WASTAGE OF OVA IN YOUNG MERINO EWES M. A. de B. BLOCKEY", M.V.Sc., R. A. PARR*,D.D.A., and B. J. RESTALL?, B.Sc., Ph.D. Introduction
Low lambing percentages of non-parous 1.5year-old ewes compared to older, parous ewes have been commonly reported in the Australian Merino (Kelley 1939; Turner and Dolling 1965; Giles 1968; de Haas and Dunlop 1969). One of the major causes is low ovulation rates with only 0 to 23% of young ewes shedding more than 1 ovum (Cahill & Blockey 1974). In addition, in some flocks, young ewes have a higher wastage of ova than older ewes. In 3 of the 11 mixedage flocks studied by Connors and Giles (1970) and Dawe et a1 (1974), the 1.5-year-old maiden ewes had a much higher rate of return to service than the mature ewes. Edgar (1962) made observations on a Romney Marsh flock in which the return to service rates of non-parous 1.5- and parous 5-year-old ewes were 30% and 16% respectively and found that the fertilisation rates of the two groups were 82% and 84% respectively. Thus in this flock the young ewes suffered a greater wastage of fertilised ova than the older ewes. There are no published studies of ovum wastage in the young Merino ewe. This paper describes a study undertaken to assess fertilisation and embryo survival rates in non-parous 1.5-year-old and parous 2.5-year-old ewes. 1czaiterbls aud Methods Animals
In early March 1971, 200 non-parous 1.5-year-old Merino ewes and 195 parous 2.5-year-old Merino ewes were transferred from a property in the Western District of Victoria to the State Research Farm, Werribee, where the experiment was undertaken. Each ewe was weighed and randomly allotted within each age group to 4 sub-flocks of 48 to 51 ewes. Within each sub-flock, ewes were ranked in order of descending bodyweight and allotted equally to either Group A (fertilisation assessment or Group B (pregnancy and embryo survival assessment). Six Dorset Horn and 2 Merino rams were selected for mating. All rams had clinically normal genitalia on palpation and had mounted a ewe within 2 minutes of introduction into a pen of ewes exhibiting oestrus. Two weeks after the end of mating ,all but 1 of the rams (Merino sub-flock 1 ) were subjected to two examinations of genitalia, semen and serving behaviour (Galloway 1966). * Department of ,Agriculture, S. S. Cameron Laboratory, Werribee Victoria 1030. t Departmdnt of A&culture, Ian Clunies Ross Laboratory, Prospect, New South Wales, 2149.
298
Conduct of Experiment One ram was joined with each sub-flock of ewes so that any ram effects would be randomised when comparing the pooled data from 1.5- and 2.5-year-old ewes. As well, any multiple sire effects such as dominance were avoided and an assessment of individual ram fertility was possible. In late March, 1971, a vasectomised ram fitted with "sire-sire" harness and crayon (Radford et d 1960) was joined to each sub-flock of ewes. The next day the 8 entire rams, fitted with crayons, replaced the vasectomised rams. Those ewes marked by vasectomised rams were removed from the sub-flocks for 2 days and then returned so that at their next oestrus they had the opportunity to experience a full duration of oestrus in the presence of a fertile ram. For the next 18 days, each sub-flock was yarded daily and ewes marked with crayons were recorded. Group A ewes underwent mid-ventral laparotomy 72 t o 96 hours after the onset of oestrus. Ova were recovered by flushing the oviducts and rostra1 portion of the uterine horn with normal saline .The criterion of fertilisation used was to examine recovered ova for cleavage of cells. At the end of the 18-day period, all 8 sub-flocks were combined. Eight vasectomised rams wearing harnesses replaced the entire rams and the flock was yarded daily to detect Group B ewes returning to service. All ewes marked by vasectomised rams underwent laparotomy on the day of return to service and the number of regressing corpora lutea noted and recorded. Those Group B ewes not returning to service underwent laparotomy between 22 and 29 post coitum, the ovaries examined for corpora lutea and corpora albicantia and the number recorded. An incision was made into the uterine horns of both pregnant and non-pregnant ewes and the embryo(s) and/or membranes were removed and examined. The assessment of viability of the embryo was similar to that described by Quinlivan et a! (1966). Statistical Analysis Fertilisation, pregnancy and embryo survival rates for each sub-flock were calculated and the data for each age group pooled. The between age group comparisons were made by chi-square analysis.
