Make sure you buy tires appropriate for the load range.
I didn't copy the rest but it's all correct, too. We went east, 15 hour drive in 2008 with 3 horses, and came back with 4. I'd never had a problem with my original tires, but we blew two tires 1/2-way home--we were driving on my TWO spares--until I realized that my tires weren't the right load range. I found a tire shop that serviced semi's. (Garmins are great for this stuff while you're on the road!!) They loaded up my trailer with the horses in it in the back of their shopt to change out all 4 and 2 new tires for spares. I'd ALWAYS had my bearings checked and greased, but I didn't even think about the weight load. This, BTW, is where trailer companies will go cheap, so CHECK YOUR BRAND NEW TRAILER TIRES.