I hate using terms like "Business Casual", "Informal", "Semi-Formal", etc. I don't know what those mean anyway. If you're wondering what to wear to our wedding, I would just suggest wearing something you'd wear to an outdoor work party or to church. Wear something you're going to be comfortable in. Keep in mind that you're going to be on a farm, so PLEASE don't wear heels or uncomfortable shoes!!! This is for your own good. I would even suggest wearing closed toed shoes (flats, ladies!) to prevent possible ant bites, etc.
Since it's starting to get a little chilly in the evenings, I would also suggest that the ladies bring sweaters. Sunglasses might also be needed in the early evening.
Cowboy boots and hats are encouraged!