H. Lukanov1*, A. Genchev1, A. Pavlov2

1Department of Animal Science, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria
2Department of Pharmacology, Animal Physiology and Physiological Chemistry, Faculty of Veterinary Medicine, Trakia University, 6000 Stara Zagora, Bulgaria.

Abstract. The experiment was performed with 8 groups of 38-week-old hens (4 control and 4 experimental groups). Control groups comprised purebred White Leghorn, Schijndelaar, Rhode Island Red and Araucana hens used for formation of experimental groups of F1 crosses between White Leghorn х Schijndelaar, White Leghorn х Araucana, Rhode Island Red х Schijndelaar and Rhode Island Red х Araucana. Twenty-four eggs were collected from each group. The aim of the study was to investigate some of the primary egg quality traits and shell colour characteristics in the CIE L*a*b* system. The following parameters were determined: egg weight (g), shape index (%), albumen index, Haugh units, yolk index, yolk colour score (by La Roche scale) and average eggshell thickness (μm). The proportions of the different egg parts were calculated for each of the groups. Higher egg weights were found in breeds selected for this trait (Rhode Island Red and White Leghorn) compared to breeds in which the selection in this direction was not purposeful (Schijndelaar and Araucana) (р<0.001). Egg weight had very weak influence on egg albumen and yolk (<1% of the total dispersion of traits). The shape index of the studied eggs varied from 78.98 in purebred Araucana group to 75.36 in White Leghorn x Araucana crosses. The eggs of Schijndelaar and White Leghorn, as well as of White Leghorn х Schijndelaar and White Leghorn х Araucana crosses had normal oval shape (SI 75.3 – 76.5%), whereas the eggs of purebred Araucana and Rhode Island Red hens as well as their crosses were more oval (SI 77 – 79%). A linear negative relationship between the shape index and yolk proportion was established (L=0.068), with correlation coefficient r =-0.260. Schijndelaar hens exhibited the lowest values of the following traits: egg weight 45.90 g, albumen index 0.05, haugh units 65.13, yolk index 0.405 and yolk colour score 11.58 (p <0.001). Among the studied groups, White Leghorn eggs had the thickest eggshells (445.39 μm), while the Rhode Island Red х Araucana crosses (392.8μm) – the thinnest. The lowest relative albumen proportion was demonstrated in the eggs of Rhode Island Red х Schijndelaar crosses (56.2). The investigation of eggshell colour showed reduction of L* values from 95.49 in white-shelled eggs to 59.71 in brown-shelled eggs (р<0.001). The shell colour index (SCI) was the highest in white eggs (White Leghorn) and in blue-green eggs of Schijndelaar which had comparable colour characteristics. The lowest SCI was established in brown eggs (Rhode Island Red).