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Time of onset and potential length of the breeding season of Icelandic sheep: Luteal activity
Animal Reproduction Science, 34 ( 1993 ) 101-109
101
0378-4320/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Time of onset and potential length of the breeding season of Icelandic sheep: Luteal activity J. Eldon Institute for Experimental Pathology, Universityof Iceland, Keldur, IS- I 12 Reykjavik, Iceland (Accepted 3 March 1993 )
Abstract
A total of 40 mature Icelandic ewes were studied during four breeding seasons. Blood samples were collected at regular intervals and the progesterone concentrations assayed. The progesterone profiles were evaluated and the time of onset, cessation and duration of the breeding season as well as the pattern of the luteal function in non-pregnant and pregnant ewes was established. On average, the onset of the breeding season occurred on 15 November and its cessation on 20 February. The length of the breeding season was 97 + 24 days. The earliest onset was in October and ovarian cyclicity ceased in April in the ewes with the longest breeding period. The onset of cyclicity spanned 53 days and its cessation 90 days. In 90% of the ewes the onset spanned 28 days and the cessation 70 days. In relation to daylength, all the ewes were cycling during the period when the centre of the sun was 5 ° above the horizon at noon with decreasing daylength until it was 5 ° above the horizon with increasing daylength (sunrise-sunset 5.5 h). The length of the ovarian cycle was 16 + 2 days. The length of the luteal phase was 11 + 2 days and was not significantly different between cycles but the mean progesterone concentration of the luteal phase of the first ovarian cycle was lower and the area under the progesterone curve was smaller (P< 0.01 ) than in successive cycles. A transient rise in progesterone (3-5 nmol 1- t for 5 days or less ) preceding a full length ovarian cycle was seen in 30% of the ewes. The average number of luteal phases during the breeding period was 5.5 + 1.5. The length of the interluteal or follicular phase was 5 + 1 days and did not vary significantly between ovarian cycles. The gestation length was 138-148 days and during the last 50-70 days of gestation progesterone increased four- to six-fold owing to placental production.
Introduction
The breeding season of sheep in the Northern Hemisphere is a well defined time period of 100-200 days starting during the last trimester of the year. With decreasing latitude the breeding season becomes extended over a larger part of the year until a defined breeding season disappears (Bronson, 1989; Lincoln et al., 1990) The Icelandic sheep is a seasonal breeder and is bred in December and early
102
:. Eldon / AnimaI Reproduction Science 34 (1993) 101-109
January so that lambing occurs in spring (D~rmundsson, 1978; Adalsteinsson, 1981 ). In light of the controlling influence of daylength on reproduction in sheep, the aim of the present study was to evaluate the time of onset and length of the breeding season as well as the luteal activity of Icelandic sheep, which live under conditions of extreme changes in seasonal daylength. The length of the luteal and interluteal phases during the oestrus cycle of non-pregnant ewes and the changes in progesterone concentration from conception to birth are described. Materials and methods
A total of 40 Icelandic ewes was studied during four breeding seasons. The ewes were housed at the Institute for Experimental Pathology at Keldur, Reykjavik (64 ° 10'N, 22°W). The ewes were kept under natural seasonal variations of daylight and housed in a bright (approximately 1000 lx at noon; no artificial light), unheated and well-ventilated barn and fed hay twice a day. Fresh water and mineral stone was always available. No rams were housed near the ewes. To evaluate the changes in progesterone concentration during pregnancy, five ewes were housed separately with a fertile ram from 1 December to 15 February. Blood was collected from the vena jugularis three times a week from the beginning of September until the end of May. Blood was also collected daily, from all ewes, for 30 days during each breeding season (no fixed period). The blood was centrifuged within 1 h of collection and the serum was stored at - 20 ° C until assayed. The serum was assayed for progesterone concentration ( 100 gl per tube) using Coat-A-Count ® solid-phase, direct (no extraction of sample) radioimmunoassay (RIA) kit (Diagnostic Products Corporation, Los Angeles, CA). Detection limits of the assay were 0.16 nmol l - ~. Inter- and intra-assay coefficients of variation (CV) were 9.3% and 7.5%, respectively for a True-Value ® control sample containing 4.8 _+0.7 nmol 1-~. The mean of 50 assays of this control sample in 50 different assay runs was 4.77 _+0.4 nmol l- ~. The date of onset of ovarian cyclicity was the sample date before progesterone rose above 3 nmol 1-~ in two successive serum samples (English et al., 1986). The length of the luteal phase was evaluated from the day on which progesterone rose above 3 nmol l-1 to the day before it fell below the same value. The interluteal phase was estimated as the interval between the luteal phases. The area under the progesterone curve was calculated using the formula fax+ bdx for each part of the curve that could be expressed by a straight line and then adding up the parts. For a numerical evaluation the number of days from the autumn equinox to the onset and cessation of the breeding season was used. The data were evaluated using Student's t-test and analysis of
103
J. Eldon / Animal Reproduction Science 34 (1993) 101-109
