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

were higher for lactating ewes during their normal estrous season (fall) compared to ewes lactating
during anestrus (spring). Labussière et al. (1993, 1996) demonstrated a significant positive
correlation between the number of CL present and the volume of milk obtained at milking in
superovulated Lacaune dairy ewes. The East Friesian is highly prolific (McKusick et al., 1999a),
and the increased number of CL combined with the large cisternal volume may allow this breed
to markedly benefit from an effect of CL on milk production.
The objective of the present experiment was to study the effect of a relatively normal number
of CL for the East Friesian breed on milk production and composition during mid-lactation. The
experiment was designed to remove the potential stimulatory effect of estradiol (E2) on milk
production by maintaining high serum progesterone (P4) concentrations during the treatment
period with the aid of intravaginal P4.
Materials and methods
Twenty-four second parity East Friesian crossbred dairy ewes in mid-lactation (77 ± 6 d,
mean ± SD) of similar body weight and daily milk production (81 ± 8 kg and 1.9 ± 0.6 kg/d,
respectively, mean ± SD) were studied during the summer of 1999. Ewes were selected from the
University of Wisconsin-Madison’s main dairy ewe flock of 350 ewes at the Spooner Agricultural
Research Station based on their lambing date and daily milk production. Ewes were housed
in four separate pens in an indoor laboratory facility on the University of Wisconsin-Madison
campus, fed a 16% crude protein grain concentrate and alfalfa haylage, and maintained under
constant anestrous light conditions (16:8 hr of light to dark).
A schematic diagram of the experimental design is shown in Figure 1. Immediately prior to
the experiment, ewes were artificially synchronized for estrus and induced to ovulate by treatment
for 14 d with a controlled intravaginal drug-releasing device containing 330 mg of P4
(CIDR, InterAg, New Zealand) from d –17 to –3, relative to expected ovulation (d 0). Twelve
hours prior to CIDR removal, 600 IU of pregnant mare serum gonadotropin (PMSG, Sioux
Biochemical, Inc., Sioux Center, IA), and 250 mg of PGF2a (cloprostenol sodium, Estrumate,
Bayer Corp., USA) were administered.
The experiment was divided into three time periods: d 0 to 5 (pre-treatment), d 6 to 18
(treatment), and d 19 to 25 (post-treatment). Laparoscopy was performed on d 4 for the purpose
of counting CL and to affirm that ovulation had occurred. On d 5, ewes were ranked according
to number of CL and alternately assigned to one of two treatments: injections at 0630 and 1630
of either saline (CLY, n = 12) or 250 mg of PGF 2α (CLN, n = 12) to allow persistence or regression,
respectively, of CL during the treatment period. In addition to the treatments, all ewes in
both treatment groups received two CIDRs for the duration of the treatment period in order to
maintain circulating P4 at normal, or above, luteal phase concentrations. On d 11, laparoscopy
was performed to confirm luteal persistence in CLY ewes and luteolysis in CLN ewes. On d 18,
CIDRs were removed and all ewes received an injection of 250 mg of PGF 2α .
Machine milking took place at 0630 and 1630 in groups of 6 ewes on a portable milking
platform with cascading head stanchions. The milking machine (Coburn Co., Inc., Whitewater,
WI and Interpuls Inc., Albinea, Italy) was set to provide 180 pulsations per minute in a 50:50
ratio with a vacuum level of 37 kPa. Individual ewe milk production and milking time were
recorded at every milking (d 1 to 25) and milk samples were collected periodically at the morning
milking (d 4, 5, 6, 7, 8, 9, 13, 14, 15, 18, 25). Average milk flow rate was calculated by