he partial replacement of soybean meal (SBM) with different protein sources (corn gluten meal; CGM, and fish meal; FM) was evaluated on the performance, blood metabolites, microbial protein synthesis, and insulin concentration in Holstein dairy calves. Forty-four Holstein female calves with average body weight (BW) 42 ± 0.81 kg and 3-d age were allocated in a completely randomized design with four treatments: 1) control diet with soybean meal as a sole protein source (SBM); 2) partial replacement (5%) of SBM with corn gluten meal (CGM); 3) partial replacement (5%) of SBM with fish meal (FM) and 4) replacement of SBM with a combination (2.5% each) of CGM and FM meal (CGM-FM). The study lasted 11 weeks with the weaning date on d-66, though the study lasted until d-80. The statistical analysis was considered as three separate periods including pre-weaning (3-66 d), post-weaning (66-80 d), and the entire period (3-80 d). The results revealed that the starter intake and average daily gain were not affected among experimental treatments (P>0.05). Replacing SBM with a combination of fish meal and corn gluten meal (CGM-FM) caused improved feed efficiency (P<0.05). Among the growth indices, wither height was enhanced in the CGM-FM treatment at weaning time (P<0.05), and in the final stage of recording, a tendency was significant for this trait (P=0.05). The results suggested that the protein digestibility slightly improved in the CGM-FM treatment compared to other treatments (P=0.06); however, it did not affect others. The microbial protein yield did not differ among experimental treatments (P>0.05). The greatest insulin concentration was found to be for CGM-FM among experimental treatments (P<0.05). In conclusion, the results showed that the partial replacement of SBM with the combination of CGM and FM in dairy calves’ starter diet improved the growth performance and contributed to a higher insulin concentration.
