Essential Oils as a Dietary Additive for Small Ruminants: A Meta-Analysis on Performance, Rumen Parameters, Serum Metabolites, and Product Quality

Simple Summary Essential oils can be used to improve animal performance as well as the health and quality of livestock products. The aim of this study was to evaluate the effects of essential oil supplementation on animal performance, ruminal fermentation, blood metabolites, and meat and milk quality of small ruminants through a meta-analysis. Supplementation with essential oils improved weight gain, milk production and composition, oxidative stability of meat, and blood serum antioxidant enzyme activity. Additionally, essential oils decreased methane emissions. This suggests that the inclusion of essential oils in the diets of small ruminants could be used to improve animal performance and the quality of meat and milk, in addition to reducing the environmental impact and oxidative stress of the animals. There is an increasing pressure to identify natural feed additives that improve the productivity and health of livestock, without affecting the quality of derived products. The objective of this study was to evaluate the effects of dietary supplementation with essential oils (EOs) on productive performance, rumen parameters, serum metabolites, and quality of products (meat and milk) derived from small ruminants by means of a meta-analysis. Seventy-four peer-reviewed publications were included in the data set. Weighted mean differences (WMD) between the EOs treatments and the control treatment were used to assess the magnitude of effect. Dietary inclusion of EOs increased (p < 0.05) dry matter intake (WMD = 0.021 kg/d), dry matter digestibility (WMD = 14.11 g/kg of DM), daily weight gain (WMD = 0.008 kg/d), and feed conversion ratio (WMD = -0.111). The inclusion of EOs in small ruminants' diets decreased (p < 0.05) ruminal ammonia nitrogen concentration (WMD = -0.310 mg/dL), total protozoa (WMD = -1.426 x 10(5)/mL), methanogens (WMD = -0.60 x 10(7)/mL), and enteric methane emissions (WMD = -3.93 L/d) and increased ruminal propionate concentration (WMD = 0.726 mol/100 mol, p < 0.001). The serum urea concentration was lower (WMD = -0.688 mg/dL; p = 0.009), but serum catalase (WMD = 0.204 ng/mL), superoxide dismutase (WMD = 0.037 ng/mL), and total antioxidant capacity (WMD = 0.749 U/mL) were higher (p < 0.05) in response to EOs supplementation. In meat, EOs supplementation decreased (p < 0.05) the cooking loss (WMD = -0.617 g/100 g), malondialdehyde content (WMD = -0.029 mg/kg of meat), yellowness (WMD = -0.316), and total viable bacterial count (WMD = -0.780 CFU/g of meat). There was higher (p < 0.05) milk production (WMD = 0.113 kg/d), feed efficiency (WMD = 0.039 kg/kg), protein (WMD = 0.059 g/100 g), and lactose content in the milk (WMD = 0.100 g/100 g), as well as lower somatic cell counts in milk (WMD = -0.910 x 10(3) cells/mL) in response to EOs supplementation. In conclusion, dietary supplementation with EOs improves productive performance as well as meat and milk quality of small ruminants. In addition, EOs improve antioxidant status in blood serum and rumen fermentation and decrease environmental impact.