Supplementing Oregano Essential Oil in a Reduced-Protein Diet Improves Growth Performance and Nutrient Digestibility by Modulating Intestinal Bacteria, Intestinal Morphology, and Antioxidative Capacity of Growing-Finishing Pigs

SIMPLE SUMMARY: Serious nitrogen pollution and shortage of protein feed resources have constrained the rapid development of the pig industry. Thus, the strategy of using a reduced-protein diet supplemented with amino acids in pig production has been widely accepted. Additionally, antibiotic growth promoters have been widely used in pig production for many years. However, their enormous and uncontrolled use has promoted bacterial resistance, leading to less effective treatment for animal diseases. Accordingly, this study investigated the effects of inclusion of oregano essential oil in a reduced-protein, amino acid-supplemented diet on growth performance, nutrient digestibility, gut health, and antioxidative capacity of growing-finishing pigs as an alternative to antibiotics. Our results suggested that adding oregano essential oil to a reduced-protein diet improved the growth performance and carcass lean percentage of pigs. In addition, long-term supplementation of oregano essential oil to a reduced-protein diet improved the intestinal bacteria, intestinal morphology, and antioxidative capacity of pigs. This study provides theoretical guidance for application of low-protein diet to guide the production of antibiotic-free feed for growing–finishing pigs. ABSTRACT: This study investigated the effects of supplementing oregano essential oil (OEO) to a reduced-protein diet on growth performance, nutrient digestibility, intestinal bacteria, intestinal morphology, and antioxidative capacity of growing-finishing pigs. Forty-eight barrows were randomly allotted to four treatments including normal-protein diet (NPD), reduced-protein, amino acid-supplemented diet (RPD), the same RPD supplemented with chlortetracycline (RPA), and RPD supplemented with OEO (RPO). The data showed that dietary OEO supplementation increased the average daily gain of pigs compared with NPD and RPD. The gain:feed in RPO- and NPD-fed pigs was higher than those in RPD- and RPA-fed pigs. Increased average daily feed intake and 10th-rib backfat thickness were detected in RPA-fed pigs. Pigs fed the RPO had higher apparent total tract digestibility (ATTD) of crude protein than those fed the other diets. The RPD and RPA treatments showed reduced counts of Lactobacillus spp. in ileal digesta of pigs. The RPA and RPO treatments also showed lower Escherichia coli counts in ileal digesta than the NPD and RPD treatments. Dietary OEO supplementation increased villous height of the jejunum and the ileal and plasma total antioxidative capacity of pigs. In conclusion, dietary OEO supplementation could improve the growth performance and nutrient digestibility of pigs by modulating intestinal bacteria, intestinal morphology, and antioxidative capacity.