Genetic variation in pure lines and crosses of large-bodied turkey lines. 1. Body weight, walking ability, and body measurements of live birds

Growth Rate Distribution and Potential Non-Linear Relationship between Body Weight and Walking Ability in Turkeys

The aim of this study was to investigate the potential non-linear relationship between growth and walking ability (WA). The phenotypic data included body weights at 12 and 20 weeks and WA at 20 weeks of age measured on 276,059 male turkeys. The growth rate at three age periods (0 to 12, 12 to 20 and 0 to 20 weeks) was calculated. Each bird was assigned to one of the quartiles of the growth rate distribution for each age period. Between the first and fourth quartiles, the incidence of score 1 (bad WA) increased by 31, 18, and 33% for the first, second, and third age periods, respectively. For good WA (scores 4, 5, and 6), the incidence decreased by 55, 66, and 72% between the first and fourth quartiles for the first, second, and third age periods, respectively. Estimates of heritability of WA ranged between 0.18 and 0.26. The genetic correlations between adjacent growth rate quartiles were high and decayed as the interval between quartiles increased. The magnitude of the variation in the incidence of walking scores and genetic correlations across the growth rate quartiles point towards a non-linear relationship between growth and mobility suggesting other factors may affect walking ability.

Gastrocnemius Muscle Structural and Functional Changes Associated with Domestication in the Turkey

Selection for increased muscle mass in domestic turkeys has resulted in muscles twice the size of those found in wild turkeys. This study characterizes muscle structural changes as well as functional differences in muscle performance associated with selection for increased muscle mass. We compared peak isometric force production, whole muscle and individual fiber cross-sectional area (CSA), connective tissue collagen concentration and structure of the lateral gastrocnemius (LG) muscle in wild and adult domestic turkeys. We also explored changes with age between juvenile and adult domestic turkeys. We found that the domestic turkey's LG muscle can produce the same force per cross-sectional area as a wild turkey; however, due to scaling, domestic adults produce less force per unit body mass. Domestic turkey muscle fibers were slightly smaller in CSA (3802 ± 2223 μm2) than those of the wild turkey (4014 ± 1831 μm2, p = 0.013), indicating that the absolutely larger domestic turkey muscles are a result of an increased number of smaller fibers. Collagen concentration in domestic turkey muscle (4.19 ± 1.58 μg hydroxyproline/mg muscle) was significantly lower than in the wild turkeys (6.23 ± 0.63 μg/mg, p = 0.0275), with visible differences in endomysium texture, observed via scanning electron microscopy. Selection for increased muscle mass has altered the structure of the LG muscle; however, scaling likely contributes more to hind limb functional differences observed in the domestic turkey.

Genetics of growth and reproduction in the turkey. 16. Effect of repeated backcrossing of an egg line to a commercial sire line

The turkey industry's view of the relative economic importance of growth and egg production has changed rapidly, and genetic changes by selection within lines may not be rapid enough to meet the changing needs. The objective of the present study was to determine the feasibility of rapidly increasing the BW of dam lines by repeated backcrossing of a dam line (E) to a commercial sire line (B). The experimental E line was selected long-term for increased egg production and was used as the model for a turkey dam line. The B line was larger (more than 3-fold) in BW at 8, 16, and 20 wk of age, had wider breasts (approximately 1.8-fold) at 16 wk of age, and had lower egg production for 180 d (about 3-fold) than the E line. Based on additive genetic variation, males in the F1 generation of the B x E cross did not differ from expected in BW at any age, but females of this cross had BW less than expected at 16 and 20 wk of age. In the F1 generation, breast width of the cross did not differ from the expected value, but egg production for 180 d was greater than expected (126.6 vs.102.3 eggs/hen). After 3 generations of backcrossing, the backcrosses exhibited a gain in 20-wk BW of 12.5 and 8.8 kg, respectively, for males and females; a gain of 5.9 and 5.3 cm in breast width at 16 wk of age for males and females, respectively; and a loss of 74.1 eggs per hen over a 180-d production period. Based on the results of the current and a previous study, limited backcrossing of a dam line to a sire line may be an economically feasible method to greatly increase the BW of dam lines without unduly sacrificing egg production. For maximum gains per generation, backcrossing probably should be used for a maximum of 2 or 3 generations.

Genetic variation in pure lines and crosses of large-bodied turkeys. 4. Body shape and carcass traits

An experimental line (F) of turkeys selected long-term for increased 16-wk BW was reciprocally crossed with a primary breeding sire line (C) from a major international turkey breeder to study the inheritance of carcass traits and body shape. The birds were weighed and killed at 16 wk of age, and various measurements of muscling, leg bones, and body shape were made. The BW of males were 16.0 and 15.1 kg, respectively, in the C and F lines. The respective BW for females were 12.2 and 11.2 kg. Additive genetic variation, as indicated by differences between the F and C line, was a more important source of variation for measurements of muscling than for measurements of leg bones. The anterior and posterior depth of the body and the ratio of these measurements differed between the F and C lines. Body cavity volume index (length x width x height) differed between lines in females and sexes combined. The weights of some internal organs (gizzard, heart, liver, pancreas, and spleen), but not others (proventriculus and lungs), differed between lines, and the total weight of these organs was greater in the C line than in the F line. However, the ratio of total demand organs (muscles and bones) to the total weight of the supply organs was less in the F line than in the C line. Heterosis was a more important source of variation in body weight and weights of the pectoralis major and pectoralis minor muscle weights in males than in females. Heterosis for some body shape measurements (keel length, body depth 1, body depth 2, body depth ratio, body cavity height, and body cavity volume index) was significant in some comparisons. Heterosis (range = 3.64 to 3.99%) for leg muscle measurements (thigh muscles, drumstick muscles, and total leg muscles) was highly significant (P < or = 0.01) based on the analysis for both sexes. Differences between the reciprocal crosses of the F and C lines were more frequent in the present study than in previous studies in which the F line was reciprocally crossed with sire lines from 2 other commercial breeders. However, when the data from the previous studies and the current study were summarized, it appeared that the only reciprocal effect that was consistent was for the weight of the drumstick muscles, indicating sex linkage or maternal effects may influence the weight of these muscles.

