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Onset of the preovulatory LH surge and of oestrus in intact ewes: Night is a preferred period
THERIOGENOLOGY
ONSET OF THE PREOVULATORY LH SURGE AND OF OESTRUS IN INTACT EWES: NIGHT IS A PREFERRED PERIOD. C. Fabre-Nys, G.B. Martin , Y. Cognie and J.C. ThiGry C.N.R.S. - I.N.R.A. - Station de Physiologie de la Reproduction 37380 NOUZILLY France Received for publication: November 11, 1983 Accepted: August 27. 1984
ABSTRACT Oestrous cycles were induced in seasonally anoestrous ewes by introducing rams into the flock and giving to the ewes one intramuscular injection of 20 mg progesterone. At the second ovulation the onset of oestrus and the preovulatory surge of luteinizing hormone (LH) were recorded. It was found that the LH surge began in significantly more ewes during the night (79%) than during the day (21 X). The onset of oestrus tended to follow a similar pattern. This temporal pattern was not related to the time of ram introduction, but may be the result of daily rhythms in ovarian activity. Furthermore, a preferred period for the LH surge indicates a preferred period for ovulation and this may be important in deciding when to begin artificial insemination. Keywords: Sheep, LH, oestrous behaviour, circadian rhythm.
INTRODUCTION In female rats, the time of the onset of oestrous behaviour depends on environmental lighting and under normal regimes of light and darkness, it occurs on the afternoon of proestrus (1). In contrast, the association between lighting schedule and oestrus in the ewe is not as consistent. This is evident from the conflicting reports: Schindler et al. (2) stated that there was no preferred period for onset ofxrus in ewes, whereas other reports that do describe preferred periods do not agree on its timing (3-6). In most of these studies the authors attributed the effect to variation in the activity of the rams they used to test for oestrous behaviour.
Acknowledgments Our thanks go to Rosemary Martin and Francois Dieumegard for their valuable assistance and to the staff of "Le Merle" for allowing US to use their animals. G.B. Martin received financial support through an Overseas Study Award from the Australian Meat Research Committee.
NOVEMBER 1984 VOL. 22 NO. 5
THERIOGENOLOGY The rat and the ewe are both spontaneous ovulators so oestrus, preovulatory surge of luteinizing hormone (LH) and ovulation are synchronized. In rats, in accordance with the observations of oestrus, the LH surge occurs on the afternoon of proestrus and the timing of this event also depends on the light/dark schedule (7). In the ewe, the LH surge also occurs within a few hours of the time of onset of oestrus (8, 9) but a preferred period for LH release during a 24 h interval has not yet been documented. Knowledge of preferred periods for the onset of oestrus, LH surge and ovulation would be important for management operations such as artificial insemination, which is most effective when done at a specific time relative to ovulation (10). We have recently developed a method of measuring oestrous behaviour in ewes that eliminates many problems associated with variation in the behaviour of the rams (11). The purpose of this study was to establish the temporal distribution of the preovulatory LH surge and of the onset of oestrus in a flock of ewes. MATERIALS AND METHODS The experimental flock consisted of 39 Merinos d'Arles ewes, aged three to four years, that were maintained on pasture on a farm in southern France. The management system made use of the ram effect (review: 12) in May just before initiation of the breeding season, one month before transport to Alps where the ewes stay during gestation. The sexual season and use of the ram effect under these conditions has been described in detail by Prud'hon and Denoy (13). The ram effect was associated with progesterone treatment as described by Lindsay et al. (14) and Cognig et al. (15). In brief, seasonally anoestrous ewes, isolated from rams for at least a month, are treated with a single intramuscular injection of 20 mg progesterone in oil. On the same day, rams are reintroduced and ewes ovulate within three days. This first ovulation, which is rarely accompanied by expression of oestrous behaviour, is followed by a normal oestrous cycle and the first period of oestrous behaviour and second ovulation are synchronized over a period of 3 to 5 days (instead of 17 days in a non synchronized flock) 19 days after male introduction (14, 15). We took advantage of this synchrony and recorded the temporal distribution of onsets of oestrus and LH surges in the flock around the time of the second ovulation. In order to eliminate any bias that the synchronizing treatment may have had on this distribution, progesterone injection and introduction of rams were done on 20 ewes at 9 h and on the remaining 19 at 18 h. Ewes and rams were kept together during the induced cycle and detailed observations began 17 days later at 9h. The observation period took four days. After a four more days, the ewes underwent laparoscopy to determine the ovulation rate. Oestrous behaviour. The ewes were subjected to a standardized test in order to measure their level of receptivity (11). In brief: each ewe was presented individually to an active ram for at least 2 min while we 490
NOVEMBER
1984 VOL. 22 NO. 5
THERIOCENOLOGY recorded her responses to each of the various "nudges" that comprise ram courtship behaviour (16). Her responses to each "nudge" were then classified as either "no immobilization" or "immobilization", this being the pattern characteristic of ewes in oestrus (16). An immobilization will ultimately allow the ram to mount the ewe, but in this test mounting is not necessary to determine the ewe's response. After a total of a least ten nudges by the ram the test was stopped and the next ewe was brought in. For each ewe a "receptivity index" was calculated: Number of nudges followed by immobilization x 100 Total number of nudges This index was measured every 8 h defined as the onset of oestrus.
