Why can't donkeys reproduce?

They have an odd number of chromosomes

Donkey's can reproduce, but the offspring of a male horse and female donkey (hinny) or a female horse and male donkey (mule) cannot produce offspring due to the fact that they are sterile and cannot produce eggs or sperm. Donkey's do not have an odd number of chromosomes, they have 62, and a horse has 64. The hinny and the mule both have 63 chromosomes, which is an odd number, and they are unable to reproduce.