Quality Characteristics, Phenotypic correlations and Principal Component Analysis of Indigenous Free Range Chicken Eggs in Lusaka, Zambia

Eggs are a biological structure intended for reproduction found in female animals. They protect and provide diet to the developing embryo. Even after hatching, eggs remain a source of nutrition for few more days. Some animals produce eggs external to their bodies like birds while others produce eggs inside their bodies. In developing countries, eggs as the most important protein source to man cannot be overemphasized because eggs contain multiple essential nutrients. As a major product from poultry, eggs are composed of egg albumin, egg yolk and the shell in 58, 31 and 11 percentages. More than half of the egg protein is in the albumin while most of the vitamins and all fats are in the egg yolk (Abanikannda et al. 2007) The egg quality depends mainly on consumer preference with respect to various traits that include freshness, mass, cleanliness, egg weight, volume, shell quality and color, yolk index, and shell thickness (Narushin 1997). In the recent past, the trait that has drawn so much attention from researchers is the egg shape (Nedomova et al. 2009; Duman et al. 2016) which is mainly determined by shape index. The shape index (SI) of the egg is the quotient of the width and the length of the egg. It is the standard by which eggs are graded. Eggs with shape index of less than 72 are classified as sharp, those with SI between 72 and 76 are classified as standard eggs while round eggs have egg index of above 76. Unusually sharp and round eggs can never be graded in AA (top grade) or A (second to AA) this is because they are difficult to pack in cartons and can easily break during freight than those classified as standard eggs (Galic et al. 2019).

The indigenous chicken egg farming in Zambia is still a developing one like in any other developing country. And there is very limited commercialization in this sector with only 0.5% of the total population of indigenous chickens being under commercialized production systems (MFL 2019). Chances of developing this sector are so limited due to among many reasons the poor genetic potential of indigenous chickens. Indigenous chickens will lay between 20 and 80 eggs per. This is very low compared to the commercial counterparts that will layup to 300 eggs per year (Guèye 2000; Wong et al. 2017; Dumas et al. 2018). This can however, be improved by design of proper breeding programs.

In order to breed towards improving the quality of eggs, there is need for breeders to understand the available phenotypes and the relationships that exist between egg quality characteristics. Studies on Several intraspecific variations in egg composition have been conducted (Hill 1995; Dzialowski & Sotherland 2004). Intraspecific variations in egg traits have been attributed to numerous factor amongst them size and nutritional standing of hen, feed handiness, heritability, order of laying and a blend of these factors (Schreiber & Lawrence 1976; Abanikannda & Leigh 2007). In order to make sense of these relationships and make informed decisions, as breeders we find ourselves in situations where we deal with large numbers of correlated traits and this data would be complex to handle. Principal component analysis (PCA) a multivariate analysis procedure becomes handy to analyze these data. PCA analyses data where observations are described by numerous inter-correlated quantifiable reliant variable (Abdi & Williams 2010). The fundamental theory of PCA is that it reduces the dimensionality of data sets with a huge quantity of related variables at the same time holding as much disparity as there can be of the existing in the data. This is attained by changing to a different set of variables, (the principal components, PCs) which are not correlated but organized so much that the highest of the dissimilarity extant in the original variables is maintained in the first few components (Jolliffe 2005). PCA has been used several times to study variations in egg traits (Sarica et al. 2012; Bing-Xue et al. 2013; Ukwu et al. 2017).

This study aimed at characterizing indigenous chicken eggs in Lusaka, Zambia and use PCA to analyze these characteristics. The results of this study would serve as an inventory of the egg production genetic potential possessed by these chickens on which decisions on the designing of breeding programs as well as selection and breeding would be based.