I use them as well. I've noticed a much better response when transplanting if I add alfalfa to the mix. I pick mine up from Tractor Supply. I think they run $10-12 for a 40 or 50 lb bag. If memory serves there is something specific they add to the soil. A fatty acid the plants want/need or something. I don't have time to go digging or I would try to point you in that direction. If you search the forum I am certain there are other threads on the topic.
And yes, make sure they are horse pellets. The rabbit ones have a load of salts and minerals added to them which can wreak havoc on your soil.
