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Effects of continuous ram exposure and early spring lambing on initiation of the breeding season in yearling crossbred ewes
Animal Reproduction Science, 19 (1989) 265-272
265
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effects of Continuous Ram E x p o s u r e and Early Spring Lambing on Initiation of the Breeding Season in Yearling Crossbred Ewes D.R. NOTTER
Department of Animal Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Accepted 28 January 1989)
ABSTRACT Notter, D.R., 1989. Effects of continuous ram exposure and early spring lambing on initiation of the breeding season in yearling crossbred ewes. Anim. Reprod. Sci., 19: 265-272. The duration of the seasonal anestrus, the postpartum interval from lambing to first estrus and the date of initiation of the breeding season were studied using four groups of yearling ewes. The ewes were 1/2-Dorset, 1/4-Rambouillet, and 1/4-Finnsheep and averaged 190 days of age at the start of the study on 26 September. Group I ewes (n = 17) were not exposed for breeding but were continuously cohabited with vasectomized rams from 26 October until the following 28 September. Group 2 (n = 19), 3 (n - 18) and 4 (n = 16) ewes were exposed to fertile rams from 26 September until 1 December and subsequently isolated from rams. Group 2 ewes were then exposed to vasectomized rams within 7 to 10 days of lambing (mean date of 20 March) whereas group 3 ewes remained isolated from males. Group 4 ewes failed to lamb and also remained isolated from rams. Lambs were weaned on 28 May and ewes from all groups were combined and placed with vasectomized rams on 4 June to determine the time required for mating to begin. Ewes were checked twice weekly for evidence of mating; serum progesterone levels were determined to confirm that observed matings were accompanied by ovulation. The mean dates of last and subsequent first estrus in group 1 ewes were 22 February and 24 August, respectively, and the mean duration of the first seasonal anestrus was 183 days. The mean postpartum interval in group 2 ewes was 115 days; the mean date of first estrus was 15 July. After combining the groups on 4 June, the median dates of first mating for groups 1, 2, 3 and 4 were 1 September, 19 July, 18 July and 20 July, respectively. Thus, ewes that were continuously cohabited with rams took an average of 45 days longer to begin mating. Ewes in the other groups did no differ, suggesting that an average of 63 days of ram exposure immediately following lambing had no negative effects on subsequent summer mating behavior.
INTRODUCTION
Introduction of rams into flocks of seasonally anestrous ewes following a period of isolation from males ("teasing") often results in ovulation and estrus
0378-4320/89/$03.50
© 1989 Elsevier Science Publishers B.V.
266 in a substantial proportion of the ewes. Endocrine patterns in ewes that respond to this "ram effect" have been described by Martin et al. (1986). Ewes that respond to teasing usually ovulate within 54 h of ram introduction but do not show estrus at first ovulation (Oldham et al., 1978). Following this ovulation, some ewes develop a normal corpus luteum (CL) and subsequently exhibit estrus at their second ovulation. Other ewes experience premature regression of the first CL, ovulate a second time (again without estrus) and then develop a normal CL which provides the elevated circulating levels of progesterone that are required to allow estrus at the subsequent (third) ovulation. Thus, flocks of teased ewes normally show two peaks of estrous activity. The first occurs 18 to 19 days after ram introduction and the second occurs about 7 days later. The response of ewes to teasing varies with the breed of the ewe (Nugent et al., 1988a) and ram (Tervit et al., 1977; Tervit and Peterson, 1978; Nugent et al., 1988b) and with the timing of ram introduction relative to the normal anestrous season (Nugent et al., 1988a). Teasing is most effective immediately before the onset of the normal breeding season in most breeds (Edgar and Bilkey, 1963; Nugent et al., 1988a). In contrast, ewes exposed to rams during periods of deep anestrus may ovulate once in response to males but subsequently return to anestrus before the second ovulation and without showing estrus (Oldham and Cognie, 1980). Proper use of the ram effect may be critical in accelerated lambing systems. To maximize annual lambing rate, ewes need to be rebred as soon as possible after lambing. However, if rams are introduced too soon after lambing, the ewes may be incapable of ovulating due to incomplete uterine involution or lactational anestrus but may still become desensitized to the ram effect, thereby unnecessarily prolonging time to rebreeding. This experiment was designed to evaluate the effect of early spring lambing and of continuous ram exposure on the mating behavior of whitefaced crossbred ewes in early summer. Specific objectives were: (1) to estimate the duration of the first anestrus in ewes continuously exposed to vasectomized rams, (2) to estimate the postpartum interval from lambing to first estrus in spring-lambing ewes, (3) to compare the mating behavior of teased ewes to that of ewes continuously exposed to vasectomized rams, and (4) to determine effects of immediate postpartum ram exposure on subsequent mating behavior. MATERIALSAND METHODS
