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

0.05) during the treatment and post-treatment periods, respectively, for CLY ewes compared to
CLN ewes (Table 1) and peaked on d 9 for CLY ewes (Figure 2B).
Average milk fat and milk protein percentage for CLY ewes was lowest (P < 0.05) during the
treatment period compared to the pre-treatment period (Table 1). Average milk fat yield for CLY
ewes was similar during the entire experiment (87.4 g/d), however average milk protein yield
was lower (P < 0.05) during the post-treatment period (70.8 g/d) compared to the pre-treatment
and treatment periods (81.7 g/d, Table 1). During the treatment period, daily milk fat and protein
yield for CLY ewes tended (P < 0.10) to increase from d 5 to13, decrease (P < 0.05) from d 13 to
d 18, and remain stable during the post-treatment period (Figure 3). Milk fat yield for CLN ewes
tended (P < 0.10) to increase from d 5 to 7, and then decreased until d 18; milk protein yield for
CLN ewes followed the same pattern, but peaked between d 6 to 8, and decreased until the end
of the experiment (Figure 3).
Serum P4 concentration was similar for CLY and CLN ewes from d 0 to 4 (2.2 ng/ml),
corresponding to CL development (Table 1 and Figure 4). Concentration of P4 for CLY ewes
peaked between d 6 and 10 (Figure 4) and was consistently greater (P < 0.0001) than for CLN
ewes until the end of the treatment period (5.3 vs. 2.9 ng/ml, respectively, Table 1); P4 concentration
for CLN ewes did not change over time during the treatment period (Figure 4). Concentration
of P4 for CLY and CLN ewes was similar during the post-treatment period (0.8 ng/ml,
Table 1) and did not differ from P4 measured on d 0 (Figure 4). Serum concentration of E2 was
greatest (P < 0.05) during estrus, decreased by d 4, and was not different from 0 during the
treatment and post-treatment periods (Figure 5). There were no differences in E2 concentration
between CLY and CLN ewes during any period (Figure 5).
Discussion
This study provides the most direct evidence, to date, that the presence of CL increases milk
production in dairy ewes. Two previous studies using superovulated Lacaune ewes (Labussière
et al., 1993, 1996) implicated the CL in milk production and provided the rationale for the
present research. Labussière et al. (1993) superovulated Lacaune ewes with various hormonal
treatments and analyzed milk yield of ewes that had 0, 1, 2, 3 to 6, or > 6 CL after the hormonal
treatments. There was a positive relationship between number of CL and milk yield with ewes in
the group with > 6 CL having significantly greater milk yield than ewes with 0 CL. In the
second study (Labussière et al., 1996), Lacaune ewes were either untreated, treated with a
progestin sponge, or treated with a progestin sponge and superovulated with FSH and LH.
Superovulated ewes had greater milk yield (+ 11.3%) and fat yield (+ 11.1%) than untreated or
progestin-treated ewes, again implicating the presence of CL in milk production of dairy ewes.
These previous studies found significant effects on milk production in the presence of a supraphysiological
number of CL and the experimental designs could not preclude the possibility that
numbers of preovulatory follicles and circulating estradiol concentrations were the cause of the
increase in milk yield. In the present study, ewes were studied during the non-breeding season so
that circulating estradiol and progesterone were related to exogenous hormonal treatments. All
ewes received the same hormonal treatments except for a PGF 2α injection on d 5 to eliminate the
CL in CLN ewes. In addition, number of CL (2.4 CL) in this study was similar to the number
that would be expected in normally ovulating East Friesian crossbred ewes during the breeding
season (average lambing rate of 220%, McKusick et al., 1999a). Within 5 d after regression of