of Pelibuey ewes respond to the small variations in photoperiod that are present at 19 813
X
N, and that under natural conditions, photoperiod appears to be the main regulator of ovarian activity at
this latitude. However, other factors such as temperature or humidity may act as modulators, and their relative importance could increase at more equatorial latitudes. q 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Pelibuey sheep; Reproductive seasonality; Photoperiod; Subtropics
1. Introduction
Photoperiod is the main environmental factor that regulates seasonal breeding in Ž
sheep and goats from northern latitudes Yeates, 1949; Thwaites, 1965; Karsch, 1984; .
Chemineau et al., 1988 . It has been considered that the effects of photoperiod on Ž
. reproductive activity decrease in lower latitudes Hafez, 1952 , so that seasonal repro-
Ž duction is absent or greatly reduced in tropical or subtropical regions Carles and
. Kipngeno, 1986; Eloy et al., 1990 , or that if present, it is mainly due to seasonal
Ž .
variations in food availability Gonzalez et al., 1991; Chemineau et al., 1995
´
In the case of Pelibuey sheep, a breed of hair sheep of African origin, initial studies Ž
conducted in Mexico suggested that reproductive seasonality was absent Castillo et al., .
1972; Valencia et al., 1975 . Some degree of seasonality was later accepted, but it was Ž
. attributed to variations in food availability Gonzalez et al., 1991; Cruz et al., 1994 ,
´
Ž .
temperature or humidity Gonzalez et al., 1992 . However, evidences were soon found
´
that the reduced ovarian activity that occurs during spring is independent from nutri- Ž
. tional status Rodrıguez-Maltos et al., 1992; Velazquez et al., 1995; Martınez, 1998 .
´ ´
´
In the first direct studies of the effects of artificial photoperiod on Pelibuey sheep, we Ž
. found that it strongly influenced ovarian activity in intact ewes Porras et al., 1998 and
Ž .
LH profiles in ovariectomized ewes implanted with oestradiol Porras, 1999 . However, Ž
. the alternate artificial photoperiods used in those studies were classic long 16L:8D or
Ž .
short 8L:16D ones, which are extremes that are not naturally present at the latitudes where Pelibuey sheep are raised. Thus, the objective of this study was to evaluate if the
ovarian activity of Pelibuey ewes is affected by variations in photoperiod that mimic those found in tropical and subtropical regions of Mexico, where the difference between
the longest and the shortest day of the year is only about 2 h. In addition, 24-h melatonin profiles were characterized at different times of the year in order to evaluate the pineal
response to small changes in photoperiod, and the annual profiles of prolactin were characterized in order to evaluate the transduction of the photoperiodic signal to other
physiological end points.
2. Materials and methods
The study was conducted from December 21, 1996 to December 21, 1998, on a farm located at 19
813
X
N. A total of 21 adult Pelibuey ewes were used, 14 of them had never
been exposed to artificial photoperiod, and they were maintained on natural photoperiod Ž
until March 21, 1997, when they were assigned to natural photoperiod control group, .
Ž .
n s 8 or to inverse photoperiod n s 6 . The other seven ewes had been on a previous Ž
. Ž
. experiment Porras et al., 1999 , and had been kept on long photoperiod 16L:8D from
October 21 to December 21, 1996, when they entered the present study and were exposed to a gradual decrease in photoperiod until reaching an equinox photoperiod
Ž .
Ž .
12L:12D on March 21, 1997, when they were assigned to natural n s 3 or inverse Ž
. photoperiod for the rest of the study n s 4 .
Independently of their previous photoperiodic history, starting on March 21, 1997, all the ewes assigned to natural photoperiod were exposed to the photoperiod prevalent at
19 813
X
N, while those on inverse photoperiod were exposed to a daily ligth:dark relation Ž
. opposite to that occurring naturally Yeates, 1949 . The photoperiodic treatments were
given in well ventilated, 7 = 3 m chambers isolated from natural light and provided with Ž
artificial light at an intensity of 350 lx at the height of the animal’s head Legan and .
Ž Karsch, 1980 . Lights were turned on and off as required by a digital switch Tork EW
. 101, Mexico City that allowed for daily adjustments. In both groups, the length of the
longest day of the year was 13 h 6 min, and that of the shortest one was 10 h 54 min, as Ž
. occurs naturally at this latitude Muhlia and Chavez, 1980 . The difference between the
´
longest and the shortest day was 2 h 12 min. The animals were isolated from males throughout the experiment, and they were
maintained on a constant plane of nutrition using oat hay, alfalfa hay, corn silage and concentrate. Heparinized blood samples were obtained from all the ewes twice a week.
The samples were immediately centrifuged and the plasma was separated and kept frozen at y20
8C until assayed for progesterone using a solid-phase radioimmunoassay Ž
. Pulido et al., 1991 . Sensitivity of the assay was 0.15 ngrml; intra- and inter-assay
variation coefficients were 5.0 and 9.5, respectively. Prolactin concentrations were determined in the samples from the second year of the study of five ewes on natural
photoperiod and five ewes on inverse photoperiod. A homologous radioimmunoassay Ž
. was used Porras, 1999 . The sensitivity of the assay was 0.25 ngrml, the intra- and
inter-assay coefficients were 5.8 and 12.5, respectively. Blood samples for melatonin determination were obtained from five ewes on natural
photoperiod and five ewes on inverse photoperiod on September 21, 1997, December 21, 1997, March 21, 1998, and June 21, 1998,. Each time the sampling protocol
consisted of a 24-h period during which samples were obtained every hour, except 2 h before and after the lights were turned on or off, when the intervals were reduced to 30
Ž .