Experimental design Seventy-five 1/2-Dorset, 1/4-Rambouillet, 1/4-Finnsheep ewe lambs were used for this study. Ewes were second-generation crossbreds produced by mating three-way-cross (Dorset X Finnsheep-Rambouillet) ewes and rams and had
267 been born in March and April. Ewe lambs averaged 190 days of age (s.d. = 13 days) and had an average weight of 40.4 kg (s.d.=4.7 kg) at the start of the study on 26 September. At this time, 55 of the ewe lambs were exposed for 66 days to ram lambs of comparable age and genetic composition in single-sire breeding pastures and were subsequently isolated from rams until lambing. The remaining 20 ewes (group 1 ) remained isolated from rams until 26 October, at which time they were joined with three vasectomized rams. These group 1 ewes remained with vasectomized rams continuously for the next 11 months to determine the timing and duration of the first anestrous period. Vasectomized rams were mature, crossbred rams possessing varying percentages of Dorset, Finnsheep, Coopworth and Barbados Blackbelly breeding. Rams were fitted with crayon-equipped marking harnesses and ewes were checked twice weekly for evidence of mating. Marks were scored with regard to the intensity of the mark. A score of 1 denoted an intense mark that apparently resulted from repeated mounts. A score of 2 denoted a single mark of any kind, and was interpreted to indicate a single a t t e m p t (perhaps unsuccessful) by a ram to mount the ewe. The color of the crayons worn by the rams was changed every 14 days ( o r a n g e - . b l u e - - , r e d - , b l a c k ) . R u m p s of the ewes were shorn every 56 days before reinitiating the color sequence. Jugular blood samples were taken from all marked ewes on the day the mark was observed and at the next two observation times. Serum from these samples was frozen, and serum progesterone concentrations were subsequently determined by radioimmunoassay (Beal et al., 1986; Nugent et al., 1988a). Marks that were not accompanied by elevated progesterone levels characteristic of a CL were assumed to not be indicative of normal estrus. Lambing began on 24 February, and the ewes that had been exposed to intact rams were divided into three groups at that time. Group 2 ewes were exposed to three vasectomized crossbred rams comparable to those with group 1 ewes as soon as the ewes and their lambs left the lambing area (usually within 7 to 10 days of birth). These ewes were used to determine the postpartum interval to first estrus and to investigate effects of immediate ram exposure on later mating behavior. Group 3 ewes also lambed but were isolated from mature rams after lambing. The average lambing date for the group 2 and 3 ewes was 20 March (range: 24 February-17 April). Sixteen of the ewes that had been exposed to intact rams did not lamb. These ewes (group 4) thus represent ewes that were isolated from rams during the winter but did not lamb. Care must be taken in interpreting subsequent performance of this group because they represent a selected, and potentially less fertile, group of ewes than those in the other groups. Group 4 ewes were 11.4 days younger ( P < 0.01) and 3.2 kg lighter ( P < 0.05) at the start of breeding than ewes that lambed, suggesting that the relative immaturity of group 4 ewes likely contributed to their failure to lamb. Lambs were weaned on 28 May. Ewes from all four groups were combined
268 on 4 June and placed with three of the vasectomized rams (two from group 1 and one from group 2). Checking of marks and bleeding continued as it had for group 1. Five ewes died or were removed for health reasons prior to this time. The final numbers of ewes in groups 1, 2, 3 and 4 were 17, 19, 18 and 16, respectively. All ewes remained with the vasectomized rams until 30 July. At that time, ewes that had been marked with an intensity score of 1 on at least two dates separated by a period consistent with a normal estrous cycle were removed from the study. Remaining ewes were subsequently removed as they met this criterion. The study terminated on 28 September with the beginning of the fall breeding season. All ewes had marked at least once by this time but nine ewes had not received a second mark. These nine ewes were assumed to have initiated their breeding season at the time of the first observed mark. The study thus allows comparison of late spring and early summer mating in yearling ewes that were maintained open with continuous ram contact (group 1 ), were allowed to lamb with immediate ram exposure (group 2 ), were allowed to lamb with continued ram isolation (group 3) or failed to lamb when given the opportunity and were isolated from rams (group 4).
