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Stimulation of seasonally anovular merino ewes by rams. II. Premature regression of ram-induced corpora lutea
Animal Reproduction Science, 1 (1978/1979) 29]--295
291
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
S T I M U L A T I O N O F S E A S O N A L L Y A N O V U L A R M E R I N O EWES B Y RAMS
II. P R E M A T U R E
REGRESSION
OF RAM-INDUCED
CORPORA
LUTEA
C.M. OLDHAM and G.B. MARTIN Department o f Animal Science and Production, Institute o f Agriculture, University o f Western Australia, Nedlands, 6009 W.A. (Australia)
(Accepted 6 September 1978)
ABSTRACT Oldham, C.M., Martin, G.B., 1978/1979. Stimulation of seasonally anovular Merino ewes by rams. II. Premature regression of ram-induced corpora lutea. Anim. Reprod. Sci., 1: 291--295. Ovulation was induced by rams in 74 of 91 seasonally anovular Merino ewes. The resulting corpora lutea (CL) were observed by laparoscopy and were found to either persist normally (38/74), or regress prematurely (36/74). In 32 of the latter ewes premature regression of the CL was followed by a second ovulation within 6 days of the introduction of rams.
INTRODUCTION T h e i n t r o d u c t i o n o f r a m s to p r e v i o u s l y isolated a n o v u l a r Merino ewes (teasing) will cause m a n y ewes t o ovulate. T h e r a m - i n d u c e d o v u l a t i o n is well s y n c h r o n i s e d , 50% o f teased ewes ovulating within 41 h o f t h e i r first c o n t a c t w i t h rams, and is generally n o t a c c o m p a n i e d b y o e s t r u s {Oldham et al., 1 9 7 9 ) . D e s p i t e t h e s y n c h r o n i s e d o v u l a t i o n , s u b s e q u e n t oestrus in Merino f l o c k s is n o t well s y n c h r o n i s e d . C h a r a c t e r i s t i c a l l y t h e r e are t w o p e a k s in activity, t h e first a p p r o x i m a t e l y 18 d a y s a f t e r the c o n t a c t w i t h r a m s , and the s e c o n d a p p r o x i m a t e l y 6 d a y s later {Schinckel, 1 9 5 4 ; L y l e and H u n t e r , 1967; Fairnie, 1976). O u r analysis o f t h e d a i l y o e s t r o u s d a t a p r e s e n t e d b y H u n t e r et al. {1971) also reveals t w o p e a k s 6 d a y s apart. F o l l o w i n g t h e r a m - i n d u c e d o v u l a t i o n , d e v e l o p m e n t in s o m e ewes o f c o r p o r a l u t e a (CLs) w i t h n o r m a l life spans w o u l d a c c o u n t f o r the first p e a k in o e s t r u s a r o u n d D a y 18. We r e a s o n e d t h a t t h e s e c o n d p e a k a r o u n d D a y 24 m a y be caused b y p r e m a t u r e regression o f t h e initial CL in s o m e ewes, foll o w e d b y s e c o n d o v u l a t i o n w i t h o u t o e s t r u s (silent o v u l a t i o n ) and CL w i t h a n o r m a l life span.
