Estimates of genetic parameters in turkeys. 3. Sexual dimorphism and its implications in selection procedures

Multivariate analysis of sexual size dimorphism in local turkeys (Meleagris gallopavo) in Nigeria

Sexual size dimorphism is a key evolutionary feature that can lead to important biological insights. To improve methods of sexing live birds in the field, we assessed sexual size dimorphism in Nigerian local turkeys (Meleagris gallopavo) using multivariate techniques. Measurements were taken on 125 twenty-week-old birds reared under the intensive management system. The body parameters measured were body weight, body length, breast girth, thigh length, shank length, keel length, wing length and wing span. Univariate analysis revealed that toms (males) had significantly (P < 0.05) higher mean values than hens (females) in all the measured traits. Positive phenotypic correlations between body weight and body measurements ranged from 0.445 to 0.821 in toms and 0.053-0.660 in hens, respectively. Three principal components (PC1, PC2 and PC3) were extracted in toms, each accounting for 63.70%, 19.42% and 5.72% of the total variance, respectively. However, four principal components (PC1, PC2, PC3 and PC4) were extracted in hens, which explained 54.03%, 15.29%, 11.68% and 6.95%, respectively of the generalised variance. A stepwise discriminant function analysis of the eight morphological traits indicated that body weight, body length, tail length and wing span were the most discriminating variables in separating the sexes. The single discriminant function obtained was able to correctly classify 100% of the birds into their source population. The results obtained from the present study could aid future management decisions, ecological studies and conservation of local turkeys in a developing economy.

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.

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.

Genetics of growth and reproduction in the turkey. 14. Changes in genetic parameters over thirty generations of selection for increased body weight

A line (F) of turkeys was selected over 30 generations for increased 16-wk BW. The base population for the F line was a randombred control population (RBC2) that was maintained without conscious selection and used to remove yearly environmental variation in the F line. Selection was effective in increasing 16-wk BW in the F line. Selection differentials based on the mean of selected parents minus mean of entire population (intended) and intended selection differentials weighted for number of offspring produced (actual) did not consistently differ, indicating that natural selection was not significantly opposing artificial selection. The realized heritability (h2) of 16-wk BW in the F line, based on the linear regression of the selection response on accumulated actual selection differentials, declined with selection; the decline appeared to be slightly different for males than females. For both sexes combined, the realized h2 was 0.309+/-0.022 (SE), 0.268+/-0.033, 0.242+/-0.026, and 0.254+/-0.007, respectively, for Generations 1 to 10, 11 to 20, 21 to 30, and 1 to 30. Genetic increases in 16-wk BW in the F line were positively associated with BW at other ages (8, 20, and 24 wk of age and at 50% production), days from stimulatory lighting to production of the first egg, and egg weight. Genetic increases were negatively associated with egg production, intensity of lay (maximum and average clutch length and rate of lay), and hatch of fertile eggs. There was no significant relationship between 16-wk BW and total days lost from broodiness or fertility. The genetic changes in some correlated traits were not consistent in all generation intervals studied, indicating that the genetic correlation between the selected trait (16 wk BW) and the correlated trait changed with selection.

Beak trimming of turkeys. 2. Effects of arc beak trimming on weight gain, feed intake, feed wastage, and feed conversion

An experiment was conducted to study the effects of arc beak trimming on feed consumption, weight gain, and feed wastage in males and females from two large-bodied strains and one medium-bodied strain of turkeys. Birds were placed in wire-floored battery cages from 3 to 8 wk of age and feed intake, weight gains, and feed spilled into dropping pans were recorded for each of the 5 wk of each trial. Feed conversion was calculated as feed consumed divided by weight gained. Sex by beak trimming interactions were not found. Beak trimming reduced feed wastage of the two large-bodied strains, but did not affected feed wastage of the medium-bodied strain, which did not consume as much feed as the large-bodied strains. The results of this study support the hypothesis that beak trimming reduces feed wastage of large-bodied turkeys.

Direct and correlated responses to long-term selection for increased body weight and egg production in turkeys

Lines of turkeys were selected long-term for increased egg production (E line; 34 generations) or increased 16-wk BW (F line; 28 generations). The E and F lines were started from randombred control populations (RBC1 and RBC2, respectively) that were also maintained to remove environmental variation among generations. Realized heritabilities (h2) +/- SE in the E line, based on regressions of response on cumulated actual selection differentials (selection differentials weighted for the number of offspring produced), for 180-d and 250-d egg production were 0.34 +/- 0.02 (17 generations) and 0.26 +/- 0.13 (8 generations), respectively. The realized h2 of 16-wk BW in the F line was 0.26 +/- 0.01. There was no consistent evidence of selection response reaching a plateau in either line. The genetic association of BW and egg production changed with selection in the E and F lines. The genetic correlation varied from near zero to strongly negative and fluctuated between these extremes in both lines even though they started from different base populations and selection criteria differed. Other correlated responses to selection for increased egg production were increased average clutch length (intensity of lay), and decreased broodiness (total days lost), egg weight, shell coloration, and rate of response to stimulatory lighting. Other correlated responses to selection for increased 16-wk BW in the F line included: increased egg weight (due to increased albumen), longer eating bouts, and decreased average clutch length, semen production, walking ability, and resistance to Pasteurella multocida and Newcastle disease virus. Selection within the E and F lines also changed the frequency of MHC haplotypes and the changes appeared to be in opposite directions in the two lines.

Range and confinement rearing of four genetic lines of turkeys. 1. Effects on growth, mortality, and walking ability

Line by environment interactions complicate interpretation of results from long-term selection experiments. To investigate the potential effects of such interactions, a study was conducted in which birds from four genetic lines were reared in two environments. Birds from four turkey lines were brooded in confinement housing. At 8 wk of age, half of the birds from each full-sib family were moved to a large range pen and the remainder were placed in a large confinement pen. Significant line by environment interactions were present only for walking ability scores of males. A general lack of significant line by environment interactions indicates that rearing environment of these lines may be altered without affecting interpretation of results from a selection experiment. In addition, the current randombred control lines continue to adequately serve as control populations for a long-term selection experiment.