Rmlfs
Fertilisation Data There were no significant differences between any of the sub-flocks in fertilisation rates (Table 1 ) . Nor was there any significant difference between age groups with 87.5% and 86.2% of the 1.5- and 2.5-year-old ewes respectively having fertilised ova. Pregnancy and Embryo Survival Data Only 59 (61.4%) of 1.5-year-old ewes were pregnant at 22 to 29 days post coitum compared Austrdian Veterinary Journal, Vol. 51. June, 1975
TABLE 1 Mating, Fertilisation, Pregnancy and Embryo Survival Data in Non-parous 1.5 year-old and Parous 2.5 Year-old
Ewes Non-parous 1.5 Year-old Ewes Subflock Ram Breed*
1 M
2
DH
Group A: Fertilisation Data No. of ewes joined 25 25 No. ewes mating in 18 days 22 24 No. of ewes from which ova were recovered 17 21 No. of ewes from which fertilised ova were recovered 15 18 Fertilisation rate 88 86 f 95% confidence limits 214 214
3
Parous 2.5 Year-old Ewes
DH
4 DH
Pooled Data
M
6 DH
DH
7
8 DH
Pooled Data
25
25
100
25
25
24
23
97
23
23
92
22
24
24
21
91
16
18
72
22
21
21
16
80
15 94
15 83
63 88
18 82
19 90
17 81
15 94
69 86
210
f16
26
f16
k12
216
fll
26
100
25
24
24
25
98
96
24
22
24
24
94
59 61
19 79
19 86
17 71
20 83
75 80
210 100
216 28
el4 24
218 25
f1.5 24
28 101
60
20
20
18
20
78
60
71
83
72
83
78
Group B: Pregnancy and Embryo Survival Data No. of ewes joined 26 24 26 24 No. of ewes mating in 18 days 26 23 25 22 No. of ewes pregnant at 22 to 29 days P c. 12 7 20 20 Pregnancy rate ( % ) 46 30 80 91 _C 95% confidence limits 2 1 9 f 1 9 -+16 a 1 2 No. of corpora lutea 27 24 26 23 No. of viable embryos at 22 to 29 days P c. 13 7 20 20 Embryo survival rate (% 1 48 29 77 87 * M = Merino, DH = Dorset Horn.
5
this ram was not examined because its paresis precluded its use in a subsequent artificial insemination program for which the other r a m s were examined. The rams mated to sub-flocks 2 to 8 had semen of a quality consistent with good fertility (Galloway 1972; D. B. Galloway 1974, personal communication) and on the occasions required readily served into an artificial vagina. Rams Throughout the mating period the ram mated The Merino ram mated to sub-flock 1 was to sub-flock 2 maintained the ability to fertilise observed once while attempting to mount a ewe ova. In both the first and second 9-day periods and appeared weak in the hindquarters repeatedly 86% of ewes mated had their ova fertilised. sagging onto its haunches during attempts to However, pregnancy rate declined as mating promount. This weakness of the hindquarters was gressed with 55% and 8% of ewes mated in noticed on several other occasions when the ram the first and second 9-day periods respectively, was confined to an experimental sheep shed for still pregnant at 22 to 29 days post ca'ttcm (X2, another experiment. Unfortunately the semen of = 4.27, p < 0.05).
to 75 (80%) of the 2.5-year-old ewes (Table 1) (X21 = 6.8, P < 0.01). The difference in embryo survival rates between the two age groups was of a similar magnitude (Table 1) (X21 = 6.15, p < 0.025). The low pregnancy rates of sub-flocks 1 and 2 (Table 1) were wholly re+ ponsible for the difference between the 2 age groups.