variance, ANOVA (Sokal and Rohlf, 1981 ). The height of the sun above the horizon at noon is from Saemundsson (1990). During the s u m m e r solstice (21 June) the centre of the sun at noon rises 49 ° above the horizon and the time from sunrise to sunset is 21 h. During the winter solstice (21 December) the centre of the sun rises 2.7 ° above the horizon at noon and the time from sunrise to sunset is 4 h. Results
Breeding season The times of onset and cessation, and the length of the breeding season are presented in Table 1 and Fig. 1. The mean date of onset was 15 November and spanned 53 days from 6 October to 28 November. Two ewes (5%) started ovarian cyclicity in October (6th and 19th) and the rest (95%) started cyclicity in November. The mean date of cessation of seasonal ovarian cyclicity was 20 February and spanned 90 days from 15 January to 15 April. Seventyfive percent of the ewes had ceased cycling by 9 March and 90% by 29 March. The mean length of the breeding season was 97 _+24 days. The interval from the onset of ovarian activity in the first ewe to start and the time of cessation in the last ewe to cease seasonal ovarian activity was 191 days. All the ewes were cycling at the same time for 48 days from 28 November to 15 January. There was no correlation between the time of onset of cyclicity in the number of days from the a u t u m n equinox and the length of the breeding season for each individual ewe. In relation to daylength and the height of the sun, the first ewe started cycling on 6 October when the centre of the sun rose 21 ° above the horizon at noon and the time from sunrise to sunset was 11 h. On the mean date of onset of cyclicity ( 15 November), the sun rose 7.5 ° above the horizon and the time from sunrise to sunset was 6.5 h. All ewes were cycling when the sun rose 5 ° above the horizon at noon (28 November, sunrise-sunset 5.5 h). When the sun had reached 5 ° at noon with increasing daylength ( 15 January), the first Table 1 The mean date of onset and cessation of ovarian cyclicity ( + SD, days) and mean ( + SD) length of the breeding season (days) 1988 n 12 Onset 14Nov.(_+7) Cessation 22Feb.(+29) Length (days) 105 ( + 2 4 ) NS, not significant.
1989
1990
1991
8 15Nov.(_+9) 13Feb.(+30) 92(+29)
8 12 13Nov.(_+10) 17Nov.(_+9) 20Feb.(_+23) 21Feb.(_+24) 103(_+27) 92 (_+23)
All years 40 15Nov.(+_8)NS 20Feb.(+25)NS 97 (_+24)NS
104
J. Eldon / Animal Reproduction Science 34 (1993) 101-109
The sunangle above the horizon at noon and the reproductive period
I
~11~ Sunangle
-o-- No. of ewes
I
50
~C, OGGG~
45
k
cycling
%
_~
/-""
100
,o
40 35
7O
30
6O
25
5O
20
4O
15
3O
10 5
20
0 Jul.
Sep.
Nov.
Jan.
Mar.
May
Fig. 1. The reproductive period. Sun angle: degrees of the centre of the sun above the horizon at noon in Reykjavlk (64 ° 10'N, 22°W). Number of ewes cycling: the percentage of ewes with cyclic ovarian activity (n--40). Onset, cessation: the onset and cessation of seasonal luteal activity.
ewes stopped cycling. The last ewe stopped ovarian cyclicity when the sun had reached 36 ° above the horizon ( 15 April, sunrise-sunset 15 h; Fig. 1 ).
Ovarian activity The length of the ovarian cycle was 16 _+2 days. There was no difference between cycles ( P > 0.05 ). The length of the luteal phase of the ovarian cycles was 11 _+2 days and the length of the interluteal phases was 5 + 1 day. There were no differences in the lengths of the luteal or interluteal phases between ovarian cycles ( P > 0.05 ). The average progesterone concentrations during the luteal phases of the first, second and third ovarian cycles of the breeding season were 6.9 _+ 1.5 nmol 1- ~, 7.5 + 1.6 nmol 1-1 and 7.7 _ 1.5 nmol 1-1, respectively (mean + SD, n = 200 ). The difference was statistically significant. The area under the progesterone curve during the luteal phase of the first ovarian cycle was less than those of the second and third ovarian cycles ( P < 0.01 ). In 40% of the ewes, the mean progesterone concentration during the luteal phases increased after the third ovarian cycle of the breeding season and decreased again during the last two ovarian cycles. In 25% of the ewes, the progesterone concentration decreased from the first ovarian cycle to the last; in 20% of the ewes it was similar during all ovarian cycles and in 15% of the ewes the mean progester-
Z F,ldon/Animal Reproduction Science 34 (1993) 101-109
105
one concentration increased from the first to the last ovarian cycle of the breeding season. There was a marked difference in the amplitude of the luteal phase progesterone curve between ewes. According to the results from the daily samples the progesterone concentration increased from 3 to 10 nmol l- 1 in 3 days during the onset of the luteal phase and fell from 15 to below 3 nmol l- 1 in 2.5 days at the end of the luteal phase. Thirty percent of the ewes had a transient rise in progesterone (3-5 nmol 1- ~for 5 days or less ) 10-30 days before regular cyclicity started. The number of luteal phases during the breeding season was 5.5_ 1.5. Three of five ewes that were housed with a ram conceived and had gestation periods of 138, 143 and 148 days. Each ewe gave birth to one lamb. Figure 2 shows the changes that occurred in the progesterone profiles during pregnancy. Progesterone rose to a plateau of 10-15 nmol l- 1, where it stayed for the first 70-80 days postconception with peaks reaching over 20 nmol 1-1. In two ewes, the progesterone level dropped to 5 nmol l - 1 for about 5 days 70-80 days post-conception. However, 80-100 days post-conception the progesterone level rose sharply to a plateau of 30-40 nmol l- 1 with peaks of up to 60 nmol 1-1. At birth the progesterone value had dropped below the sensitivity of the assay. The drop from peak value took 5-6 days. Progesterone nmol/I 70
tI I
Ewe _no:__
60 50
I-*-33
/
t ~
40
,
,
c 30 20 10 0 0
10
20
30
40
50
60
70
80
90
100 110 120 130 140
50
Days from conception
Fig. 2. The progesterone profiles during the gestation period of Ewes 30, 31 and 33: C, time of conception; D, drop in progesterone concentration to 5 nmol l-1 (see text); B, birth.
