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The mean values of shortening force at fracture of egg found out during the five-month observation period ranged between 0.17 and 0.19 mm in 231 hens of seven initial laying lines and strength of eggshell ranged from 29.05 N to 36.46 N. The mean values of shortening force at fracture decreased with age of hens from 0.18–0.22 mm at 28 wk of age to 0.12–0.15 mm at 46 wk of age. Similarly, strength of eggshell decreased from 31.35–37.68 N to 27.91–34.50 N in same time period. Index of shape, ratio of eggshell and albumen to total egg weight also decreased with increasing age of hens. The highest ratio of abnormal eggs was found in both Bar Plymouth Rock lines (BPR A and BPR B) – 7.5% and 8.3% resp. The lowest ratio was found in lines Rhode Island Red (RIR C) – 2.1% and Susex Light (SU) – 2.4 %. The ratio of cracked eggs ranged from 0.9 % to 3.9 %, the ratio of double-yolked eggs was 0.5%–3.2% and the ratio of eggs without shell was 0.4 %–1.5 %. The calculated coefficients of correlation between shortening of egg and index of shape were mainly positive with rp = 0.41 (P < 0.05) and rp = 0.49 (P < 0.01). The correlations between shortening of egg and total number of eggs were mainly negative with rp = −0. 34 (P < 0.05) and rp = −0. 46 (P < 0.01). Similar correlations existed also between shortening of egg and ratio of cracked eggs, i.e. rp = −0. 44 (P < 0.05) and rp = −0. 46 (P < 0.01). The correlations between shortening and total number of abnormal eggs were mainly negative rp = −0. 39 (P < 0.05) and rp = −0. 53 (P < 0.01). The relationships between strength of eggshell and shortening of egg are characterized by positive correlations – rp = 0. 40 (P < 0.05) and rp = 0.55, (P < 0.01). Positive correlations existed also between strength of eggshell and index of shape (from rp = 0. 37 (P < 0.05) to rp = 0.59 (P < 0.01)). Correlations between strength of eggshell and ratio of eggshell ranged from rp = 0.44 (P < 0.01) to rp = 0.74 (P < 0.01). Mainly negative correlations were found out between strength of eggshell and number of eggs rp = −0. 34 (P<0.05) and rp = −0. 44 (P < 0.01), similarly between the strength of eggshell and cracked eggs rp = −0. 40 (P < 0.05) and rp = −0. 52 (P < 0.01). Negative correlations existed between strength of eggshell and total number of abnormal eggs (rp = −0.45 to rp = −0.53 (P < 0.01)). This means that both eggshell shortening and strength may be used when selecting lines for egg quality

The relationship of shortening and strength of eggshell to some egg quality indicators and egg production in hens of different initial laying lines*