This study aim was to test the effects of supplementing forage with varying particle sizes on performance and ruminal pH of calves. Holstein calves (n = 40; 48.3 ± 3.2 kg of BW) from 15 to 90d of age were randomly assigned to 1 of 4 treatments (n = 10/treat; 5 males and 5 females): 1) starter without forage supplementation (CON), 2) starter with wheat straw chopped at 1mm geometrical mean particle length (GMPL) (FPS), 3) starter with wheat straw at 4 mm GMPL (MPS), 4) starter with wheat straw at 7 mm GMPL (LPS). The calves had ad libitum access to water, forage, and starter throughout the study. All calves were offered milk 7L/d from 15 to 25 old, 6L/d from 26 to 35d old, 5L/day from 36 to 45d old, and 3L/day from 45d old until weaning at 56d old. Starter and forage intake were recorded daily. Calves were weighed every 10d. ADG and feed efficiency was calculated for 3 periods: pre-weaning, post-weaning, and overall. Rumen pH was recorded at 35 and 90d of age at 4, 8 and 24h after morning feeding via stomach tube. Data were analyzed using the MIXED procedure of SAS with time as repeated measures for performance variables. Furthermore, a contrast between the control group and all groups fed forage was examined. During the post-weaning and overall periods starter, forage, and total DM intake were not affected by GMPL of wheat straw, but CON calves had lower starter and DM intake (P = 0.05) than calves offered forage. Through all 3 periods, ADG did not differ by forage particle size, but CON calves had lower overall ADG than calves fed wheat straw (0.806 kg/d, CON; 0.896 kg/d, MPS; 0.902 kg/d, LPS and 0.930 kg/d, FPS; P = 0.05). Feed efficiency did not differ across treatments. Ruminal pH was lower in CON than calves supplemented with forage at 8h after feeding on d 35 (5.50, CON vs 6.08, FPS; 6.16, MPS and 6.01, LPS; P = 0.05), 4h after feeding on d 90 (5.32, CON vs. 5.73, FPS; 5.91, MPS and 5.63, LPS; P = 0.01), and 8h after feeding on d 90 (5.48, CON vs 5.97, FPS; 6.02, MPS and 5.77, LPS; P = 0.02). In conclusion, supplementing wheat straw increased starter intake, improve calf performance, and modified ruminal pH dynamics of dairy calves but no difference was found for different GMPL treatments.