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

of the alveoli (Carulo, 1971; Heap et al., 1986; Knight, 1994; Peaker and Blatchford, 1988). Our
results support a stimulatory effect of OT on milk secretion because CLY ewes continued to
maintain superior milk production, milk flow rate, and milk fat and protein yield compared to
CLN ewes during the post-treatment period. Differentiation of the cellular mechanism(s) involved
in the increase in milk production during the treatment and post-treatment periods will
obviously require further research.
Our experimental design did not allow us to eliminate the possibility that the increase in
circulating P4 was the cause of the increase in milk production. Continuous provision of exogenous
P4 from the 2 CIDRs produced circulating P4 concentrations equivalent to or higher than
what is normally reported in the ewe during the luteal phase (2.5 to 3 ng/ml, Murdoch and Van
Kirk, 1998) even after induced (d 5 in CLN ewes) or spontaneous (before d 18 in CLY ewes)
luteolysis. However, circulating P4 was greater in CLY than CLN ewes between d 6 and 14 of
the treatment period. Our primary concern was to maintain similar circulating E2 concentrations
in the 2 groups because E2 has been found to be directly mitogenic in mammary tissue and could
have been responsible for the previously observed increases in milk production in superovulated
ewes (Labussière et al., 1993, 1996). There was no difference in circulating E2 concentrations
between the two treatment groups at any of the times that were evaluated in our experiment.
Although the effects of P4 cannot be ruled out, exogenous P4 administration was found to have
no effect on lactation performance in the ewe (Smith and Inskeep, 1970) or rat (Herrenkohl,
1972).
A number of characteristics of the East Friesian breed may make it more compatible with
improved milk production by the presence of CL. The East Friesian breed has a large cisternal
storage capacity as compared to other dairy sheep breeds (Bruckmaier et al., 1997; McKusick et
al., 1999b) and this may facilitate the storage of milk in the cistern away from the alveoli during
the intermilking interval. Additionally, the large cistern may improve milk flow rate during
machine milking (McKusick and Marnet, 2001, unpublished data). Moreover, the East Friesian
is a prolific breed and therefore substantial numbers of CL would be expected during the normal
breeding season. Baseline and peak serum OT concentrations are increased in breeds with high
fecundity (Schams et al., 1982). Therefore, the positive effect of the presence of CL on milk
production in East Friesian sheep may be an advantage for dairy sheep producers who organize
their lactation season to be coincident with the natural estrous season. The improvements in milk
production produced by CL in East Friesian ewes may also be produced in other dairy sheep,
goats, or cattle; although, this will need to be specifically tested in these animals. Unfortunately
the stimulatory effects of the CL may not be maintained throughout pregnancy because luteal OT
decreases to minimal concentrations by d 18 of pregnancy in the ewe (Marnet and Combaud,
1995). Thus, practical implementation of improvements in milk yield by CL will require substantial
subsequent research.
Conclusions
We conclude that the presence of corpora lutea during lactation can increase milk production
in dairy ewes. Although the exact physiological mechanism remains unclear, the results of the
present experiment indicate that this effect is not due to estradiol. Instead, we provide evidence
that a hormonal milieu created in the presence of corpora lutea, quite possibly as a result of
ovarian oxytocin secretion, allows for better transfer of milk from the alveoli to the cistern