This study examined the effects of oregano essential oil (OEO) and sodium butyrate (SB) dissolved in milk on Holstein suckling calves' performance, and blood and fermentation parameters. Twenty-four newborn male calves (3 to 5 days old) weighing an average of 36 ± 2 kg were randomly divided into four equal groups. Experimental treatments included: 1- basal diet (control), 2- basal diet + 5 g SB, 3- basal diet + 5 g OEO, and 4- basal diet + 5 g SB and 5 g OEO. Every two weeks, calves were weighed, and their skeletal growth indices were measured. In addition, the daily feed intake was determined. Blood samples were taken on days 7, 21, 42, and 56, and analyzed for determination of serum glucose, albumin, total protein, β-hydroxybutyrate (BHBA), total cholesterol, triglycerides, and urea concentration. On the 21st, 42nd, and 56th days of the experiment, rumen fluid was collected to determine pH, ammonia nitrogen (NH3-N), and vola-tile fatty acids (VFAs) concentrations. The SB and SB + OEO treatments increased feed intake compared to the control (P=0.06). Neither weight gain (P=0.11) nor feed conversion ratio (FCR, P=0.45) were influ-enced by treatments. However, some skeletal growth characteristics, including body length, withers height, and hip height, were enhanced by SB and OEO (P=0.05). Except protein, which was reduced by all treat-ments (P=0.002), there were no significant differences in blood parameters among treatments. Although treatments with SB and OEO increased the concentrations of acetic acid, propionic acid, butyric acid, and total VFAs in rumen fluid (P<0.0001), rumen fluid pH (P=0.27) or NH3-N (P=0.70) concentration were influenced by none of treatments. In conclusion, our results demonstrated that although SB and OEO treat-ments improved rumen VFAs profiles, they had no significant effect on calf performance traits. Based on these results, investigation of the effects of using these treatments as a strategy in rumen development and early weaning of calves is suggestable. تفاصيل المقالة