A ustrdian Veterinary Journal, Vol. 5 1 . June. 1975
299
Time of Ova Wastage Thirteen, 1.5-year-old and 11, 2.5-year-old ewes returned to service 15 to 20 days post cuitum (Table 2 ) . These ewes were likely to have suffered fertilisation failure or to have lost a fertilised Ovum before day 12 post coitum (Edey 1967). The difference between the fertilisation rate measured in Group A and the proportion of ewes not returning to service 15 to 20 days post coitum (20 day non-return rate) in Group B gives an estimate of the early (preday 12) embryo loss. For each age group, these figures are: 88% and 86% for 1.5-year-old ewes and 86% and 88% for the 2.5-year-old ewes. These figures indicate that little loss of fertilised ova occurred prior to day 12 post coitum. The number of ewes in each sub-flock which had extended cycle lengths or had degenerating embryonic material at hysterotomy are shown in Table 2. Embryonic mortality would have occurred after day 12 in these ewes. Also shown in Table 2 are 13 ewes (12 of which were 1.5 years old) that had no embryonic remnants at hysterotomy and had not returned to service. Five of the ewes had an active corpus luteum and a regressing corpus albicans. In the 8 remaining ewes an active corpus luteum was present and no corpus albicans was seen. The close similarity between the fertilisation rate and the 20-day non-return rate would suggest that whether these 13 ewes maintained the corpus luteum of pregnancy or not, they all suffered late (after day 12) embryonic mortality. Discussion
The high fertilisation rate in each sub-flock (81 to 9 4 % ) indicates that during the 18-day mating period all rams were inseminating ewes with highly fertile semen. Despite the uniformly high ram fertility and the fact that all 8 sub-flocks of ewes were mated under the same climatic and similar nutritional conditions, sub-flocks 1 and 2 had low pregnancy rates (46 and 30%) whilst the other 6 sub-flocks had first service pregnancy rates between 71% and 91%. A similar pattern of prenatal loss has been reported in mature Merino ewes following variations between the number of times individual ewes were served during oestrus (Mattner and Braden 1967). In 2 flocks mated to single rams, groups of ewes served once, twice, or three or more times had high fertilisation rates (83% to 100%) . However, in flock 1, the groups of ewes receiving 1, 2 or 3-t- services had 20-day pregnancy rates of 50, 63 and 91% respectively and in flock 2, the 30 to 60-day pregnancy rates for 300
Australian Veterinary Journal, Vol. 51. June, 1975
the same classes of ewes were 65, 84 and 97% respectively. Thus in these flocks one service from a fertile ram was sufficient to effect fertilisation whilst survival of the fertilised ovum was dependent on the number of times a ewe was served. The time of service relative to ovulation also affects pregnancy rate (Dzuik 1970). Ewes served twice in the period 11 to 15 hours before ovulation, had a pregnancy rate of 81% compared to a pregnancy rate of between 32 and 43% for ewes served twice in any other 4-hour period prior to ovulation. An explanation for the results of Mattner and Braden (1967) is that ewes served thrice or more had a greater chance of being served at this optimum time than ewes served only once. It is possible that in the present study ewes in sub-flocks 1 and 2 were served fewer times than the ewes in the other sub-flocks and that it was the low number of services received by ewes in sub-flocks 1 and 2 that resulted in the low pregnancy rates. The two factors likely to influence the number of times a ewe is served during oestrus are the duration of that oestrus and the service activity of rams. Ewes in sub-flocks 1 and 2 were nonparous 1.5-year-old ewes and these ewes have a significantly shorter duration of oestrus than parous 2.5-year-old ewes (Lambourne 1956; Blockey and Cumming 1970; Blockey 1971), and are served fewer times during oestrus (Lambourne 1956). Of particular relevance to this discussion is the fact that in the study of Blockey (1971), 63% of the parous 2.5-year-old ewes were still in oestrus 15 hours before ovulation compared to only 31% of the maiden 1.5-yearold ewes (p < 0.05). These data would suggest that in all flocks non-parous 1.5-year-old ewes would have a lower first service pregnancy rate than the older, parous ewes. Yet in the present study sub-flocks 3 and 4 of maiden 1.5-year-old ewes had pregnancy rates (80 and 9 1 % ) comparable with those of the parous 2.5-year-old ewes (71 to 8 6 % ) . Also in 8 of the 11 Merino flocks studied by Connors and Giles (1970) and Dawe et al (1974), the proportion of maiden and mature ewes returning to service in each flock differed by no more than 10%. However, in the other 3 flocks, the young ewes had return to service rates from 19 to 49% higher than the older ewes. Thus in some flocks but not in others, non-parous 1.5-year-old ewes suffer a greater wastage of ova than parous, older ewes. The sporadic occurrence of this higher wastage of ova in young ewes might be explained in terms of service activity of rams. This is highly variable both within ram flocks and between ram Australian Veterinary Journal, Vol. 51. June, 1975
