The present study was intended to evaluate the effect of forage source (alfalfa hay; ALF vs. corn silage; CS) along with a supplemental fat source (soybean oil; SO vs. rumen-inert palm fatty acids; PF) on growth performance, nutrient digestibility, and ruminal fermentation in dairy calves. Forty-eight new-born Holstein female calves (3-d-old) were assigned to one of four treatments: 1) alfalfa hay with soybean oil (ALF-SO); 2) alfalfa hay with palm fatty acids (ALF-PF); 3) corn silage with soybean oil (CS-SO); 4) corn silage with palm fatty acids (CS-PF). Starter diets had equal amounts of forage (100 g/kg dry matter; DM) and fat source (30 g/kg DM). Calves were fed a constant amount of milk (d 1 to 63) and had ad-libitum access to water and starters (d 1 to d 83). The lowest and greatest starter intakes during the pre-weaning period occurred in ALF-SO and CS-PF, respectively. This coincided with forage × fat source interaction for average daily gain (ADG) during pre-weaning. The forage source affected total DM intake and ADG over the entire period, body weight (BW) at weaning, and final BW with greater values in calves that received CS compared to ALF. The concentration of total short chain fatty acids and butyrate were increased, whereas concentration of acetate and acetate:propionate ratio were decreased in the rumen of calves fed CS compared to ALF. Feeding CS increased urinary excretion of allantoin and, as a trend, total purine derivatives (PD) and estimated microbial protein synthesis in comparison with ALF. The fat source affected starter intake, ADG, and BW post-weaning with the highest values in PF. The digestibility of neutral detergent fiber, crude protein and, as a trend, organic matter were higher in calves fed PF compared to SO. Calves fed PF had lower ruminal ammonia-N concentration and urinary N excretion, and greater urinary excretion of allantoin and total PD. Calves receiving SO had lower ruminal protozoa population. In conclusion, supplementing
