This study investigated the effects of feeding starter diets containing either 18% or 22% crude protein (CP) content (DM basis) and rumen undegradable protein to rumen degradable protein (RUP: RDP) ratio on growth performance, nutrient digestibility, ruminal fermentation, blood metabolites and urinary purine derivatives in dairy calves. A total of forty-eight 3 d-old Holstein female dairy calves with 40.2 ± 2.5 kg of initial BW were randomly assigned in a completely randomized block design (block was considered as the progressive assignment of animals to experimental treatments) to a 2 × 2 factorial arrangement of treatments (12 calves/treatment). Thus, 4 treatments were evaluated: 1) starter diet with 18% CP and low RUP: RDP ratio (low ratio (LR) = 26:74; 18CP-LR); 2) starter diet with 18% CP and high RUP: RDP ratio (high ratio (HR) = 35:65; 18CP-HR); 3) starter diet with 22% CP and low RUP: RDP ratio (22CP-LR); 4) starter diet with 22% CP and high RUP: RDP ratio (22CP-HR) on DM bases. Starter was fed for all experimental groups in the physical form of finely ground (mean particle size = 0.69 mm). Calves received 4 L of the milk/d from d 3 to 10, 7 L/d from d 11 to 40, 4 L/d from d 41 to 49, and 2.5 L/d from 50 to 53, and then all calves were weaned but remained in the experiment until d 83 of age. The results showed that overall ADG, weaning BW, feed efficiency (FE) were greater in 22% CP than in 18% CP. However, increasing the starter CP content from 18% to 22% of DM did not influence overall starter feed intake, milk intake, total DMI, postweaning ADG, and postweaning FE of calves. No effect of RUP: RDP ratio was observed for starter feed intake, milk intake, total DMI, preweaning ADG, FE, and g CP per Mcal of ME. The RUP intake and postweaning ADG were greater for calves fed the HR diets compared to those fed the LR diets. The digestibility of NDF was greater and the digestibility of OM tended to be greater and the ruminal concentrations of total short-chain fa
