Resumen:
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The objective of this study was to estimate genetic
parameters for milk protein fraction contents, milk
protein composition, and milk coagulation properties
(MCP). Contents of αS1-, αS2-, β-, γ-, and κ-casein ...[+]
The objective of this study was to estimate genetic
parameters for milk protein fraction contents, milk
protein composition, and milk coagulation properties
(MCP). Contents of αS1-, αS2-, β-, γ-, and κ-casein (CN),
β-lactoglobulin (β-LG), and α-lactalbumin (α-LA) were
measured by reversed-phase HPLC in individual milk
samples of 2,167 Simmental cows. Milk protein composition
was measured as percentage of each CN fraction
in CN (αS1-CN%, αS2-CN%, β-CN%, γ-CN%, and
κ-CN%) and as percentage of β-LG in whey protein (β-
LG%). Rennet clotting time (RCT) and curd firmness
(a30) were measured by a computerized renneting meter.
Heritabilities for contents of milk proteins ranged from
0.11 (α-LA) to 0.52 (κ-CN). Heritabilities for αS1-CN%,
κ-CN%, and β-CN% were similar and ranged from 0.63
to 0.69, whereas heritability of αS2-CN%, γ-CN%, and
β-LG% were 0.28, 0.18, and 0.34, respectively. Effects
of CSN2-CSN3 haplotype and BLG genotype accounted
for more than 80% of the genetic variance of αS1-CN%,
β-CN%, and κ-CN% and 50% of the genetic variance of
β-LG%. The genetic correlations among the contents of
CN fractions and between CN and whey protein fractions
contents were generally low. When the data were
adjusted for milk protein gene effects, the magnitude
of the genetic correlations among the contents of milk
protein fractions markedly increased, indicating that
they undergo a common regulation. The proportion
of β-CN in CN correlated negatively with κ-CN% (r
= −0.44). The genetic relationships between CN and
whey protein composition were trivial. Low milk pH
correlated with favorable MCP. Genetically, contents
and proportions of αS1- and αS2-CN in CN were positively
correlated with RCT. The relative proportion of
β-CN in CN exhibited a genetic correlation with RCT
of −0.26. Both the content and the relative proportion
of κ-CN in CN did not correlate with RCT. Weak curds
were genetically associated with increased proportions
in CN of αS1- and αS2-CN, decreased contents of β-CN
and κ-CN, and decreased proportion of κ-CN in CN.
Negligible effects on the estimated correlations between
a30 and κ-CN contents or proportion in CN were observed
when the model accounted for milk protein gene
effects. Increasing β-CN and κ-CN contents and relative
proportions in CN and decreasing the content and
proportions of αS1-CN and αS2-CN and milk pH through
selective breeding exert favorable effects on MCP.
Key words: milk protein composition , milk coagulation
property , heritability , genetic correlation
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