This study aimed to investigate the effects of feeding dairy calves starter diets containing corn grain processed by different methods (ground versus steam-flaked; GC vs. SFC) and either 18% or 21% crude protein (CP) contents (DM basis) on growth performance, digestibility, ruminal fermentation, urinary purine derivatives (PD), and blood metabolites of dairy calves. Holstein dairy calves (39.3 ± 1.9 kg of BW, n = 12 calves per treatment, 6 males and 6 females) were randomly distributed to experimental diets in a 2 × 2 factorial arrangement of treatments. The four dietary treatments were (1) starter diet containing GC and 18% dietary CP (GC-18CP; geometric mean particle size; GMPS = 0.73 mm); (2) GC and 21% dietary CP (GC-21CP; GMPS = 0.71 mm); (3) SFC and 18% dietary CP (SFC-18CP; GMPS = 2.21 mm); and (4) SFC and 21% dietary CP (SFC-21CP; GMPS = 2.16 mm). Calves were weaned on d 63 and remained in the study until d 83 of age. The starter feed intake did not differ among treatments; however, average daily gain (ADG) and feed efficiency (FE) were improved and final BW was higher for SFC diets compared with GC diets. The organic matter (OM) and non-fibrous carbohydrate (NFC) digestibilities were greater for calves fed the SFC diets than for those fed the GC diets. The ruminal total volatile fatty acids concentrations and the molar proportions of propionate and butyrate were greater, and the molar proportion of acetate and NH3-N concentrations were lower, for calves fed the SFC diets than for those fed the GC diets. The excretion of allantoin and total PD, and subsequently microbial protein synthesis, were greater for calves fed the SFC diets than the GC diets. The total urinary nitrogen excretion (UN) and its proportion of N intake were lower for calves fed the SFC diets than the GC diets. The blood concentrations of insulin (pre- and postweaning), glucose (postweaning) and beta-hydroxybutyrate (preweaning) were greater and blood urea nitrogen (preweaning) was lower fo
