Heritability of test day somatic cell counts and its relationship with milk yield and protein percentage in dairy ewes.

A total of 6620 monthly test day records of SCC, milk yield, and protein percentage from single lactations of 2374 Spanish Churra ewes from 10 flocks was used to estimate genetic and environmental parameters. A subset of 4278 records containing data from healthy udders (SCC < or = 250,000 cells/ml) was also analyzed. Genetic parameters were estimated by REML using an animal model. Herd test date, parity, and lactation stage contributed significantly to variation of most variables, and birth type significantly affected milk yield only. The SCC increased markedly as parity number and stage of lactation increased. Heritabilities (+/- SE) for test day milk yield, log SCC, protein percentage, and log SCC (< or = 250,000 cells/ml) were 0.18 (0.03), 0.09 (0.02), 0.16 (0.03), and 0.03 (0.02), respectively. The corresponding repeatabilities were 0.54, 0.38, 0.38, and 0.10. Genetic correlations of log SCC with milk yield and protein percentage were -0.23 and 0.18, respectively. Phenotypic correlations were -0.15 and 0.16. Genetic and environmental reduction of SCC for dairy ewes could be achieved using practices similar to those for dairy cows. The negative genetic correlation between milk yield and SCC suggested that selection for increased milk yield alone is expected to result in a decrease in SCC.