I grew up on a dairy farm. Amazingly enough, I didn't have more severe lung involvement than I did.
The only things to be careful about is the dust from the hay, and animal feces. Take along a hand sanitizer and your rescue med inhaler and you should be fine. If you have allergic rhinitis (aka hay fever) you might want to take some claritin or allegra before you visit just to keep your allergies from flaring up.
Other than that, have fun!