Weaned Holstein steer calves fed an oregano essential oil and cobalt blend to enhance growth performance

Background Post-weaning calves are vulnerable to nutritional and environmental stress caused by abrupt dietary transitions, which may impair growth, immunity, gut health, and later performance. With the restriction of antibiotic growth promoters, plant essential oils have gained attention as natural feed additives. Oregano essential oil (OEO) combined with cobalt in a blend (OEOB) has shown potential to improve calf growth and ruminal function, but its optimal inclusion rate for post-weaned Holstein steer calves remains unclear. Therefore, this study evaluated the effects of varying OEOB inclusion rates in weaned calves. Sixty-four healthy Holstein steer calves aged 70 ± 5 days and weighing 91.6 ± 0.93 kg were blocked by body weight (BW) and randomly assigned to one of four treatments in a randomized complete block design (RCBD), with 16 calves per treatment. The OEOB powder was manually mixed into the calves’ TMR at 0, 4, 6, or 8 g/day. The TMR consisted of pelleted feed and oat hay at a ratio of 9:1. The OEOB powder was added once daily at 07:30 h. The 70-day trial included 14 days of adaptation and 56 days of data collection. Growth performance, body frame measurements, blood biochemical, immune, antioxidant and heat-stress indicators, rumen fermentation, nutrient digestibility, and post-incubation recoverable bacterial colony counts were assessed. Results With increasing OEOB inclusion rate, BW, BW gain, average daily gain (ADG), dry matter intake (DMI), withers height gain, and apparent total-tract dry matter (DM), crude protein (CP), and neutral detergent fiber (NDF) digestibility increased linearly ( P < 0.01), whereas feed efficiency, heart girth gain, abdominal girth gain, and acid detergent fiber (ADF) digestibility increased linearly ( P < 0.05). Serum superoxide dismutase (SOD) showed significant linear, quadratic, and cubic dose responses to increasing OEOB inclusion rate ( P < 0.05). Total antioxidant capacity (T-AOC) and immunoglobulin A (IgA) showed both linear and quadratic responses, whereas immunoglobulin G (IgG) increased linearly and malondialdehyde (MDA) decreased linearly ( P < 0.05). Feeding increasing amounts of OEOB resulted in linear decreases in ruminal total volatile fatty acid (VFA) concentration and the acetate-to-propionate ratio, whereas propionate concentration increased linearly ( P < 0.05). Feeding increasing amounts of OEOB also resulted in linear decreases in post-incubation recoverable Escherichia coli and Salmonella/Shigella colony counts ( P < 0.01). Conclusions Feeding OEOB at inclusion rates up to 8 g/d can improve growth performance, antioxidant status, gut health, total tract apparent nutrient digestibility, and shift ruminal fermentation, and alter post-incubation recoverable Post-incubation recoverable bacterial colony counts in weaned calves. Future studies evaluating higher OEOB supplementation levels are needed to determine the optimal or maximum effective supplementation level.