and an
index of 85% was
Preovulatory LH surge. Blood was sampled every 4 h beginning like the behavioural tests, at 9 h 17 days after synchronizing treatment, until 12 h after the onset of oestrus. The plasma was separated and stored until it was assayed for LH. The details of the radioimmunoassay we used have been previously described (17). For the purposes of analysis the time of onset of the LH surge was defined as the time of the sample before the peak in which the concentration was at least twice the basal level (the mean concentration of all previous samples). Statistical analysis. Differences between day and night in the frequencies of the onset of oestrus and onset of the LH surge were tested for significance by Chi-square. RESULTS The synchronization treatment was highly effective and more than 80% of the flock came into oestrus during the observation period. Ovulation rates and the proportions of the flock showing oestrus and an LH surge during the observation period are presented in Table 1. Table 1
Responses of ewes to a synchronizing treatment (ram effect and progesterone) given in the morning or the afternoon. The data for oestrus and the LH surge were collected 17-20 days after the introduction of rams and the ovulation rates (mean number of ovulations per ewes ovulating) were measured 4 days later.
Treatment time
Ewes in oestrus
Ewes with LH surge
Ewes ovulating
Ovulation rate
9h
17120
18120
18120
1.33
18h
16/19
16119
17/18a
1.24
a Ovaries not observed in 1 ewe.
NOVEMBER
1984VOL.22NO.S
491
THERIOGENOLOCY The time during which synchronization treatments were performed did not affect the proportion of ewes responding to treatment or their ovulatory responses 17 days later. The data in Table 2 show the temporal distribution of the onsets of oestrus and the LH surges. It can be seen that the LH surges began in a significantly (PcO.01) larger proportion of the ewes (79%) during the night i.e. between 21 h and 5 h.
Table 2
Temporal distribution of onset of oestrus and onset of the LH surge. The figures represent data collected over all four days of the observation period.