flocks (Mattner et al 1971a, b; Knights and Lindsay 1973). It is probable that the rams of high service activity are capable of at least 3 services to ewes with either a short or long oestrus whilst rams of low service activity are incapable of even 3 services to ewes with even a long oestrus. It is logical to assume that there are also rams of intermediate service activity, that is, capable of 3 or more services to parous, 2.5year-old and older ewes with an oestrus length of 2 0 . 5 2 1.2 (S.E.) hours but capable of only 1 or 2 services to non-parous 1.5-year-old ewes with an oestrus length of 12.6+ 1.1 (S.E.) hours (Blockey and Gumming 1970). Fertile rams with this level of service activity would be expected to impregnate a high proportion of mature ewes but a lower proportion of young ewes. In the present study no detailed observations were made on the service activity of rams. HOWever, there are indications that the rams mated to sub-flocks 1 and 2 did have low service activity. The Merino mated to sub-flock 1 frequently showed weakness in the hindquarters which was associated with poor mating dexterity and the low and progressively declining pregnancy rate in sub-flock 2 suggests that the ram mated to this group of ewes initially had poor service activity which decreased as mating progressed. Whether these 2 rams were of low or intermediate service activity must remain unknown. There is no published information on the stage at which embryonic mortality occurs in young Merino ewes and the only studies on the time of embryonic death in mature Merino ewes gives conflicting results. Mattner and Braden (1967) found that virtually all of the 20% loss of fertilised ova in their sheep occurred prior to day 12 post coitum whilst in 3 separate experiments Entwhistle (1970) showed that embryonic death varied from 16 to 21% and occurred after day 12 post coitum. In the present study, all the 27% and 6% embryonic mortality suffered by the 1.5- and 2.5-year-old ewes respectively, appears to have occurred after day 12 post coiturn. Eleven of the 24, non-parous 1.5-yearold ewes losing embryos had extended cycle lengths whilst the remaining 13 ewes had not returned to service by 24 to 29 days post coitum. There are 2 possible explanations for the occurrence of the latter. After embryonic death and resorption these ewes ovulated but either did not display oestrus or exhibited oestrus with insufficient intensity for the 8 vasectomised rams to detect it. Alternatively despite the lack of embryonic remnants the corpus luteum of pregnancy may have failed to regress by 24 to 29 days post 301
coitum. Of greater relevance is the fact that some of these ewes may not have re-mated before the end of a 6 to 8 week mating period. In addition those ewes that did return to service after a long cycle would have a worse lambing performance than ewes returning to service after a normal cycle length (Edey 1970). It is concluded that a high incidence of late embryonic mortality, associated with failure to re-mate or depressed fertility in those that do re-mate, could be a major contributing factor to the low lambing percentages experienced by some flocks of non-parous 1.5-year-old Merino ewes. Summary
Eight sub-flocks each of 48 to 51 Merino ewes were mated for 18 days with 1 ram to each subflock. Four of the sub-flocks were of non-parous 1.5-year-old ewes and the other 4 were of parous 2.5-year-old ewes. Fertilisation rates for the subflocks of 1.5-year-old ewes varied from 83 to 94%, and for the sub-flocks of 2.5-year-old ewes ranged from 81 to 94%. By day 29 post coitum 42% and 56% of 1.5-year-old ewes in sub-flocks 1 and 2 respectively were no longer pregnant. Embryonic mortality was low in other sub-flocks of 1.5- and 2.5-year-old ewes. Virtually all embryonic death occurred after day 12 post coitum. The sporadic occurrence of high ova wastage in maiden 1.5-year-old ewes in this experiment and in others is discussed in relation to the short duration of oestrus in young ewes and the great variation in service activity of rams. Acknowledgments
