Effects of protein content and rumen-undegradable to rumen-degradable protein ratio in finely ground calf starters on growth performance, ruminal and blood parameters, and urinary purine derivatives

摘要

This study investigated the effects of feeding finelyground starter diets containing either 18 or 22% crudeprotein (CP) content [dry matter (DM) basis] andhigh or low ratios of rumen-undegradable protein torumen-degradable protein (RUP: RDP) on growth performance,nutrient digestibility, ruminal fermentation,blood metabolites, and urinary purine derivatives indairy calves. A total of 48 three-day-old female Holsteindairy calves with 40.2 ± 2.5 kg of initial body weight(BW) were randomly assigned in a complete randomizedblock design to a 2 × 2 factorial arrangement oftreatments (12 calves/treatment). Treatments were asfollows: (1) finely ground starter diet (mean particle size= 0.69 mm) with 18% CP and low RUP: RDP ratio [lowratio (LR) = 26:74; 18CP-LR]; (2) finely ground starterdiet with 18% CP and high RUP: RDP ratio [high ratio(HR) = 35:65; 18CP-HR]; (3) finely ground starter dietwith 22% CP and low RUP: RDP ratio (22CP-LR);(4) finely ground starter diet with 22% CP and highRUP: RDP ratio (22CP-HR) on DM bases. Blockingwas based on the day of treatment assignment, andtreatments were randomly assigned within each block.Calves received 4 L of milk daily from d 3 to 10, 7L/d from d 11 to 40, 4 L/d from d 41 to 49, and 2.5L/d from d 50 to 53, and then all calves were weanedbut remained in the experiment until d 83 of age. Theresults showed that overall average daily gain (ADG),weaning BW, and feed efficiency (FE) were greater in22% CP treatments than in 18% CP. Increasing thestarter CP content from 18 to 22% of DM did not influenceoverall starter feed intake, milk intake, total drymatter intake (DMI), postweaning ADG, and FE ofcalves. No effect of RUP: RDP ratio was observed forstarter feed intake, milk intake, total DMI, preweaningADG, FE, and grams of CP per megacalorie of metabolizableenergy. The RUP intake and postweaning ADGwere greater for calves fed the HR diets than for thosefed the LR diets. The digestibility of neutral detergentfiber was greater, and the digestibility of OM tendedto be greater, and the ruminal concentrations of totalshort-chain fatty acids (SCFA), acetate proportion, andacetate-to-propionate ratio were greater in 22% CP thanin 18% CP. A 2-way interaction between starter proteincontent and time was observed for total ruminal SCFA,acetate proportion, and acetate-to-propionate ratio, indicatingthat starter CP concentration had more effecton ruminal parameters. Preweaning urinary purine derivatives,preweaning microbial protein synthesis, andpostweaning urinary nitrogen were greater for calvesfed the 22CP diets than for those fed the 18CP dietsbut were not affected by the different RUP: RDP ratios.The concentrations of blood glucose and insulin weregreater in 22% CP than in 18% CP diets. The bloodinsulin concentration was greater when calves receivedthe HR diets compared with the LR diets. Therefore,we conclude that greater starter protein content canhave beneficial effects on growth performance, probablythrough increased microbial protein synthesizedand preweaning blood insulin concentration; however,a greater RUP: RDP ratio showed marginal effects ongrowth performance during the postweaning period.