Number of ewea Night Day ___________-___--___-____^_______________ 5h 9h 21 h lh 13 h 17 h
LH surge
12
5
10
1
a
15
3
7134
27/34a Oestrus
3
11
7
P
Onset of oestrus is also more frequent at night, but it was not statistically significant. DISCUSSION To the best of our knowledge, this is the first evidence showing that night is the preferred period for the onset of the LH surge in ewes. Whether this phenomenon would be observed during other seasons, treatments or in the absence of any with other synchronizing synchronizing treatments has yet to be determined. There is a tendency for women and female baboons to have midcycle LH surges at night (18, 19), and two reports describe a higher incidence of onset of oestrus in the ewe in early evening (5, 6). In general this agrees with our data. Unfortunately we were not able to measure the behaviour of the ewes frequently enough and the consequent lack of precision probably prevented us from observing a significant temporal distribution. The existence of this distribution is supported by our observation of the preferred period for the onset of the LH surge and the well established liaison between the surge and the onset of oestrus in ewes (8). In the ovariectomized ewe, it has been shown that between an injection of oestradiol benzoate (50 ug in subsequent surge of LH is constant and independent of injection (ZO), so it seems unlikely that the temporal 492
NOVEMBER
the interval oil) and the the time of distribution
1984 VOL. 22 NO. 5
THERIOGENOLOGY
of the LH surge is caused by circadian changes in sensitivity of the feed-back action of central system to the positive nervous oestradiol. However, we must be cautious in comparing observations of the cyclic ewes with the responses of ovariectomized ewes to an injection of oestrogen. It is possible, for example, that if the doses injected were too high it would cover up subtle changes in hypothalamic sensitivity. Nevertheless, the system in the ewe contrasts sharply with that in the ovariectomized rat where a preovulatory surge is seen every day at the same time even when implants are used to provide continuous high levels of oestrogen (21). In the ewe, the temporal distribution of the LH surge may be explained by a light-associated rhythm of ovarian activity rather than by a circadian rhythm in sensitivity to oestrogen. This ovarian activity rhythm, during the follicular phase, would increase the likelihood of the preovulatory oestrogen rise being secreted at a particular time of the day. Our data indicate that the time of the day that ewes are exposed to the ram had no effect on their responses at the time of the second ovulation. However it is still possible that there was an effect in the short term, around the time of the first ovulation, and that it was lost during the following cycle. Our practical implications. findings have some important Artificial insemination is most efficient when done at a particular moment relative to ovulation (10). The ewes are usually tested for oestrus twice daily, in the morning and in the afternoon, and the first positive test is taken as the onset of oestrus and the time for insemination is decided accordingly. If the ewes were to ovulate at random throughout the day this method would be the most practical. However, our data indicate that ewes should mostly ovulate at night since the interval between the onset of the LH surge and ovulation is 22 h to 26 h (22, 23). It may be worthwhile to test the ewes more frequently at night to ensure a more accurate detection of the onset of oestrus. This would lead to a more accurate estimate of the moment of ovulation and thus a more appropriate time of insemination. Another implication concerns the choice of the best time for treatment that stimulates the ovarian function like PMSG. It has recently been shown that there are differences between day and night in the responsiveness of ewes to treatment by PMSG (24). This supports also the idea of a rhythm in ovarian activity.
NOVEMBER
1984 VOL. 22 NO. 5
493
THERIOGENOLOGY
REFERENCES 1.
Schwartz, N.B. A model for the regulation of ovulation in the rat. Rec. Prog. Horm. Res. -25: l-43 (1969).
2.
Schindler, H. and Amir, D. Length of oestrus, duration of phenomena related to ocstrus, and ovulation time in the local fat-tailed Awassi ewe. J. Agri. Sci. (Camb). -78: 151-156 (1972).
3.
Hulet, C.V., Blackwell, R.L., Ercanbrack, S.K., Price, D.A. and Wilson, L.O. Mating behavior in the ewe. J. Anim. Sci. -21: 870-874 (1962).
4.
Blockey, M.A. de B. and Cumming, I.A. Mating behaviour of Merino ewes. Proc. Aust. Sot. Anim. Prod. 8: 344-347 (1970).
5.
Cognie, Y., Mariana, J.C. and Thimonier, J. Etude du moment chez la trait&e par un d'ovulation brebis normale ou progestagene associe ou non a une injection de PMSG. Ann. Biol. Anim. Biochim. Biophys. -10: 15-24 (1970).
6.
Cahill, L.P., Blockey, M.A. de B. and Parr, R.A. Effects of mating behaviour and ram libido on the fertility of young ewes. Aust. J. Exp. Agric. Anim. Husb. -15: 337-341 (1975).
7.
Rodgers, C.H. Timing of sexual behavior in the female rat. Endocrinology. -86: 1181-1183 (1970).
8.
Land, R.B., Pelletier, J., Thimonier, J. and Mauleon, P. A quantitative study of genetic differences in the incidence of oestrus, ovulation and plasma luteinizing hormone concentration in the sheep. J. Endocrinol. -58: 305-317 (1973).
9.
