Plant secondary metabolites as feed additives in calves for antimicrobial stewardship

Abstract

Irrational usage of antibiotic feed additives in livestock industry, especially as growth promoters, has become a global challenge due to the unintended zoonotic impact through resistance transfer. Herbal plant extracts or their secondary metabolites with antimicrobial properties may possess similar effects of various antibiotic feed additives in impeding the pathogenic microbial growth and enhancing the health, growth, and production performance. Majority of the calf-plant extract associated research works were primarily geared towards phenolic compounds and essential oils, with a few related to terpenes, saponins, tannins, flavonoids, organic acids, complex carbohydrates, and non-protein amino acids. The plant secondary metabolites possess diverse antimicrobial mechanisms, including cell membrane disruption, enzyme inhibition, substrate deprivation, and prevention of bacterial colonization, to name a few. Further, the unification of different levels and types of plant metabolites before supplementing to calves renders synergistic effects, which aids in several beneficial responses such as increased bioavailability, improved efficiency, and reduced undesirable effects and effective dosage. Nevertheless, the employment of herbal extracts or plant metabolites as antimicrobial feed additives to calves retain many challenges regarding the dosages, levels, adaptation lengths, herbal or herb-drug interactions, analytical methods, and public health safety. The present review focuses on assembling existing data on the plant-based antimicrobial components, individual secondary plant metabolites or plant extracts, which are already in use or having the efficacy in benefitting calves on supplementation.