292 MATERIALS AND METHODS F r o m a c o m m e r c i a l flock o f 1 2 0 0 ewes w h i c h had b e e n isolated f r o m r a m s during t h e p r e c e d i n g 2 m o n t h s , 113 4-year-old Merino ewes were selected at r a n d o m o n l l t h O c t o b e r 1977. T h e n e x t d a y t h e y w e r e e x a m i n e d b y l a p a r o s c o p y u n d e r local anaesthesia b u t w i t h o u t t r a n q u i l i z a t i o n ( O l d h m n et al., 1976a), to select t h o s e ewes w h i c h w e r e anovular. T h e results are s h o w n in T a b l e I. TABLE I Ovarian activity, Day 0 *Classification of ewes at laparoscopy, Day 0 No result Recent ovulatory activity (cycling ewes) No recent ovulatory activity (anovular ewes)
No. of ewes 6 16 91
*Presence of either a current Corpus Luteum or past Corpus Albicans was taken as evidence of recent ovulatory activity. F o l l o w i n g l a p a r o s c o p y ( D a y 0) t h e ewes were j o i n e d w i t h five vasectom i s e d Merino r a m s in a 20 h a p a d d o c k . T h e r a m s were fitted with "sire s i n e " harnesses and c r a y o n s ( R a d f o r d et al., 1960). C r a y o n m a r k s o n the r u m p s o f t h e ewes w e r e r e c o r d e d on D a y s 3, 10 and 14 and t h e r e a f t e r e v e r y 7 d a y s until D a y 35. T h e ovaries o f the 91 ewes which were a n o v u l a r on D a y 0 w e r e e x a m i n e d again o n D a y s 3 and 10. On D a y 3 the ewes ovulating in r e s p o n s e to teasing w e r e identified and t h e site or sites o f o v u l a t i o n o n the surfaces o f their ovaries w e r e l o c a t e d and a g e d a c c o r d i n g to t h e p r o c e d u r e o f O l d h a m and L i n d s a y (1979). A t l a p a r o s c o p y {Day 10) t h e ewes w h o s e r a m - i n d u c e d CLs had regressed p r e m a t u r e l y were identified. In ewes w i t h n o r m a l ovarian cycles the CLs were greater t h a n seven d a y s old and l o c a t e d in the s a m e p o s i t i o n s on the ovaries as d e s c r i b e d on D a y 3. In ewes w h i c h had h a d a b n o r m a l cycles the CLs were 1--2 d a y s old and l o c a t e d in n e w p o s i t i o n s on the ovaries. In m o s t o f these cases c o r p o r a a l b i c a n t i a (CA) were o b s e r v e d at the sites p r e v i o u s l y o c c u p i e d b y CLs o n D a y 3. T h e f a t e o f the r a m - i n d u c e d CLs in p r e v i o u s l y a n o v u l a r ewes is s h o w n in t h e k e y in T a b l e II. At l a p a r o s c o p y o n D a y 3, 56 ewes had o v u l a t e d (A). On D a y 10 a f u r t h e r 18 had o v u l a t e d at a t i m e e s t i m a t e d to be s o o n a f t e r D a y 3 (AA.B}. S e v e n t e e n ewes (AA.BB) failed to r e s p o n d to rams.
293 TABLE II The fate of the ram-induced CLs in previously anovular Merino ewes No. of Ewes (n = 91) Key to ewe classification based on ovarian observations on Days 0, 3 and 10 of the experiment A.
+ CL Day 3 (67--72 h after Rams in) B. + C L D a y 10 C. CL > 7 days old CC. CL 1--2 days old D. + CA Day 3 DD. --CA Day 3 BB. --CL Day 10 but + CA Day 3
AA. --CL Day 3 B. + CL Day 10 C. CL > 7 days old CC. CL 1--2 days old D. + CA (not obs. Day 3) DD. --CA* BB.
--CL Day 10
56 52 28 24 20
4 35 18 10 8 5 3 17
*These ewes are included in the analysis as ewes ovulating twice within 10 days of teasing. Their inclusion in this category is supported by photographic evidence that some CAs and even some waning CLs from current ovulations are invisible at laparoscopy (Oldham and Lindsay, 1979).