Bindon, B.M., Blanc, M.R., Pelletier, J., Terqui, M. and Thimonier, J. Periovulatory gonadotrophin and ovarian steroid patterns in sheep of breeds with differing fecundity. J. Reprod. Fertil. -55: 15-25 (1979).
10.
Colas G., Thimonier J., Courot M. and Ortavant R. Fertilite, prolificite et fecondite pendant la saison sexuelle des brebis inseminees artificiellement apres traitement 1 l'acetate de fluorogestone. Ann. Zootech. -22: 441-451 (1973).
11.
Fabre-Nys, C. Development and use of a method for quantifying female sexual behaviour in ewes. Appl. Anim. Ethol. (in preparation).
12.
Martin, G.B. and Scaramuzzi, R.J. The induction of oestrus and ovulation in seasonally anovular ewes by exposure to rams. J. Steroid. Biochem. -19: 869-875 (1983).
494
NOVEMBER
1984 VOL. 22 NO. 5
THERIOGENOLOGY 13.
Prud'hon, M. and Denop, I. LEfets de 1' introdrrction de bisli~r~ vasectomises dans un troupeau MPrinos d'Arles, quinze _jours avant le debut de la lutte de printemps, sur l'appariticn des oestrus, la frequence des erreurs de d@tectinn des ruts et 1:r frrtilite des brebis. Ann. Zootech. -_ 18: 95-106 (1969).
14 .
I.indsay, D.R., Cognie, Y. and Signoret-, .i.P. Methode simplifiec de maytrise de l'oestrus chez la hrehis. Ann. Zootech. 31: i_-P:' (1982).
15.
CogniP, Y., Gray, Pearce, D.T. and breeding in sheep Prod. _14: 519-522
16.
Banks, E.Ef. Some aspects of sexual behavior in domestic sheep (Avis aries). Behaviour. -_ 23 : 249-279 (1464).
17.
Pelletier, J., Kann, G., Dolais, .I. and Rosselin, G. Dosage radioimmunnlogique de l'hormone IutGinisante plasmatique chez le mouton. Mise au point de la tecllnique de dosage. C.R. Acad. 266. Sci., Paris (D), -_ : 2291-2294 (1968).
18.
Hagino, N. and Yamaoka, S. Role of biorhythm of sleep and wakefulness in the periodicity of gonadotropin secretion in the baboon (non-human primate). Tn Biologi.cal Rhythms in Neuroendocrine Activity E:d. M. Kawakami, Igaku Shoin I.tti , Tokyo, pp 326-337 (1974).
19.
Seibel, M.M., Shine, W., Smith, D-M., and Taymor, M.L. Biological rhythm of the luteinizing hormone surge in women. Fertil . Steril. -37: 709-711 11962).
20.
~Jackson, G.L., Thurmon, J. and Nelson, D. Estrogen-induced release of LII in the ovariectomized ewe: independence of time of day. Biol. Reprod. -13: 358-362 (1975).
21.
Legan, S.J., Coon, G.A. and Karsch, F.J. Role of estrogen as rat. initiator of daily 1.H surges in the ovariectomized F:ndocrinolI,gy.-96: 50-56 (19?5).
??.
Wheatley, l..S. and Radford, H.M. Luteinizing hormone secretion cycle of the ewe as determined b) during the oestrous radioimmunoassay. J. Reprod. Fertil. -19: 211-214 (1969).
23.
Cumming, I.A., Buckmaster, J.M., Blockey M.A. de B., R.W. Constancy Goding J.R., Winfield, C.G. and Baxter, interval between luteinizing hormone release and ovulation the ewe. Riol. Reprod. -9: 24-29 (1973).
24.
S.&J., I,indsay, U.R., Oldham, C.M., Signoret, .J.P. A new approach to controlled using the “ram effect". I'roc. AlIst. Sot. Anim. (1982).
0 fin
Cognie, Y., Martinet, J. and Schirar, A. Ovarian responses of in the ewes to hormonal stimulatiou seasonallv anoestrous morning or the evening. 10th. Jnt. Congr. on Anim. Reprod. and J.A. June 10-14, Urbana-Champaign, U.S.A. (1984).
