In 1965 Pernell Roberts left this series & his part as eldest son Adam, saying he'd grown tired of the role
He wanted Bonanza to be "a little more grown up," (Mike Douglas Show
,... [DOC] [TLE] Pernell Roberts - Biography - IMDb [PAR] Best recalled as the eldest son and first member of the "Bonanza" Cartwright clan 
to ... oldest and best-educated son Adam in the Bonanza (1959) series in 1959.