The authors wish to thank Dr R. H. Watson for advice and encouragement, Dr D. B. Galloway for the breeding soundness examinations of the rams, Mr P. Langdon and Mrs M. Perry for skilled and willing technical assistance throughout the experiment, and Messrs L. P. Cahill, A. W. Makin, A. H. Williams, R. W. Baxter, R. Kearins
302
and P. Rozea for assistance with the hysterotomies. This work was financed by the Wool Research Trust Fund. Ref erenm Blockey, M. A. de B. (1971)-''Studies on Infertility in Young Ewes", M.V.Sc. Thesis, University of Melbourne, Melbourne, Victoria. Blockey, M. A. de B. and Cumrning, I. A. (1970)Proc. Aust. SOC. Anim. Prod. 8: 344. Cahill, L. F. and Blockey, M. A. de B. (1974)-Proc. Aust. Soc. Anim. Prod. 10: 258. Connors, R. W. and Giles, J. R. (1970)-Proc. Aust. SOC. Anim. Prod. 8: 331. Dawe, S. T., Archer, W. R., Bennett, N. W., Brunskill, A.. Cahill. J . R.. Donnellv. F. B.. Roberts. B. C. and Trimmer, B. I. (1974)-&oc. A z h . Soc. Anim. Prod. 10: 274. de Haas, H. J. and Dunlop, A. A. (1969)-Ausr. J . agric. RFS.20: 549. Dziuk, P. J. (1970)-J. Reprod. Fert. 22: 277. Edey, T. N. (19671-4. Reprod. Fert. 13: 437. Edey, T. N. (1970)-J. ugric. Sci., Camb. 74: 199. Edgar. D. G. (1962)--3. Reprod. Fcrt. 3: 50. Entwhistle, K. W. (1970)-Proc. Aust. Soc. Anim. Prod. 8: 3 5 3 . Galloway, D. B. (1966)-Aust. vet. J . 42: 7 9 . Galloway, D. B. (1972)-A.M.R.C. Review N o . 10. Giles. J. R. (1968)-Ausr. -3. exp. Agric. Anim. Husb. 8: 149. Kelley, R. B. (1939b-Aust. vet. J . 15: 184. Knights, T. W. and Lindsay, D. R. (1973)--Aust. J . agric. Res. 24: 579. Lambourne, L. J. (1956)-Proc. Ruakura Fmrs' C o n f . Week. p. 16. Mattner, P. E. and Braden, A. W. H. (1967)-Aust. J . exp. Agric. Anim. Husb. 7 : 110. Mattner, P . E., Braden, A. W. H. and George, J. M. (1971a)-J. Reprod. Fert. 2 4 149. Mattner, P. E., Braden, A. W. H. and George, J. M. (197lb)-Aust. J . exp. Agric. Anim. Husb. 11: 473. Quinlivan, T. D., Martin, C . A., Taylor, W. B. and Cairney, I. M. (1966)--3. Reprod. Fert. 11: 379. Radford, H. M., Watson, R. H. and Wood, G. F. ( 1 9 6 0 ) -Aust. vet. J . 36: 57. Turner, H. G. and Dolling, C. H. S. (1965)--Ausr. J . agric. Res. 1 6 699. (Received for publication I October 1974)
Australian Veterinary Journal, Vol. 51. June, 1975
Low lambing percentages of non-parous 1.5year-old ewes compared to older, parous ewes have been commonly reported in the Australian Merino (Kelley 1939; Turner and Dolling 1965; Giles 1968; de Haas and Dunlop 1969). One of the major causes is low ovulation rates with only 0 to 23% of young ewes shedding more than 1 ovum (Cahill & Blockey 1974). In addition, in some flocks, young ewes have a higher wastage of ova than older ewes. In 3 of the 11 mixedage flocks studied by Connors and Giles (1970) and Dawe et a1 (1974), the 1.5-year-old maiden ewes had a much higher rate of return to service than the mature ewes. Edgar (1962) made observations on a Romney Marsh flock in which the return to service rates of non-parous 1.5- and parous 5-year-old ewes were 30% and 16% respectively and found that the fertilisation rates of the two groups were 82% and 84% respectively. Thus in this flock the young ewes suffered a greater wastage of fertilised ova than the older ewes. There are no published studies of ovum wastage in the young Merino ewe. This paper describes a study undertaken to assess fertilisation and embryo survival rates in non-parous 1.5-year-old and parous 2.5-year-old ewes. 1czaiterbls aud Methods Animals
In early March 1971, 200 non-parous 1.5-year-old Merino ewes and 195 parous 2.5-year-old Merino ewes were transferred from a property in the Western District of Victoria to the State Research Farm, Werribee, where the experiment was undertaken. Each ewe was weighed and randomly allotted within each age group to 4 sub-flocks of 48 to 51 ewes. Within each sub-flock, ewes were ranked in order of descending bodyweight and allotted equally to either Group A (fertilisation assessment or Group B (pregnancy and embryo survival assessment). Six Dorset Horn and 2 Merino rams were selected for mating. All rams had clinically normal genitalia on palpation and had mounted a ewe within 2 minutes of introduction into a pen of ewes exhibiting oestrus. Two weeks after the end of mating ,all but 1 of the rams (Merino sub-flock 1 ) were subjected to two examinations of genitalia, semen and serving behaviour (Galloway 1966). * Department of ,Agriculture, S. S. Cameron Laboratory, Werribee Victoria 1030. t Departmdnt of A&culture, Ian Clunies Ross Laboratory, Prospect, New South Wales, 2149.