RESULTS AND DISCUSSION In 38 o f 74 (A.B.C. a n d A A . B . C ) o f t h e r e s p o n d i n g ewes t h e r a m - i n d u c e d CLs p e r s i s t e d f o r a n o r m a l p e r i o d . This was c o n f i r m e d w h e n 33 o f t h e s e ewes w e r e m a r k e d b y t h e r a m s b e t w e e n D a y s 14 a n d 21. In a t e a s e d f l o c k , ewes like t h e s e w o u l d c o n t r i b u t e t o a p e a k o f o e s t r o u s a c t i v i t y a r o u n d D a y 18. In 32 o f 74 ( A . B . C C a n d A A . B . C C ) o f t h e r e s p o n d i n g ewes t h e r a m - i n d u c e d CLs r e g r e s s e d p r e m a t u r e l y , t o be f o l l o w e d b y a s e c o n d o v u l a t i o n a n d CL w h i c h p e r s i s t e d f o r a n o r m a l p e r i o d . A l l t h e s e ewes w e r e m a r k e d b y t h e r a m s b e t w e e n D a y s 21 a n d 28. In a t e a s e d f l o c k , ewes like t h e s e w o u l d c o n t r i b u t e t o a s e c o n d p e a k o f o e s t r o u s a c t i v i t y a r o u n d D a y 24. In t h e r e m a i n i n g ewes t h a t r e s p o n d e d t o r a m s t h e CLs r e g r e s s e d prem a t u r e l y , as s h o w n b y a C A on D a y 10, b u t t h e r e was n o s e c o n d o v u l a t i o n . O e s t r u s was d i s p l a y e d b y o n l y t w o o f t h e 74 ewes o v u l a t i n g in r e s p o n s e t o r a m s b e t w e e n D a y s 0 a n d 3. In o n e o f t h e s e ewes t h e i n i t i a l CL p e r s i s t e d n o r m a l l y a n d in t h e o t h e r it r e g r e s s e d p r e m a t u r e l y . O n l y o n e o f t h e 32 ewes o v u l a t i n g t w i c e b e t w e e n D a y 0 a n d D a y 10 dis-
294
played oestrus at her second ovulation. The incidence of seasonal anovulation in the flock before teasingand the magnitude of the ovulation response of seasonally anovular ewes to teasing are similar to values previously reported for commercial Merino ewes in Western Australia during October and November (Oldham et al., 1976b, Oldham et al., 1979). Short oestrous cycles (6--7 days) and premature regression of CLs have been noted in ewes beginning oestrous activity during lactation (Land, 1971; Restall, 1971), and in ewes suffering from chronic clover disease (N.R. Adams and C.M. Oldham, unpublished data, 1978). Oldham and Lindsay (1979) present photographic evidence of a 6-day oestrous cycle associated with apparently normal development of the CL until the fourth day of the cycle. Walton et al. (1977) observed a surge of LH followed by a short period of high concentration of progesterone in the plasma of Clun-Forest ewes, immediately before the first ovulation of the breeding season. The authors suggested that the progesterone came from luteinized follicles but it seems likely from our evidence that the source of progesterone was a CL with a short life span. Progesterone priming is necessary if behavioural oestrus is to accompany ovulation in ewes at the first ovulation of the breeding season (Robinson, 1959) or at the ram-induced ovulation in anoestrous Merino ewes (Hunter et al., 1971). When Hunter et al. (1971) administered progestagen to ewes prior to teasing there was only one peak of oestrus compared with two peaks in the control group, This suggests to us that a progestational phase not only facilitates behavioural oestrus but also prevents premature regression of the ram-induced CL. If the CLs with a short life span in our experiment did secrete progesterone, the amount secreted or duration of production was insufficient to ensure behavioural oestrus at the second ovulation some 6 days after the rams were introduced. However, it may have been sufficient to ensure persistence of the CL which followed the second ovulation. ACKNOWLEDGEMENTS
We wish to acknowledge the financial support of the Australian Meat Research Committee, the technical assistance of P.T. Doyle, P.B. Gherardi, A.M. Paterson and I. Pope, and the constructive criticism of D.R. Lindsay.
REFERENCES Fairnie, I.J., 1976. Organisation of artificial breeding programmes of sheep in Western Australia. In G.T. Tomes, D.E. Robertson and R.J. Lightfoot (Editors), Proc. Int. Sheep Breeding Congr. W.A.I.T. Press, Perth, pp. 500--508.