Statistical analysis Average duration of first anestrus under continuous ram exposure was determined for group 1 ewes. Average postpartum interval to first estrus was likewise determined from group 2 ewes. Time to first mating following combining of the four ewe groups on 4 June was calculated for each ewe. The first mating was specifically defined as the first mark that was accompanied by an ovulation and followed by a second mark and ovulation within 14 to 18 days. Differences among groups in the distribution of time to first mating after 4 June were tested with the L e e - D e s u statistic (Lee and Desu, 1972). This nonparametric statistic compares waiting periods from some initial time until the occurrence of some signal event. Calculations were performed using procedures described by Hull and Nie (1981). RESULTS
Timing and duration of first anestrus All but two of the 17 group 1 ewes mated within 2.5 week of the start of the study. Thus, 88% of these ewe lambs apparently reached puberty by 229 + 3 days of age. The mean date of last estrus for group 1 ewes was 22 February (range 23 J a n u a r y - 3 1 March). Under continuous ram cohabitation, the mean date of onset of the subsequent breeding season was 24 August (range: 13 J u l y -
269 28 September) and the mean duration of anestrus was 183 days (range: 136223 days). The correlation between date of last estrus and date of first subsequent estrus was - 0 . 1 3 and was not significant. Intermittent estrous activity was observed during the seasonal anestrus. Between 12 May and 5 June, low-intensity marks (intensity score of 2) were observed on 8 of 17 group 1 ewes (47%). No marks with a score of 1 were recorded. Subsequent analysis of circulating progesterone indicated that 5 of the ewes ovulated (peak progesterone of > 0.75 ng/ml) and one additional ewe had an elevation in circulating progesterone to 0.58 ng/ml. In contrast, only one ewe received a low-intensity mark in the month of April and no marks of any kind were observed from 9 June until 13 July. No reason for this flurry of low-intensity mating in M a y was apparent. Fenceline contact with other rams did not occur. Two ewes mated and ovulated in July (13 and 16 July) hm apparently returned to anestrus before initiating regular cycles on 24 August and 14 September, respectively. Both these ewes had also received low-intensity marks and ovulated in late May.
Postpartum interval to first estrus W h e n exposed to vasectomized rams within 7 to 10 days of lambing, the mean date of first mating for group 2 ewes was 15 July (range: 15 May-17 September) and the mean p o s t p a r t u m interval was 115 days (range: 61-183 days ). P o s t p a r t u m interval was not correlated with lambing date (r = - 0.19, P = 0.40 ). Only one group 2 ewe mated early (on 29 May) but then apparently returned to anestrus until a later date (13 July). Three other ewes that mated before 1 June at less than 80 days postpartum all maintained a continuous pattern of estrus at regular intervals until removal from the study on 30 July.
Effect of acute ram introduction on initiation of mating Patterns of occurrence of first mating after combining all groups on 4 June are shown in Fig. 1. Median dates of first mating for ewes in groups I through 4 were 1 September, 19 July, 18 July and 20 July, respectively. Thus, group 1 ewes required an average of 45 days longer ( P < 0.01) to begin cycling than ewes in the other groups; approximately 40% of the ewes in the other groups had mated before the beginning of mating by group 1 ewes. The timing of the first mating did not differ for ewes in groups 2, 3 and 4. although group 4 ewes (which did not lamb and were isolated from rams ) tended to show less variation in time to first mating. Among ewes that lambed, immediate ram exposure for an average of 63 days during and immediately after lactation appeared to have no effect on subsequent mating ability in June and July.
270
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Fig. 1. Cumulative percentage of ewes in each of the four groups that mated after being combined on 4 June. DISCUSSION
This study demonstrates that continuous exposure of open yearling ewes to rams during late fall, winter and early spring prolongs the onset of mating relative to that which is achievable by ram isolation and subsequent reintroduction. Cohabitation of rams and ewes during anestrus prolonged time to first mating by over 6 weeks compared to ewes that had been isolated from rams. Several studies have used continuous cohabitation of ewes and vasectomized rams to characterize breed and individual differences in timing and duration of anestrus (Dufour, 1974; Lax et al., 1979; Jenkins and Ford, 1982 ). However, results of such characterizations must be strictly interpreted in terms of the experimental conditions. The percentage of ewes that are observed to cycle under conditions of continuous ram exposure likely represents the minimum fraction that could be expected to mate at that time under more favorable conditions. Use of the ram effect could result in much higher frequencies of cycling and the extent to which population differences in frequency of cycling under continuous ram exposure correlate to population differences in ability to respond to ram introduction is not known. The mean postpartum interval for spring-lambing ewes in this study was 115 days. Hoefler and Hallford (1987) reported a similar mean postpartum interval of 121 days for March-lambing fine-wool ewes in the southwestern U.S.A., and also reported that the postpartum interval did not differ between dry and lactating ewes. The latter observation supports the concept that there is not an important lactational anestrus in the sheep. The nonsignificant cor-
271
relation between lambing date and postpartum interval in this study suggests that the postpartum and seasonal anestrus are independent. For ewes that lambed exposure to vasectomized rams immediately after lambing had no negative impact on time of subsequent mating. Twenty-one percent of the group 2 ewes had already begun to cycle by the time the groups were combined, and the subsequent time to first mating in group 2 ewes was similar to that of ewes that had been isolated from rams. Thus, use of immediate ram exposure after lambing to attempt to minimize lambing interval is not contraindicated. Four of the group 2 ewes remated less than 80 days after lambing and during lactation. We do not know if these ewes would have conceived, but these results do suggest potential for rapid rebreeding in some springlambing ewes in this population. In a similar study, Amir and Gacitua ( 1987 ) reported a mean postpartum interval of 120 days in February-lambing Assaf ewes in Israel, but likewise found that 45% of these ewes returned to estrus relatively rapidly, with a mean postpartum interval of only 71 + 8 days. Group 4 ewes did not differ in mating behavior from ewes that lambed. Several studies have reported that winter-lambing ewes may be more likely to mate and conceive in spring breeding than ewes that remained open during winter. Speedy and FitzSimons (1977) reported that Finnsheep X Dorset and Greyface ewes in Scotland that had lambed 45 to 60 days before breeding had an 11% higher conception rate in August than ewes that were more than 60 days postpartum. Similarly, Notter and Copenhaver (1980) found that conception rates in Finnsheep crossbred ewes in April in the U.S.A. were 23% higher for ewes that had lambed in January and early February than for ewes that were open during the winter (65 vs 42% ). In both these studies, and in the current study, the open ewes represented a selected sample of ewes (i.e., those that failed to conceive as ewe lambs the previous fall). Thus, results from these groups must be interpreted with caution. In the current study, however, no inferiority of open ewes under ram isolation was observed. Dzabirski and Notter (1989) likewise evaluated the effect of time since lambing on lambing rates of mature unselected Dorset ewes bred in May in the U.S.A. Ewes that had previously lambed in late December and early January did not differ in lambing rate from those that had not lambed since the previous October (74% lambing for both groups). Thus, yearling ewes that lamb in March and April do not appear to differ in spring mating behavior from ewes that have been open but isolated from rams during the winter. ACKNOWLEDGEMENTS
The author wishes to express his appreciation to C. Burnsteel, L. Massoudi, L. O'Neal and M. Phelan for collection of marking data and blood and to L. Massoudi for conduct of the progesterone assays.
