Tiina Ikonen

Repeatability estimates for milk coagulation traits and non-coagulation of milk in Finnish Ayrshire cows

Effects of systematic environmental factors and milk production and quality traits on milk coagulation properties (MCP), and on repeatability of those traits were estimated from 979 milk samples collected once a month over a period of 2 years from 83 Finnish Ayrshire cows. Estimation was based on a multitrait animal model and REML methodology. In addition, persistence of non-coagulation of milk in individual cows, and factors associated with it were established from a sub sample of 24 cows producing non-coagulating (NC) milk at least once. MCP were at their best during the first lactation, at the beginning and at the end of lactation, and during grazing seasons. Variation in MCP with systematic environmental factors was partly due to variation in composition and quality of milk, especially in pH and ln (somatic cell count, SCC). Coefficients of repeatability for milk coagulation time and curd firmness were 0.65 and 0.68. These estimates were of the same magnitude as those for protein content, but were higher than those for daily milk yield, fat content, pH, and SCC. Based on the repeatability estimates for the milk coagulation traits and effects of the environmental factors, cows should be sampled at least three times during a lactation to estimate reliably breeding values for the milk coagulation traits. A total of 10% of the milk samples did not coagulate in 30 min after addition of rennet. Cows that produced NC milk at least once (30% of the cows) could be classified into those that produced NC milk only a few times during a lactation and those that produced NC milk at almost every sampling. Based on logistic regression analyses, peak and mid-lactation, high milk yield, low protein and fat content and high pH increased the risk of non-coagulation of milk.

Chromosomal Regions Underlying Noncoagulation of Milk in Finnish Ayrshire Cows

About 10% of Finnish Ayrshire cows produce noncoagulating milk, i.e., milk that does not form a curd in a standard 30-min testing time and is thus a poor raw material for cheese dairies. This phenomenon is associated with peak and midlactation, but some cows produce noncoagulating milk persistently. A genomewide scan under a selective DNA pooling method was carried out to locate genomic regions associated with the noncoagulation of milk. On the basis of the hypothesis of the same historical mutation, we pooled the data across sires. Before testing pools for homogeneity, allele intensities were corrected for PCR artifacts, i.e., shadow bands and differential amplification. Results indicating association were verified using daughter design and selective genotyping within families. Data consisted of 18 sire families with 477 genotyped daughters in total, i.e., 12% of each tail of the milk coagulation ability. Data were analyzed using interval mapping under maximum-likelihood and nonparametric methods. BMS1126 on chromosome 2 and BMS1355 on chromosome 18 were associated with noncoagulation of milk across families on an experimentwise 0.1% significance level. By scanning gene databases, we found two potential candidate genes: LOC538897, a nonspecific serine/threonine kinase on chromosome 2, and SIAT4B, a sialyltransferase catalyzing the last step of glycosylation of κ-casein on chromosome 18. Further studies to determine the role of the candidates in the noncoagulation of milk are clearly needed.

Associations between milk protein genotypes and fertility traits in Finnish Ayrshire heifers and first lactation cows

Abstract Effects of composite β–κ-casein genotypes and β-lactoglobulin genotypes on age at first insemination and length of service period of 17,059 Finnish Ayrshire heifers, and on days from calving to first insemination and length of service period of 17,869 first lactation cows were estimated. A mixed linear model under an animal model was assumed. The effect of the β–κ-casein genotypes on days from calving to first insemination (DFI) was statistically significant. The difference in DFI between the rare extreme β–κ-casein genotypes A 1 A 2 BB and A 1 A 2 EE was about 19 days (0.75 phenotypic standard deviation), but the standard errors of the effects of these genotypes were large. Between the most frequent β–κ-casein genotypes the differences in DFI were negligible. The other reproduction traits studied were not affected by composite β–κ-casein genotypes or β-lactoglobulin genotypes. Based on the results presented in this study, selection based on β–κ-casein and β-lactoglobulin polymorphism should thus have no substantial impact on fertility of Finnish Ayrshire heifers and cows.