Genetic analysis for mastitis resistance and somatic cell score in French Lacaune dairy sheep

Genetic analysis for mastitis resistance was studied in 957 first lactation Lacaune ewe lambs of an experimental INRA flock composed of 2 divergent lines for milk yield and in 3,811 first lactation (19,200 test day) Lacaune ewe lambs recorded in 38 flocks. The arithmetic average somatic cell count (SCC) was 375,000 cell/ml. The clinical mastitis frequency (5%) was lower than the occurrence of estimated subclinical mastitis (10%) and appeared not to be genetically related to milk yield unlike subclinical mastitis. Thus, at the moment, selection for mastitis resistance may be reduced to selection against subclinical mastitis using somatic cell score (SCS). Heritability estimates for test day SCS (similar to 0.02 to 0.15) were clearly lower than those for lactation average SCS (similar to 0.15 to 0.18) and increased with day in milk in connection with the increase in infected ewes. This result suggests that the usual test day model versus lactation model is not sufficient to substantially improve the accuracy of genetic evaluation for SCS, since the main objective is, in fact, to account for subclinical infections.