This article summarizes current experimental knowledge on the efficacy, possible mechanisms and feasibility in the application of phytogenic products as feed additives for food-producing animals. Phytogenic compounds comprise a wide range of plant-derived natural bioactive compounds and essential oils are a major group. Numerous studies have demonstrated that phytogenic compounds have a variety of functions, including antimicrobial/antiviral, antioxidative and anti-inflammation effects and improvement in the palatability of feed and gut development/health. However, the mechanisms underlying their functions are still largely unclear. In the past, there has been a lack of consistency in the results from both laboratory and field studies, largely due to the varied composition of products, dosages, purities and growing conditions of animals used. The minimal inhibitory concentration (MIC) of phytogenic compounds required for controlling enteric pathogens may not guarantee the best feed intake, balanced immunity of animals and cost-effectiveness in animal production. The lipophilic nature of photogenic compounds also presents a challenge in effective delivery to the animal gut and this can partially be resolved by microencapsulation and combination with other compounds (synergistic effect). Interestingly, the effects of photogenic compounds on anti-inflammation, gut chemosensing and possible disruption of bacterial quorum sensing could explain a certain number of studies with different animal species for the better production performance of animals that have received phytogenic feed additives. It is obvious that phytogenic compounds have good potential as an alternative to antibiotics in feed for food animal production and the combination of different phytogenic compounds appears to be an approach to improve the efficacy and safety of phytogenic compounds in the application. It is our expectation that the recent development of high-throughput and “omics” technologies can significantly advance the studies on the mechanisms underlying phytogenic compounds’ functions and, therefore, guide the effective use of the compounds.

A study was conducted on 500 juvenile rainbow trout (122 ± 4 g) fed either a control diet or a treatment diet containing 300 mg/kg of a microencapsulated blend of organic acids and essential oils to elucidate effects on intestinal morphology and microbiome. Proximal intestinal villi length was significantly increased in fish fed the treatment diet. Despite no differences in gut inflammation scores, edema, lamina propria inflammation and apoptosis were completely absent in the distal intestine of fish fed the treatment diet. Next-generation sequencing of the 16S rDNA showed no differences in alpha and beta diversity, and gut bacteria were mainly composed of Firmicutes, Bacteroidetes and Proteobacteria. On the genus level, LefSe analysis of indicator OTUs showed Bacteroides, Sporosarcina, Veillonella, Aeromonas and Acinetobacter were associated with the control diet whereas Streptococcus, Fusobacterium and Escherichia were associated with the treatment diet. Aeromonas hydrophila and Acinetobacter spp. are opportunistic pathogens and several strains have been found to be resistant to antibiotics. The increase in villi length and reduction of specific pathogens indicates that feeding a microencapsulated blend of organic acids and essential oils improves gut health and may serve as a part of an effective strategy to reduce antibiotic use in aquaculture.

Use of antibiotics and other chemicals to combat disease outbreaks has been a bottleneck for the sustainable growth of shrimp industry. Among various replacements proposed, organic acid (OA) and their salts (OS) are commonly used by farmers and feed millers. However, in free forms, their requirement is very high (2–3 kg/MT) as they tend to disassociate before reaching the hindgut. The dosage can be reduced by microencapsulation of the ingredients. In this study, a 63-day trial was conducted to assess the effects of OA and OS (COMP) microencapsulated (ENCAP) with fat (HF), fat + alginate (HA), wax esters (WE) and HA + WE (HAWE) on performance, digestive enzymes, immunity and resistance to Vibrio parahaemolyticus. A positive control (PC, 200 g/kg fishmeal-FM) and a negative control (NC, 130 g/kg FM) diet were formulated. Eight other diets were formulated, supplementing an NC diet with microencapsulated OA (OAHF, OAHA, OAWE, OAHAWE) and OS (OSHF, OSHA, OSWE, OSHAWE). Among the ENCAPs, significant difference was observed in serum malondialdehyde (p = 0.026), where HF showed the lowest level (6.4 ± 0.3 mmol/L). Significant interactions between COMP and ENCAP were observed in lipid deposition (p = 0.047), serum alkaline phosphatase, acid phosphatase, hepatopancreatic and serum phenol oxidase (p < 0.0001). Despite no differences, 96-h mortality during pathogenic Vibrio parahaemolyticus challenge in all treatment diets (45–56%) was lower compared to the NC diets (63%). In conclusion, use of HF microencapsulated OA diets could provide improved performance and disease resistance that could contribute to the reduction of antibiotic use by the shrimp industry.

The effects of graded levels of hydrolyzed yeast (HY) supplementation (0.0, 0.5, 1.0, and 2.0 g/kg, i.e., Control, HY0.5, HY1.0, HY2.0, respectively) on growth performance, gut health, and immune responses of juvenile Nile tilapia ( Oreochromis niloticus ) were assessed in this study. The experiment was conducted in a completely randomized design for 14 weeks, where the treatments were distributed in 16 300‐L tanks with four replicates each. Despite no significant differences, the final body weight and weight gain were numerically higher in treatments containing HY (44.7 and 34.7 g, 43.5 and 33.5 g, and 45.5 and 35.5 g in HY0.5, HY1.0, and HY2.0, respectively). Feed efficiency (FE) was improved linearly ( p < 0.05) with increasing dietary HY level (0.65, 0.70, and 0.72, respectively). Similarly, there was also a significant linear relationship between protein deposition (PD), as well as protein and energy retention efficiency (ERE), with the increasing dietary HY level. Among the blood parameters, only the hematocrit (HCT) value was significantly lower in HY1.0 and HY2.0 compared to the control and HY0.5 treatments. Gut histology showed significantly higher villi length in fish fed HY2.0 diets (795 ± 89.6 µm) compared to those fed the control diet (504 ± 80.7 µm). The average surface volume (SV) of the villi was also higher in tilapia fed HY0.5, HY1.0, and HY2.0 diets (0.025, 0.026, and 0.038 mm ³ , respectively) compared to the control diet (0.021 mm ³ ). All four target genes were significantly upregulated in HY1.0 and HY2.0 treatments. The expression of the genes supporting growth and ATP production, insulin‐like growth factor‐1 (IGF‐1) and glyceraldehyde‐3‐phosphate (G‐3‐P), respectively, was significantly improved, as well as the expression of the immune‐related gene, hepcidin. The expression of ghrelin also showed a significant increase with increasing HY levels. It can be concluded that the HY supplementation improved feed utilization, gut health, nutrient absorption capacity, and immunity in Nile tilapia.