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J. Eldon / Animal Reproduction Science 34 (1993) 101-109
Discussion The average date of the onset of the breeding season was 15 November. This is 23 days earlier than reported by D~rmundsson ( 1978 ) for ewes kept under 'field conditions' and accompanied by vasectomised rams equipped with a raddle harness and crayon. The use of vasectomised rams should have advanced the onset of the breeding season, as other authors have reported that the introduction of rams towards the end of the seasonal anoestrous period significantly advances the breeding season in ewes (Martin et al., 1986; Robinson et al., 1991; J. Eldon, unpublished data, 1991 ). The use of artificial light during the morning and afternoon, as is done under 'field conditions' in Iceland, disrupts the effects of natural photoperiod and may delay the onset of the breeding season. The fact that the ewes do not display oestrus preceding the first and even the second ovulation, may also be a reason for the difference between these two studies (Hammond, 1944; I'Anson and Legan, 1988; Schirar et al., 1989; Wayne et al., 1989). It is generally accepted that the decrease in daylength has an influence on the onset of seasonal ovarian activity either directly (Jackson et al., 1989; Khalid and Jackson, 1991 ) or indirectly (Malpaux and Karsch, 1990 ). The small difference between years in average date of onset of ovarian cyclicity and the fact that 90% of the ewes started cycling within 28 days in November, emphasise the close synchronisation between the chronological change in daylength and the physiological response of the reproductive system to that change. The lack of significance between years in the times of onset and cessation of the breeding season and its duration, contrasts with the findings of Lees (1966), who found a significant variation between seasons in the time of onset of breeding activity in British breeds of sheep. The cessation of the breeding season spanned 90 days with a mean date of 20 February. This shows that the cessation of ovarian cyclicity owing to the onset of photorefractoriness during an increase in daylength is a slower process than the onset of ovarian cyclicity in response to decrease in daylength. The average length of the breeding season was 97 _+24 days. All the ewes were cycling for 48 days during the period while the angle of the centre of the sun at noon was 5 ° or less above the horizon. This period has the shortest daylength and consequently the longest night, which, through its effect on melatonin synthesis in the pineal gland, is probably the controlling factor rather than the angle of the sun during daylight (English et al., 1986; Malpaux et al., 1988; Krause and Dubocovich, 1990). D~,rmundsson ( 1978 ) reported the duration of the breeding season in Icelandic sheep to be 146 _+16 days, which is 49 days or three ovarian cycles longer than in the present study. The reasons for this difference are probably the same as those discussed earlier for the difference in the onset of the breeding season, i.e. the use of artificial light and ram effect. In comparison, the duration of the breeding season of the
J. Eldon / Animal Reproduction Science 34 (1993) 101-109
107
European wild sheep (Mouflon, Ovis musimon) and the chief breeds in Norway (Cheviot, Dala, Rygja and Old Norwegian) is 3 months (October-December) and the duration of the breeding season of six British breeds is 166 days with 9.1 oestrus periods (Hafez, 1952). The length of the ovarian cycle ( 16 _+2 days) was in agreement with earlier studies for other sheep breeds (Hammond, 1944; Zarco et al., 1988; Wathes et al., 1989 ). There was no difference in the length of the luteal phases of the ovarian cycles ( 11 _ 2 days). The difference in the area under the progesterone curve during the luteal phases of the first three ovarian cycles of the breeding period is a result of differences in progesterone concentration. Ashworth et al. (1989) stated that corpora lutea which secrete low amounts of progesterone may be formed from follicles with impeded morphology and/or physiology. Low progesterone of short duration contributes to reduced functional capacity of luteal tissue during the onset of ovarian activity in cows following post-partum anoestrus (Eldon, 1988, 1991 ). A transient rise in progesterone was found in 30% of the ewes. I'Anson and Legan (1988) stated that most ewes had several luteinising hormone (LH) surges preceding the first full-length luteal phase and that the first LH surge induced a transient rise in progesterone. The pattern of luteal function was diverse between ovarian cycles and between ewes. This could be a factor affecting embryo survival during early gestation (Ashworth et al., 1989). The duration of the rise in luteal phase progesterone to a plateau at the beginning of the luteal phase was 3 days, as was the duration of the drop from this level to below luteal phase concentration. In comparison, in the dairy cow, the rise in progesterone to a plateau takes 5 days while the drop from the plateau to below luteal phase value took 3 days (Eldon, 1991 ). The speed of the progesterone rise and its amplitude is an indication of the activity of the corpus luteum, as the speed of the drop reflects the functional status of the mechanism ofluteolysis (Kindahl et al., 1980, 1981 ). All three pregnant ewes had similar changes in progesterone profile. The drop in progesterone approximately 70 days post-conception is probably a result of decreased output by the corpus luteum and may act as a signal to the placenta to start progesterone production. The sharp rise in progesterone 80100 days post-conception to values four- to six-fold greater than during the luteal phase is due to substantial production by the placenta (Norris et al., 1989 ). The fall in progesterone concentration from peak level to levels below the sensitivity of the assay took 5-6 days, a slightly shorter time than the 610 days reported by Jainudeen and Hafez (1980), a discrepancy which may represent species differences.
