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

CAN THE OVARY INFLUENCE MILK PRODUCTION IN DAIRY EWES?
Brett C. McKusick 1 , Milo C. Wiltbank 2 , Roberto Sartori 2 , Pierre-Guy Marnet 3 ,
and David L. Thomas 1
Departments of Animal Sciences 1 and Dairy Science 2
University of Wisconsin-Madison, Madison
3 Institut National de la Recherche Agronomique, UMR Production du Lait,
Rennes, France
Abstract
In dairy ewes with large cisternal storage capacity, such as the East Friesian, we hypothesize
that milk transfer between milkings from the alveoli to the cisterns, and therefore overall milk
production and composition, might be improved by the hormonal milieu created by the presence
of corpora lutea (CL). Furthermore, we wanted to evaluate this effect with the number of CL
that would be typically present during the estrous season and without the potential stimulatory
effect of estradiol (E2) on milk production. Mid-lactation East Friesian crossbred ewes (n = 24)
were synchronized for estrus with intravaginal progesterone (CIDR), PGF 2α , and gonadotropins.
Following ovulation, CL were counted via laparoscopy on d 4, and the ewes were studied during
three time periods (pre-treatment: d 0 to 5; treatment: d 6 to 18; post-treatment: d 19 to 25). On
d 5, ewes received a treatment of either saline (CLY, n = 12) or PGF 2α (CLN, n = 12) to allow CL
persistence or regression, respectively. Additionally, all ewes received two CIDRs during the
treatment period to provide high concentrations of plasma progesterone (P4). All ewes received
PGF2 2α on d 18. Milk yield and milk flow rate were recorded daily, milk samples were obtained
periodically for analyses of milk fat and protein, and jugular blood samples were collected for P4
and E2 immunoassay. During the treatment period, CLY ewes had higher daily milk yield (1.56
vs. 1.44 kg/d) and milk flow rate (244 vs. 208 ml/min) and produced more milk fat (92.2 vs. 81.1
g/d) and milk protein (83.7 vs. 77.5 g/d) compared to CLN ewes, respectively. These trends
were maintained during the post-treatment period, despite luteolysis in CLY ewes. Thus, milk
production was increased in East Friesian ewes due to the presence of CL, consistent with other
reports of a putative role of luteal oxytocin (OT) in milk transfer between milkings and/or a
direct effect of OT on secretory epithelium.
Introduction
In species with large cisternal storage capacity, such as the dairy ewe, milk transfer from the
alveoli to the cistern between milkings may improve milk yield by reducing the concentration of
negative feedback inhibitors of lactation in the area of the alveoli (Wilde et al., 1987), and may
improve efficiency of milk removal due to increased udder filling (Bruckmaier, 2001) or by
allowing for increased cisternal intramammary pressure, thereby increasing milk flow rate
(McKusick and Marnet, 2001, unpublished data). Compared to most other dairy sheep breeds,
the East Friesian has relatively larger cisterns (Bruckmaier et al., 1997; McKusick et al., 1999b)
and this may provide increased milk storage between milkings.
Although the mechanism by which milk is transferred between milkings from the alveoli to
the cistern is not completely clear, it has been hypothesized that oxytocin (OT) secreted from the
corpus luteum (CL) might be responsible for myoepithelial contraction and could play a significant
role in dairy ewes. Marnet et al. (1998) showed that baseline plasma OT concentrations
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