1. Divergent selection for attachment strength between the shell membrane and the calcium shell was performed in a White Leghorn strain. Multivariate analysis was used to estimate genetic parameters for shell membrane measurements and shell thickness. The aim was to investigate the possibility of improving shell strength in laying hens by selecting for increased attachment strength. 2. A significant direct selection response for attachment strength was achieved. There was a favourable correlated selection differential in the frequency of cracked eggs which resulted in a nonsignificant but favourable, correlated response. Selection for a strong attachment resulted in a thinner egg shell, and vice versa. Heritabilities were all relatively high (0.30 to 0.70). Most of the genetic correlations were in agreement with the achieved correlated responses. 3. Two factors that probably contributed to the responses in egg shell thickness were an unfavourable genetic correlation between attachment strength and shell thickness, and natural selection against changes in attachment strength, in the form of increased mortality during incubation and hatching. Problems connected with breeding for increased shell strength are discussed.

Summary The data consisted of information on egg production traits (age at first egg, egg number and egg weight), and reproduction traits such as percentage fertile eggs (PF), percentage hatched of fertile eggs (PHF) and percentage hatched of total egg set (PHT) were also recorded for the selected parents. The experiment was carried out for 10 generations of selection for increased egg number (EN), egg weight (EW), egg mass (EM) along with a control line (C). Although line C had high PF, PHF and PHT in most generations compared to the selected lines, it was not significantly better than the EN and EM lines at the last generation. Differences in PF, PHF and PHT were also observed among the selected lines. The lowest performances of PF, PHF and PHT was observed in line EW. (Co)variances and resulting correlations were estimated between egg production and reproduction traits using a multivariate animal model. The result showed that the estimates of heritability values for egg production traits were moderate to high and low for reproduction traits. The genetic correlation between PF and egg weight traits in line EW was negative and positive in line EM. PHF was positively correlated with egg number in line EN and negatively correlated with egg weight in line EW. Genetic trends calculated from the animal model showed that direct selection for increased egg number in line EN, egg weight in line EW and egg mass in line EM has resulted in substantial genetic gain over generations. Moreover, in lines selected for egg number and egg mass there were consistent positive genetic trends of PHF and PHT along with the selected trait. Selection for increased egg weight, however, negatively affected PF, PHF and PHT over generations. Zusammenfassung Wird Reproduktion durch Selektion auf Legeleistung verschlechtert? Produktionsdaten waren Alter bei Legebeginn, Eizahl und Eigewicht, Reproduktion wurde durch %fruchtbarer Eier (PF) %Schlupf fruchtbarer Eier (PHF) und %Gesamtschlupf (PHT) bei selektierten Elterntieren erfasst. In drei Linien wurde durch 10 Generationen entweder auf Eizahl (EN), Eigewicht (EW) oder Eimasse (EM) selektiert und eine Kontollinie paralell vermehrt. Obwohl C überlegen in Reproduktion war, fehlte statistische Signifikanz in Vergleich zu EN und EM in der letzten Generation. Die selektierten Linien unterschieden sich ebenfalls in Reproduktionseigenschaften, wobei diese in Linie EW am schlechtesten waren. (Co)Varianzen wurden mittels eines multivariaten Modells zwischen Produktions‐ und Reproduktionsmerkmalen geschätzt. Die genetische Korrelation zwischen EW und PF war in EW negativ aber positiv in EM. Heritabilitaten waren mittel bis hoch für Produktions‐ und gering für Reproduktionsmerkmale. PHF war mit Eizahl in der EN Linie positiv und negativ mit EW in der EW Linie korreliert. Die mittels Tiermodell geschätzten genetischen Trends zeigten erhebliche Gewinne bei direkt selektierten Merkmalen in den drei Linien und in Linien EN und EM konsistent positive Trends für PHF und PHT, während in Linie EW sich PF, PHF u...

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