Kelly,
What do your pastures look like, other than the pine? What's your pastures history? Has it ever been cropped? I've dealt with mineral issues before, and they can be tough. If you aren't providing salt and mineral, I would suggest you start. I like redmond, it's really good stuff (especially their salt). Your extension office can help determine if you're in a selenium deficient area, if so consider redmond selenium salt 30.
http://www.redmondnatural.com/products
Do you rotate pastures often, or set stock graze?
If heat is an issue, maybe move from highland to galloway. Very similar animal, plenty of high quality genetics available, moderate frame, and more heat tolerant than the highland.