Maintenance of shell colour in free range laying hens

Abstract

Loss of shell colour in brown-egg layers is a shell quality issue in Australia and is periodically associated with free range egg production. It has been suggested that exposure to sunlight on the range may affect shell colour. Birds from the same free range flock, exhibiting shell colour deterioration, were either maintained in the free range or transferred to cages in an experimental facility. Egg quality was measured immediately following transfer to cages and then up to five weeks following transfer. Shell colour was measured by shell reflectivity, protoporphyrin IX levels and staining of the cuticle with MST cuticle blue dye. Transfer of birds to cages improved shell colour compared with birds remaining in situ. The lighter coloured shells of the birds that remained in free range were due to lower cuticle cover and less protoporphyrin in the eggshell of that group. The hypothesis that loss of shell colour was related to sunlight and levels of vitamin D was tested experimentally by supplementing diets with different levels of vitamin D and its metabolites. When birds received doses of 526, 666, 993 and 1926 IU per day of vitamin D, there was a steady linear numeral decrease in shell reflectivity with increasing vitamin D dose. However, this change was not statistically significant. The a* spectrum following staining with cuticle blue dye was higher for the groups receiving 666 and 1926 IU vitamin D per day than for the control group, with the 993 IU vitamin D per day group intermediate. Therefore, it appears that the loss of brown egg shell colour observed in free range flocks cannot be attributed to exposure to light resulting in increased levels of vitamin D in the birds as has been suggested previously.