Sex proportion as a covariate increases the statistical test power in growth performance based experiments using as-hatched broilers

Abstract

The availability of sexed day-old broiler chicks is becoming an issue as feather sexing is no longer possible. This has great implications for broiler researchers as the use of randomly distributed mixed-sex birds may result in a greater between-pen variation and thus less statistical power than the use of single-sex birds. The objective of this study was to evaluate the effect of including sex proportion as a covariate in an analysis of covariance (ANCOVA) on the statistical power compared to analysis of variance (ANOVA) where sex was not considered. The statistical parameters examined include mean square error (MSE), the F-statistic, model fit, model significance and observed power. A total of 4 separate experiments that used mixed-sex broilers with unequal numbers of male and female birds per pen were conducted during which performance of the birds was measured. The male % in each pen was recorded during each experiment and corrected for mortality. The performance results were analysed by ANOVA and the statistical parameters were then compared to ANCOVA where sex proportion was included as a covariate. The results showed that a set of assumptions first needed to be met to run ANCOVA. In addition, if the ANOVA results show a high level of model significance and power, then ANCOVA may not be necessary. In other circumstances where the assumptions are met and model significance and observed power are low, the inclusion of sex proportion as a covariate in the analysis will help to reduce MSE, increase the F-statistic value and improve the model significance, model fit and observed power. Therefore, it is suggested that sex proportion should be considered as a covariate in ANCOVA to improve statistical power in nutritional experiments when male and female broilers are unequally and randomly distributed in pens.

Fig 1. The benefit of the inclusion of male % as a covariate in the statistical analysis model.

a. FCR during d 21–35 was plotted against the male %; b. the statistical analysis outcome without the inclusion of male % as a covariate; c. the statistical analysis outcome with the inclusion of male % as a covariate. "solid line" Treatment with antibiotic; "dashed line" Treatment with a mixture of additives; "filled circle" FCR of pens fed diet with antibiotic supplementation; "unfilled circle" FCR of pens fed diet with a mixture of additives supplementation.