This study was conducted to investigate the effectsof the ratio of rumen-degradable protein (RDP) torumen-undegradable protein (RUP) and corn processingmethod on production performance, nitrogen (N)efficiency, and feeding behavior of high-producingHolstein dairy cows. Twelve multiparous Holstein cows(second parity; milk yield = 48 ± 3 kg/d) were assignedto a replicated 4 × 4 Latin square design with a 2 × 2factorial arrangement of treatments. Factor 1 was cornprocessing method [ground corn (GC) or steam flakedcorn (SFC) with a flake density of about 390 g/L], andfactor 2 was RDP: RUP ratio [low ratio (LR) = 60:40;high ratio (HR) = 65:35] based on crude protein (%).The crude protein concentrations were kept constantacross the treatments (16.7% of DM). No significantinteractions of main treatment effects occurred for lactationperformance data. Cows fed 2 different RDP:RUP ratios exhibited similar dry matter intake (DMI),but those fed SFC showed decreased feed intake comparedwith those receiving GC (25.1 ± 0.48 vs. 26.2 ±0.47 kg/d, respectively). Cows fed HR diets producedmore milk than did those fed LR diets (44.4 ± 1.05 vs.43.2 ± 1.05 kg/d, respectively). Milk fat content decreasedbut milk protein content increased in cows fedSFC compared with those fed GC. Feed efficiency (i.e.,milk yield/DMI) was enhanced with increasing ratioof RDP: RUP (1.68 ± 0.04 vs. 1.74 ± 0.04 for LR andHR, respectively). Apparent N efficiency was higher incows fed HR than in those fed LR (30.4 ± 0.61 vs.29.2 ± 0.62, respectively). Compared with cows fed theGC-based diet, those receiving SFC exhibited lowervalues of N intake, N-NH3 concentration, and fecal Nexcretion. Cows receiving SFC-based diets spent moretime ruminating (min/kg of DMI) than did those fedGC. Although these results showed no interaction effectsof RDP: RUP ratio and corn processing method onperformance, higher RDP: RUP ratios and ground corncan be effective feeding strategies for feed to lactatingcows receiving high-concentr
