Growth Performance, Immuno-Oxidant Status, Intestinal Health, Gene Expression, and Histomorphology of Growing Quails Fed Diets Supplemented with Essential Oils and Probiotics

This study aimed to evaluate how natural dietary supplements, including essential oils (EOs) and probiotics, influence the growth performance, carcass traits, serum components, gut function, gene expression, and jejunal histomorphology of growing quails. A total of 240 unsexed 7-day-old growing Japanese quails were randomly assigned to four experimental groups (n = 60 per group), with each group further divided into six replicates (10 quails per replicate). The control group (S₀) received a basal diet without incorporating any additives, while the experimental groups were supplemented with (i) essential oils (S₁); (ii) probiotics (S₂); or (iii) a mixture of EOs and probiotics (S₃) at a level of 1.5 kg/ton and 0.55 g per kg diet, respectively, and the ratio of the mixture of EOs and probiotics was approximately 2.73:1. The results showed that, from 7 to 35 days of age, S₃ quails showed increased growth performance, carcass weight, and serum total protein with a decreased lipid profile, outperforming the individual supplementation of either additive (<i>p</i> < 0.05). Importantly, EOs or probiotics enhanced immune-antioxidant status in growing quails, particularly those who were fed both EOs and probiotics, showing significantly increased levels of the serum immune parameters IgY and IgM as well as boosting T-AOC, SOD, and GPx levels when MDA content was lowered compared to S₀ quails (<i>p</i> < 0.05). Additionally, in quails fed a mixture of EOs and probiotics, the primary pro-inflammatory factors TNF-α, IL-1β, and IL-6 were downregulated, and the anti-inflammatory factors TGF-β and IL-10 were elevated compared to the S₀ group (<i>p</i> < 0.05). In this context, there was a notable increase in growth (IGF-I, myogenin, and AvUCP), immunological (IL-2, IL-4, IL-6, and AVBD), antioxidant (SOD, CAT, GPx, and ATOX1), and intestinal absorption (VEGF, MUC2, GLUT2, calbindin, and FABP6) markers in quails supplemented with EOs and/or probiotics when compared to the control group (<i>p</i> < 0.05). The combination of EOs and probiotics had the most noticeable impact on the markers' expression patterns compared to either additive alone (<i>p</i> < 0.05). Consistent with our results, quails given both EOs and probiotics showed significantly greater villi in terms of height and width when compared to the control group in intestinal histomorphology, the primary measure of intestinal wellness. In conclusion, quail diets could benefit from the use of EOs or probiotics as natural growth promoters since they improve growth performance, blood parameters associated with protein and lipid profiles, immune-antioxidant status and inflammation, and marker gene expression profiles of growth, immune, antioxidant, and intestinal absorption.