Copyright © 2018 by Asian-Australasian Journal of Animal Sciences. Objective: The impact of forage feeding strategy on growth performance, ruminal fermentation and nutrient digestibility in postweaning calves was investigated. Methods: Fortyfive female Holstein calves (body weight [BW] = 79.79±0.38 kg) were enrolled in the 35d study at one week after weaning and randomly assigned to one of three dietary treatments. All diets were fed as total mixed ration containing 60% (dry matter [DM] basis) of basal starter feed and 40% (DM basis) of forage, but varied in composition of forage source including i) alfalfa (40% DM, AH); ii) alfalfa hay (26.7% DM)+oat hay (13.3% DM; OH); iii) alfalfa hay (26.7% DM)+corn silage (13.3% DM; WS). Results: Dry matter intake was not different among treatment groups (p>0.05). However, BW (p<0.05) and average daily gain (p<0.05) of calves fed AH and OH were greater than WSfed calves, whereas heart girth was greater in OHfed calves than those fed AH and WS (p<0.05). Ruminal fermentation parameters including proportion of butyric acid, acetatedtopropionate ratio, concentration of total volatile fatty acid, protozoal protein, bacterial protein, and microbial protein in rumen were the highest in OH (p<0.05) and the lowest in WS. Compared with the AH and WS, feeding oat hay to postweaning calves increased crude protein digestibility (p<0.05), and decreased duration of diarrhea (p<0.05) and fecal index (p<0.05). Conclusion: Our results suggested that partially replacing alfalfa hay with oat hay improved ruminal fermentation, nitrogen utilization, and reduced incidence of diarrhea in postweaning dairy calves.