Acknowledgement I am indebted to Dr. E. Gunnarson for his technical help. This study was supported by the Icelandic Science Fund.
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J. Eldon / Animal Reproduction Science 34 (1993) 101-109
References Aralsteinsson, S., 1981. Saurkindin, landi6 opj66in (The sheep, the country and the people). Bjallan, Reykjavik, 119 pp. Ashworth, C.J., Sales, D.I. and Wilmut, I., 1989. Evidence of an association between the survival of embryos and the periovulatory plasma progesterone concentration in the ewe. J. Reprod. Fertil., 87: 23-32. Bronson, F.H., 1989. Mammalian Reproductive Biology. University of Chicago Press, IL, 325 PP. D~rmundsson, O.R., 1978. Studies on the breeding season of Icelandic ewes and ewe lambs. J. Agric. Sci., 90: 275-281. Eldon, J., 1988. The post-partum ovarian activity of the Icelandic dairy cow. J. Vet. Med. Assoc., 35: 277-284. Eldon, J., 1991. Post-partum and post-conceptional ovarian activity of dairy cows: Evaluation based on progesterone profiles. Acta. Vet. Scand., 32: 377-386. English, J., Poulton, A.L., Arendt, J. and Symons, A.M., 1986. A comparison of the efficiency of melatonin treatments in advancing oestrus in ewes. J. Reprod. Fertil., 77: 321- 327. Hafez, E.S.E., 1952. Studies on the breeding season and reproduction of the ewe. J. Agric. Sci., 42:189-265. Hammond, J., 1944. On the breeding season in the sheep. J. Agric. Sci., 34: 97-105. I'Anson, H. and Legan, S.J., 1988. Does the first LH surge of the breeding season initiate the first full-length cycle in the ewe? J. Reprod. Fertil., 82: 761-767. Jackson, G.L., Jansen, H.T., Kuehl, D.E. and Shanks, R.D., 1989. Time of the sidereal year affects responsiveness to the phase-resetting effects of photoperiod in the ewe. J. Reprod. Fertil., 85: 221-227. Jainudeen, M.R. and Hafez, E.S.E., 1980. Gestation, prenatal physiology and parturition. In: E.S.E. Hafez (Editor), Reproduction in Farm Animals. Lea and Febiger, Philadelphia, PA, pp. 247-283. Khalid, M. and Jackson, G.L., 1991. Exposure of ewes to long-day photoperiods before the winter solstice can disrupt refractoriness to short days. Anim. Reprod. Sci., 25: 225-232. Kindahl, H., Lindell, J-O. and Edqvist, L-E., 1980. On the control of prostaglandin release during the bovine estrous cycle. Effects of cloprostenol injections. Nord. Veterinaermed., 32: 467-473. Kindahl, H., Lindell, J-O. and Edqvist, L-E., 1981. Release of prostaglandin F2, during the oestrous cycle. Acta Vet. Scand. Suppl., 77: 143-158. Krause, D.N. and Dubocovich, M.L., 1990. Regulatory sites in the melatonin system of mammals. Trends in Neurosciences, 13: 464-470. Lees, J.L., 1966. Variations in the time of onset of the breeding season in Clun ewes. J. Agric. Sci., 67: 173-179. Lincoln, G.A., Lincoln, C.E. and McNeilly, A.S., 1990. Seasonal cycles in the blood plasma concentration of FSH, inhibin and testosterone, and testicular size in rams of wild, feral and domesticated breeds of sheep. J. Reprod. Fertil., 88: 623-633. Malpaux, B. and Karsch, F.J., 1990. A role for short days in sustaining seasonal reproductive activity in the ewe. J. Reprod. Fertil., 90: 555-562. Malpaux, B., Robinson, J.E., Brown, M.B. and Karsch, F.J., 1988. Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe. J. Reprod. Fertil., 83: 461-470. Martin, G.B., Oldham, C.M., Cognir, Y. and Pearce, D.T., 1986. The physiological responses of anovulatory ewes to the introduction of rams--a review. Livest. Prod. Sci., 15:219-247. Norris, T.A., Weesner, G.D., Shelton, J.M., Harms, P.G. and Forrest, D.W., 1989. Biological