272 REFERENCES Amir, D. and Gacitua, H., 1987. Sexual activity of Assaf ewes after October and February lambings. Theriogenology, 27: 377-382. Beal, W.E., Onthank, D.C. and Zirkle, S.M., 1986. Effects of human chorionic gonadotropin and cyclic adenosine monophosphate on progesterone secretion by the ovine placenta. J. Anim. Sci., 63: 184-188. Dzabirski, V. and Notter, D.R., 1989. Effects of breed and time since lambing on spring estrous activity in mature ewes. Anim. Reprod. Sci., 19: 99-108. Dufour, J.J., 1974. The duration of the breeding season of four breeds of sheep. Can. J. Anim. Sci., 54: 389-392. Edgar, D.G. and Bilkey, D.A., 1963. The influence of rams on the onset of the breeding season in ewes. Proc. N.Z. Soc. Anim. Prod., 23: 79-87. Hoefler, W.C. and Hallford, D.M., 1987. Influence of suckling status and type of birth on serum hormone profiles and return to estrus in early-postpartum spring-lambingewes. Theriogenology, 27: 887-895. Hull, C.H. and Nie, N.H., 1981. Survival: life table analysis. In: SPSS Update 7-9. McGraw-Hill, New York, NY, pp. 205-219. Jenkins, T.G. and Ford, J.J., 1982. Estrous characteristics of Finnsheep crossbred and Morlam ewes. J. Anim. Sci., 55: 741-744. Lax, J., French, L.R., Chapman, A.B., Pope, A.L. and Casida, L.E., 1979. Length of breeding season for eight breed groups of sheep in Wisconsin. J. Anim. Sci., 49: 939-942. Lee, E. and Desu, M., 1972. A computer program for comparing K samples with right-censored data. Comput. Programs Biomed., 2: 315-321. Martin, G.B., Oldham, C.M., Cognie, 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. Notter, D.R. and Copenhaver, J.S., 1980. Performance of Finnish Landrace crossbred ewes under accelerated lambing. I. Fertility, prolificacy and ewe productivity. J. Anim. Sci., 5 l: 1033-1042. Nugent, R.A., III, Notter, D.R. and Beal, W.E., 1988a. Effects of ewe breed and ram exposure on estrous behaviour in May and June. J. Anim. Sci., 66: 1363-1370. Nugent, R.A., III, Notter, D.R. and McClure, W.H., 1988b. Effects of ram pre-exposure and ram breed on fertility of ewes in summer breeding. J. Anim. Sci., 66: 1622-1626. Oldham, C.M. and Cognie, Y., 1980. Do ewes continue to cycle after teasing? Proc. Aust. Soc. Anim. Prod., 13: 82-85. Oldham, C.M., Martin, G.B. and Knight, T.W., 1978. Stimulation of seasonally anovular ewes by rams. I. Time from introduction of rams to the preovulatory LH surge and ovulation. Anim. Reprod. Sci., 1: 283-290. Speedy, A.W. and FitzSimons, J., 1977. The reproductive performance of Finnish Landrace X Dorset Horn and Border Leicester × Scottish Blackface ewes mated three times in 2 years. Anim. Prod., 24: 189-196. Tervit, H.R. and Peterson, A.J., 1978. Testosterone levels in Dorset and Romney rams and the effectiveness of these breeds in stimulating early onset of estrus in Romney ewes. Theriogenology, 9: 271-278. Tervit, H.R., Havik, P.G. and Smith, J.F., 1977. Effect of breed of ram on the onset of the breeding season in Romney ewes. Proc. N.Z. Soc. Anim. Prod., 37: 142-148.
