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Strain differences in embryonic and early chick growth Iton, Laurence Eric


The extent to which egg weight modifies genetic differences in body-weight of the embryo and chick and the correlation between embryonic growth rate and post-hatching growth rate were investigated. This was done by studying the relationships between (1) egg weight and body weight of embryo and chick; (2) strain and body weight of embryo and chick; (3) strain and percentage growth rate of embryo and chick; and (4) the relationship between embryonic growth rate and post-hatching growth rate. This study was conducted on five strains and one strain-cross. Two of the strains were bred for meat, (White Plymouth Rock and White Cornish); two were bred for high egg production, (White Leghorns); and one was an intermediate type, (White New Hampshire). The strain-cross was derived from mating males of one White Leghorn strain with females of the other. Individual egg weights were recorded for over 2,200 eggs. Between nine and eighteen embryos of each category were weighed from each of two incubators from the ninth to the eighteenth day of incubation. Between fifteen and thirty-three chicks of each category were weighed at hatching and at weekly intervals for three weeks. Analyses of variance of embryonic weights, chick weights, and embryonic or chick weights expressed as percentages of egg weight were done. Analyses of variance were also done on embryonic and post-hatching growth rates. Coefficients of correlation (r) between egg weight and embryonic or chick weight and also coefficients of regression of chick weight on egg weight were computed. The correlation between embryonic and post-hatching growth rates was estimated. From the results of the above tests it was concluded that: (1) Differences in embryonic weights among the strains were due to differences in inherent genetic factors; (2) Egg weight exerted a temporary measurable influence on embryonic and chick weight, the effect being greatest at hatching; (3) Differences in post-hatching growth rate among the strains were probably due to differences in nutritional factors which contributed to a more efficient utilization of nutrients by the heavy type chicks; and (4) Approximately 65 per cent of the variation in post-hatching growth rate to three weeks of age was dependent on the variation in growth rate during the nine- to fourteen-day incubation period. The estimate of correlation between growth rate during these two periods was, however, not precise.

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