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

the CL there were significant differences in milk yield between ewes that had or did not have CL.
Thus, the experimental design of the present study allowed us to produce strong evidence in
support of the hypothesis that the presence of a normal number of CL increases milk production;
however, the mechanism(s) responsible for this increase are not conclusively demonstrated.
It seems logical that the hormonal milieu produced by the CL is responsible for the increase
in milk production and we postulate that an expected increase in circulating OT may be the key
hormonal change that produces the observed increase in milk production. The corpus luteum has
been found to contain high amounts of both mRNA and protein for OT (Jones and Flint, 1988)
and circulating OT concentrations are much greater in ewes with CL than in ewes without CL
(Labussière et al., 1993; Marnet et al., 1998; Schams et al., 1982). Although OT was not measured
in the current study, other researchers have consistently found significantly elevated basal
OT concentrations in ewes with CL as compared to ewes without CL. For example, circulating
OT was extremely low during the 2 d prior to estrus (near lower limit of assay of 3 pg/ml), but
increased to high concentrations (30-60 pg/ml) between d 5 and 10 after estrus (Schams et al.,
1982). Similarly, superovulated ewes with > 6 CL had basal circulating OT concentrations of >
100 pg/ml as compared to 25 to 50 pg/ml in ewes with 1 CL (d 4 to 12 of the estrous cycle) and
these concentrations were greater than the < 10 pg/ml concentration found in ewes with no CL
(Labussière et al., 1993). Numerous studies have shown that exogenous OT treatment increases
milk production in ruminants (Heap et al., 1986; Knight, 1994) and this occurs whether OT is
administered before or after milking (Ballou et al., 1993). Daily treatment of Mehraban ewes
(Zamiri et al., 2001) with 2 IU of OT from d 15 of lactation dramatically increased total milk
yield (+ 55.5%) due to greater average daily milk yield as well as longer lactation length (175 vs.
143 d). Thus, a likely physiological model to explain our results is that the presence of CL
increases circulating OT and this hormone increases milk production through possibly multiple
physiological mechanisms.
Oxytocin is critical for milk transfer from the alveoli to the cistern. Mice with knockout of
the OT gene do not have milk ejection during suckling and pups die soon after birth unless
exogenous OT is provided, whereas other aspects of reproduction and parturition appear normal
in these mice (Young et al., 1996). Very low concentrations of OT are necessary to elicit milk
ejection in ruminants (Gorewit et al., 1983). During machine milking of dairy ewes, plasma OT
concentrations peak between 70 and 200 pg/ml and this OT is associated with ejection of milk
from the alveoli to the cistern (Bruckmaier et al., 1997; Marnet et al., 1998). There is little
scientific literature on the role of OT during the intermilking interval in ruminants. In the goat
(Peaker and Blatchford, 1988) and ewe (McKusick et al., 2001, unpublished data), milk begins to
accumulate in the cistern immediately following milking and continues to fill the cistern in a
linear fashion until 16 to 24 hr, after which milk production begins to reach a plateau. Compared
with the cow, the ewe and goat have relatively larger cisternal storage capacity resulting in
substantial extra-alveolar storage of milk (Bruckmaier et al., 1997; Knight et al., 1994). Movement
of milk from the alveoli to the cistern would result in less alveolar pressure (Labussière,
1988), lower concentrations of local feedback inhibitors of lactation (Wilde et al., 1987), and less
alveolar milk fat (Levy, 1964; Williamson et al., 1995), all conditions that could increase milk
secretion. In addition, there may be direct effects of OT on milk synthesis by mammary epithelial
cells (Ballou et al., 1960; Benson et al., 1960; Ollivier-Bousquet and Courrier, 1976); although,
it is difficult to rule out that stimulatory effects are the result of more efficient removal of
residual alveolar milk by OT and therefore decreased feedback inhibition of lactation in the area