Sperm production measures
Semen evaluation is regularly employed in modern poultry industry to determine the cocks to be used artificial insemination (AI) programme. The use of poor quality spermatozoa in AI programme has been reported to reduce fertility, increases embryo mortality and makes the hen to rely on the stored spermatozoa from the sperm storage tubules (Thurston 1995). Traditional semen quality assessment procedures include the determination of motility, viability, concentration, and morphology of spermatozoa as well as semen volume and colour. Most of these semen quality parameters correlate with the spermatozoa fertilizing capacity (Donoghue and Wishart et al. 2000). The ability of the spermatozoa to fertilize the egg is dependent on the structural and functional components of spermatozoa membrane lipids. Avian spermatozoa membrane is high in polyunsaturated fatty acids (PUFAs) and this makes it vulnerable to lipid peroxidation by oxygen free radicals produced by mitochondrial and plasma membrane (Ogbuewu et al., 2010). The antioxidant defense activity of sperm is low, but enzymatic and non-enzymatic antioxidants in the seminal plasma help to protect sperm from reactive oxygen species (Zini et al., 2009). However, over production of these free radicals decrease the antioxidant defense activity of the sperm thereby leading to low sperm motility, viability, DNA fragmentation and protein denaturation. Several attempts have been made to improve the quality of spermatozoa in poultry using plant materials with antioxidant properties with mixed results (Khan et al. 2012; Nahed et al. 2014). Thus, there is need to establish the effect of tropical plants with antioxidant activity such as ginger on semen quality of poultry.
In the current study, 4 primary studies (60 comparisons) were employed in this meta-analysis to determine the efficacy of ginger supplementation on semen quality and information generated are summarized in Table 1. The meta-analysis findings for each sperm production measure are summarized in forest plots are displayed in figures 1a, b, c, d and e. Dietary ginger supplementation significantly increased semen volume (d = 0.868; 95%CI = 0.195 to 1.542) and sperm motility (d = 0.581; 95%CI = 10.229 to 1.446) from zero hence suggesting that ginger improve some semen production outcomes (Figures 1a and b). Dietary ginger supplementation increased sperm concentration (d = 2.437; 95%CI = 0.467 to 1.603), live spermatozoa (d = 1.142; 95%CI = 0.391 to 1.516) and reduced the number of spermatozoa with abnormal cells (d = -1.450; 95%CI = -1.953 to -0.948) from zero (Figures 1c, d and e), although the differences were not significant. The results indicate that ginger reduced the percentage spermatozoa with abnormal cells in chickens. The significant reduction in the percentage of spermatozoa with abnormal cells in the current study could be attributed to the ability of antioxidants contained in ginger to protect sperm membranes from free radical attack during in vitro and in vivo storage (Zhang et al. 2009; Zhao et al. 2011).
Analysis for heterogeneity is important in the meta-analysis because it tests the amount of variance within the group of studies compared with the within-study variation. A high I2 suggests the difference between individual study outcomes is greater (or more variable) than expected. The difference in treatment response may actually be caused by differences due to other factors, including duration of feeding, strains, dietary form of ginger and inclusion level. Heterogeneity was small (75%) for the majority of spermatozoa production responses with spermatozoa concentration having the highest heterogeneity, 98.2%. Overall, the number of studies used in spermatozoa production measures was small and study power was low, and as result of this we could not subject them to meta-regression to explore the source of heterogeneity.