NOVEMBER
1984VOL.22N0.S
495
ONSET OF THE PREOVULATORY LH SURGE AND OF OESTRUS IN INTACT EWES: NIGHT IS A PREFERRED PERIOD. C. Fabre-Nys, G.B. Martin , Y. Cognie and J.C. ThiGry C.N.R.S. - I.N.R.A. - Station de Physiologie de la Reproduction 37380 NOUZILLY France Received for publication: November 11, 1983 Accepted: August 27. 1984
ABSTRACT Oestrous cycles were induced in seasonally anoestrous ewes by introducing rams into the flock and giving to the ewes one intramuscular injection of 20 mg progesterone. At the second ovulation the onset of oestrus and the preovulatory surge of luteinizing hormone (LH) were recorded. It was found that the LH surge began in significantly more ewes during the night (79%) than during the day (21 X). The onset of oestrus tended to follow a similar pattern. This temporal pattern was not related to the time of ram introduction, but may be the result of daily rhythms in ovarian activity. Furthermore, a preferred period for the LH surge indicates a preferred period for ovulation and this may be important in deciding when to begin artificial insemination. Keywords: Sheep, LH, oestrous behaviour, circadian rhythm.
INTRODUCTION In female rats, the time of the onset of oestrous behaviour depends on environmental lighting and under normal regimes of light and darkness, it occurs on the afternoon of proestrus (1). In contrast, the association between lighting schedule and oestrus in the ewe is not as consistent. This is evident from the conflicting reports: Schindler et al. (2) stated that there was no preferred period for onset ofxrus in ewes, whereas other reports that do describe preferred periods do not agree on its timing (3-6). In most of these studies the authors attributed the effect to variation in the activity of the rams they used to test for oestrous behaviour.
Acknowledgments Our thanks go to Rosemary Martin and Francois Dieumegard for their valuable assistance and to the staff of "Le Merle" for allowing US to use their animals. G.B. Martin received financial support through an Overseas Study Award from the Australian Meat Research Committee.
NOVEMBER 1984 VOL. 22 NO. 5
THERIOGENOLOGY The rat and the ewe are both spontaneous ovulators so oestrus, preovulatory surge of luteinizing hormone (LH) and ovulation are synchronized. In rats, in accordance with the observations of oestrus, the LH surge occurs on the afternoon of proestrus and the timing of this event also depends on the light/dark schedule (7). In the ewe, the LH surge also occurs within a few hours of the time of onset of oestrus (8, 9) but a preferred period for LH release during a 24 h interval has not yet been documented. Knowledge of preferred periods for the onset of oestrus, LH surge and ovulation would be important for management operations such as artificial insemination, which is most effective when done at a specific time relative to ovulation (10). We have recently developed a method of measuring oestrous behaviour in ewes that eliminates many problems associated with variation in the behaviour of the rams (11). The purpose of this study was to establish the temporal distribution of the preovulatory LH surge and of the onset of oestrus in a flock of ewes. MATERIALS AND METHODS The experimental flock consisted of 39 Merinos d'Arles ewes, aged three to four years, that were maintained on pasture on a farm in southern France. The management system made use of the ram effect (review: 12) in May just before initiation of the breeding season, one month before transport to Alps where the ewes stay during gestation. The sexual season and use of the ram effect under these conditions has been described in detail by Prud'hon and Denoy (13). The ram effect was associated with progesterone treatment as described by Lindsay et al. (14) and Cognig et al. (15). In brief, seasonally anoestrous ewes, isolated from rams for at least a month, are treated with a single intramuscular injection of 20 mg progesterone in oil. On the same day, rams are reintroduced and ewes ovulate within three days. This first ovulation, which is rarely accompanied by expression of oestrous behaviour, is followed by a normal oestrous cycle and the first period of oestrous behaviour and second ovulation are synchronized over a period of 3 to 5 days (instead of 17 days in a non synchronized flock) 19 days after male introduction (14, 15). We took advantage of this synchrony and recorded the temporal distribution of onsets of oestrus and LH surges in the flock around the time of the second ovulation. In order to eliminate any bias that the synchronizing treatment may have had on this distribution, progesterone injection and introduction of rams were done on 20 ewes at 9 h and on the remaining 19 at 18 h. Ewes and rams were kept together during the induced cycle and detailed observations began 17 days later at 9h. The observation period took four days. After a four more days, the ewes underwent laparoscopy to determine the ovulation rate. Oestrous behaviour. The ewes were subjected to a standardized test in order to measure their level of receptivity (11). In brief: each ewe was presented individually to an active ram for at least 2 min while we 490
NOVEMBER
1984 VOL. 22 NO. 5
THERIOCENOLOGY recorded her responses to each of the various "nudges" that comprise ram courtship behaviour (16). Her responses to each "nudge" were then classified as either "no immobilization" or "immobilization", this being the pattern characteristic of ewes in oestrus (16). An immobilization will ultimately allow the ram to mount the ewe, but in this test mounting is not necessary to determine the ewe's response. After a total of a least ten nudges by the ram the test was stopped and the next ewe was brought in. For each ewe a "receptivity index" was calculated: Number of nudges followed by immobilization x 100 Total number of nudges This index was measured every 8 h defined as the onset of oestrus.
