The objective of this trial was to evaluate the impact of different NDF content of the solid diet on indicators of rumen development and calf behavior. Thirty-five Holstein calves were housed in suspended individual cages without bedding, blocked according to sex, birth date and weight and distributed in one of the treatments: 1) starter concentrate with 22% NDF (22NDF); 2) starter concentrate with 31% NDF, replacing part of the corn with soybean hull (31NDF); and 3) starter concentrate with 22% NDF and free access to coast-cross hay (22NDFhay). Calves received 4 L/d of milk replacer until gradual weaning at 56d. Metabolic indicators were evaluated weekly, while ruminal characteristics were evaluated at wk 4, 6, 8. Animal behavior was weekly evaluated by focal instantaneous time sampling, every 5 min, during 10 h. Data were analyzed as repeated measures over time using the MIXED procedure of SAS. Hay supplementation decreased total SCFA (22NDF = 133.3, 31NDF = 133.4 vs. 22NDFhay = 104.0 ± 10.62 mM), but molar proportion of individual SCFA were not affected (P > 0.05). The higher rumen N-NH3 was found in 22NDF calves (22NDF = 17.6 vs. 31NDF = 9.9, 22NDFhay = 10.7 ± 1.64 mg/dL). The 22NDFhay calves presented the longer time of solid food intake (22NDF = 45.8, 31NDF = 41.1, vs. 22NDFhay = 68.8 ± 8.25 min/10h) and rumination (22NDF = 41.8, 31NDF = 17.8, vs. 22NDFhay = 48.9 ± 8.95 min/10h). Raising the NDF content in the concentrate was not efficient in stimulating rumination. 31NDF and 22NDFhay diets may have promoted greater rumen N-NH3 utilization, resulting in decreased concentration. 22NDF diet did not cause lower ruminal pH in comparison to diets with high NDF content (P > 0.05). However, blood metabolites (glucose, lactate, BHB, total protein and urea) show no effect of different solid diet NDF content on rumen development (P > 0.05). Yet, age affected all blood parameters suggesting the metabolic transition (P < 0.01). The NDF increase in the concentrate can stimulate ruminal development earlier than the supply of hay. Although the supply of hay stimulates rumination.