Genetic variation in pure lines and crosses of large-bodied turkey lines. 3. Growth-related measurements on live birds

An experimental line (F) selected over 34 generations for increased 16-wk BW was reciprocally crossed with a primary breeding sire line (C) from a large international turkey breeder to study the inheritance of growth-related traits measured on live birds. All genetic groups were grown intermingled in confinement with sexes reared in different houses. The traits measured included BW at 8, 16, and 20 wk of age and shank length, shank width, shank depth, breast width, and walking ability scores at 16 wk of age. Walking ability was rated from 1 to 5 with 1 representing birds with no leg defects and no difficulty walking and 5 indicating birds with extreme lateral deviations of the legs and great difficulty walking. Ratings of 2, 3, and 4 represented intermediate values. The F line had a different growth pattern than the C line with the F line being larger than the C line at 8 wk of age, but the reverse was true at 16 and 20 wk of age. The difference in BW between the C and F lines increased from 16 to 20 wk of age. The C line had wider breasts than the F line at 16 wk of age. The F line had longer shanks than the C line. Shank width was larger in the C line than the F line for females but not males. No line difference in shank depth was observed. Walking ability scores at 16 wk of age were lower (better) in the C line than in the F line for males but not females. Significant heterosis in BW of the crosses of the F and C lines was observed at all ages in males (range = 3.3 to 5.6%) and only at 8 wk of age in females (3.6%). These results were similar to an earlier study in which the F line was crossed with a primary-breeding sire line from 2 other international turkey breeders. No significant heterosis in the crosses of the C and F line was observed for breast width and shank measurements. Heterosis was significant for walking ability scores of females (-3.0%) but not males. Reciprocal effects, a measure of sex linkage and maternal influences, were noted only for shank length and the direction of the difference was not the same in the 2 sexes.

A comparison of growth and development patterns in diverse genotypes of broilers. 1. Male broiler growth

Selection for breast muscle yield and BW in commercial broilers has resulted in genotypes far different from broilers processed in the past. When comparative studies with commercial lines are conducted, it is often difficult to differentiate between carcass effects resulting from direct genetic selection vs. correlated effects that partially reflect genetic changes in BW. The objective of the present experiment was to compare growth and development characteristics of male broilers from commercial lines exhibiting similar rates of BW gain based on a percentage of 8-wk BW but exhibiting different carcass traits. Male broilers from 2 commercial genotypes exhibiting increased breast muscle yield (A and B) were compared with broilers from a commercial line that was "unimproved" with respect to conformation (C). All birds were fed a commercial-type broiler starter diet throughout the study and were processed at 8 wk of age for carcass comparisons. No differences were observed among lines for breast-free BW or weight of the abdominal fat pad. The absolute and relative weights of pectoralis major and pectoralis minor breast muscles were heavier in lines A and B than C (P < 0.001). Length (C > A > B; P < 0.001), width (B > C >A; P < 0.001), and depth (A = B > C; P < 0.001) of the pectoralis major were different between lines. Drum and thigh weights were heavier in lines A and B than C (P < 0.001). There were genotype differences in tibia length (C > A = B, P < 0.001) and femur length (C > A = B, P < 0.001). The comparison of line C with selected lines A and B, suggested that line C may provide a useful model for studying carcass development between commercial genotypes with similar growth patterns.

Genetic variation in pure lines and crosses of large-bodied turkey lines. 2. Carcass traits and body shape

An experimental line (F) of turkeys was reciprocally crossed with sire lines (designated A and B) from each of two commercial breeders in order to study the inheritance of carcass traits and body shape. The birds were weighed and killed at 17 wk of age, and various measurements of muscling, leg bones, and body shape were made. Additive genetic variation, as indicated by line differences, was an important source of variation in most traits. The only traits that did not exhibit a line difference in any comparison were weight of the drumstick muscles, tarsometatarsal width, keel length, and Body Depth 2 (body depth at a point 2.54 cm anterior to the posterior end of the keel). Heterosis of live BW was greater in males than in females. Heterosis in the weight of the pectoralis major and p. minor muscles were similar to those of live BW, but heterosis in the weight of the leg muscles were higher than that of live BW. The average heterosis for bone measurements (length of the femur, tarsometatarsal, and tibiotarsal bones and tarsometatarsal width) was very low and ranged from -0.3 to 1.4%. For measurements of body shape [keel length, Body Depth 1 (body depth measured at the cranial process of the keel), Body Depth 2, ratio of Body Depth 1 to Body Depth 2, length, width, and height of the body cavity, and body cavity volume index), heterosis was low and the average ranged from -2.7 to 2.6%. Based on an analysis of both sexes combined, the commercial sire lines differed in BW, weights of the p. minor and drumstick muscles, weights of the tarsometatarsal, femur, and tibiotarsal bones, lengths of the tarsometatarsal and femur bones, and various measurements of body shape (Body Depth 1, body depth ratio, body cavity length and height, and body cavity volume index). Relative to the commercial sire lines, the F line was smaller, had less breast and leg muscling, and, in general, larger leg bones when sexes were combined. The carcass of the F line was deeper than that of the commercial sire lines when measured at the cranial process of the keel but not at 2.54 cm anterior to the posterior portion of the keel. Body cavity height and body cavity volume index were larger in the F line than in the commercial sire lines.