295 Hunter, G.L., Belonje, P.C. and Van Niekerk, C.H., 1971. Synchronized mating and lambing in spring-bred Merino sheep: The use of progestogen-impregnated intra-vaginal sponges and teaser rams. Agroanimalia, 3: 133--140. Land, R.B., 1971. The incidence of oestrus during lactation in Finish Landrace, Dorset Horn and Finn-Dorset sheep. J. Reprod. Fertil., 24 : 345--352. Lyle, A.D. and Hunter, G.L., 1967. Teasing groups of ewes at staggered intervals as a means of levelling the ram mating load and flock lambing rate. S. Afr. J. Agric. Sci., 10 : 597--608. Oldham, C.M. and Lindsay, D.R., 1979. Laparoscopy in the ewe: a photographic record of the ovarian activity of ewes experiencing normal or abnormal oestrous cycles. Anita. Reprod. Sci. (accepted for publication). Oldham, C.M., Knight, T.W. and Lindsay, D.R., 1976a. A comparison of the effects on reproductive performance in sheep, of two methods of estimation of ovulation rate. Aust. J. Exp. Agric. Anita. Husb., 16: 24--27. Oldham, C.M., Knight, T.W. and Lindsay, D.R., 1976b. An explanation for the reduced fertility in Merino ewes at the first oestrus of the breeding season. Proc. Aust. Soc. Anim. Prod., 11: 129--132. Oldham, C.M., Martin, G.B. and Knight, T.W., 1979. Stimulation of seasonally anovular Merino ewes by rams. I. Time from introduction of the rams to the pre-ovulatory LH surge and ovulation. Anita. Reprod. Sci., 1 : 283--290. Radford, H.M., Watson, R.H. and Wood, G.F., 1960. A crayon and associated harness for the detection of mating under field conditions. Aust. Vet. J., 36: 57--66. Restall, B.J., 1971. The effect of lamb removal on reproductive activity in Dorset Horn × Merino ewes after lambing. J. Reprod. Fertil., 24: 145--146. Robinson, T.J., 1959. The oestrous cycle of the ewe and doe. In: H.H. Cole and P.T. Cupps (Editors), Reproduction in Domestic Animals Vol. I. Academic Press, N e w York and London, pp. 291--333. Schinckel, P.G., 1954. The effect of the ram on the incidence and occurrence of oestrus in ewes. Aust. Vet. J., 30 (7): 189--195. Walton, J.S., McNeilly, J.R., McNeilly, A.S. and Cunningham, F.J., 1977. Changes in concentrations of follicle-stimulating hormone, luteinizing hormone, prolactin and progesterone in the plasma of ewes during the transition from anoestrus to breeding activity. J. Endocrinol., 75: 127--136.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
S T I M U L A T I O N O F S E A S O N A L L Y A N O V U L A R M E R I N O EWES B Y RAMS
II. P R E M A T U R E
REGRESSION
OF RAM-INDUCED
CORPORA
LUTEA
C.M. OLDHAM and G.B. MARTIN Department o f Animal Science and Production, Institute o f Agriculture, University o f Western Australia, Nedlands, 6009 W.A. (Australia)
(Accepted 6 September 1978)
ABSTRACT Oldham, C.M., Martin, G.B., 1978/1979. Stimulation of seasonally anovular Merino ewes by rams. II. Premature regression of ram-induced corpora lutea. Anim. Reprod. Sci., 1: 291--295. Ovulation was induced by rams in 74 of 91 seasonally anovular Merino ewes. The resulting corpora lutea (CL) were observed by laparoscopy and were found to either persist normally (38/74), or regress prematurely (36/74). In 32 of the latter ewes premature regression of the CL was followed by a second ovulation within 6 days of the introduction of rams.
