Effects of Rhodobacter sphaeroides protein on growth performance, nutrient digestibility, rumen fermentation, and serum metabolites in dairy calves during the pre- and postweaning periods

This study used Rhodobacter sphaeroides protein (RSP) as an innovative dietary protein source for Holstein dairy calves. We investigated its effects on growth performance, nutrient digestibility, rumen fermentation, and serum metabolites, antioxidants, immunoglobulins, and inflammatory factors. A total of 45 female Holstein calves (36.54 ± 1.68 kg of BW; 1 d of age) were randomly assigned to 3 treatment groups and fed either a control diet (CON) or diets with 5.89% or 11.49% RSP (RS1 and RS2, respectively) on a diet DM basis for 14 wk. The inclusion of RSP linearly increased BW and ADG, whereas it quadratically decreased DMI. There was a quadratic effect for gain-to-feed ratio and hip height, with higher mean values for the RS1 group. In addition, the dietary RSP tended to result in a linear increase in the overall withers height and postweaning heart girth. The dietary RSP reduced linearly the digestibility of DM and OM and tended to reduce linearly the digestibility of NDF and ADF. The levels of rumen butyrate showed a linear decrease in RSP calves. There was a quadratic effect for serum aspartate aminotransferase, with the highest value being for RS2 group. Increasing dietary RSP quadratically affected serum triglyceride levels, with the highest value being for the RS1 group. Meanwhile, serum alanine aminotransferase levels were quadratically decreased by RSP, with the RS1 group having the lowest levels. A quadratic trend effect was observed for serum albumin and total protein, with the highest values being in the RS1 group. The levels of lactate dehydrogenase, BUN, malondialdehyde, IL-10, and immunoglobulins decreased linearly with RSP. In contrast, total antioxidant capacity and the levels of superoxide dismutase, catalase, tumor necrosis factor-α, IL-8, and IL-6 increased linearly. In addition, the serum levels of low- and high-density lipoprotein, cholesterol, IL-4, IL-22, and glutathione peroxidase tended to increase linearly with the inclusion of dietary RSP. In conclusion, dietary inclusion of RSP in dairy calves reduced DMI, nutrient digestibility, and serum immunoglobulins levels, particularly at higher inclusion rates, whereas it improved ADG and antioxidant capacity.