Usually if they use Roundup, it is a contact only herbicide. If you are getting as much drift as you say you are, they shouldn't be treating if it's that windy. If you do notice drift coming over, you can water after the app and there should be less effects from roundup. It doesn't stay in the soil.
How close is your garden to the pasture they treat? If you know when they are planning to treat, you can call the TDA and get advice from them on the conditions allowed to do the application.you can also look up the product and read the label. There is a lot of good information and if they are using it, they are required to follow the label.
Hope this helps a little.
