If beasts of burden were so great, we wouldn't have tractors
The Amish still use draft horses, for religious reasons. The horses take up barn space, require feed and grooming and veterinary care, none of which the tractor needs. It's true they reproduce themselves, but baby tractors don't need to be raised and trained. And the animals don't produce enough fertilizer on their own to make that a significant benefit.
You can live by farming the old-fashioned way, but you can't send your kid to college.
Regards--
Mary Anne