Effect of somatic cell count on longevity in dairy ewes using survival analysis

The aims of the present study were to compare estimated breeding values (EBV) for milk yield using different testing schemes with a test-day animal model and to evaluate the effect of different testing schemes on the ranking of top sheep. Alternative recording schemes that use less information than that currently obtained with a monthly test-day schedule were employed to estimate breeding values. A random regression animal mixed model that used a spline function of days in milk was fitted. EBVs obtained with alternative recording schemes showed different degrees of Spearman correlation with EBVs obtained using the monthly recording scheme. These correlations ranged from 0.77 to 0.92. A reduction in accuracy and intensity of selection could be anticipated if these alternative schemes are used; more research in this area is needed to reduce the costs of test-day recording. Mastitis incidence has an important effect on culling decision. Somatic cell count (SCC) has been recognized as a very useful indicator of mastitis. The aim of this study was, therefore, to evaluate the effect of SCC on culling decision in Valle del Belice ewes. The data used for the analysis accounted for 4,430 lactations of 2,071 ewes. Data for SCC were collected by the University of Palermo between 1998 and 2006 in 11 flocks. Longevity was defined on a lactation basis, from one lambing to the next instead of from the first lambing to culling. SCC information were divided in 5 classes and included in the analysis as a time dependent covariate changing at each stage for lactation, which changed at 60, 120, 180, and 270 dim. A Cox model was used for the analysis. The percentage of right-censored data was 59.4%. A ewe with a test-day in the highest class of phenotypic level of SCC had a significant 1.21 times greater hazard risk of being culled when compared to the risk for the class with lowest SCC level (SCC<600,000); in general al classes had greater hazard risks than the one in the lowest level. As a conclusion, the phenotypic level of SCC played a role in culling decision in Valle del Belice ewes.