Ž .
min. Red light 1 lx was used for sampling during dark periods Malpaux et al., 1987 . Ž
. The assay used
Malpaux et al., 1987 had a sensitivity of 4 pgrml; intra- and
inter-assay variation coefficients were 7.5 and 13.7. The duration of the nightly melatonin elevation was defined as the period during which its concentrations were
above 16 pgrml, i.e. four times the sensitivity of the assay. Ovulation was assumed to occur 4 days before the first of two or more consecutive
Ž .
samples with progesterone concentrations over 1 ngrml Rodrıguez-Maltos et al., 1992 .
´
A luteal phase was comprised by all the consecutive samples above 1 ngrml. An ovulatory season was defined as a period of regular ovulations, with less than 15 days
between the end of a luteal phase and the next ovulation. It was considered to begin on
C. Cerna
et al.
r Animal
Reproduction
Science 60
– 61
2000 511
– 525
514
Table 1 Mean dates for the onset and the end of the first ovulatory season and for the onset of the second ovulatory season in Pelibuey ewes maintained under natural or
inverse photoperiod, according to their previous photoperiodic history Values are mean datestandard error.
Ž .
Ž .
Ž .
For a given variable row , values with different literal superscripts a, b, c are statistically different P - 0.05 . Ewes without previous exposure to artificial photoperiod
Ewes with previous exposure to artificial photoperiod Ž
. Ž
. Ž
. Natural photoperiod
Inverse photoperiod ns6 Natural photoperiod ns 3
Inverse photoperiod ns 4 Ž
. control group, ns8
First oÕulatory season
c b
a a
Ž .
Ž .
Ž .
Ž .
Ž .
Onset date July 31, 1997
5.8 days June 16, 1997
7.0 days April 3, 1997
7.5 days March 22, 1997
13.5 days
b a
c a
Ž .
Ž .
Ž .
Ž .
Ž .
End date February 21, 1998
6.7 days November 19, 1997
31.4 days March 15, 1998
2.3 days December 25, 1997
26.5 days Second oÕulatory season
b a
b a
Ž .
Ž .
Ž .
Ž .
Ž .
Onset date July 19, 1998
7.0 days March 4, 1998
7.6 days July 27, 1998
17.2 days February 8, 1998
32.3 days Excludes two ewes with a first ovulatory season that extended until the end of the study.
Excludes one ewe that ovulated continuously for more than 18 months until it died.
C. Cerna
et al.
r Animal
Reproduction
Science 60
– 61
2000 511
– 525
515 Table 2
Interval from longest day to first ovulation, from shortest day to last ovulation, and duration of the ovulatory and anovulatory season in Pelibuey ewes exposed to different photoperiodic treatments
Values are meanstandard error. Ž
. Ž
. Ž
. For a given variable row , values that do not share at least one literal superscripts a, b, c are different P - 0.05 .
Ewes without previous exposure Ewes with previous exposure
to artificial photoperiod to artificial photoperiod
Natural photoperiod Inverse photoperiod
Natural photoperiod Inverse photoperiod
Ž .
Ž .
Ž .
Ž .
control group, ns8 ns6
ns 3 ns 4
First year Ž .
Length h of longest day 13 h 6 min
12 h 16 h
16 h
a b
b ab
Ž .
Interval day from longest day to first ovulation 406
876 837
7113 Ž .
Length h of shortest day 10 h 54 min
10 h 54 min 10 h 54 min
10 h 54 min
a c
b c
Ž .
Ž .
Ž .
Interval day from shortest day to last ovulation 467
14431 ns 4
842 17026
ns 3
a b
b
Ž .
Length day of first ovulatory season 2049
28988 34510
28632
b ab
b a
Ž .
Ž .
Ž .
Length day of first anovulatory season 1487
6335 ns 4
13419 4321
ns 3 Second year
Ž . Length h of longest day
13 h 6 min 13 h 6 min
13 h 6 min 13 h 6 min
a a
a a
Ž .
Ž .
Ž .
Interval day from longest day to first ovulation 287.0
7328 ns 4
3617.2 4932
ns 3 Does not include two animals that continued ovulating until the end of the study.
Does not include one ewe that continued ovulating until it died. Two animals in this group continued ovulating at the end of the study.
the day of the first ovulation and to finish with the end of the last luteal phase. An anovulatory season was defined as a period of 20 or more days with basal progesterone
concentrations. Ž
. An analysis of variance that considered the effects of photoperiod natural or inverse
Ž .
and the previous history with or without previous exposure to artificial photoperiod
Fig. 1. Individual ovulatory seasons in Pelibuey ewes maintained under natural or inverse photoperiod. The animals on the inverse photoperiod group were kept on natural photoperiod until March 21, 1997, when they
were transferred to a photoperiod with a light:dark relation opposite to that found naturally at 19 813
X
N. Each bar represents an ovulatory season for the ewe identified by the number at the left. The tick marks are located
on the 15th day of each month.
was used to compare the dates for the onset and end of each ovulatory season, the intervals from the longest day to first ovulation and from the shortest day to the last
ovulation, as well as the duration of the first ovulatory season, and that of the anovulatory season. The duration of melatonin elevations was compared by a two-way
Ž .
analysis of variance treatment and date . Prolactin concentrations were compared using an analysis of variance that considered the effects of treatment and month, with the
animal nested within the treatment.
Fig. 2. Individual ovulatory seasons in Pelibuey ewes that had been previously exposed to 3 months of long Ž
. 16L:8D photoperiod before gradually reducing it starting on December 22, 1996 in order to reach an equinox
Ž
X
. photoperiod on March 21, 1997. Thereafter, the ewes were assigned to natural or inverse
19 813 N
photoperiod. Each bar represents an ovulatory season for the ewe identified by the number at the left. Ewe N 8
20 had uninterrupted ovarian cycles until its death from causes not related to the study. The tick marks are located on the 15th day of each month.
3. Results
