Abstract
In a system based on a test-day alternate recording, a total of 1,227,949 test-day records for somatic cell counts (SCC) and milk yield obtained throughout 12 years (1998–2009) from 196,496 ewes in 156 Manchega flocks belonging to the National Association of Manchega Breeders (AGRAMA, Spain) were used to study the environmental variations of the SCC and its relationship with milk yield. Two subsets containing test-day SCC and milk yield data were also analyzed: SCC≤300×103cells/ml, and SCC>300×103cells/ml.Analysis of variance showed significant effects for the ewe, season, age of ewe, lactation stage, number of lambs born, milking shift, and for the flock×year random effect on variables studied. Average values obtained for somatic cell score (SCS) and milk yield were 4.09 and 1156ml/d, respectively. The SCS increased 7.6% through lactation and was highest in winter. The age of ewe effect was highly significant for SCS and milk yield, both increasing with the age, even for the group of ewes with a SCC≤300×103cells/ml. Twin-birth ewes showed higher SCS (5.0%) and milk yield (9.8%) than single-birth ewes. SCS for p.m. milking was significantly higher than for the a.m. milking. Phenotypic correlation between milk yield and SCS was negative (−0.09), which reflects the reduction in the production ability of ewes with higher SCS. Average differences in milk yield between test-day subsets studied were 11.8%; the greatest differences were in autumn (14.5%) and the lowest in spring (9.8%). Differences in milk yield between the ewes with SCC≤300×103cells/ml and ewes with SCC>300×103cells/ml also increased through lactation. Thus, in the first test-day record differences in milk yield reached 5.4%, and the difference increased up to 11.8% in the 4th test-day record. Our results suggest that it seems necessary to take into account the effect of non-infectious factors to interpret the SCC results of recording systems.
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