265
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effects of Continuous Ram E x p o s u r e and Early Spring Lambing on Initiation of the Breeding Season in Yearling Crossbred Ewes D.R. NOTTER
Department of Animal Science, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A.) (Accepted 28 January 1989)
ABSTRACT Notter, D.R., 1989. Effects of continuous ram exposure and early spring lambing on initiation of the breeding season in yearling crossbred ewes. Anim. Reprod. Sci., 19: 265-272. The duration of the seasonal anestrus, the postpartum interval from lambing to first estrus and the date of initiation of the breeding season were studied using four groups of yearling ewes. The ewes were 1/2-Dorset, 1/4-Rambouillet, and 1/4-Finnsheep and averaged 190 days of age at the start of the study on 26 September. Group I ewes (n = 17) were not exposed for breeding but were continuously cohabited with vasectomized rams from 26 October until the following 28 September. Group 2 (n = 19), 3 (n - 18) and 4 (n = 16) ewes were exposed to fertile rams from 26 September until 1 December and subsequently isolated from rams. Group 2 ewes were then exposed to vasectomized rams within 7 to 10 days of lambing (mean date of 20 March) whereas group 3 ewes remained isolated from males. Group 4 ewes failed to lamb and also remained isolated from rams. Lambs were weaned on 28 May and ewes from all groups were combined and placed with vasectomized rams on 4 June to determine the time required for mating to begin. Ewes were checked twice weekly for evidence of mating; serum progesterone levels were determined to confirm that observed matings were accompanied by ovulation. The mean dates of last and subsequent first estrus in group 1 ewes were 22 February and 24 August, respectively, and the mean duration of the first seasonal anestrus was 183 days. The mean postpartum interval in group 2 ewes was 115 days; the mean date of first estrus was 15 July. After combining the groups on 4 June, the median dates of first mating for groups 1, 2, 3 and 4 were 1 September, 19 July, 18 July and 20 July, respectively. Thus, ewes that were continuously cohabited with rams took an average of 45 days longer to begin mating. Ewes in the other groups did no differ, suggesting that an average of 63 days of ram exposure immediately following lambing had no negative effects on subsequent summer mating behavior.
INTRODUCTION
Introduction of rams into flocks of seasonally anestrous ewes following a period of isolation from males ("teasing") often results in ovulation and estrus
0378-4320/89/$03.50
© 1989 Elsevier Science Publishers B.V.
266 in a substantial proportion of the ewes. Endocrine patterns in ewes that respond to this "ram effect" have been described by Martin et al. (1986). Ewes that respond to teasing usually ovulate within 54 h of ram introduction but do not show estrus at first ovulation (Oldham et al., 1978). Following this ovulation, some ewes develop a normal corpus luteum (CL) and subsequently exhibit estrus at their second ovulation. Other ewes experience premature regression of the first CL, ovulate a second time (again without estrus) and then develop a normal CL which provides the elevated circulating levels of progesterone that are required to allow estrus at the subsequent (third) ovulation. Thus, flocks of teased ewes normally show two peaks of estrous activity. The first occurs 18 to 19 days after ram introduction and the second occurs about 7 days later. The response of ewes to teasing varies with the breed of the ewe (Nugent et al., 1988a) and ram (Tervit et al., 1977; Tervit and Peterson, 1978; Nugent et al., 1988b) and with the timing of ram introduction relative to the normal anestrous season (Nugent et al., 1988a). Teasing is most effective immediately before the onset of the normal breeding season in most breeds (Edgar and Bilkey, 1963; Nugent et al., 1988a). In contrast, ewes exposed to rams during periods of deep anestrus may ovulate once in response to males but subsequently return to anestrus before the second ovulation and without showing estrus (Oldham and Cognie, 1980). Proper use of the ram effect may be critical in accelerated lambing systems. To maximize annual lambing rate, ewes need to be rebred as soon as possible after lambing. However, if rams are introduced too soon after lambing, the ewes may be incapable of ovulating due to incomplete uterine involution or lactational anestrus but may still become desensitized to the ram effect, thereby unnecessarily prolonging time to rebreeding. This experiment was designed to evaluate the effect of early spring lambing and of continuous ram exposure on the mating behavior of whitefaced crossbred ewes in early summer. Specific objectives were: (1) to estimate the duration of the first anestrus in ewes continuously exposed to vasectomized rams, (2) to estimate the postpartum interval from lambing to first estrus in spring-lambing ewes, (3) to compare the mating behavior of teased ewes to that of ewes continuously exposed to vasectomized rams, and (4) to determine effects of immediate postpartum ram exposure on subsequent mating behavior. MATERIALSAND METHODS
Experimental design Seventy-five 1/2-Dorset, 1/4-Rambouillet, 1/4-Finnsheep ewe lambs were used for this study. Ewes were second-generation crossbreds produced by mating three-way-cross (Dorset X Finnsheep-Rambouillet) ewes and rams and had