INTRODUCTION T h e i n t r o d u c t i o n o f r a m s to p r e v i o u s l y isolated a n o v u l a r Merino ewes (teasing) will cause m a n y ewes t o ovulate. T h e r a m - i n d u c e d o v u l a t i o n is well s y n c h r o n i s e d , 50% o f teased ewes ovulating within 41 h o f t h e i r first c o n t a c t w i t h rams, and is generally n o t a c c o m p a n i e d b y o e s t r u s {Oldham et al., 1 9 7 9 ) . D e s p i t e t h e s y n c h r o n i s e d o v u l a t i o n , s u b s e q u e n t oestrus in Merino f l o c k s is n o t well s y n c h r o n i s e d . C h a r a c t e r i s t i c a l l y t h e r e are t w o p e a k s in activity, t h e first a p p r o x i m a t e l y 18 d a y s a f t e r the c o n t a c t w i t h r a m s , and the s e c o n d a p p r o x i m a t e l y 6 d a y s later {Schinckel, 1 9 5 4 ; L y l e and H u n t e r , 1967; Fairnie, 1976). O u r analysis o f t h e d a i l y o e s t r o u s d a t a p r e s e n t e d b y H u n t e r et al. {1971) also reveals t w o p e a k s 6 d a y s apart. F o l l o w i n g t h e r a m - i n d u c e d o v u l a t i o n , d e v e l o p m e n t in s o m e ewes o f c o r p o r a l u t e a (CLs) w i t h n o r m a l life spans w o u l d a c c o u n t f o r the first p e a k in o e s t r u s a r o u n d D a y 18. We r e a s o n e d t h a t t h e s e c o n d p e a k a r o u n d D a y 24 m a y be caused b y p r e m a t u r e regression o f t h e initial CL in s o m e ewes, foll o w e d b y s e c o n d o v u l a t i o n w i t h o u t o e s t r u s (silent o v u l a t i o n ) and CL w i t h a n o r m a l life span.
292 MATERIALS AND METHODS F r o m a c o m m e r c i a l flock o f 1 2 0 0 ewes w h i c h had b e e n isolated f r o m r a m s during t h e p r e c e d i n g 2 m o n t h s , 113 4-year-old Merino ewes were selected at r a n d o m o n l l t h O c t o b e r 1977. T h e n e x t d a y t h e y w e r e e x a m i n e d b y l a p a r o s c o p y u n d e r local anaesthesia b u t w i t h o u t t r a n q u i l i z a t i o n ( O l d h m n et al., 1976a), to select t h o s e ewes w h i c h w e r e anovular. T h e results are s h o w n in T a b l e I. TABLE I Ovarian activity, Day 0 *Classification of ewes at laparoscopy, Day 0 No result Recent ovulatory activity (cycling ewes) No recent ovulatory activity (anovular ewes)
No. of ewes 6 16 91
*Presence of either a current Corpus Luteum or past Corpus Albicans was taken as evidence of recent ovulatory activity. F o l l o w i n g l a p a r o s c o p y ( D a y 0) t h e ewes were j o i n e d w i t h five vasectom i s e d Merino r a m s in a 20 h a p a d d o c k . T h e r a m s were fitted with "sire s i n e " harnesses and c r a y o n s ( R a d f o r d et al., 1960). C r a y o n m a r k s o n the r u m p s o f t h e ewes w e r e r e c o r d e d on D a y s 3, 10 and 14 and t h e r e a f t e r e v e r y 7 d a y s until D a y 35. T h e ovaries o f the 91 ewes which were a n o v u l a r on D a y 0 w e r e e x a m i n e d again o n D a y s 3 and 10. On D a y 3 the ewes ovulating in r e s p o n s e to teasing w e r e identified and t h e site or sites o f o v u l a t i o n o n the surfaces o f their ovaries w e r e l o c a t e d and a g e d a c c o r d i n g to t h e p r o c e d u r e o f O l d h a m and L i n d s a y (1979). A t l a p a r o s c o p y {Day 10) t h e ewes w h o s e r a m - i n d u c e d CLs had regressed p r e m a t u r e l y were identified. In ewes w i t h n o r m a l ovarian cycles the CLs were greater t h a n seven d a y s old and l o c a t e d in the s a m e p o s i t i o n s on the ovaries as d e s c r i b e d on D a y 3. In ewes w h i c h had h a d a b n o r m a l cycles the CLs were 1--2 d a y s old and l o c a t e d in n e w p o s i t i o n s on the ovaries. In m o s t o f these cases c o r p o r a a l b i c a n t i a (CA) were o b s e r v e d at the sites p r e v i o u s l y o c c u p i e d b y CLs o n D a y 3. T h e f a t e o f the r a m - i n d u c e d CLs in p r e v i o u s l y a n o v u l a r ewes is s h o w n in t h e k e y in T a b l e II. At l a p a r o s c o p y o n D a y 3, 56 ewes had o v u l a t e d (A). On D a y 10 a f u r t h e r 18 had o v u l a t e d at a t i m e e s t i m a t e d to be s o o n a f t e r D a y 3 (AA.B}. S e v e n t e e n ewes (AA.BB) failed to r e s p o n d to rams.
