Estimation of Variance Components From the OPCAP Data

G. VanderVoort and J.P. Gibson

Background

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 interest.

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 carcass traits.

Methods

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:

TLEAN = kg lean in three primals
LLEAN = kg lean in loin
LEA = loin eye area measured on carcass (cm2)
LONGMD = live animal longitudinal real-time ultrasound muscle depth (mm)
LONGFAT = live animal longitudinal real-time ultrasound fat depth (mm)
ADJBF = A-mode backfat on live animal adjusted to 100 kg liveweight (mm)
PRBFAT = fat depth from carcass probe (mm)
MARB = marbling score of loin
COLOUR = Ag Canada colour score of loin
DRIPL = drip loss of loin (%)
Results

The table below lists heritabilities, genetic and phenotypic correlations of carcass yield traits, and indirect estimates of carcass composition and phenotypic s.d.

Table 1. Heritabilities (on diagonal), genetic (above) and phenotypic correlations (below diagonal) and phenotypic s.d. (below table).
TLEAN
LLEAN
LEA
LONGMD
LONGFAT
ADJBF
PRBFAT
MARB
COLOUR
DRIPL
TLEAN
0.580
0.901
0.752
0.521
-0.730
-0.773
-0.740
-0.481
-0.300
0.249
LLEAN
0.800
0.489
0.841
0.631
-0.657
-0.702
-0.693
-0.498
-0.229
0.276
LEA
0.650
0.664
0.457
0.722
-0.532
-0.489
-0.483
-0.431
-0.351
0.344
LONGMD
0.310
0.376
0.461
0.360
-0.038
-0.191
-0.039
-0.083
-0.062
0.145
LONGFAT
-0.650
-0.554
-0.420
-0.110
0.593
0.945
0.975
0.298
-0.03
-0.011
ADJBF
-0.696
-0.589
-0.447
-0.178
0.727
0.568
0.933
0.366
0.195
-0.181
PRBFAT
-0.682
-0.580
-0.445
-0.211
0.712
0.72
0.530
0.310
0.020
0.025
MARB
-0.256
-0.241
-0.227
-0.113
0.174
0.198
0.191
0.192
0.256
-0.311
COLOUR
-0.098
-0.073
-0.106
-0.099
0.040
0.050
0.062
0.113
0.213
-0.539
DRIPL
0.116
0.127
0.176
0.126
-0.058
-0.067
-0.054
-0.105
-0.364
0.211
Phenotypic s.d.
0.876
0.443
5.090
4.494
2.279
2.24
3.23
0.659
0.547
3.53

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.

Implications

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.