G. VanderVoort and J.P. Gibson
Important tools for improving several traits
together are multiple trait genetic evaluations and selection
indexes. To develop these for carcass yield and quality traits,
we require estimates of the variances and covariances among these
traits, and covariances of these traits with other traits of economic
The OPCAP provides information on a number
of traits of interest for which very little or no genetic information
is available elsewhere. Here, we show key estimates obtained
from the OPCAP data for use in national genetic evaluations for
The REML VCE (Groeneveld, 1996) package
was used to obtain multiple trait estimates of variance components.
Parameters for the first seven traits in Table 1 below were from
a single simultaneous analysis. Parameters for other traits came
from various subsets of data. Details of the models used are given
in Appendix 3. Traits analyzed were:
The table below lists heritabilities, genetic and phenotypic correlations of carcass yield traits, and indirect estimates of carcass composition and phenotypic s.d.
Heritabilities (on diagonal), genetic (above) and phenotypic correlations
(below diagonal) and phenotypic s.d. (below table).
With the exception of longitudinal muscle
depth, all lean and fat measures had high heritabilities in the
range 0.5 to 0.6. By adjusting to constant carcass or three primal
weights, total lean and loin lean are essentially measures of
proportions of lean. Their high heritability is therefore consistent
with previous estimates for measures of carcass leanness (rather
than carcass or lean weight, which generally has a lower heritability).
Estimates of heritability for marbling, colour and drip loss,
at around 0.2, were somewhat lower than previously reported, but
still indicate sufficient genetic variation to be able to make
genetic change if desired. The high negative genetic correlation
between drip loss and colour is not due to the PSS gene since
other analyses of this data accounting for PSS genotype found
a similar correlation. As might be expected, marbling showed
moderate genetic correlations with measures of carcass leanness
and fatness. Colour and drip loss showed moderate genetic correlations
with lean traits, but not with fat traits.
In virtually all cases, genetic correlations
were substantially higher than phenotypic correlations. This
indicates that underlying biological relationships are stronger
than would be suggested by simple observations between animals.
Much of the difference probably reflects the substantial difficulties,
and consequent errors, of measuring these traits.
There is substantial genetic variation in all three carcass quality traits (marbling and colour score and loin drip loss) which will allow continued genetic improvement. The antagonistic genetic correlations between measures of leanness and quality (marbling, colour and drip loss) indicate that meat quality will likely deteriorate with continued selection for leanness indexes unless direct selection on meat quality is practiced. Interestingly, the lack of a correlation of colour and drip loss with fat measurements suggest that it is only direct change in leanness (i.e. increase in actual muscle mass) that is negatively associated with quality. If true, indirect selection for leanness through selection for reduced backfat only may have little impact on quality. Conversely, improving our accuracy of estimates of leanness by use of ultrasonic muscle depths and muscle areas or direct carcass measures of leanness, while improving genetic change in leanness, may accelerate reductions in quality unless steps are taken to improve quality directly.