and an
index of 85% was
Preovulatory LH surge. Blood was sampled every 4 h beginning like the behavioural tests, at 9 h 17 days after synchronizing treatment, until 12 h after the onset of oestrus. The plasma was separated and stored until it was assayed for LH. The details of the radioimmunoassay we used have been previously described (17). For the purposes of analysis the time of onset of the LH surge was defined as the time of the sample before the peak in which the concentration was at least twice the basal level (the mean concentration of all previous samples). Statistical analysis. Differences between day and night in the frequencies of the onset of oestrus and onset of the LH surge were tested for significance by Chi-square. RESULTS The synchronization treatment was highly effective and more than 80% of the flock came into oestrus during the observation period. Ovulation rates and the proportions of the flock showing oestrus and an LH surge during the observation period are presented in Table 1. Table 1
Responses of ewes to a synchronizing treatment (ram effect and progesterone) given in the morning or the afternoon. The data for oestrus and the LH surge were collected 17-20 days after the introduction of rams and the ovulation rates (mean number of ovulations per ewes ovulating) were measured 4 days later.
Treatment time
Ewes in oestrus
Ewes with LH surge
Ewes ovulating
Ovulation rate
9h
17120
18120
18120
1.33
18h
16/19
16119
17/18a
1.24
a Ovaries not observed in 1 ewe.
NOVEMBER
1984VOL.22NO.S
491
THERIOGENOLOCY The time during which synchronization treatments were performed did not affect the proportion of ewes responding to treatment or their ovulatory responses 17 days later. The data in Table 2 show the temporal distribution of the onsets of oestrus and the LH surges. It can be seen that the LH surges began in a significantly (PcO.01) larger proportion of the ewes (79%) during the night i.e. between 21 h and 5 h.
Table 2
Temporal distribution of onset of oestrus and onset of the LH surge. The figures represent data collected over all four days of the observation period.
