Complex sentence: In 2000 he was sold to the Harris Farm operation in California where he stood for the next three years; and thereafter stood at the Key Ranch breedership in Texas where in 2010 he completed his stallion career.
Split into simpler sentences: In 2000 he was sold to the Harris Farm operation in California where he stood for the next three years. He was then relocated to the Key Ranch breedership in Texas where in 2010 he completed his stallion career.