Introduction High productivity and feed efficiency are the bane modern livestock industry

High productivity and feed efficiency are the bane modern livestock industry, which can be achieved using sub therapeutic doses of antibiotics in chicken nutrition. Although, chickens raised using antibiotics achieved better performance, the harmful effects associated with its regular use in poultry feed include the development of host and cross drug resistant bacteria strains which pose a real public health concern globally (FAO, 2016; Österberg et al., 2016). In an attempt to circumvent such negative problems and maintain health and productivity, several scientists and researchers have called for the development of natural products (e.g. ginger) with the ability to replace antibiotics in animal feed.
Ginger (Zingiber officinale), a tropical herbaceous plants is a rich source of active chemical substances which have been documented to improve poultry performance (Tollba et al. 2010; Khan et al. 2012; Ogbuewu et al., 2017). Antioxidant and antihypercholesterolemic activity as well as other activities of ginger has been reported in the literature (Sekiwa et al. 2000; Tapsell et al. 2006; Ali et al. 2008) and this has been connected to the actions of its various active ingredients such as gingerol, gingerdiol (Fuhrman et al., 2000). Ginger is loaded in essential oils and important micro minerals (Ogbuewu et al. 2014) and could be used as feed additive source for poultry and livestock. Globally, ginger is utilized as a flavouring agent in human diet (Khan et al. 2012), and in concocting traditional herbal medicine (Chrubasik et al. 2005; Ali et al. 2008).
Studies abound on the effects of dietary ginger supplementation on performance indices in poultry (Khaki et al. 2009; Ghasemi and Taherpour, 2015). The positive effect of dietary ginger supplementation on feed efficiency and nutrient utilization have been established and this may be due to the stimulating action of ginger chemicals on gastric secretion and enzymes (Patel and Srinivasan 2000). Presently, there is no consensus among the studies on the performance enhancing effects of ginger supplementation on chickens (Khan et al. 2012; Nahed et al. 2014; Ogbuewu et al. 2017). Some authors have observed non-significant effect of ginger supplementation on the production and performance indices of chickens (El-Deek et al. 2002; Zhang et al. 2009), whereas others (Farinu et al. 2004; Khan et al. 2012; Ogbuewu et al. 2017) have reported significant improvement effect of dietary ginger supplementation. Conversely, Al-Homidan (2005) observed reduced productive indices of chickens on different dietary ginger supplementation levels. The disparity among these studies may be resolved using meta-analytic approach.
Meta-analysis is an advanced statistical method that combines the results of independent investigation to identify pattern among the production variables as well as determine the sources of variation among those results. Meta-analysis aggregate information leading to a higher statistical power and provide more strong overall estimate of treatment effect that ordinarily could not obtained from the effect size derived from any of the candidate study (Van Houwelingen et al. 2002). Presently, information on the meta-analysis of effect of dietary ginger supplementation on the production indices of chickens are lacking in the literature. Therefore, the objectives of this meta-analysis were: (i) to close the knowledge gap on the effect of dietary ginger supplementation on production response of chickens using data from published studies, and (ii) to determine how drivers such as supplementation level, presentation form, duration of supplementation and chicken type used will moderate the responses of chickens to dietary ginger supplementation.