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One hundred and four hens o f parential stock Astra S were kept in individual cages. Two eggs per hen were taken for analyses at 30, 38, 46, 54 and 62 weeks o f hens' age. The total number o f 845 eggs were tested. Shell surface area (SSA) was calculated according to the formuła : SSA = egg w e i g h t 0 6 6 7 x 4.67, and shell density according to the ratio o f weight o f dried shell wi thout membranes to SSA. Ca and M g contents in dried shell wi thout membranes were determined using atomie spectrophotometry absorption methods. Egg weight inereased throughout the laying period, whereas egg shell weight only up to 46 weeks. After 46 weeks shell thickness declined significantly (P<0 .01) . The highest breakage strength was found at 38 weeks, however, the highest shell density was at 46 and 54 weeks. The highest levels o f Ca and M g in 1 g o f shell and their total contents in the shell were at 38 weeks. The Mg:Ca ratio reached the maximum values at 54 weeks. D u r i n g the laying period not only the total amont o f Ca and M g in shells was undergoing changes but also the chemical composition o f shells. These traits proved to be strictly related to the egg shell breakage strength. K E Y W O R D S : egg shell ąua l i ty , hen, minerals I N T R O D U C T I O N After having attained the peak o f laying by hens, the ąual i ty o f the egg shell is becoming more important than inereasing the weight o f the egg. Recently, deterioration o f egg shell ąual i ty has been observed in many countries. In the USA, for instance, the number o f broken eggs in May 1991 amounted to 1224 min, exceeding by 8% the number reported for May 1990 (Washington Bureau, 1991). This phenomenon results from an attempt to attain high laying performance wi th little consideration for egg shell ąual i ty . Egg shell damage problems have become more noticeable sińce hens are being kept in battery cages (Oosterwoud, 1987). © Institute o f A n i m a l Physiology and N u t r i t i o n 52 B. K A M I Ń S K A A N D B. S K R A B A The studies by Lewis and Perry (1987), K a m i ń s k a (1990) as well as by Skraba and K a m i ń s k a (1991) have shown that there are strains in which the egg shell weight increases only up to 44-46 weeks o f age, after which i t is maintained at the same level while the egg weight keeps increasing. This evidently leads to a reduction of the thickness of the shell and thus to its lesser percentage of the egg weight, both traits being highly correlated with breakage strength. The aim of this study was to investigate changes occurring during the laying period in the parameters which are regarded as relevant to the egg breakage strength. M A T E R I A L S A N D M E T H O D S The mater ia ł for the study included 104 hens of parental stock Astra A , aged 28 to 64 weeks, kept in individual cages. Hens were fed ad l ibi tum, the feed containing 14.5% crude protein and 3.5% Ca. Daily feed intake was about 140 g, which resulted in consumption of about 21 g protein and 4.9 g Ca per day. Daily individual laying and egg weight were recorded. Egg analyses were made 5 times, viz., at 30, 38,46,54 and 62 weeks of hens' age. I t was planned to collect two eggs from each hen 3-4 days preceding analysis, however, in practice this was not always possible. That is why the total number of tested eggs was 845. The following parameters were determined: egg weight, weight of fresh shells (after a thorough cleaning from albumen), weight of dried shells without membranes, thickness at 3 points (at sharp and blunt ends and at eąua to r ) of fresh shells without membranes, breakage strenght measured by the ąuasi-s tat ic compression techniąue . Ca and M g levels in the shell without membranes were determined using atomie spectrophotometry absorption methods in a Carl Zeiss Jena A A S I N apparatus. The following values were calculated: mean shell thickness, percent of fresh and dried shell, Mg:Ca ratio. Shell surface area was calculated according to the formuła recommended by Hugnes (1984) and used by Lewis and Perry (1987) as follows: egg w e i g h t 0 6 6 7 x 4.67, and shell density as dried shell weight:shell surface area.

Changes in egg shell ąuality during the first year of laying in hens