Genetics and breeding of a black-bone and blue eggshell chicken line. 1. Body weight, skin color, and their combined selection

Abstract

An experimental population of chickens was developed from the cross between 2 indigenous Chinese breeds, Dongxiang blue eggshell and Jiangshan black-bone. This breeding was aimed at eventually combining dark heavy black-bone body and blue eggshell, into a single dual-purpose breed. BW was recorded and skin L∗, a∗, and b∗ color parameters were measured by a Chroma Meter at several ages (56, 105, 150, 200, 250, and 300 d). At 250 d, 3 independent observers classified skin darkness using a 3-level visual scale (1 = light, 2 = intermediate, 3 = dark). The 7-level average visual skin darkness, calculated for each chicken, was highly correlated (−0.658 and −0.612 in females and males, respectively) with skin L∗ (lightness), indicating that the accurately measured L∗ is reliable and useful reverse expression of visual skin darkness of black-bone chickens. Mean BW and skin L∗ of both sexes increased with age, to 2,063 and 1,522 g in males and females, respectively, at 300 d, and to 63 and 55 L∗ units in males and females, respectively, at 250 d. The population's full-pedigree allowed estimating heritability and genetic correlations between traits. The heritability estimates of BW were similar in both sexes, increasing from around 0.25 at 56 d, to 0.53 to 0.60 at 150 d, and 0.57 to 0.62 at 300 d. Over the 5 ages, heritability estimates of skin L∗ were moderate to high, ranging from 0.45 to 0.58 in females, and from 0.31 to 0.65 in males, and the genetic correlations between BW and L∗ ranged mostly from 0.20 to 0.45. These low-to-moderate correlations between high BW and high L∗ (low darkness) are unfavorable; hence they were combined into an index, standardized BW minus standardized L∗, allowing future selection for high BW with low L∗. With high heritability of this index, 0.487 (females at 300 d) and 0.410 to 0.555 (males at 150 d or older), simultaneous improvements in BW and skin darkness appear to be feasible. The methodology used in this study can be useful in chicken populations experimentally bred for combination of high BW and other body characteristics.

Genetics and breeding of a black-bone and blue eggshell chicken line. 2. Laying patterns and egg production in two consecutive generations

“…Selection of the G4 parents (of G5) was done by an index that included %L300 and EW200 as egg production traits (along with BW and skin darkness, Wang et al, 2021 ). Accordingly, mean EW200 of the dams and the sires were significantly higher than the mean of all G4 hens (40.9 g) and with Mid-parents = 41.7 g, the significant SelDif was 0.8 g ( Table 3 ).…”

“…The BG line has been bred at the Hangzhou Academy of Agricultural Sciences ( Wang et al, 2021 ). Briefly, it originated from the segregating progeny of the cross between 2 Chinese indigenous breeds, Dongxiang blue eggshell chicken and Jiangshan black-bone chicken.…”

“…From the 441 G4 females that were alive at 300 d of age (300 d), 154 were selected to serve as dams of G5. They had the top values of an index with 2 carcass-related traits: body weight ( BW ) at 300 d (BW300) minus skin lightness (L*) at 250 d ( Wang et al, 2021 ), and 2 egg production traits: % lay to 300 days (%L300) plus mean egg weight at 200 d (EW200). The index consisted of the standardized phenotypic values the 4 traits, with the following relative weights: BW = 30%, L* = 20%, %L = 35%, EW=15%.…”

“…The inbreeding coefficient was determined in each generation of the BG line and found to be too low to affect performance neither to bias the estimates of genetic parameters ( Wang et al, 2021 ). Estimates of heritability (h 2 ) of each trait, and the genetic correlation (r G ) and phenotype correlation (r P ) between traits, were calculated in each generation using the individual animal model of ASReml 4.1 software ( https://asreml.kb.vsni.co.uk/ ) with pedigree information (G3 parents of G4 hens, G3 grandparents and G4 parents of G5 hens) as random effects, and hatch as fixed effect.…”

“…The general interest in populations derived from indigenous breeds is increasing, following the efforts to improve their performances for meat production ( Yang and Jiang, 2005 ) or for dual-purpose production ( Ibrahim et al, 2019 ; Ibrahim, 2020 ), and to broaden the genetic basis of commercial chicken lines by capturing useful genetic variants that exist only in indigenous breeds ( Athrey, 2020 ). The first paper in this series ( Wang et al, 2021 ) presented and discussed the genetic parameters of body weight and skin darkness at various ages in the G4 generation of the BG line, and their potential use in selecting females and males for these 2 carcass-related traits. This paper covers the genetic parameters of laying pattern and egg production in 2 consecutive generations, G4 and G5, and the prospects to improve these traits by selective breeding.…”