The laying hens were housed in conventional cages, enriched cages, litter system and outdoor system according to Council Directive 1999/74/EC. Three hundred twenty-two ISA brown laying hens were used in the experiment. A total of 196 hens
were housed (550 cm2 per hen) in conventional cages (four-floor system). Enriched cages met the requirements of EU Directive 1999/74/EC (750 cm2 per hen) and 80 hens were housed in these cages. Shavings, an automatic nest and an automatic drinking system were used in the litter system. Thirty-four laying hens were housed in this system (17 laying hens per 4 m2). In the outdoor system, 12 hens were housed as they would be kept on a typical small farm. The hens had 5 m2 of litter with a place for scratching, 3 nests, and an outdoor run (2 m × 5 m). The eggs were collected from the hens between week 39 and 75 of age, at 39, 50, 59, 68 and 75 weeks of age. 15 eggs from each housing system and in the whole range of weights were collected at each of those weeks, and a total of 75 eggs from each system were analyzed. The laying hens were fed