Number of ewea Night Day ___________-___--___-____^_______________ 5h 9h 21 h lh 13 h 17 h
LH surge
12
5
10
1
a
15
3
7134
27/34a Oestrus
3
11
7
P
Onset of oestrus is also more frequent at night, but it was not statistically significant. DISCUSSION To the best of our knowledge, this is the first evidence showing that night is the preferred period for the onset of the LH surge in ewes. Whether this phenomenon would be observed during other seasons, treatments or in the absence of any with other synchronizing synchronizing treatments has yet to be determined. There is a tendency for women and female baboons to have midcycle LH surges at night (18, 19), and two reports describe a higher incidence of onset of oestrus in the ewe in early evening (5, 6). In general this agrees with our data. Unfortunately we were not able to measure the behaviour of the ewes frequently enough and the consequent lack of precision probably prevented us from observing a significant temporal distribution. The existence of this distribution is supported by our observation of the preferred period for the onset of the LH surge and the well established liaison between the surge and the onset of oestrus in ewes (8). In the ovariectomized ewe, it has been shown that between an injection of oestradiol benzoate (50 ug in subsequent surge of LH is constant and independent of injection (ZO), so it seems unlikely that the temporal 492
NOVEMBER
the interval oil) and the the time of distribution
1984 VOL. 22 NO. 5
THERIOGENOLOGY
of the LH surge is caused by circadian changes in sensitivity of the feed-back action of central system to the positive nervous oestradiol. However, we must be cautious in comparing observations of the cyclic ewes with the responses of ovariectomized ewes to an injection of oestrogen. It is possible, for example, that if the doses injected were too high it would cover up subtle changes in hypothalamic sensitivity. Nevertheless, the system in the ewe contrasts sharply with that in the ovariectomized rat where a preovulatory surge is seen every day at the same time even when implants are used to provide continuous high levels of oestrogen (21). In the ewe, the temporal distribution of the LH surge may be explained by a light-associated rhythm of ovarian activity rather than by a circadian rhythm in sensitivity to oestrogen. This ovarian activity rhythm, during the follicular phase, would increase the likelihood of the preovulatory oestrogen rise being secreted at a particular time of the day. Our data indicate that the time of the day that ewes are exposed to the ram had no effect on their responses at the time of the second ovulation. However it is still possible that there was an effect in the short term, around the time of the first ovulation, and that it was lost during the following cycle. Our practical implications. findings have some important Artificial insemination is most efficient when done at a particular moment relative to ovulation (10). The ewes are usually tested for oestrus twice daily, in the morning and in the afternoon, and the first positive test is taken as the onset of oestrus and the time for insemination is decided accordingly. If the ewes were to ovulate at random throughout the day this method would be the most practical. However, our data indicate that ewes should mostly ovulate at night since the interval between the onset of the LH surge and ovulation is 22 h to 26 h (22, 23). It may be worthwhile to test the ewes more frequently at night to ensure a more accurate detection of the onset of oestrus. This would lead to a more accurate estimate of the moment of ovulation and thus a more appropriate time of insemination. Another implication concerns the choice of the best time for treatment that stimulates the ovarian function like PMSG. It has recently been shown that there are differences between day and night in the responsiveness of ewes to treatment by PMSG (24). This supports also the idea of a rhythm in ovarian activity.
NOVEMBER
1984 VOL. 22 NO. 5
493
THERIOGENOLOGY
REFERENCES 1.
Schwartz, N.B. A model for the regulation of ovulation in the rat. Rec. Prog. Horm. Res. -25: l-43 (1969).
2.
Schindler, H. and Amir, D. Length of oestrus, duration of phenomena related to ocstrus, and ovulation time in the local fat-tailed Awassi ewe. J. Agri. Sci. (Camb). -78: 151-156 (1972).
3.
Hulet, C.V., Blackwell, R.L., Ercanbrack, S.K., Price, D.A. and Wilson, L.O. Mating behavior in the ewe. J. Anim. Sci. -21: 870-874 (1962).
4.
Blockey, M.A. de B. and Cumming, I.A. Mating behaviour of Merino ewes. Proc. Aust. Sot. Anim. Prod. 8: 344-347 (1970).
5.
Cognie, Y., Mariana, J.C. and Thimonier, J. Etude du moment chez la trait&e par un d'ovulation brebis normale ou progestagene associe ou non a une injection de PMSG. Ann. Biol. Anim. Biochim. Biophys. -10: 15-24 (1970).
6.
Cahill, L.P., Blockey, M.A. de B. and Parr, R.A. Effects of mating behaviour and ram libido on the fertility of young ewes. Aust. J. Exp. Agric. Anim. Husb. -15: 337-341 (1975).
7.
Rodgers, C.H. Timing of sexual behavior in the female rat. Endocrinology. -86: 1181-1183 (1970).
8.
Land, R.B., Pelletier, J., Thimonier, J. and Mauleon, P. A quantitative study of genetic differences in the incidence of oestrus, ovulation and plasma luteinizing hormone concentration in the sheep. J. Endocrinol. -58: 305-317 (1973).
9.
Bindon, B.M., Blanc, M.R., Pelletier, J., Terqui, M. and Thimonier, J. Periovulatory gonadotrophin and ovarian steroid patterns in sheep of breeds with differing fecundity. J. Reprod. Fertil. -55: 15-25 (1979).
10.
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