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activity of LH during the peripartum period and the estrous cycle of the ewe. Domest. Anita. Endocrinol., 6: 25-33. Robinson, J.J., Wigzell, S., Aitken, R.P., Wallace, J.M., Ireland, J. and Robertson, I.S., 1991. The modifying effects of melatonin, ram exposure and plane of nutrition on the onset of ovarian activity, ovulation rate and the endocrine status of ewes. Anim. Reprod. Sci., 26: 73-91. Schirar, A., Meusnier, C., Paly, J., Levasseur, M.C. and Martinet, J., 1989. Resumption of ovarian activity in post-partum ewes: Role of the uterus. Anim. Reprod. Sci., 19: 79-89. Sokal, R.R. and Rohlf, F.J., 1981. Biometry, 2nd edn. W.H. Freeman, New York, 859 pp. S~emundsson, Th., 1990. Almanak um ~irid 1990 (A calendar for the year 1990). University of Iceland, 96 pp. Wathes, D.C., Ayad, V.J., McGoff, S.A. and Morgan, K.L., 1989. Effect of active immunization against oxytocin on gonadotrophin secretion and the establishment of pregnancy in the ewe. J. Reprod. Fertil., 86: 653-664. Wayne, N.L., Malpaux, B. and Karsch, F.J., 1989. Social cues play a role in timing onset of the breeding season of the ewe. J. Reprod. Fertil., 87:707-713. Zarco, L., Stabenfeldt, G.H., Quirke, J.F., Kindahl, H. and Bradford, G.E., 1988. Release of prostaglandin F-2a and the timing of events associated with luteolysis in ewes with oestrous cycles of different lengths. J. Reprod. Fertil., 83:517-526.
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0378-4320/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Time of onset and potential length of the breeding season of Icelandic sheep: Luteal activity J. Eldon Institute for Experimental Pathology, Universityof Iceland, Keldur, IS- I 12 Reykjavik, Iceland (Accepted 3 March 1993 )
Abstract
A total of 40 mature Icelandic ewes were studied during four breeding seasons. Blood samples were collected at regular intervals and the progesterone concentrations assayed. The progesterone profiles were evaluated and the time of onset, cessation and duration of the breeding season as well as the pattern of the luteal function in non-pregnant and pregnant ewes was established. On average, the onset of the breeding season occurred on 15 November and its cessation on 20 February. The length of the breeding season was 97 + 24 days. The earliest onset was in October and ovarian cyclicity ceased in April in the ewes with the longest breeding period. The onset of cyclicity spanned 53 days and its cessation 90 days. In 90% of the ewes the onset spanned 28 days and the cessation 70 days. In relation to daylength, all the ewes were cycling during the period when the centre of the sun was 5 ° above the horizon at noon with decreasing daylength until it was 5 ° above the horizon with increasing daylength (sunrise-sunset 5.5 h). The length of the ovarian cycle was 16 + 2 days. The length of the luteal phase was 11 + 2 days and was not significantly different between cycles but the mean progesterone concentration of the luteal phase of the first ovarian cycle was lower and the area under the progesterone curve was smaller (P< 0.01 ) than in successive cycles. A transient rise in progesterone (3-5 nmol 1- t for 5 days or less ) preceding a full length ovarian cycle was seen in 30% of the ewes. The average number of luteal phases during the breeding period was 5.5 + 1.5. The length of the interluteal or follicular phase was 5 + 1 days and did not vary significantly between ovarian cycles. The gestation length was 138-148 days and during the last 50-70 days of gestation progesterone increased four- to six-fold owing to placental production.
