The search for efficacious alternative growth promoters (AGPs) continues alike in all major meat-producing countries where antibiotics have been banned and where such restrictions are still waiting to be imposed. A scrutiny of the different classes of alternatives brings to light the different advantages and disadvantages that each of them poses[1, 2]. Phytogenics remain the least popular amongst these classes despite having withstood the test of time for the longest, exerting consistent results across disparate geographies and even species. One reason for this relative unpopularity is the lack of an understanding of the exact mechanisms; phytogenics work at all possible levels through overlapping mechanisms and do not have pinpoint mechanisms of action as some other classes of antibiotic replacers do. Phytogenics work best when offered regularly over a period of time and they offer a multitude of benefits that other classes do not: they can be used at a blanket inclusion level that does not require fine-tuning, they have very wide margins of safety, they are usually the most pelletization-friendly and stable, they act both directly and indirectly on all three components i.e. the feed, the host and the gut microbiota, and they offer multiple benefits beyond growth promotion and immunity.
A blend of crude herbal ingredients and essential oils with differing yet complementary activities can offer the best of results, however, stringent quality control especially with regards to phytoactive markers, is warranted for ensuring batch-to-batch consistency (Fig. 1). The complex physicochemical matrix provided by the herbal ingredients also protects the phytoactive constituents, which usually results in the best stability when compared to other classes of AGPs (Fig. 2).
The use of such supplements offers distinct and multi-stage benefits. Parts of plants like garlic, ginger, and fenugreek have beenpopularmateriamedica in ethnomedicinal and ethnoveterinary practices for improving digestive and assimilative functions. In the feed, the active phytoconstituents prevent and inhibit the growth of microorganisms, including fungi. Extant literature strongly supports the ability of common herbal ingredients, such as garlic and ginger, to sequester, inactivate and degrade toxins present in feed [5, 6]. In the host, the benefits are not restricted to the gut. Besides their direct ability to reduce inflammaging, indirect reductions in tissue wear due to strong modulating activity on gut microbes  result in better histological characteristics of the gut epithelium (Fig 3). These improvements in gut architecture, along with modulation of far-flung molecular targets in the hypothalamus, muscles, liver, and adipose tissues support better growth performance (Table 2) [8-11]; benefits are also evident in terms of better lipid profiles, lower oxidative stress, and improved keeping quality of meat. Lectins present in some of the constituents, such as garlic, further act to stimulate humoral, cellular, and mucosal immunity in the host .
Finally, modern systems biology tools are also shedding light on the mechanisms by which a phytogenic growth promoter can possibly act to improve growth, feeding performance, immunity, and overall health. Reverse pharmacological investigations into the mechanism of a phytogenic growth promoter  identified the salient biomolecular targets and biological pathways thereofviz. CCKR signaling, B- and T-cell activation, inflammation-mediated cytokine response, GnRH pathway, heterotrimeric G-protein signaling, PDGF signaling, integrin signaling, to name a few (Figure 4). The modulation of these pathways, which play salient roles in host-pathogen interactionsand cellular sensing of gut environment, upkeep of immunity, gut barrier health, and cell renewal and proliferation, reasons for the pleiotropic benefits realized by the dietary supplementation of a natural growth promoter. Additionally, the selective inhibition of many biomolecular targets within the pathogens, which are essential for their survival and pathogenesis, by prominent phytoconstituents has also been established through molecular docking studies .
It can be inferred that natural growth promoters can offer several benefits over other alternative growth promoters, resulting in better growth performance and feeding efficiency, immunity and overall health, and improved meat attributes. Stringent quality control remains critical for the consistency of such results at the farmlevel.
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