267 been born in March and April. Ewe lambs averaged 190 days of age (s.d. = 13 days) and had an average weight of 40.4 kg (s.d.=4.7 kg) at the start of the study on 26 September. At this time, 55 of the ewe lambs were exposed for 66 days to ram lambs of comparable age and genetic composition in single-sire breeding pastures and were subsequently isolated from rams until lambing. The remaining 20 ewes (group 1 ) remained isolated from rams until 26 October, at which time they were joined with three vasectomized rams. These group 1 ewes remained with vasectomized rams continuously for the next 11 months to determine the timing and duration of the first anestrous period. Vasectomized rams were mature, crossbred rams possessing varying percentages of Dorset, Finnsheep, Coopworth and Barbados Blackbelly breeding. Rams were fitted with crayon-equipped marking harnesses and ewes were checked twice weekly for evidence of mating. Marks were scored with regard to the intensity of the mark. A score of 1 denoted an intense mark that apparently resulted from repeated mounts. A score of 2 denoted a single mark of any kind, and was interpreted to indicate a single a t t e m p t (perhaps unsuccessful) by a ram to mount the ewe. The color of the crayons worn by the rams was changed every 14 days ( o r a n g e - . b l u e - - , r e d - , b l a c k ) . R u m p s of the ewes were shorn every 56 days before reinitiating the color sequence. Jugular blood samples were taken from all marked ewes on the day the mark was observed and at the next two observation times. Serum from these samples was frozen, and serum progesterone concentrations were subsequently determined by radioimmunoassay (Beal et al., 1986; Nugent et al., 1988a). Marks that were not accompanied by elevated progesterone levels characteristic of a CL were assumed to not be indicative of normal estrus. Lambing began on 24 February, and the ewes that had been exposed to intact rams were divided into three groups at that time. Group 2 ewes were exposed to three vasectomized crossbred rams comparable to those with group 1 ewes as soon as the ewes and their lambs left the lambing area (usually within 7 to 10 days of birth). These ewes were used to determine the postpartum interval to first estrus and to investigate effects of immediate ram exposure on later mating behavior. Group 3 ewes also lambed but were isolated from mature rams after lambing. The average lambing date for the group 2 and 3 ewes was 20 March (range: 24 February-17 April). Sixteen of the ewes that had been exposed to intact rams did not lamb. These ewes (group 4) thus represent ewes that were isolated from rams during the winter but did not lamb. Care must be taken in interpreting subsequent performance of this group because they represent a selected, and potentially less fertile, group of ewes than those in the other groups. Group 4 ewes were 11.4 days younger ( P < 0.01) and 3.2 kg lighter ( P < 0.05) at the start of breeding than ewes that lambed, suggesting that the relative immaturity of group 4 ewes likely contributed to their failure to lamb. Lambs were weaned on 28 May. Ewes from all four groups were combined
268 on 4 June and placed with three of the vasectomized rams (two from group 1 and one from group 2). Checking of marks and bleeding continued as it had for group 1. Five ewes died or were removed for health reasons prior to this time. The final numbers of ewes in groups 1, 2, 3 and 4 were 17, 19, 18 and 16, respectively. All ewes remained with the vasectomized rams until 30 July. At that time, ewes that had been marked with an intensity score of 1 on at least two dates separated by a period consistent with a normal estrous cycle were removed from the study. Remaining ewes were subsequently removed as they met this criterion. The study terminated on 28 September with the beginning of the fall breeding season. All ewes had marked at least once by this time but nine ewes had not received a second mark. These nine ewes were assumed to have initiated their breeding season at the time of the first observed mark. The study thus allows comparison of late spring and early summer mating in yearling ewes that were maintained open with continuous ram contact (group 1 ), were allowed to lamb with immediate ram exposure (group 2 ), were allowed to lamb with continued ram isolation (group 3) or failed to lamb when given the opportunity and were isolated from rams (group 4).
Statistical analysis Average duration of first anestrus under continuous ram exposure was determined for group 1 ewes. Average postpartum interval to first estrus was likewise determined from group 2 ewes. Time to first mating following combining of the four ewe groups on 4 June was calculated for each ewe. The first mating was specifically defined as the first mark that was accompanied by an ovulation and followed by a second mark and ovulation within 14 to 18 days. Differences among groups in the distribution of time to first mating after 4 June were tested with the L e e - D e s u statistic (Lee and Desu, 1972). This nonparametric statistic compares waiting periods from some initial time until the occurrence of some signal event. Calculations were performed using procedures described by Hull and Nie (1981). RESULTS
Timing and duration of first anestrus All but two of the 17 group 1 ewes mated within 2.5 week of the start of the study. Thus, 88% of these ewe lambs apparently reached puberty by 229 + 3 days of age. The mean date of last estrus for group 1 ewes was 22 February (range 23 J a n u a r y - 3 1 March). Under continuous ram cohabitation, the mean date of onset of the subsequent breeding season was 24 August (range: 13 J u l y -
269 28 September) and the mean duration of anestrus was 183 days (range: 136223 days). The correlation between date of last estrus and date of first subsequent estrus was - 0 . 1 3 and was not significant. Intermittent estrous activity was observed during the seasonal anestrus. Between 12 May and 5 June, low-intensity marks (intensity score of 2) were observed on 8 of 17 group 1 ewes (47%). No marks with a score of 1 were recorded. Subsequent analysis of circulating progesterone indicated that 5 of the ewes ovulated (peak progesterone of > 0.75 ng/ml) and one additional ewe had an elevation in circulating progesterone to 0.58 ng/ml. In contrast, only one ewe received a low-intensity mark in the month of April and no marks of any kind were observed from 9 June until 13 July. No reason for this flurry of low-intensity mating in M a y was apparent. Fenceline contact with other rams did not occur. Two ewes mated and ovulated in July (13 and 16 July) hm apparently returned to anestrus before initiating regular cycles on 24 August and 14 September, respectively. Both these ewes had also received low-intensity marks and ovulated in late May.