298
Conduct of Experiment One ram was joined with each sub-flock of ewes so that any ram effects would be randomised when comparing the pooled data from 1.5- and 2.5-year-old ewes. As well, any multiple sire effects such as dominance were avoided and an assessment of individual ram fertility was possible. In late March, 1971, a vasectomised ram fitted with "sire-sire" harness and crayon (Radford et d 1960) was joined to each sub-flock of ewes. The next day the 8 entire rams, fitted with crayons, replaced the vasectomised rams. Those ewes marked by vasectomised rams were removed from the sub-flocks for 2 days and then returned so that at their next oestrus they had the opportunity to experience a full duration of oestrus in the presence of a fertile ram. For the next 18 days, each sub-flock was yarded daily and ewes marked with crayons were recorded. Group A ewes underwent mid-ventral laparotomy 72 t o 96 hours after the onset of oestrus. Ova were recovered by flushing the oviducts and rostra1 portion of the uterine horn with normal saline .The criterion of fertilisation used was to examine recovered ova for cleavage of cells. At the end of the 18-day period, all 8 sub-flocks were combined. Eight vasectomised rams wearing harnesses replaced the entire rams and the flock was yarded daily to detect Group B ewes returning to service. All ewes marked by vasectomised rams underwent laparotomy on the day of return to service and the number of regressing corpora lutea noted and recorded. Those Group B ewes not returning to service underwent laparotomy between 22 and 29 post coitum, the ovaries examined for corpora lutea and corpora albicantia and the number recorded. An incision was made into the uterine horns of both pregnant and non-pregnant ewes and the embryo(s) and/or membranes were removed and examined. The assessment of viability of the embryo was similar to that described by Quinlivan et a! (1966). Statistical Analysis Fertilisation, pregnancy and embryo survival rates for each sub-flock were calculated and the data for each age group pooled. The between age group comparisons were made by chi-square analysis.
Rmlfs
Fertilisation Data There were no significant differences between any of the sub-flocks in fertilisation rates (Table 1 ) . Nor was there any significant difference between age groups with 87.5% and 86.2% of the 1.5- and 2.5-year-old ewes respectively having fertilised ova. Pregnancy and Embryo Survival Data Only 59 (61.4%) of 1.5-year-old ewes were pregnant at 22 to 29 days post coitum compared Austrdian Veterinary Journal, Vol. 51. June, 1975
TABLE 1 Mating, Fertilisation, Pregnancy and Embryo Survival Data in Non-parous 1.5 year-old and Parous 2.5 Year-old
Ewes Non-parous 1.5 Year-old Ewes Subflock Ram Breed*
1 M
2
DH
Group A: Fertilisation Data No. of ewes joined 25 25 No. ewes mating in 18 days 22 24 No. of ewes from which ova were recovered 17 21 No. of ewes from which fertilised ova were recovered 15 18 Fertilisation rate 88 86 f 95% confidence limits 214 214
3
Parous 2.5 Year-old Ewes
DH
4 DH
Pooled Data
M
6 DH
DH
7
8 DH
Pooled Data
25
25
100
25
25
24
23
97
23
23
92
22
24
24
21
91
16
18
72
22
21
21
16
80
15 94
15 83
63 88
18 82
19 90
17 81
15 94
69 86
210
f16
26
f16
k12
216
fll
26
100
25
24
24
25
98
96
24
22
24
24
94
59 61
19 79
19 86
17 71
20 83
75 80
210 100
216 28
el4 24
218 25
f1.5 24
28 101
60
20
20
18
20
78
60
71
83
72
83
78
Group B: Pregnancy and Embryo Survival Data No. of ewes joined 26 24 26 24 No. of ewes mating in 18 days 26 23 25 22 No. of ewes pregnant at 22 to 29 days P c. 12 7 20 20 Pregnancy rate ( % ) 46 30 80 91 _C 95% confidence limits 2 1 9 f 1 9 -+16 a 1 2 No. of corpora lutea 27 24 26 23 No. of viable embryos at 22 to 29 days P c. 13 7 20 20 Embryo survival rate (% 1 48 29 77 87 * M = Merino, DH = Dorset Horn.
