Composite β-κ-casein genotypes and their effect on composition and coagulation of milk from Estonian Holstein cows

Abstract

The objective of this study was to estimate the effect of composite β-κ-CN genotypes on milk coagulation and composition traits, and on the additive genetic variation of these traits in Estonian Holstein dairy cattle. A total of 23,970 milk samples, repeated measurements from the first to third lactation from 2,859 Estonian Holstein cows from 78 herds across the country, were analyzed for milk yield, milk fat and protein percentages, somatic cell count, and milk coagulation properties (milk coagulation time and curd firmness). Each cow had at least 3 measurements per lactation. Two single-trait random regression animal models were fitted for the traits studied. The first model considered fixed effects of year-season of sampling and year-season of calving, calving age (nested within lactation), sample age (only for milk coagulation traits) and days in milk, and random herd, additive genetic, and permanent environmental effects. The animal and permanent environmental effects were modeled over the lactation period by using Legendre polynomials. The second model had the additional fixed β-κ-casein effect in the form of a third-order Legendre polynomial. The 2 most frequent β-κ-casein composite genotypes were A2A2AA and A1A2AA, both with prevalence greater than 20%. Percentages of the remaining 31 genotypes were less than 8%, including 20 genotypes with percentages less than 1%. The β-κ-casein genotype-specific lactation curves were significantly different for milk coagulation traits and milk protein percentage. The B variant of κ-casein showed a favorable effect on both milk coagulation traits, whereas the IB haplotype had an increasing effect on curd firmness and protein percentage. Inclusion of the β-κ-casein genotype effects in the model resulted in decreases in the mean additive genetic variations for milk coagulation time and curd firmness of 12.9 and 51.1%, respectively.

Structural differences between bovine A1 and A2 β-casein alter micelle self-assembly and influence molecular chaperone activity

“…Milk coagulation is a complicated process that is governed not only by the milk protein variants but also by protein and mineral contents (Fox and McSweeney, 1998). Recent research by multiple research groups has confirmed that milk homozygous for A 2 β-CN (usually in combination with the A variant of κ-CN) is the predominant β-CN variant found in poorly or noncoagulating milk in multiple cow breeds (Jensen et al, 2012a,b;Vallas et al, 2012;Poulsen et al, 2013;Gustavsson et al, 2014).…”

Selection for milk fat and milk protein composition

“…Protein variants of k-casein have been shown to be associated with the renneting time of milk (e.g. Van den Berg et al, 1992;Comin et al, 2008;Vallas et al, 2012). The association between the k-casein B variant and a shorter renneting time might be because of the differences in amino-acid composition between the A and B variants resulting in different physical or chemical properties.…”

Heritability and repeatability of milk coagulation properties predicted by mid-infrared spectroscopy during routine data recording, and their relationships with milk yield and quality traits

“…However, direct phenotyping for renneting ability would result in higher genetic gain because both related traits (Pretto et al, 2012) and protein genotypes Penasa et al, 2010;Vallas et al, 2012) are not able to give the same genetic change. For instance, genetic correlations of MCP were found negligible with milk yield but moderate to strong with protein and casein percentages and SCC, generally moderate with fat percentage, and strong with milk pH.…”