Postpartum interval to first estrus W h e n exposed to vasectomized rams within 7 to 10 days of lambing, the mean date of first mating for group 2 ewes was 15 July (range: 15 May-17 September) and the mean p o s t p a r t u m interval was 115 days (range: 61-183 days ). P o s t p a r t u m interval was not correlated with lambing date (r = - 0.19, P = 0.40 ). Only one group 2 ewe mated early (on 29 May) but then apparently returned to anestrus until a later date (13 July). Three other ewes that mated before 1 June at less than 80 days postpartum all maintained a continuous pattern of estrus at regular intervals until removal from the study on 30 July.
Effect of acute ram introduction on initiation of mating Patterns of occurrence of first mating after combining all groups on 4 June are shown in Fig. 1. Median dates of first mating for ewes in groups I through 4 were 1 September, 19 July, 18 July and 20 July, respectively. Thus, group 1 ewes required an average of 45 days longer ( P < 0.01) to begin cycling than ewes in the other groups; approximately 40% of the ewes in the other groups had mated before the beginning of mating by group 1 ewes. The timing of the first mating did not differ for ewes in groups 2, 3 and 4. although group 4 ewes (which did not lamb and were isolated from rams ) tended to show less variation in time to first mating. Among ewes that lambed, immediate ram exposure for an average of 63 days during and immediately after lactation appeared to have no effect on subsequent mating ability in June and July.
270
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Fig. 1. Cumulative percentage of ewes in each of the four groups that mated after being combined on 4 June. DISCUSSION
This study demonstrates that continuous exposure of open yearling ewes to rams during late fall, winter and early spring prolongs the onset of mating relative to that which is achievable by ram isolation and subsequent reintroduction. Cohabitation of rams and ewes during anestrus prolonged time to first mating by over 6 weeks compared to ewes that had been isolated from rams. Several studies have used continuous cohabitation of ewes and vasectomized rams to characterize breed and individual differences in timing and duration of anestrus (Dufour, 1974; Lax et al., 1979; Jenkins and Ford, 1982 ). However, results of such characterizations must be strictly interpreted in terms of the experimental conditions. The percentage of ewes that are observed to cycle under conditions of continuous ram exposure likely represents the minimum fraction that could be expected to mate at that time under more favorable conditions. Use of the ram effect could result in much higher frequencies of cycling and the extent to which population differences in frequency of cycling under continuous ram exposure correlate to population differences in ability to respond to ram introduction is not known. The mean postpartum interval for spring-lambing ewes in this study was 115 days. Hoefler and Hallford (1987) reported a similar mean postpartum interval of 121 days for March-lambing fine-wool ewes in the southwestern U.S.A., and also reported that the postpartum interval did not differ between dry and lactating ewes. The latter observation supports the concept that there is not an important lactational anestrus in the sheep. The nonsignificant cor-
271
relation between lambing date and postpartum interval in this study suggests that the postpartum and seasonal anestrus are independent. For ewes that lambed exposure to vasectomized rams immediately after lambing had no negative impact on time of subsequent mating. Twenty-one percent of the group 2 ewes had already begun to cycle by the time the groups were combined, and the subsequent time to first mating in group 2 ewes was similar to that of ewes that had been isolated from rams. Thus, use of immediate ram exposure after lambing to attempt to minimize lambing interval is not contraindicated. Four of the group 2 ewes remated less than 80 days after lambing and during lactation. We do not know if these ewes would have conceived, but these results do suggest potential for rapid rebreeding in some springlambing ewes in this population. In a similar study, Amir and Gacitua ( 1987 ) reported a mean postpartum interval of 120 days in February-lambing Assaf ewes in Israel, but likewise found that 45% of these ewes returned to estrus relatively rapidly, with a mean postpartum interval of only 71 + 8 days. Group 4 ewes did not differ in mating behavior from ewes that lambed. Several studies have reported that winter-lambing ewes may be more likely to mate and conceive in spring breeding than ewes that remained open during winter. Speedy and FitzSimons (1977) reported that Finnsheep X Dorset and Greyface ewes in Scotland that had lambed 45 to 60 days before breeding had an 11% higher conception rate in August than ewes that were more than 60 days postpartum. Similarly, Notter and Copenhaver (1980) found that conception rates in Finnsheep crossbred ewes in April in the U.S.A. were 23% higher for ewes that had lambed in January and early February than for ewes that were open during the winter (65 vs 42% ). In both these studies, and in the current study, the open ewes represented a selected sample of ewes (i.e., those that failed to conceive as ewe lambs the previous fall). Thus, results from these groups must be interpreted with caution. In the current study, however, no inferiority of open ewes under ram isolation was observed. Dzabirski and Notter (1989) likewise evaluated the effect of time since lambing on lambing rates of mature unselected Dorset ewes bred in May in the U.S.A. Ewes that had previously lambed in late December and early January did not differ in lambing rate from those that had not lambed since the previous October (74% lambing for both groups). Thus, yearling ewes that lamb in March and April do not appear to differ in spring mating behavior from ewes that have been open but isolated from rams during the winter. ACKNOWLEDGEMENTS
The author wishes to express his appreciation to C. Burnsteel, L. Massoudi, L. O'Neal and M. Phelan for collection of marking data and blood and to L. Massoudi for conduct of the progesterone assays.
