Dairy Sheep Symposium - the Department of Animal Sciences ...

percentage is not expected to result in a genetic change in SCC under the conditions in Churra
sheep. The negative correlations found between SCC and milk production have also been found
in other investigations in cattle (Ng-Kawi-Hang et al., 1984; Emanuelson et al., 1988) and sheep
(Gonzalo et al., 1994; Lagriffoul et al., 1994; Fuertes et al., 1998).
Table 9. Genetic correlations (below diagonal) and phenotypic correlations (above
diagonal) among lactation variables
Milk yield
Log SCC
Protein percentage
Milk yield Log SCC
-0.15
-0.47
-0.16
0.03
Protein
percentage
In view of the relatively low heritability and the moderate repeatability values of SCC, the
inclusion of repeated records seems advisable in order to increase the accuracy of prediction of
genetic values in the same way as it is used in cattle (Da et al., 1992; Zhang et al., 1994). An Animal
Model with repeatability seems particularly interesting in sheep due to the reduced number of
daughters per ram.
It is not possible for the moment to establish a relationship between SCC and genetic resistance
to mastitis. Further investigations are required in order to clarify this relationship. However
from the results obtained till the present moment SCC heritability seems to be low indicating a
great environmental influence and thus response to selection will not be high.
Other Criteria
Feed Efficiency
Cost of feed represents a major expense for most dairy sheep production. In sheep dairy
breeding programs, economic gain will be only obtained if an increase in milk yield maintains or
increases feed efficiency. The verification of this point was carried out in an investigation performed
in Lacaune sheep which results indicated that dairy breeding based only on an increase in
milk income appeared as fully acceptable regarding indirect responses in feed efficiency (Marie
et al., 1996). This experiment involved a divergent selection on milk dry matter yield (fat +
protein) and ad libitum feeding. After four years of experiment the high line´s ewes produced,
22% more milk compared to the low line, without increase in body weight, ingested 7% more
feed and targeted more adequately their body reserves towards milk production. Thus Gross
Efficiency was improved (0.36 vs 0.31). These results indicate that an increase of individual milk
yield will be followed by a progress in the economic margin per animal (Barillet et al., 2001).
Disease Resistance
Animal diseases represent a major cost to sheep producers, through production losses and the
direct costs of treatment and control. The danger of relying entirely on antibiotics or chemicals
was foreseen in the early 1970s, and scientists set about determining the potential for genetic
selection of the sheep host to reduce the susceptibility to diseases. A clear understanding of
disease and the animal’s defense systems is required for alternative approaches to disease control
(Hill, 1999). The onset of disease is often the result of the interaction between an individual
animal’s genotype and the environment to which the animal is exposed. If an animal has a
-0.26
0.12