Fat Deposition in a Broiler Sire Strain I. Phenotypic and Genetic Variation in, and Correlations Between, Abdominal Fat, Body Weight, and Feed Conversion

Variability in body composition was examined weekly from 3 to 10 weeks of age in chickens from a broiler sire strain. Genetic aspects of body weight, quantity of abdominal fat, and feed conversion were examined in pedigree birds of the same strain at 6 weeks of age. Variability in abdominal fat relative to body weight (RAF), total percentage of carcass fat, and body weight (BW) was not influenced by sex or age. The phenotypic correlation (rp) between RAF and BW was generally positive, but low (between .20 and .68); that between RAF and total percentage of carcass fat was invariably positive and higher. The heritability estimated from sib analyses for body weight after ad libitum feeding was 0 (h2sires) and 1 (h2dams), for RAF .41 (h2sires) and 1 (h2dams), for feed conversion .39 (h2sires) and .53 (h2dams), and for body weight after restricted feeding .31 (h2sires) and .57 (h2dams). The genetic correlations were estimated as correlation between mean performance of progeny of each sire (rsires) or dam (rdams). The correlation between family mean of RAF and BW was −.01 (rsires) or .04 (rdams), between RAF and feed conversion .19 (rsires) and .29 (rdams), and between RAF and body weight after restricted feeding −.12 (rsires) and −.24 (rdams). The heritability indicates that selection against RAF is possible. The genetic correlations indicate that selection based on feed conversion or on body weight after restricted feeding could result in less fat deposition under ad libitum feeding conditions.