great lakes dairy sheep symposium - the Department of Animal ...

et al., 2001b), these results being in agreement with the dairy cattle literature (Rupp and
Boichard, 2003). Moreover, the genetic correlations between LSCS and several udder-type traits
such as teat placement or udder depth were moderately favourable as already shown in cattle and
ranged from -0.10 to -0.32 (Legarra and Ugarte, 2005, Barillet et al., 2006). There is evidence
that selection on milk traits only would lead in the long term to “baggy” udders more difficult to
milk by machine and more susceptible to mastitis. Therefore it is relevant to include in the
breeding objective functional traits related to udder morphology (udder scores) and udder health
(SCS) for dairy sheep breeds profiting for several decades from efficient selection on milk traits
only. Since 2005, the Lacaune breed has been selected on a total merit index giving the same
relative weights to milk production and udder morphology and health (Barillet et al., 2006). It is
expected that SCC of the ewes will decrease by 54 % in 10 years, corresponding roughly to a
genetic decrease of about a third of the bulk milk SCC.
Currently the selection for mastitis resistance is based on a linear decrease of milk SCC. To
validate this strategy, in the INRA Lacaune flock, an experiment based on SCC divergent lines
has been in progress since 2003 (Rupp et al., 2006) to study the consequences of such a SCC
selection on different components of the host’s resistance to IMI . The preliminary results are
promising: in 2005, the production during the first lactation of the two SCC lines was similar:
closed to 0.9 liters per milking, while the SCS difference was 1.6, i.e. 400,000 cells/ml for the
low SCC line versus 850,000 cells/ml for the high SCC line. Moreover, low SCC line ewes
showed no clinical cases of mastitis, lower infection rate and better ability to recover from
infections during lactation.
4.4. Conclusion
At the flock level, the control of SCC has to be based on several complementary approaches:
- lessening of the infection pressure: decrease of the new infections by culling the most
infected ewes (if possible) or carrying out a “selective” drying-off treatment. An
“exhaustive individual SCC picture” by sampling all the milked ewes would be
recommended to select the sheep to be culled or treated. The Figure 11 illustrates the
evolution of bulk tank SCC between 2001 and 2002 for flocks involved in a control
program in 2001. There were 2 levels of “control”: normal or simplified differing from
the realization or not of an exhaustive SCC (all the other actions being comparable). It
can easily be noticed the decrease of SCC after the iSCC results: the sheep with high
SCC had been dried off (or culled) during the year 2001. For the year 2002, a better SCC
bulk level was noticed for the flocks involved on the normal action, the decrease of SCC
being about 100,000 cells/ml or minus 20 %.
- the limitation of new infections using all the preventive possible actions, at a reasonable
cost, in accordance with the management of the flock.
- the genetic control for the breeds profiting for several decades from an efficient selection
on milk traits only.
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