T. Sweeney et D. Ocallaghan, BREEDING-SEASON AND OVULATION RATE IN EWES TREATED WITH LONG DAYS IN SPRING FOLLOWED BY A MELATONIN IMPLANT AND EXPOSURE TO A RAM, Animal Science, 62, 1996, pp. 507-512
While the onset of the breeding season in ewes can be advanced up to 7
weeks following exposure to long days (44 days) before lambing, a mel
atonin implant from lambing and exposure to a ram from the summer sols
tice, this advance is reduced by the absence of a yam. The aim of this
experiment teas to determine if earlier exposure to the ram and/or ex
posure to more long days before lambing would further advance the onse
t of the breeding season. A further aim was to compare the ovulation r
ate and duration of the breeding season between ewes with a normal and
advanced onset of the breeding season due to this treatment regime. P
regnant ewes lambing in mid March were allocated to treatments as foll
ows: (I) controls, maintained on natural photoperiod (2) 44 LD + mel,
exposed to 44 long days (18L:6D; LD) before lambing followed by a 700-
mg melatonin implant after lambing; (3) 90 LD + mel, as in treatment 2
, bur exposed to 90 LD before lambing; (4) 90 LD + mel + rant, as in t
reatment 3, including continuous exposure to a vasectomized from from
10 April. Reproductive state was assessed from serum progesterone conc
entrations in blood collected at 3- to 4-day intervals from 10 April a
nd ovulation rate was determined during the luteal phase of the first
five oestrous cycles in control ewes and ewes exposed to 90 LD + mel ram. Ewes exposed to 90 LD + mel + ram commenced the breeding season
10 weeks earlier (median date 25 July, range 28 days; P < 0.001) than
controls (median date 8 October, range 68 days). Overall, the onset of
the breeding season was not advanced in ewes given 44 or 90 LD + mel
in the absence of a ram, but a biphasic response was evident, with 40
to 50% of ewes beginning the breeding season at the same time as ewes
on 90 LD + mel + ram. Ovulation rate was similar between control ewes
and ewes exposed to 90 LD + mel + ram (P > 0.05). The duration of the
breeding season was longer in ewes exposed to 90 LD + mel + ram (210 (
s.e. 15) days; P < 0.002) than in control ewes (158 (s.e. 10) days), w
ith no difference in date of the end the the breeding season (P > 0.05
). Exposure of ewes to 90 LD before lambing in March, followed by a me
latonin implant and exposure to the ram from 10 April advanced the ons
et and extended the duration of the breeding season without an overall
effect on ovulation rate. The ram is an important component of this s
ignal sequence.