Introduction
The breeding season of sheep in the Northern Hemisphere is a well defined time period of 100-200 days starting during the last trimester of the year. With decreasing latitude the breeding season becomes extended over a larger part of the year until a defined breeding season disappears (Bronson, 1989; Lincoln et al., 1990) The Icelandic sheep is a seasonal breeder and is bred in December and early
102
:. Eldon / AnimaI Reproduction Science 34 (1993) 101-109
January so that lambing occurs in spring (D~rmundsson, 1978; Adalsteinsson, 1981 ). In light of the controlling influence of daylength on reproduction in sheep, the aim of the present study was to evaluate the time of onset and length of the breeding season as well as the luteal activity of Icelandic sheep, which live under conditions of extreme changes in seasonal daylength. The length of the luteal and interluteal phases during the oestrus cycle of non-pregnant ewes and the changes in progesterone concentration from conception to birth are described. Materials and methods
A total of 40 Icelandic ewes was studied during four breeding seasons. The ewes were housed at the Institute for Experimental Pathology at Keldur, Reykjavik (64 ° 10'N, 22°W). The ewes were kept under natural seasonal variations of daylight and housed in a bright (approximately 1000 lx at noon; no artificial light), unheated and well-ventilated barn and fed hay twice a day. Fresh water and mineral stone was always available. No rams were housed near the ewes. To evaluate the changes in progesterone concentration during pregnancy, five ewes were housed separately with a fertile ram from 1 December to 15 February. Blood was collected from the vena jugularis three times a week from the beginning of September until the end of May. Blood was also collected daily, from all ewes, for 30 days during each breeding season (no fixed period). The blood was centrifuged within 1 h of collection and the serum was stored at - 20 ° C until assayed. The serum was assayed for progesterone concentration ( 100 gl per tube) using Coat-A-Count ® solid-phase, direct (no extraction of sample) radioimmunoassay (RIA) kit (Diagnostic Products Corporation, Los Angeles, CA). Detection limits of the assay were 0.16 nmol l - ~. Inter- and intra-assay coefficients of variation (CV) were 9.3% and 7.5%, respectively for a True-Value ® control sample containing 4.8 _+0.7 nmol 1-~. The mean of 50 assays of this control sample in 50 different assay runs was 4.77 _+0.4 nmol l- ~. The date of onset of ovarian cyclicity was the sample date before progesterone rose above 3 nmol 1-~ in two successive serum samples (English et al., 1986). The length of the luteal phase was evaluated from the day on which progesterone rose above 3 nmol l-1 to the day before it fell below the same value. The interluteal phase was estimated as the interval between the luteal phases. The area under the progesterone curve was calculated using the formula fax+ bdx for each part of the curve that could be expressed by a straight line and then adding up the parts. For a numerical evaluation the number of days from the autumn equinox to the onset and cessation of the breeding season was used. The data were evaluated using Student's t-test and analysis of
103
J. Eldon / Animal Reproduction Science 34 (1993) 101-109
variance, ANOVA (Sokal and Rohlf, 1981 ). The height of the sun above the horizon at noon is from Saemundsson (1990). During the s u m m e r solstice (21 June) the centre of the sun at noon rises 49 ° above the horizon and the time from sunrise to sunset is 21 h. During the winter solstice (21 December) the centre of the sun rises 2.7 ° above the horizon at noon and the time from sunrise to sunset is 4 h. Results
Breeding season The times of onset and cessation, and the length of the breeding season are presented in Table 1 and Fig. 1. The mean date of onset was 15 November and spanned 53 days from 6 October to 28 November. Two ewes (5%) started ovarian cyclicity in October (6th and 19th) and the rest (95%) started cyclicity in November. The mean date of cessation of seasonal ovarian cyclicity was 20 February and spanned 90 days from 15 January to 15 April. Seventyfive percent of the ewes had ceased cycling by 9 March and 90% by 29 March. The mean length of the breeding season was 97 _+24 days. The interval from the onset of ovarian activity in the first ewe to start and the time of cessation in the last ewe to cease seasonal ovarian activity was 191 days. All the ewes were cycling at the same time for 48 days from 28 November to 15 January. There was no correlation between the time of onset of cyclicity in the number of days from the a u t u m n equinox and the length of the breeding season for each individual ewe. In relation to daylength and the height of the sun, the first ewe started cycling on 6 October when the centre of the sun rose 21 ° above the horizon at noon and the time from sunrise to sunset was 11 h. On the mean date of onset of cyclicity ( 15 November), the sun rose 7.5 ° above the horizon and the time from sunrise to sunset was 6.5 h. All ewes were cycling when the sun rose 5 ° above the horizon at noon (28 November, sunrise-sunset 5.5 h). When the sun had reached 5 ° at noon with increasing daylength ( 15 January), the first Table 1 The mean date of onset and cessation of ovarian cyclicity ( + SD, days) and mean ( + SD) length of the breeding season (days) 1988 n 12 Onset 14Nov.(_+7) Cessation 22Feb.(+29) Length (days) 105 ( + 2 4 ) NS, not significant.
1989
1990
1991
8 15Nov.(_+9) 13Feb.(+30) 92(+29)
8 12 13Nov.(_+10) 17Nov.(_+9) 20Feb.(_+23) 21Feb.(_+24) 103(_+27) 92 (_+23)
All years 40 15Nov.(+_8)NS 20Feb.(+25)NS 97 (_+24)NS
104
J. Eldon / Animal Reproduction Science 34 (1993) 101-109
The sunangle above the horizon at noon and the reproductive period
I
~11~ Sunangle
-o-- No. of ewes
I
50
~C, OGGG~
45
k
cycling
%
_~
/-""
100
,o
40 35
7O
30
6O
25
5O
20
4O
15
3O
10 5
20
0 Jul.
Sep.
Nov.
Jan.
Mar.
