Originally Posted by Ladybug2001
They are infertile due to the different number of chromosomes which is a result of two different speices creating an offspring. For istance, horses have 64 chromosomes and donkeys have 62, resulting in mules having 63 chromosomes. There are very rare cases in which female mules can produce an offspring with a purebred horse or donkey, but it is rare.
This is what I understand, someone will correct me if I'm wrong.
This is correct. As of about 10 years ago there were only 2 mule mares that were able breed and none of their offspring were able to breed. No other scientifically proven mules were able to breed.
And I agree with Speed Racer that you should get your vet to give Lutalyse. It isn't 100% effective but hopefully it will work.