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
×
103 cells/ml, and SCC > 300
×
103 cells/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 1156 ml/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
×
103 cells/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
×
103 cells/ml and ewes
with SCC > 300
×
103 cells/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.