May
Fig. 1. The reproductive period. Sun angle: degrees of the centre of the sun above the horizon at noon in Reykjavlk (64 ° 10'N, 22°W). Number of ewes cycling: the percentage of ewes with cyclic ovarian activity (n--40). Onset, cessation: the onset and cessation of seasonal luteal activity.
ewes stopped cycling. The last ewe stopped ovarian cyclicity when the sun had reached 36 ° above the horizon ( 15 April, sunrise-sunset 15 h; Fig. 1 ).
Ovarian activity The length of the ovarian cycle was 16 _+2 days. There was no difference between cycles ( P > 0.05 ). The length of the luteal phase of the ovarian cycles was 11 _+2 days and the length of the interluteal phases was 5 + 1 day. There were no differences in the lengths of the luteal or interluteal phases between ovarian cycles ( P > 0.05 ). The average progesterone concentrations during the luteal phases of the first, second and third ovarian cycles of the breeding season were 6.9 _+ 1.5 nmol 1- ~, 7.5 + 1.6 nmol 1-1 and 7.7 _ 1.5 nmol 1-1, respectively (mean + SD, n = 200 ). The difference was statistically significant. The area under the progesterone curve during the luteal phase of the first ovarian cycle was less than those of the second and third ovarian cycles ( P < 0.01 ). In 40% of the ewes, the mean progesterone concentration during the luteal phases increased after the third ovarian cycle of the breeding season and decreased again during the last two ovarian cycles. In 25% of the ewes, the progesterone concentration decreased from the first ovarian cycle to the last; in 20% of the ewes it was similar during all ovarian cycles and in 15% of the ewes the mean progester-
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one concentration increased from the first to the last ovarian cycle of the breeding season. There was a marked difference in the amplitude of the luteal phase progesterone curve between ewes. According to the results from the daily samples the progesterone concentration increased from 3 to 10 nmol l- 1 in 3 days during the onset of the luteal phase and fell from 15 to below 3 nmol l- 1 in 2.5 days at the end of the luteal phase. Thirty percent of the ewes had a transient rise in progesterone (3-5 nmol 1- ~for 5 days or less ) 10-30 days before regular cyclicity started. The number of luteal phases during the breeding season was 5.5_ 1.5. Three of five ewes that were housed with a ram conceived and had gestation periods of 138, 143 and 148 days. Each ewe gave birth to one lamb. Figure 2 shows the changes that occurred in the progesterone profiles during pregnancy. Progesterone rose to a plateau of 10-15 nmol l- 1, where it stayed for the first 70-80 days postconception with peaks reaching over 20 nmol 1-1. In two ewes, the progesterone level dropped to 5 nmol l - 1 for about 5 days 70-80 days post-conception. However, 80-100 days post-conception the progesterone level rose sharply to a plateau of 30-40 nmol l- 1 with peaks of up to 60 nmol 1-1. At birth the progesterone value had dropped below the sensitivity of the assay. The drop from peak value took 5-6 days. Progesterone nmol/I 70
tI I
Ewe _no:__
60 50
I-*-33
/
t ~
40
,
,
c 30 20 10 0 0
10
20
30
40
50
60
70
80
90
100 110 120 130 140
50
Days from conception
Fig. 2. The progesterone profiles during the gestation period of Ewes 30, 31 and 33: C, time of conception; D, drop in progesterone concentration to 5 nmol l-1 (see text); B, birth.
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Discussion The average date of the onset of the breeding season was 15 November. This is 23 days earlier than reported by D~rmundsson ( 1978 ) for ewes kept under 'field conditions' and accompanied by vasectomised rams equipped with a raddle harness and crayon. The use of vasectomised rams should have advanced the onset of the breeding season, as other authors have reported that the introduction of rams towards the end of the seasonal anoestrous period significantly advances the breeding season in ewes (Martin et al., 1986; Robinson et al., 1991; J. Eldon, unpublished data, 1991 ). The use of artificial light during the morning and afternoon, as is done under 'field conditions' in Iceland, disrupts the effects of natural photoperiod and may delay the onset of the breeding season. The fact that the ewes do not display oestrus preceding the first and even the second ovulation, may also be a reason for the difference between these two studies (Hammond, 1944; I'Anson and Legan, 1988; Schirar et al., 1989; Wayne et al., 1989). It is generally accepted that the decrease in daylength has an influence on the onset of seasonal ovarian activity either directly (Jackson et al., 1989; Khalid and Jackson, 1991 ) or indirectly (Malpaux and Karsch, 1990 ). The small difference between years in average date of onset of ovarian cyclicity and the fact that 90% of the ewes started cycling within 28 days in November, emphasise the close synchronisation between the chronological change in daylength and the physiological response of the reproductive system to that change. The lack of significance between years in the times of onset and cessation of the breeding season and its duration, contrasts with the findings of Lees (1966), who found a significant variation between seasons in the time of onset of breeding activity in British breeds of sheep. The cessation of the breeding season spanned 90 days with a mean date of 20 February. This shows that the cessation of ovarian cyclicity owing to the onset of photorefractoriness