Materials and Methods
Genetic Stocks
The F line was started from a randombred control population by mass selection only for increased 16-week body weight. Details of the main tenance of the F line and response to selection have been given previously (Nestor, 1977b, 1984; Nestor et al., 1996, 2000). The F line had been selected for 30 generations at the time of the present study.
A sample of a sire line from each of two major commercial world turkey breeders was obtained as unpedigreed eggs. The commercial sire lines, designated A and B, were reproduced in the first generation by selecting 15 females and 10 males from each sire line with minimal selection pressure for increased 16-week body weight and increased 16-week breast width to try to maintain the lines at current performance levels. Approximately 50% of the best males were selected whereas no selection was practiced in females. The pure sire lines were reproduced by artificially inseminating 15 females with pooled semen from the 10 males. The A and B lines were then reciprocally crossed with the F line.
Offspring from the pure lines and reciprocal crosses were produced in two hatches, each of which represented a 2-week collection of eggs. The F line was produced by weekly artificially mating 36 sires to 72 dams with each sire being mated to two dams. The number of offspring produced was 91 males and 122 females. The pure A and B lines were produced from 6 dams. The number of sires used in the production of the A and B lines was 11 and 9, respectively. Each week, the sires used for artificial insemination of each hen was changed so that as large a genetic base as possible was obtained. The number of offspring obtained for the A line was 25 males and 35 females and for the B line was 23 males and 24 females. To produce crosses involving F-line sires and A- and B-line dams, the same sires were used to mate dams from both lines and were sires that were used to reproduce the F line. Different F-line sires were used to inseminate each dam each week, making a total of 28 F-line sires being used in each cross. The number of offspring from each cross and sex subgroup ranged from 18 to 38 with an average of 29 per group. For the reciprocal cross, nine F-line dams were used in each cross. The same sires used in the production of the A and B lines were also used in the production of the crosses and the sires assigned to each dam were changed weekly to insure as wide a genetic base as possible. The number of offspring for the crosses and sex subgroups involving F line dams ranged from 27 to 40 with an average of 33.
Management of Birds and Measurements Made
The birds were grown, sexes separate, in confinement in separate houses. All birds were provided a declining protein five-ration system (Naber and Touchburn, 1970) based on the schedule for males. Continuous lighting was provided from hatching to 6 weeks of age, at which time the length of the light day was reduced to 12 hours/day. At 16 weeks of age, the amount of light per day was reduced to 10 hours and remained at this level until 20 weeks of age.
Body weight was recorded at 8, 16, and 20 weeks of age. At 16 weeks of age, measurements of shank length, shank width (laterally at the dew claw), shank depth (perpendicular at the dew claw), and breast width were made. Breast width was measured at 6.35 cm of body depth at a point approximately 3.18 cm from the anterior point of the keel. Walking ability at 16 weeks of age was estimated by the method of Nestor et al. (1985) in which each bird was given a score of 1 to 5 with 1 representing birds whose legs did not have any defects and had no difficulty walking , and 5 indicating birds whose legs exhibited extreme lateral deviations and had great difficulty walking. Ratings of 2, 3, and 4 represented intermediate values.
Statistical Analysis
The data were analyzed within sexes for the crosses of the F and A lines and F and B lines using the General Linear Models procedure of SASÒ (SAS Institute, 1988) with genetic group (pure lines and crosses) and hatch as sources of variation. Orthogonal contrasts (SAS Institute, 1988) were used to estimate additive genetic effects (contrast of A vs B, F vs A and F vs B), heterotic effect (contrast of average of the parental lines with the average of the reciprocal crosses), and sex-linked or maternal effects (contrast of reciprocal crosses). The data were analyzed in one analysis but for simplicity the results of the F and A and F and B line comparisons will be presented separately.