The cost-effective rearing of heifers depends on the provision of adequate nutrients at an appropriate age to meet nutrient requirements for growth and production. The objective of the present study was to investigate how the time of switching to a rumen-undegradable protein (RUP) source affects the ruminal fermentation and growth performance of Holstein dairy calves. A total of 36 Holstein female calves were randomly allocated to one of three dietary treatments on day 28 or 42 of life. In the first 28 days of life, they were on the same soybean meal (SBM)-based starter diet containing 22.2% crude protein with 37% RUP. The RUP content of the experimental diet was increased to 48% by a total replacement of the original SBM with xylose-treated soybean meal (XSBM). The following treatments were used: (1) starter diet containing SBM from d 28 (SBM28); (2) starter diet containing XSBM from d 28 (XSBM28); and (3) starter diet containing XSBM from d 42 (XSBM42). All the calves were fed a constant amount of milk (4 L/d), weaned on day 60, and then remained in the experiment until day 70 of life. The results showed that neither feed intake nor BW were influenced by the treatments in the pre-weaning period (days 28 to 60). The calves fed the XSBM28 and XSBM42 diets had greater daily gain and gain-to-feed ratio compared to those on the SBM28 diet post-weaning from day 61 to 70 (p < 0.05). The inclusion of XSBM in the starter diets from days 28 and 42 increased the calves’ height pre- and post-weaning (p < 0.05). The ruminal pH and concentrations of butyrate and iso-butyrate were higher at weaning when the calves were fed the XSBM diets. The calves fed the XSBM diets had less ammonia-N and total short-chain fatty acids at day 70 of life (p < 0.05) and a lower neutral detergent fiber digestibility in the post-weaning period (p < 0.05). The fecal scores and blood metabolites were not different among the treatments. The results showed that increasing the RUP content from 37% to 48% in the starter diets by a total replacement of SBM with XSBM during the pre-weaning period may improve the growth performance around weaning. A later replacement at 42 days of life appears equally effective compared to a replacement at 28 days of life.