during an increase in daylength is a slower process than the onset of ovarian cyclicity in response to decrease in daylength. The average length of the breeding season was 97 _+24 days. All the ewes were cycling for 48 days during the period while the angle of the centre of the sun at noon was 5 ° or less above the horizon. This period has the shortest daylength and consequently the longest night, which, through its effect on melatonin synthesis in the pineal gland, is probably the controlling factor rather than the angle of the sun during daylight (English et al., 1986; Malpaux et al., 1988; Krause and Dubocovich, 1990). D~,rmundsson ( 1978 ) reported the duration of the breeding season in Icelandic sheep to be 146 _+16 days, which is 49 days or three ovarian cycles longer than in the present study. The reasons for this difference are probably the same as those discussed earlier for the difference in the onset of the breeding season, i.e. the use of artificial light and ram effect. In comparison, the duration of the breeding season of the
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European wild sheep (Mouflon, Ovis musimon) and the chief breeds in Norway (Cheviot, Dala, Rygja and Old Norwegian) is 3 months (October-December) and the duration of the breeding season of six British breeds is 166 days with 9.1 oestrus periods (Hafez, 1952). The length of the ovarian cycle ( 16 _+2 days) was in agreement with earlier studies for other sheep breeds (Hammond, 1944; Zarco et al., 1988; Wathes et al., 1989 ). There was no difference in the length of the luteal phases of the ovarian cycles ( 11 _ 2 days). The difference in the area under the progesterone curve during the luteal phases of the first three ovarian cycles of the breeding period is a result of differences in progesterone concentration. Ashworth et al. (1989) stated that corpora lutea which secrete low amounts of progesterone may be formed from follicles with impeded morphology and/or physiology. Low progesterone of short duration contributes to reduced functional capacity of luteal tissue during the onset of ovarian activity in cows following post-partum anoestrus (Eldon, 1988, 1991 ). A transient rise in progesterone was found in 30% of the ewes. I'Anson and Legan (1988) stated that most ewes had several luteinising hormone (LH) surges preceding the first full-length luteal phase and that the first LH surge induced a transient rise in progesterone. The pattern of luteal function was diverse between ovarian cycles and between ewes. This could be a factor affecting embryo survival during early gestation (Ashworth et al., 1989). The duration of the rise in luteal phase progesterone to a plateau at the beginning of the luteal phase was 3 days, as was the duration of the drop from this level to below luteal phase concentration. In comparison, in the dairy cow, the rise in progesterone to a plateau takes 5 days while the drop from the plateau to below luteal phase value took 3 days (Eldon, 1991 ). The speed of the progesterone rise and its amplitude is an indication of the activity of the corpus luteum, as the speed of the drop reflects the functional status of the mechanism ofluteolysis (Kindahl et al., 1980, 1981 ). All three pregnant ewes had similar changes in progesterone profile. The drop in progesterone approximately 70 days post-conception is probably a result of decreased output by the corpus luteum and may act as a signal to the placenta to start progesterone production. The sharp rise in progesterone 80100 days post-conception to values four- to six-fold greater than during the luteal phase is due to substantial production by the placenta (Norris et al., 1989 ). The fall in progesterone concentration from peak level to levels below the sensitivity of the assay took 5-6 days, a slightly shorter time than the 610 days reported by Jainudeen and Hafez (1980), a discrepancy which may represent species differences.
Acknowledgement I am indebted to Dr. E. Gunnarson for his technical help. This study was supported by the Icelandic Science Fund.
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activity of LH during the peripartum period and the estrous cycle of the ewe. Domest. Anita. Endocrinol., 6: 25-33. Robinson, J.J., Wigzell, S., Aitken, R.P., Wallace, J.M., Ireland, J. and Robertson, I.S., 1991. The modifying effects of melatonin, ram exposure and plane of nutrition on the onset of ovarian activity, ovulation rate and the endocrine status of ewes. Anim. Reprod. Sci., 26: 73-91. Schirar, A., Meusnier, C., Paly, J., Levasseur, M.C. and Martinet, J., 1989. Resumption of ovarian activity in post-partum ewes: Role of the uterus. Anim. Reprod. Sci., 19: 79-89. Sokal, R.R. and Rohlf, F.J., 1981. Biometry, 2nd edn. W.H. Freeman, New York, 859 pp. S~emundsson, Th., 1990. Almanak um ~irid 1990 (A calendar for the year 1990). University of Iceland, 96 pp. Wathes, D.C., Ayad, V.J., McGoff, S.A. and Morgan, K.L., 1989. Effect of active immunization against oxytocin on gonadotrophin secretion and the establishment of pregnancy in the ewe. J. Reprod. Fertil., 86: 653-664. Wayne, N.L., Malpaux, B. and Karsch, F.J., 1989. Social cues play a role in timing onset of the breeding season of the ewe. J. Reprod. Fertil., 87:707-713. Zarco, L., Stabenfeldt, G.H., Quirke, J.F., Kindahl, H. and Bradford, G.E., 1988. Release of prostaglandin F-2a and the timing of events associated with luteolysis in ewes with oestrous cycles of different lengths. J. Reprod. Fertil., 83:517-526.