To use agriculture as an excuse to reduce your Property Taxes in Texas, you must show agricultural use, but to avoid sales tax on equipment, you only need to live on 4+ acres, and not use the equipment for other purposes, especially commercial.
I've never paid sales tax on a single mower I've bought, and was even offered a no sales tax deal on an ATV from the local dealer. I passed on that one because I knew I'd want to take it to the lake, etc. for recreation.