Measurements of Traits, Selection Procedures, and Statistics

This study was part of a breeding program using multitrait BLUP methodology where the prime interest was to achieve a line that has a slaughtering age at 5 week of age. Age at point of inflection (IPA) derived from Gompertz growth curve, cumulative feed conversion ratio (FCR) between 3 to 5 weeks of age, and carcass yield (CY) at 5 weeks of age were the traits of interest, as well as body weight (BW) at 5 week of age. To obtain the estimates of individual growth curve parameters, all birds were weighed weekly from hatching to 8 wk of age. The Gompertz nonlinear regression model (1) was used to estimate the growth curve of each.

where y_t is the weight at age t, β₀is the asymptotic (mature) weight parameter, β₁ is the scaling parameter (constant of integration) and β₂ is the instantaneous growth rate (per day) parameter (Narinç et al., 2014). The age at inflection point (IPA) was obtained as follows (2):

To record cumulative feed intake between 3 to 5 weeks of age, the given feed for each bird was weighed and then reweighed after 14 days of ad libitum consumption. The cumulative feed conversion ratios were individually defined, relating cumulative feed intake to body weight gain. The birds that have been produced to determine carcass yields for their parents in each generation were fasted for 4 hr, weighed, and slaughtered at 5 week of age. Hot carcasses were eviscerated manually, chilled in an ice-water tank, drained for 15 min at 4°C temperature, and cold carcasses were weighed. The multi-trait animal model was utilized to estimate breeding values of birds. The following linear mixed effects model was used in the analysis:

where y is the vector of observations, β is a vector of fixed effects, and u is a vector of random genetic effects. The X and Z are known design matrices relating phenotypic records to β and u, respectively. The e is a vector of random errors. Total number of animals in the pedigree file was 1836 where 452 of them were parents without records. Within each generation, 25 percent of males and females with highest breeding value were selected to produce next generations. Realized genetic parameters (heritabilities and genetic correlations), selection responses, and genetic trends were obtained.