Genetic and environmental effects on carcass characteristics of Southdown × Romney lambs: II. Genetic and phenotypic variation

Abstract

Approximately 4,400 crossbred lambs from Southdown sires and Romney ewes were slaughtered at approximately 18, 23, and 28 wk of age over a 16-yr period. Live weights, carcass measurements, and chemical percentages were analyzed to estimate genetic and phenotypic parameters. Heritabilities of postweaning weights and gains were about .20. Heritabilities of fat and water percentages were about .35 adjusted for age. Heritability of kidney fat percentage was .53. Heritabilities of fat depth and muscle measurements ranged from .21 to .37. Crutch depth (h2 = .73) and cannon bone length (h2 = .74) were the most highly heritable carcass measurements. The genetic correlation between carcass fat and fat-free weight was .47 when lambs were slaughtered at a constant age. Fat-free weight was nearly uncorrelated with percentages of fat, water, and protein when lambs were slaughtered at the same age. Carcass measurements increased accuracy of selection for fat-free weight at a constant age very little compared with using only carcass weight. However, this does not mean that additional measurements are useless. The addition of carcass measurements to the selection criteria would result in correlated responses in chemical composition that more closely resembled direct selection for fat-free weight. Carcass weight would be of little value when used by itself to reduce fat weight adjusted for carcass weight. Direct measurement of carcass composition resulted in 1.6 to 2.6 times more predicted response for reduced fat weight than any combination of carcass weight and one fat depth measurement.

Genetic parameters for lamb carcass traits assessed by video image analysis, EUROP classification and in vivo measurements

“…High heritability estimates of in vivo conformation score of the hind limb have been reported, ranging from 0.31 to 0.62 (Thorsteinsson & Björnsson 1982, Janssens & Vandepitte 2004, Wolf & Jones 2007. ML was found to be highly heritable (0.52) in accordance with other studies that have reported extremely high heritability estimates for this trait, ranging from 0.64 to 0.82 (Thorsteinsson & Björnsson 1982, Bennett et al 1991.…”

“…Studies have shown that fat measurements on various parts on the carcass, although highly correlated, cannot be treated as one trait. Bennett et al (1991) found a high genetic correlation (0.83) between measurements of fat thickness on the back (C) and on the rib (J); similarly Thorsteinsson and Björnsson (1982) also reported a high correlation (0.62). The genetic correlation of 0.99 found here between GR and pEUROPf indicates that the GR fat measurement totally controls the EUROP fat score given by the abattoir graders.…”

Genetic and phenotypic parameter estimates for selection to improve lamb carcass traits

“…The estimate of the present study (0.31) is within the range of estimates in the review of Wolf & Smith (1983), and agrees well with Bennett at al. (1991).…”

“…Therefore, partial genetic correlations, having taken account of HCW, were computed using SAS PROC CANCORR (1985) with the estimated genetic covariance matrices from the multivariate mixed model analyses as input. The partial correlations provide a measure of the linear genetic relationship between traits after HCW has been accounted for, which is not the same as parameters adjusted to a constant HCW (Koch et al 1982;Bennett et al 1991).…”

“…The traits considered as the selection objective traits in this study, LEAN and FA T, were expressed in kg rather than as a percentage of HCW because the aim was to partition growth rate into these components. The two traits, LEAN and FAT, have not been widely studied and Wolf et al (1981), who estimated the heritability of LEAN/(day of age) to be 0.23, and (2) Bennett et al (1991), who estimated the heritability of fat-free weight to be 0.21 (fat-free weight was defined as HCW-fat weight). Botkin et al (1969) and Bennett et al (1991) used the product of HCW and %fat from chemical analysis as fat weight, obtaining estimates of heritability for fat weight of 0.44 and 0.29, respectively.…”