This study was conducted to investigate the effects of 2 forage sources supplemented from different time points on the growth performance of Holstein bull calves. Forty Holstein bull calves with similar body weights and birthdate were selected and randomly assigned to 5 treatments with 8 replicates each: control (CON) calves were fed starter feed only, and the other 4 treatments consisted of the same starter feed plus 2 forage sources started from different time points: chopped alfalfa hay started at the 15th day (AW) and the 3rd day (AD), chopped oat hay started at the 15th day (OW) and the 3rd day (OD). All calves were weaned at 56 d, and the study finished one week later. In the pre-weaning period, the starter feed intake was not significantly affected by forage supplement. Moreover, starter feed and total dry matter intake were increased in the first week of post-weaning by feeding forage from the 15th day and providing oat hay. In the pre-weaning period, the average daily gain of calves supplemented forage from the 15th day tended to be higher compared with that from the 3rd day (P = 0.079). In the first week of post-weaning, the average daily gain of calves fed forage from the 15th day was significantly higher compared with that from the 3rd day (P = 0.045) and CON calves (P = 0.032). Meanwhile, the body length of calves pre-weaning improved by feeding oat hay (P = 0.071), and the heart girth of calves fed forage from the 15th day tended to be greater than CON calves (P = 0.089) in the first week post-weaning. The contents of LDH, AKP, GOT and GPT in the serum of calves were improved by supplementing oat hay and providing forage from the 15th day (P < 0.05). The incidence of diarrhea in CON calves were numerically higher than the calves provided forage. In conclusion, forage supplementing did not adversely affect the growth and intake of calves in the pre-weaning period, and the growth performance of calves was improved in the first week of post-weaning by providing forage from the milk-feeding period, especially offering oat hay from the 15th day.