Oregano Essential Oil as a Natural Plant Additive Affects Growth Performance and Serum Antibody Levels by Regulating the Rumen Microbiota of Calves.

This experiment aimed to investigate whether supplementation of calves with different doses of oregano essential oil (OEO) could promote the development of the gastrointestinal tract and enhance the immune ability of calves by regulating the rumen microbiota. Twenty-four 70-day-old healthy and disease-free Holstein male calves were randomly divided into four groups, with the control group fed a basal diet, and the treatment group provided 4 g, 6 g, and 8 g of oregano essential oil per day in addition to the basal diet. After the 14-day pre-test, a 56-day formal test was conducted. At days 0 and 56 of the standard test period, calves were weighed, the average daily weight gain of calves during the test period was calculated, and serum samples were collected to measure the concentration of immunoglobulins (IgA, IgG, and IgM) in the serum; at day 56 of the formal test period, rumen fluid was collected from the calves, and 16SrRNA was sequenced to analyze changes in the rumen microbiota of the calves. The changes in the rumen microbiota of calves were analyzed by 16SrRNA sequencing. The results of the study showed that (1) OEO supplementation in calves significantly increased end weight and average daily gain ( p < 0.05); (2) OEO supplementation in calves significantly increased serum concentrations of immunoglobulins IgA and IgM ( p < 0.05); (3) OEO supplementation in calves significantly increased the abundance and diversity of rumen microbial organisms ( p < 0.05); (4) OEO supplementation in calves significantly regulates the relative abundance of some species, and biomarkers with significant differences were screened by LEfSe analysis: g_Turicibacter, g_Romboutsia , f_Peptostreptococcaceae, f_Clostridiaceae, g_Clostridium_sensu_stricto_1 , o_Clostridiales, g_unclassified_f_Synergistaceae , c_Coriobacteriia, o_Coriobacteriales, f_Atopobiaceae, g_Olsenella , p_Actinobacteriota, g_Defluviitaleaceae_UCG-011 , f_Defluviitaleaceae, o_Corynebacteriales, g_Corynebacterium , f_Corynebacteriaceae, g_Shuttleworthia , f_Hungateiclostridiaceae, o_norank_c_Clostridia, g_Saccharofermentans , g_Streptococcus , f_Streptococcaceae, g_unclassified_o_Oscillospirales , and f_unclassified_o_Oscillospirales ( p < 0.05, LDA ≥ 3); and (5) OEO supplementation in calves significantly enriched the metabolism of cofactors and vitamins pathway ( p < 0.05). (6) Using Superman's correlation analysis, we screened unclassified_c_Clostridia , Shuttleworthia , and Christensenellaceae_R-7_group, three beneficial strains for calves. (7) Daily supplementation with 8g of OEO significantly affected rumen microbiota regulation in calves.