Complex sentence: In 1995, they believed the population of first-generation horses was sufficient, the breed studbook was closed to outside horses whose parents were not of the Henson breed, and purebred breeding continued, exclusively on horses of second and following generations.
Split into simpler sentences: In 1995, they believed the population of first-generation horses was sufficient, the breed studbook was closed to outside horses whose parents were not of the Henson breed. Purebred breeding continued, exclusively on horses of second and following generations.