Breeding for late egg shell quality in the domestic hen
Titel:
Breeding for late egg shell quality in the domestic hen
Auteur:
Rodda, D. D.
Verschenen in:
British poultry science
Paginering:
Jaargang 13 (1972) nr. 1 pagina's 45-60
Jaar:
1972-01
Inhoud:
An experiment is reported, in which shell weight per unit surface area was measured for eggs laid by pedigreed pullets at 33, 54 and 65 weeks of age, using a Rhode Island Red strain and a White Leghorn strain. Specific gravity and albumen quality were also measured and records of individual egg production were available. The heritability of “shell thickness “ (i.e. shell weight per unit surface area) in the Rhode Island Reds declined from 0.57 in the first period to 0.33 in the last period. The genetic correlation between thickness in the first period and thickness in the last period was 0.90. The relative selection efficiencies expected by basing selection on early shell thickness with the object of improving late shell thickness were greater than 1.0, indicating that early selection would be the best method of improving shell quality in this strain. In the Leghorn flock, estimates of the heritability of shell thickness were low (0.07 to 0.20) and variable. The genetic correlations between thickness measured in different periods were generally lower than for the Rhode Island Reds, and relative selection efficiencies varied widely. The Leghorns had much thicker shells than the Rhode Island Reds at all times of the year but showed little prospect of further improvement in shell thickness whichever time of measurement was used as a basis for selection. Specific gravity was found to be highly genetically correlated with shell thickness and so would be a good estimator of shell thickness, although more eggs per pullet must be measured to give the same accuracy as direct measurement of shell weight per unit area. There were no adverse genetic correlations between thickness and the other traits measured, except perhaps in the case of albumen quality. It is concluded that the lack of any general improvement in end-of-year shell strength is probably not due to low heritability, to adverse genetic correlations or to inaccurate methods of measuring shell thickness, but to a lack of selection pressure.
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