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

Table 4. Descriptive statistics of the milk traits
Trait Mean Min. Max. CV(%)
Milk yield (L30-L120), l 95 40 272 33.37 35.27
Milk yield (L0-L120), l 137 50 338 47.46 34.60
Fat content, g l -1
71.1 23.3 120.1 12.44 17.49
Protein content, g l -1
59.9 43.1 86.9 5.66 9.45
Casein content, g l -1
47.6 33 71.9 4.94 10.38
Serum protein content, g l -1
12 7.2 14.9 1.07 8.94
Lactose content, g l -1
42.6 28.2 57.9 4.34 10.18
MU 131 66.9 207 16.11 12.30
Total solids content, g l -1
182.6 124.4 256 16.12 8.83
PH 6.62 6.13 7.54 0.17 2.62
LnSCC 12.15 9.31 16.19 1.32 10.84
LILCY 26.71 12.19 49.73 4.22 15.78
LnSCC: lactation mean of somatic cell (in their natural logarithmic form). LILCY: lactation
mean of individual laboratory cheese yield.
Several analyses on genetic and environmental parameters for milk yield and milk composition
have been performed in Churra sheep (Gonzalo et al., 1994; El-Saied et al., 1998, 1999).
Results indicated a high importance among the environmental factors of variation of “flock-yearseason”
with high significant effects on milk yield and also milk composition traits. It is also to
be noted that a negative phenotypic correlation was found between milk yield and any of the
milk components with the exception of lactose, the latter also showing negative correlations with
the rest of milk components.
Heritability estimates resulted in a low value (0.08) for the trait “individual cheese yield” and
this together with its high variation degree indicates that it is not useful as a selection criterion. A
low heritability (0.08) was also obtained for fat content, which was attributed to several factors
such as the high variability in nutrition, milking systems and non-identified factors. This low
estimate obtained for fat content is possibly related to the low heritability obtained for cheese
yield. However, these results should be considered as provisional and similar studies should be
performed in other sheep breeds, particularly in those showing larger fat content heritabilities.
Regarding genetic correlations, the two estimates of total milk yield (30-120 days and 0-120
days) seem to represent the same variable on the basis of the correlation found between them as
well as among them and the milk components. Protein fractions appear markedly correlated
among themselves (0.97 to 1) and also with fat content (0.77 to 0.82). Variables with a greater
relationship with cheese yield were total solids content (0.82), casein content (0.81) and protein
content (0.79). Is should also be mentioned the high and negative correlation obtained between
cheese yield and milk yield estimates (-0.53 and -0.54).
Other results indicated a significant effect of the lactation phase on milk composition clearly
influencing the aptitude of milk for cheese processing. In the same way, older ewes have resulted to
produce milk with a higher content in casein, proteins and total solids indicating a better aptitude
for cheese fabrication.
Regarding the possibility of using casein content instead of protein content in selection,
heritabilities obtained were very much alike and the genetic correlation between both traits was