In the dairy calf feeding, supplementation of forage to the starter feed is commonly practiced. However, data are insufficient about howchanges in particle size (PS) of forage affect calf performance and behavior in a free-choice forage provision system. This study aimed to assess the effects of supplementing wheat straw varying in PS on performance, skeletal growth characteristics, ruminal pH, nutritional behaviors, and blood metabolites of dairy calves. Forty-eight Holstein calves (43.8 ± 3.2 kg of BW) from d 15 of age were randomly assigned to one of the four treatments (n=12/treatment; six males and six females): (1) starter withoutwheat strawsupplementation (CON), (2) CON supplemented with wheat straw chopped at 1 mm geometrical mean particle length (GMPL) (fine PS), (3) CON supplemented with wheat straw at 4 mm GMPL (medium PS), and (4) CON supplemented with wheat straw at 7 mm GMPL (long PS). The calves were given ad libitum access to feed and water throughout the study. All calveswere weaned on d 56 of age and continued the experiment until d 90. The starter, wheat straw, and total solid feed intakes were not affected by GMPL of wheat straw; however, CON calves had a lower solid feed-, total DM- and NDF-intake than calves offered wheat straw. Further, ADG, weaning, and final BWs as well as feed efficiency were similar between treatments. No difference was observed in growth rate of hip height, hip width, body barrel, wither height, and heart girth among treatments, however, wheat straw supplemented calves tended to have greater body length at weaning. Blood concentration of serum β-hydroxy butyrate was greater in wheat straw supplemented calves compared with CON calves on d 56 and 90. Calves supplemented with wheat straw spent more time eating starter and forage, lying and ruminating and less time for standing and non-nutritional behaviors compared with the CON calves on d 49 and 63 of the study.Moreover, calves offeredwheat strawhad greater ruminal pH than
