<p>It is well known that male and female broilers differ in their growth performance with males having a higher body weight gain (BWG), feed intake (FI) and lower feed conversion ratios (FCR). Due to these differences researchers use single-sex broilers in their experiments in order to minimise the variation introduced when using both sexes. However, recently researchers are finding it increasingly difficult to source sexed broilers due to the fact that feather sexing is no longer possible meaning mixed-sex birds are often used. With the absence of feather sexing, it becomes important to find alternative sexing methods that can accurately sex birds in a quick and cost effective manner. Being able to sex birds prior to placement means equal numbers of males and females can be placed in each pen and the next step would be to evaluate which rearing method (single-sex or equally mixed-sex) will result in the best performance CV % and flock uniformity. If sexing is not possible prior to placement and pens consist of unequal numbers of male and female birds then a method needs to be investigated to account for the variation introduced by the sex effect in order to improve observed power and reduce bias. The current thesis also examines the possible physiological reasons for the differences in growth performance between the sexes including potential differences in nutrient utilization and gut microbiota populations.</p> <p>Chapter 1 presents a brief introduction of the problem, alternatives to overcome the absence of feather sexing, some underlying reasons for the performance differences between the sexes, and the primary objectives to conduct this study.</p> <p>Chapter 2 presents the summary of the literature related to broiler sexing, performance differences between male and female broilers, the effect of rearing method on performance CV% and flock uniformity and lastly some physiological reasons for the performance differences including nutrient requirements, nutrient transporter gene expression as well as gut microbiota populations.</p> <p>Chapter 3 looked at developing an alternate sexing method to feather-sexing that is accurate, cost effective and will allow for the sexing of birds within a couple of days. This method is based on high resolution melting (HRM) analysis which is used to detect DNA variations present in the gene chromodomain helicase DNA binding 1 protein (CHD1) on the Z and W chromosomes (CHD1Z and CHD1W, respectively) of chickens. </p> <p>In addition, a simplified DNA extraction protocol, which made use of the basal part of chicken feathers, was developed to speed up the sexing procedure.</p> <p>Chapter 4 investigated the performance response of broilers reared as single or equally mixed-sex to standard and reduced crude protein (CP) diets. For the overall period of the trial (d 0-35) there was a significant effect (P < 0.001) of rearing method and CP level on feed intake (FI) and feed conversion ratio (FCR). There was also a significant interaction between rearing method and CP level for BWG during d 0-35 (P < 0.01). There was a significant interaction between CP level and sex on d 34 BW (P < 0.01) where the reduced CP diet decreased the BW of both males and females, but to a greater extent the BW of the female birds. This study suggests that male and female broilers have different CP requirements, and rearing birds as equally mixed-sex results in the lowest CV% for performance parameters and best BW uniformity compared to single-sex birds.</p> <p>Chapter 5 objective 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 in the case where male and female birds are unevenly distributed according to sex within each pen. Where the assumptions for ANCOVA 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.</p> <p>Chapter 6 evaluated the effect of sex on nutrient transporter gene expression and gut microbiota populations. There was a significant interaction between CP level and sex on the expression of CAT2 (P = 0.02) and PEPT2 (P = 0.026) where the genes were significantly upregulated in females but only when the RCP diet was fed. Female birds had significantly higher expression of the PepT-2 gene compared to the males. There was also a difference in the relative abundance of microbiota found in the caecal content of the broilers in this experiment and lastly the differential composition of microbiota between the different treatments was also significantly different. These findings suggest females may be better able to adapt to low CP diets by upregulating the expression of certain AA transporters compared to males, and lastly, sex has an effect on the caecal microbial population and these differences contribute towards the performance differences between male and female broilers most likely through their different abilities to digest feed components.</p>