293 TABLE II The fate of the ram-induced CLs in previously anovular Merino ewes No. of Ewes (n = 91) Key to ewe classification based on ovarian observations on Days 0, 3 and 10 of the experiment A.
+ CL Day 3 (67--72 h after Rams in) B. + C L D a y 10 C. CL > 7 days old CC. CL 1--2 days old D. + CA Day 3 DD. --CA Day 3 BB. --CL Day 10 but + CA Day 3
AA. --CL Day 3 B. + CL Day 10 C. CL > 7 days old CC. CL 1--2 days old D. + CA (not obs. Day 3) DD. --CA* BB.
--CL Day 10
56 52 28 24 20
4 35 18 10 8 5 3 17
*These ewes are included in the analysis as ewes ovulating twice within 10 days of teasing. Their inclusion in this category is supported by photographic evidence that some CAs and even some waning CLs from current ovulations are invisible at laparoscopy (Oldham and Lindsay, 1979).
RESULTS AND DISCUSSION In 38 o f 74 (A.B.C. a n d A A . B . C ) o f t h e r e s p o n d i n g ewes t h e r a m - i n d u c e d CLs p e r s i s t e d f o r a n o r m a l p e r i o d . This was c o n f i r m e d w h e n 33 o f t h e s e ewes w e r e m a r k e d b y t h e r a m s b e t w e e n D a y s 14 a n d 21. In a t e a s e d f l o c k , ewes like t h e s e w o u l d c o n t r i b u t e t o a p e a k o f o e s t r o u s a c t i v i t y a r o u n d D a y 18. In 32 o f 74 ( A . B . C C a n d A A . B . C C ) o f t h e r e s p o n d i n g ewes t h e r a m - i n d u c e d CLs r e g r e s s e d p r e m a t u r e l y , t o be f o l l o w e d b y a s e c o n d o v u l a t i o n a n d CL w h i c h p e r s i s t e d f o r a n o r m a l p e r i o d . A l l t h e s e ewes w e r e m a r k e d b y t h e r a m s b e t w e e n D a y s 21 a n d 28. In a t e a s e d f l o c k , ewes like t h e s e w o u l d c o n t r i b u t e t o a s e c o n d p e a k o f o e s t r o u s a c t i v i t y a r o u n d D a y 24. In t h e r e m a i n i n g ewes t h a t r e s p o n d e d t o r a m s t h e CLs r e g r e s s e d prem a t u r e l y , as s h o w n b y a C A on D a y 10, b u t t h e r e was n o s e c o n d o v u l a t i o n . O e s t r u s was d i s p l a y e d b y o n l y t w o o f t h e 74 ewes o v u l a t i n g in r e s p o n s e t o r a m s b e t w e e n D a y s 0 a n d 3. In o n e o f t h e s e ewes t h e i n i t i a l CL p e r s i s t e d n o r m a l l y a n d in t h e o t h e r it r e g r e s s e d p r e m a t u r e l y . O n l y o n e o f t h e 32 ewes o v u l a t i n g t w i c e b e t w e e n D a y 0 a n d D a y 10 dis-
294
played oestrus at her second ovulation. The incidence of seasonal anovulation in the flock before teasingand the magnitude of the ovulation response of seasonally anovular ewes to teasing are similar to values previously reported for commercial Merino ewes in Western Australia during October and November (Oldham et al., 1976b, Oldham et al., 1979). Short oestrous cycles (6--7 days) and premature regression of CLs have been noted in ewes beginning oestrous activity during lactation (Land, 1971; Restall, 1971), and in ewes suffering from chronic clover disease (N.R. Adams and C.M. Oldham, unpublished data, 1978). Oldham and Lindsay (1979) present photographic evidence of a 6-day oestrous cycle associated with apparently normal development of the CL until the fourth day of the cycle. Walton et al. (1977) observed a surge of LH followed by a short period of high concentration of progesterone in the plasma of Clun-Forest ewes, immediately before the first ovulation of the breeding season. The authors suggested that the progesterone came from luteinized follicles but it seems likely from our evidence that the source of progesterone was a CL with a short life span. Progesterone priming is necessary if behavioural oestrus is to accompany ovulation in ewes at the first ovulation of the breeding season (Robinson, 1959) or at the ram-induced ovulation in anoestrous Merino ewes (Hunter et al., 1971). When Hunter et al. (1971) administered progestagen to ewes prior to teasing there was only one peak of oestrus compared with two peaks in the control group, This suggests to us that a progestational phase not only facilitates behavioural oestrus but also prevents premature regression of the ram-induced CL. If the CLs with a short life span in our experiment did secrete progesterone, the amount secreted or duration of production was insufficient to ensure behavioural oestrus at the second ovulation some 6 days after the rams were introduced. However, it may have been sufficient to ensure persistence of the CL which followed the second ovulation. ACKNOWLEDGEMENTS
We wish to acknowledge the financial support of the Australian Meat Research Committee, the technical assistance of P.T. Doyle, P.B. Gherardi, A.M. Paterson and I. Pope, and the constructive criticism of D.R. Lindsay.
