When the results come back from the lab, it will not be slander. You can post those results all over the internet, you can post copies in every feed store in town, (something like eight or nine, not counting tack and clothing stores, grocery stores. Anywhere there is a bulletin board, including Craigslist too. \\
You can also complain to the
BBB
Dept of Agriculture
Animal Control
Code Enforcement, for various other, serious and obvious violations, easily visible from the street.
You can completely bury her and run her out of business. She has been operating outside the laws and ordinances for decades now. This might be her last year in business if you do things right.