A.F. Groen

Relationship between somatic cell count and functional longevity assessed using survival analysis in Italian Holstein–Friesian cows

The relationship between somatic cell scores (SCS) and longevity was assessed for Italian Holstein–Friesian dairy cows using survival analysis. The data of somatic cell count (SCC) test day information of 512 979 cows were provided by nine provinces from Northern Italy. Pedigree information was extracted from the national herd book. Two survival models were run: a model similar to the one used for genetic evaluation on length of productive life, and a second one adding the phenotypic level of ten classes of test-day SCS of the cow. A cow with a test-day in the highest class of phenotypic level for SCS had more than three times greater risk of being culled when compared to risk for the class with the lowest SCS level. The genetic correlation between the risk of being culled and SCS was estimated to be 0.31, based on the correlation between sire EBV. The rank of bulls resulting from the genetic evaluation on survival did not change significantly with the inclusion of SCS in the model (rank correlation of 0.98). It is concluded that the phenotypic level of SCS plays an important role in culling decision of farmers of Italian Holstein–Friesian dairy cows and SCS is genetically associated to the functional longevity of cows.

Genetic Correlation Patterns Between Somatic Cell Score and Protein Yield in the Italian Holstein-Friesian Population

Genetic parameters for somatic cell score (SCS) in the Italian Holstein-Friesian population were estimated addressing the pattern of genetic correlation with protein yield in different parities (first, second, and third) and on different days in milk within each parity. Three approaches for parameter estimation were applied using random samples of herds from the national database of the Italian Holstein Association. Genetic correlations for lactation measures (305-d protein yield and lactation SCS) were positive in the first parity (0.31) and close to zero in the second (0.01) and third (0.09) parities. These results indicated that larger values of SCS were genetically associated with increased production. The second and third sets of estimates were based on random regression test-day models, modeling the shape of lactation curve with the Wilmink function and fourth-order Legendre polynomials, respectively. Genetic correlations from both random regression models showed a specific pattern associated with days in milk within and across parities. Estimates varied from positive to negative in the first and second parity, and from null to negative in the third parity. Patterns were similar for both random regression models. The average overall correlation between SCS and protein yield was zero or slightly positive in the first lactation and ranged from zero to negative in later lactations. Correlation estimates differed by parity and stage of lactation. They also demonstrated the dubiousness of applying a single genetic correlation measure between SCS and protein in setting selection strategies. Differences in magnitude and the sign of genetic correlations between SCS and yields across and within parities should be accounted for in selection schemes.

A stochastic model for the derivation of economic values and their standard deviations for production and functional traits in dairy cattle

Abstract The objective of this paper was to present a model of a dairy cattle production system for the derivation of economic values and their standard deviations for both production and functional traits under Danish production circumstances. The stochastic model used is dynamic, and simulates production and health in a dairy herd. Because of indirect effects between traits, the phenotypic levels of (related) traits can change as a result of genetic changes. Economic values for milk production and body weight were 0.28 and −0.76u2009€/kg per cow-year respectively. For incidence of milk fever, mastitis, retained placenta and laminitis economic values were −402.1, −162.5, −79.0 and −210.2u2009€/incidence per cow-year. The economic values for involuntary culling rate, stillbirth and conception rate were –6.66, −1.63, and 1.98u2009€/% per cow-year, respectively and the economic value for days from calving to first heat was −0.94u2009€/day per cow-year. Standard deviations of economic values expressing variation in realised profit of a farm before and after a genetic change were computed using a linear Taylor series expansion. Expressed as coefficient of variation, standard deviations of economic values based on 1000 replicates ranged between 0.07 (milk production) to 16 (retained placenta).