REFERENCES Fairnie, I.J., 1976. Organisation of artificial breeding programmes of sheep in Western Australia. In G.T. Tomes, D.E. Robertson and R.J. Lightfoot (Editors), Proc. Int. Sheep Breeding Congr. W.A.I.T. Press, Perth, pp. 500--508.
295 Hunter, G.L., Belonje, P.C. and Van Niekerk, C.H., 1971. Synchronized mating and lambing in spring-bred Merino sheep: The use of progestogen-impregnated intra-vaginal sponges and teaser rams. Agroanimalia, 3: 133--140. Land, R.B., 1971. The incidence of oestrus during lactation in Finish Landrace, Dorset Horn and Finn-Dorset sheep. J. Reprod. Fertil., 24 : 345--352. Lyle, A.D. and Hunter, G.L., 1967. Teasing groups of ewes at staggered intervals as a means of levelling the ram mating load and flock lambing rate. S. Afr. J. Agric. Sci., 10 : 597--608. Oldham, C.M. and Lindsay, D.R., 1979. Laparoscopy in the ewe: a photographic record of the ovarian activity of ewes experiencing normal or abnormal oestrous cycles. Anita. Reprod. Sci. (accepted for publication). Oldham, C.M., Knight, T.W. and Lindsay, D.R., 1976a. A comparison of the effects on reproductive performance in sheep, of two methods of estimation of ovulation rate. Aust. J. Exp. Agric. Anita. Husb., 16: 24--27. Oldham, C.M., Knight, T.W. and Lindsay, D.R., 1976b. An explanation for the reduced fertility in Merino ewes at the first oestrus of the breeding season. Proc. Aust. Soc. Anim. Prod., 11: 129--132. Oldham, C.M., Martin, G.B. and Knight, T.W., 1979. Stimulation of seasonally anovular Merino ewes by rams. I. Time from introduction of the rams to the pre-ovulatory LH surge and ovulation. Anita. Reprod. Sci., 1 : 283--290. Radford, H.M., Watson, R.H. and Wood, G.F., 1960. A crayon and associated harness for the detection of mating under field conditions. Aust. Vet. J., 36: 57--66. Restall, B.J., 1971. The effect of lamb removal on reproductive activity in Dorset Horn × Merino ewes after lambing. J. Reprod. Fertil., 24: 145--146. Robinson, T.J., 1959. The oestrous cycle of the ewe and doe. In: H.H. Cole and P.T. Cupps (Editors), Reproduction in Domestic Animals Vol. I. Academic Press, N e w York and London, pp. 291--333. Schinckel, P.G., 1954. The effect of the ram on the incidence and occurrence of oestrus in ewes. Aust. Vet. J., 30 (7): 189--195. Walton, J.S., McNeilly, J.R., McNeilly, A.S. and Cunningham, F.J., 1977. Changes in concentrations of follicle-stimulating hormone, luteinizing hormone, prolactin and progesterone in the plasma of ewes during the transition from anoestrus to breeding activity. J. Endocrinol., 75: 127--136.