Great Lakes Dairy Sheep Symposium - the Department of Animal ...

Table 9 - Coefficients of correlation and statistical significance for simple regressions
developed separately for each production class.
Production class
Milk yield (g/d) < 400 400-799 800-1199 1200-1600
Number of data 63 260 97 20
Dependent Variable
Milk fat content (%)
Independent Variable
Milk yield (g/d) ns - 0.28 c ns - 0.39 a
Dietary NDF (% DM) ns - 0.24 c - 0.44 c - 0.60 b
NDF intake (% BW) - 0.21 a - 0.35 c - 0.48 c - 0.89 c
Energy balance (UFL/d) - 0.24 a - 0.28 c - 0.42 c - 0.79 c
a P < 0.10; b P < 0.01; c P < 0.001; ns = P > 0.1
In the ewes with the highest milk yield, one of the major factors affecting milk fat
concentration was their EB. In the same class, NDF intake (% of BW) was positively
associated with energy intake (% of BW) (r = + 0.92). As energy intake increased, as a result
of the increase in pasture (and NDF) intake, the EB improved, probably reducing body fat
mobilization and the availability of long chain FA for the mammary gland. As a consequence,
milk fat concentration decreased. It is important to highlight that for each experimental
treatment the amount of concentrate supplied was the same for all animals, while pasture was
fed ad libitum. Therefore, the ewes with the highest milk yield had to eat more pasture (and
fiber) than the animals with lower milk yield and requirements. The negative correlation
between milk fat concentration and NDF concentration and intake (Table 9 and Figures 4 and
5) is in clear contrast to the positive association between milk fat concentration and NDF
previously mentioned (Emery, 1988; Bencini and Pulina, 1997).
In the ewes with low milk yield, factors other than precursor availability were important
and milk fat concentration was probably largely affected by individual characteristics. This
explains why the association between milk fat and the predictors decreased, as milk yield
decreased (Table 9).
Cannas and Avondo (2002) found that if all the ewes with less than 0.65 kg/d of milk
yield were excluded from their database, so that the minimum value was equal to the one of the
database of Bocquier and Caja (1993; 2001), the relationship between milk fat concentration and
EB would have the same slope of equation [4]:
[5] milk fat concentration (%) = 6.99 - 1.22 EB (UFL/d) (r 2 = 0.14; P< 0.0001)
The lower coefficient of determination of equation [5] compared to equation [4] is
probably the result of a wider range in the data set used for its development (650-1600 g/d of
milk in equation [5] vs. 0.65-3.5 l/d in equation [4]). The fact that the slope of the equation was
identical in the two equations suggests that the milk fat concentration decreases by 12.2 g/l for
each UFL/d of variation in EB regardless the breed considered. The difference in intercept might
be due to the different distribution of the data in the two datasets or might suggest breed
differences in the average milk fat concentration. Once these differences are accounted for, milk
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