Genetic parameters for somatic cell score and its genetic by qjo17259


									Genetic parameters for somatic cell score and its                 parities for SCS were all greater than 0.79 and in
genetic association with milk production and udder                general agreed well with previously reported estimates
conformation                                                      (Mrode and Swanson, 1996). The strength of the genetic
                                                                  correlations tended to weaken as the distance between
R.D. Evans1 and D.P. Berry2                                       the parities increased. The phenotypic correlations,
  Irish Cattle Breeding Federation, Highfield House,              although positive were weaker than their respective
Bandon, Co. Cork. 2Teagasc, Moorepark Research                    genetic correlations agreeing with previous studies
Centre, Co. Cork.                                                 (Mrode and Swanson, 1996). The phenotypic
                                                                  correlations also weakened as the distance between
Introduction                                                      parities increased. The genetic correlations between
The strong genetic association between somatic cell               milk yield, milk composition, udder conformation and
count (SCC) and clinical mastitis (Mrode and Swanson,             SCS in the first three parities are summarised in Table
1996) has led to the inclusion of SCC in many national            1. Although the genetic correlations were weak, there
breeding objectives as an indicator trait for clinical            was a tendency, especially in first lactation, for cows of
mastitis. Somatic cell count also has an economic value           high genetic merit for milk production to be genetically
in its own right based on the tiered milk pricing                 predisposed to high SCS. Genetic correlations between
operated by Irish milk processors (Berry and Amer,                milk composition and SCS were all near zero. The
2006). The objectives were to determine the variance              genetic correlations between udder conformation and
parameters of somatic cell score (SCS; the lactation              SCS suggest a greater genetic predisposition to higher
mean of the natural log of test-day SCC) across parities          SCS in cows with deeper udders and a loose fore udder
and its genetic association with milk production and              attachment. There was a tendency for positive genetic
udder conformation.                                               correlations between udder support and SCS although
                                                                  the correlations were not significantly different from
Materials and Methods                                             zero. The strength of the genetic correlation between
A total of 4,258,553 lactation records were extracted             both udder depth and fore udder attachment with SCS
from the Irish Cattle Breeding Federation (ICBF)                  intensified as parity number increased. Selection index
database including information on cow number, herd,               calculations indicated no advantage of including udder
calving date, parity, age at calving (in months), length          related type traits in a selection index as genetic
of lactation, previous calving interval, subsequent               predictors of SCS.
calving interval, lactation milk yield, fat yield, protein
yield, geometric mean SCC and number of SCC test-                 Table 1. Heritability estimates for production traits and
day records used to calculate the geometric mean. Edits           udder conformation traits (first parity) and their genetic
included removing cow records with no sire identified             correlation with SCS in the first three parities.
(n=1,069,163), lactation records with a lactation length                                                   rg with SCS
>600 days (n=36,294), and records with a milk yield               Trait              Heritability Parity1 Parity2 Parity3
<1000 or >15000 kg (n=5,613). Further edits included
restricting age at calving to be within the ranges 21 to          Milk yield             0.42        0.19*      0.06   0.09
40 months, 30 to 60 months, 40 to 80 months, 50 to 85             Fat %                  0.52         0.04      0.00   -0.05
months and 60 to 100 months for parities 1 to 5,                  Protein %              0.55         0.03      0.00   0.04
respectively. Following these edits 2,052,048 records             Udder depth           0.29        -0.19* -0.24* -0.38*
remained. Lactation average SCS was calculated as the             Udder support          0.15         0.10      0.06   0.04
natural log of SCC. Data from 128,511 lactations with             Fore udder
information on type traits were also extracted from the           attachment            0.13        -0.14*     -0.14 -0.19*
ICBF database. The pedigree file included 1,864,126               Rear udder
animals. The data were analysed using a univariate                height                 0.26         0.06      0.06   0.11
repeatability sire model in ASREML (Gilmour et al.,               Teat placement
2005). The model used included a herd-year-season-                front view             0.19        0.14*      0.12   0.12
parity contemporary group, year x month, breed, age,              Teat length            0.29        -0.02      0.06   0.18
days in calf, days dry, and a random sire (relationships
                                                                  *Correlations whose absolute value was more than
included), cow and residual effect. Multi-trait analyses
                                                                  twice the respective standard error
were also undertaken whereby SCS in each parity from
one to three was treated as a separate trait. Selection           Conclusions
index calculations were undertaken to evaluate the                The heritability of SCS suggests genetic progress for
possibility to augment the reliability of estimated               reduced SCC is possible if included in an overall dairy
breeding values for SCS with data on udder                        breeding objective. Udder conformation was not useful
conformation.                                                     as an indirect predictor of SCS in a national breeding
Results and Discussion
The univariate repeatability analysis revealed a sire,            References
cow and residual variance for SCS of 0.0165, 0.2131,              Berry, D.P. & Amer, P.R. (2006) This proceedings
and 0.3920, respectively. Hence, the heritability and             Gilmour, A.R., Cullis, B.R., Welham,S.J. & Thompson
repeatability were 0.106 (s.e.=0.0049) and 0.449                  R., (2005). ASReml User Guide.
(s.e.=0.0045), respectively. These estimates are                  Mrode, R.A., & Swanson, G.J.T. (1996). Anim. Breed.
consistent with international estimates (Mrode and                Abstr. 164,847-857.
Swanson, 1996). The genetic correlations among

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