272 REFERENCES Amir, D. and Gacitua, H., 1987. Sexual activity of Assaf ewes after October and February lambings. Theriogenology, 27: 377-382. Beal, W.E., Onthank, D.C. and Zirkle, S.M., 1986. Effects of human chorionic gonadotropin and cyclic adenosine monophosphate on progesterone secretion by the ovine placenta. J. Anim. Sci., 63: 184-188. Dzabirski, V. and Notter, D.R., 1989. Effects of breed and time since lambing on spring estrous activity in mature ewes. Anim. Reprod. Sci., 19: 99-108. Dufour, J.J., 1974. The duration of the breeding season of four breeds of sheep. Can. J. Anim. Sci., 54: 389-392. Edgar, D.G. and Bilkey, D.A., 1963. The influence of rams on the onset of the breeding season in ewes. Proc. N.Z. Soc. Anim. Prod., 23: 79-87. Hoefler, W.C. and Hallford, D.M., 1987. Influence of suckling status and type of birth on serum hormone profiles and return to estrus in early-postpartum spring-lambingewes. Theriogenology, 27: 887-895. Hull, C.H. and Nie, N.H., 1981. Survival: life table analysis. In: SPSS Update 7-9. McGraw-Hill, New York, NY, pp. 205-219. Jenkins, T.G. and Ford, J.J., 1982. Estrous characteristics of Finnsheep crossbred and Morlam ewes. J. Anim. Sci., 55: 741-744. Lax, J., French, L.R., Chapman, A.B., Pope, A.L. and Casida, L.E., 1979. Length of breeding season for eight breed groups of sheep in Wisconsin. J. Anim. Sci., 49: 939-942. Lee, E. and Desu, M., 1972. A computer program for comparing K samples with right-censored data. Comput. Programs Biomed., 2: 315-321. Martin, G.B., Oldham, C.M., Cognie, 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. Notter, D.R. and Copenhaver, J.S., 1980. Performance of Finnish Landrace crossbred ewes under accelerated lambing. I. Fertility, prolificacy and ewe productivity. J. Anim. Sci., 5 l: 1033-1042. Nugent, R.A., III, Notter, D.R. and Beal, W.E., 1988a. Effects of ewe breed and ram exposure on estrous behaviour in May and June. J. Anim. Sci., 66: 1363-1370. Nugent, R.A., III, Notter, D.R. and McClure, W.H., 1988b. Effects of ram pre-exposure and ram breed on fertility of ewes in summer breeding. J. Anim. Sci., 66: 1622-1626. Oldham, C.M. and Cognie, Y., 1980. Do ewes continue to cycle after teasing? Proc. Aust. Soc. Anim. Prod., 13: 82-85. Oldham, C.M., Martin, G.B. and Knight, T.W., 1978. Stimulation of seasonally anovular ewes by rams. I. Time from introduction of rams to the preovulatory LH surge and ovulation. Anim. Reprod. Sci., 1: 283-290. Speedy, A.W. and FitzSimons, J., 1977. The reproductive performance of Finnish Landrace X Dorset Horn and Border Leicester × Scottish Blackface ewes mated three times in 2 years. Anim. Prod., 24: 189-196. Tervit, H.R. and Peterson, A.J., 1978. Testosterone levels in Dorset and Romney rams and the effectiveness of these breeds in stimulating early onset of estrus in Romney ewes. Theriogenology, 9: 271-278. Tervit, H.R., Havik, P.G. and Smith, J.F., 1977. Effect of breed of ram on the onset of the breeding season in Romney ewes. Proc. N.Z. Soc. Anim. Prod., 37: 142-148.