Materials and Methods
Genetic Stocks
The F line was started from a randombred control population by mass selecting only for increased 16-week body weight. Details of the maintenance of the F line and response to selection have been given previously (Nestor, 1977, 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 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 in an attempt to maintain the lines at current performance levels. Approximately 50% of the best males were selected whereas no selection was practiced in females. The A and B lines were then reciprocally crossed with the F line.
Offspring from the pure lines and reciprocal crosses were produced in a single hatch that 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 used from the F line was 6 males and 18 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 used was 10 males and 4 females for the A line and 6 males and 8 females for the B line. 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 to reproduce the F line. Different F-line sires were used to inseminate each dam each week. The number of offspring from each cross and sex subgroup ranged from 3 to 13 with an average of 9.8 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 7 to 16 with an average of 11.5.
Management of Birds and Measurements Made
The birds were grown, sexes separate, in confinement. All birds were provided a declining protein four-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.
During the 17th week of age, the birds were fasted overnight (a procedure which, according to Salmon ,1979, does not affect eviscerated carcass yield), weighed, and killed. Birds were killed by severing the jugular vein. Birds were then scalded at 68oC for about 30 seconds and plucked by a mechanical picker. Males were killed one day and females the next day.
After plucking, the birds were placed flat on their back on a smooth surface and a blind subjective breast rating was assigned to each bird as follows: 1) breast was long and wide and width was similar anteriorly as posteriorly with no curvature of the keel bone and there were no defects; 2) breast was wide and the width of the breast declined slightly from anterior to posterior, the keel bone was not as long for birds that were rated 1 but there was no curvature of the keel bone and there were no defects; 3) breast was not as wide and declined in width from anterior to posterior with some curvature of the keel bone, and there were no other defects; 4) breasts were narrow, declined greatly in breast width from anterior to posterior or there was moderate curvature of the keel or a combination of these and there were some minor defects; and 5) breast was narrow and the keel bone was short with extreme curvature and there were major defects. Breast defects included breast blisters and lateral deviations of the keel.
Following the subjective breast ratings, measurements of breast width and keel bone length were made using calipers. 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. Two measurements of body depth were made using Bird's (1945) instrument. The first measurement was made at the cranial process of the keel and the second measurement was made 2.54 cm anterior to the posterior end of the keel. A body depth ratio was calculated by dividing Body Depth 1 by Body Depth 2.
The birds were eviscerated following measurements of the intact carcass. The head, neck, and left leg were removed from the carcass and discarded. The right leg was removed, the shank was separated from the rest of the leg, and both parts of the leg were placed in a freezer bag with skin intact, and frozen for later dissection. The length of the dorsal part of the body cavity anterior to posterior was measured using a ruler. The right Pectoralis major and P. minor muscles were removed and weighed. To obtain the total weights of these muscles, the weights obtained were multiplied by two. The carcasses were then carefully cut into halves using a bandsaw. The height of the internal body cavity was measured with a ruler at the cranial process of the keel. To measure one-half of body cavity width, a straight edge was placed on the cut portion at cranial process of the keel and width was taken as the maximum value from the straight edge to the body cavity wall. The value obtained was multiplied by two to obtain the total body cavity width. An index of volume of the body cavity was calculated by multiplying measurements of length, width, and height.
The frozen legs were thawed and the toes were removed from the shank and discarded. After boiling for 10 minutes, the tarsometatarsal bone was separated from the soft tissue. The width of the tarsometatarsal bone was measured laterally at the dew claw. The drumstick was separated from the thigh and the skin on both parts was removed. Large subcutaneous fat deposits were removed and muscles were separated from the bones. The drumstick and thigh muscles were weighed separately. The bones were boiled for about 5 minutes and the remaining soft tissue was carefully removed from the bones. The weights of the tibiotarsal and tarsometatarsal bones were obtained. The length of the tibiotarsal bone was measured from the tuberosity of the tibia to the tarsal part of the tibia. Length of the tarsometatarsal bone was measured from the tarsal point to the third metatarsal bone.
Statistical Analysis
The data were analyzed using the General Linear Models procedure of SAS® (SAS Institute, 1988) within sexes for the effect of genetic group and for genetic group and sex when all of the data were combined. 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). Percentage heterosis was obtained by dividing the average of the reciprocal crosses by the average of the parental lines and multiplying the result by 100.