Plant secondary metabolites as feed additives in calves for antimicrobial stewardship

Highlights

• Plant secondary metabolites may improve immunity and growth performance of calves.

• Phenolics and essential oils are the mostly used natural feed additives in calves.

• Combining plant metabolites may give beneficial owing to their synergistic effect.

• Using a combination of plant metabolites may counter the microbial resistance.

• Feeding extracts to calves may be checked for suitability and herb-drug interactions.

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.

Introduction

Calfhood diseases trigger a significant impact on the frugality of cattle farming activities because of direct losses apart from the long-term effects on the performance (Lorenz et al., 2011). In this regard, calf health could be highlighted as one of the most significant livestock health issues being faced by livestock farmers. Calf mortality is a global challenge that needs to be addressed for sustainable livestock farming. Of the 5% mortality rate in pre-weaned calves, 56% is attributed to digestive problems and scours with remaining caused by respiratory problems (USDA, 2016). The relevant policymakers are involved with implications in preventing the calfhood diseases by supplementing the antimicrobial compounds through diet. Antibiotics have been used at sub-therapeutic levels to encourage the growth and minimize the morbidity and mortality throughout the animals’ life cycle. Employing antibiotics as feed additives could be traced back to 1950’s (Kertz et al., 2017). However, prolonged application of antibiotic feed additives in livestock sector could cause the continuous adaptation of microbes to the antibiotics, thus generating resistance against antimicrobial agents (Schwarz et al., 2017). Resistance to indispensable antibiotics such as broad-spectrum beta-lactams, aminoglycosides, carbapenems, and fluoroquinolones, is of increasing magnitude among zoonotic pathogens (Zhang et al., 2018). Apropos of the antimicrobial resistance phenomenon, the antibiotic supplementation is worth reevaluating; hence, its usage has been banned in the European Union since January 2006 (Directive 1831/2003/CEE, European Commission, 2003). In this theme, the perspectives in calf nutrition towards antibiotic feed additives have been changing with a shift to the encouraged use of plant secondary metabolites (PSMs) or natural herbal extracts.

Plant secondary metabolites are an exceedingly large group of compounds with small molecular weights, which are meant for protective purposes against insects, microbes, and herbivores, in addition, to adapt to adverse environmental conditions. In nature, these PSM’s play important roles as antibacterials, antifungals, antivirals, herbicides, and insecticides (Miguel, 2010); among them, antibacterial activity is one of the greatest notable contributing features for animal agriculture. Plant secondary metabolites are usually considered as anti-nutritional factors for calves and monogastric species. Nevertheless, few investigations have revealed that some of them would beneficially affect the host metabolism and performance when used at appropriate levels. The global herbal products market size has been continuously growing and predicted to reach $111 billion by the end of 2023, with a compound annual growth rate of ∼ 7.2% during 2017-2023 (Elghandour et al., 2018). This abrupt hike in herbal market could be taken as a shred of evidence for increasing consumer preference towards plant secondary metabolites as feed additives. The implications pertaining to herbal research in adult ruminants could not be adopted for calves, as younger calves, especially those fed on milk replacer, behave as monogastric species due to the presence of an incompletely formed or non-functional rumen with a well-developed abomasum. As an example, the ionophore antibiotics may show less beneficial effects in young calves compared to adult ruminant species, presumably because of the low microbial population in undeveloped rumen (Akbarian-Tefaghi et al., 2018). Besides the antimicrobial benefits, supplementation of plant extracts or secondary metabolites may deliver flavor to poor quality or low appetence calf starters, which might further enhance the ingesting competence and growth performance (Montoro et al., 2011). Furthermore, few phytonutrients can exert a prebiotic effect in post-weaning calves by escaping the rumen microbial degradation and reaching the small intestine in biologically active form (Wolfswinkel, 2016). The purpose of this review paper is to synthesize an elaborative summary of the published calf-PSM related peer-reviewed papers and discuss their modes of action, different synergic mechanisms, and challenges in dietary supplementation to calves.