, get some beet pulp, TSC has it in pellet form here, not shreds, but I have found that it soaks out just as good as the shreds. I soak it two hours before feeding , can be a pain, but it keeps the weight on my ancient ones.I also give them all in one or a+m feed , you can also wet soak this , easier to chew for them than hay. You are kind to help this poor old horse ;) Good luck, hope the lice go away, be sure to powder and bleach your brush ! My old horses shed out funny , clumpy, but they do shed to sleek haired, cushings don't really shed out.