Discussion
Additive genetic variation, as indicated by line differences in the current study, was an important source of variation in most traits. The only traits in the present study that did not exhibit a significant difference between lines in the comparison of Line A vs Line B, Line A vs. Line F, or Line B vs Line F were the weight of the drumstick muscles, tartsometatarsal width, keel length, and Body Depth 2. Based on the variation of full sibs, Johnson and Asmundson (1957) observed that the heritability of drumstick muscles was high, ranging from 0.36 to 0.65 in males and 0.09 to 0.47 in females. The realized heritability of shank width was 0.33 ± 0.05 (SE) in a single-trait selection experiment of Nestor et al. (1985). There have been no reports of the inheritance of the tarsometatarsal bone width. Early estimates of heritability of keel length measured near 24 weeks of age and based on selected populations averaged 0.48 (Nestor et al., 1967). In the study of Nestor et al. (1967), heritability estimates of keel length at 24 weeks of age were higher in a randombred control population than in several selected lines. There have been no reported estimates of heritability of body depth at the point of measurement of Body Depth 2. From the reports in the literature, additive genetic variation in the weight of the drumstick muscles, tartsometatarsal width, and keel length is likely an important source of variation but the lines utilized in the current study did not differ in these traits.
The average percentage heterosis for 17-week body weight observed in the present study was 7.2 for males and 2.4 for females. The average heterosis for both sexes was 4.8%. Nestor et al. (2001) also observed, in different hatches of the same crosses, that heterosis for 16-week body weight was higher in males than in females with an average of 3.3%. The average heterosis in the present study for both crosses and sexes for weight of the combined, in Body Depth 1 (measured at the cranial process of the keel) in females and sexes combined, in body depth ratio for sexes combined, and in body cavity height and body cavity volume of sexes combined. In all cases where a significant difference between the commercial sire lines existed in body shape, Line A had larger values than Line B. In comparison to Line A, the F line had a greater Body Depth 1, body depth ratio, and body cavity height for males and sexes combined and body cavity volume for sexes combined. The F line had greater Body Depth 1, body depth ratio, body cavity length, body cavity height, and body cavity volume than the B line in all analysis. Differences between the F and B lines were larger than those between the F and A lines. Differences in body cavity width between the F and B lines was inconsistent in the two sexes.
The F line may have been taller than the commercial sire lines. Based on sexes combined, the total length of the three leg bones (tibiotarsal, femur, and tarsometatarsal) was 52.3, 50.2, and 50.1cm for the F, A, and B lines, respectively. However, total length of all leg bones was not analyzed statistically.
Differences between reciprocal crosses can be due to sex linkage or maternal effects. For males, the difference between reciprocal crosses was significant (P < 0.05) for only body cavity width in the cross of Lines A and F and Body Depth 1 in the crosses of Line B and F. Because there were 44 possible comparisons, the 2 significant differences may have been due to chance. For females, differences between reciprocal crosses were more frequent. The weights of the leg muscles (drumstick and thigh) were consistently larger (significant in three of four comparisons) in the reciprocal cross in which the dam was from the commercial sire line. Significant reciprocal effects for bone measurements of females occurred for tarsometatarsal weight, tarsometatarsal width, and tibiotarsal weight in the cross of Lines A and F and for femur weight in the crosses of Lines B and F and, in all cases in which a significant difference occurred, values were larger when the dam was from the commercial sire line. Reciprocal effects might be expected to be more frequent in females than males if they were due to sex linkage because the female in the heterogametic sex. However, reciprocal effects should be viewed with caution in the present study due to the small number of birds involved.
In summary, heterosis of live body weight at 17 weeks of age was a more important source of variation in males than females in crosses involving three large-bodied lines of turkeys. When averaged over both crosses and sexes, heterosis of the weight of breast muscles was similar to that of live body weight. Heterosis of weight of leg muscles was more and heterosis of weight of leg bones was less than that of live body weight. Heterosis was not an important source of variation for measurements of body shape. The commercial lines utilized in the present study differed in body weight and both commercial sire lines were larger for sexes combined than the F line. For sexes combined, the commercial sire lines had heavier breast and thigh muscle weights but tended to have lighter leg bone weights than the F line. The F line had greater body depth at the cranial process of the keel and greater body cavity volume than the commercial sire lines.