This study investigated the effects of feeding finely ground starter diets containing either 18 or 22% crude protein (CP) content [dry matter (DM) basis] and high or low ratios of rumen-undegradable protein to rumen-degradable protein (RUP: RDP) on growth performance, nutrient digestibility, ruminal fermentation, blood metabolites, and urinary purine derivatives in dairy calves. A total of 48 three-day-old female Holstein dairy calves with 40.2 ± 2.5 kg of initial body weight (BW) were randomly assigned in a complete randomized block design to a 2 × 2 factorial arrangement of treatments (12 calves/treatment). Treatments were as follows: (1) finely ground starter diet (mean particle size = 0.69 mm) with 18% CP and low RUP: RDP ratio [low ratio (LR) = 26:74; 18CP-LR]; (2) finely ground starter diet with 18% CP and high RUP: RDP ratio [high ratio (HR) = 35:65; 18CP-HR]; (3) finely ground starter diet with 22% CP and low RUP: RDP ratio (22CP-LR); (4) finely ground starter diet with 22% CP and high RUP: RDP ratio (22CP-HR) on DM bases. Blocking was based on the day of treatment assignment, and treatments were randomly assigned within each block. The results showed that overall average daily gain (ADG), weaning BW, and feed efficiency (FE) were greater in 22% CP treatments than in 18% CP. Increasing the starter CP content from 18 to 22% of DM did not influence overall starter feed intake, milk intake, total dry matter intake (DMI), postweaning ADG, and FE of calves. Therefore, we conclude that greater starter protein content can have beneficial effects on growth performance, probably through increased microbial protein synthesized and preweaning blood insulin concentration; however, a greater RUP: RDP ratio showed marginal effects on growth performance during the postweaning period.