5
this ram was not examined because its paresis precluded its use in a subsequent artificial insemination program for which the other r a m s were examined. The rams mated to sub-flocks 2 to 8 had semen of a quality consistent with good fertility (Galloway 1972; D. B. Galloway 1974, personal communication) and on the occasions required readily served into an artificial vagina. Rams Throughout the mating period the ram mated The Merino ram mated to sub-flock 1 was to sub-flock 2 maintained the ability to fertilise observed once while attempting to mount a ewe ova. In both the first and second 9-day periods and appeared weak in the hindquarters repeatedly 86% of ewes mated had their ova fertilised. sagging onto its haunches during attempts to However, pregnancy rate declined as mating promount. This weakness of the hindquarters was gressed with 55% and 8% of ewes mated in noticed on several other occasions when the ram the first and second 9-day periods respectively, was confined to an experimental sheep shed for still pregnant at 22 to 29 days post ca'ttcm (X2, another experiment. Unfortunately the semen of = 4.27, p < 0.05).
to 75 (80%) of the 2.5-year-old ewes (Table 1) (X21 = 6.8, P < 0.01). The difference in embryo survival rates between the two age groups was of a similar magnitude (Table 1) (X21 = 6.15, p < 0.025). The low pregnancy rates of sub-flocks 1 and 2 (Table 1) were wholly re+ ponsible for the difference between the 2 age groups.
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Time of Ova Wastage Thirteen, 1.5-year-old and 11, 2.5-year-old ewes returned to service 15 to 20 days post cuitum (Table 2 ) . These ewes were likely to have suffered fertilisation failure or to have lost a fertilised Ovum before day 12 post coitum (Edey 1967). The difference between the fertilisation rate measured in Group A and the proportion of ewes not returning to service 15 to 20 days post coitum (20 day non-return rate) in Group B gives an estimate of the early (preday 12) embryo loss. For each age group, these figures are: 88% and 86% for 1.5-year-old ewes and 86% and 88% for the 2.5-year-old ewes. These figures indicate that little loss of fertilised ova occurred prior to day 12 post coitum. The number of ewes in each sub-flock which had extended cycle lengths or had degenerating embryonic material at hysterotomy are shown in Table 2. Embryonic mortality would have occurred after day 12 in these ewes. Also shown in Table 2 are 13 ewes (12 of which were 1.5 years old) that had no embryonic remnants at hysterotomy and had not returned to service. Five of the ewes had an active corpus luteum and a regressing corpus albicans. In the 8 remaining ewes an active corpus luteum was present and no corpus albicans was seen. The close similarity between the fertilisation rate and the 20-day non-return rate would suggest that whether these 13 ewes maintained the corpus luteum of pregnancy or not, they all suffered late (after day 12) embryonic mortality. Discussion
The high fertilisation rate in each sub-flock (81 to 9 4 % ) indicates that during the 18-day mating period all rams were inseminating ewes with highly fertile semen. Despite the uniformly high ram fertility and the fact that all 8 sub-flocks of ewes were mated under the same climatic and similar nutritional conditions, sub-flocks 1 and 2 had low pregnancy rates (46 and 30%) whilst the other 6 sub-flocks had first service pregnancy rates between 71% and 91%. A similar pattern of prenatal loss has been reported in mature Merino ewes following variations between the number of times individual ewes were served during oestrus (Mattner and Braden 1967). In 2 flocks mated to single rams, groups of ewes served once, twice, or three or more times had high fertilisation rates (83% to 100%) . However, in flock 1, the groups of ewes receiving 1, 2 or 3-t- services had 20-day pregnancy rates of 50, 63 and 91% respectively and in flock 2, the 30 to 60-day pregnancy rates for 300
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the same classes of ewes were 65, 84 and 97% respectively. Thus in these flocks one service from a fertile ram was sufficient to effect fertilisation whilst survival of the fertilised ovum was dependent on the number of times a ewe was served. The time of service relative to ovulation also affects pregnancy rate (Dzuik 1970). Ewes served twice in the period 11 to 15 hours before ovulation, had a pregnancy rate of 81% compared to a pregnancy rate of between 32 and 43% for ewes served twice in any other 4-hour period prior to ovulation. An explanation for the results of Mattner and Braden (1967) is that ewes served thrice or more had a greater chance of being served at this optimum time than ewes served only once. It is possible that in the present study ewes in sub-flocks 1 and 2 were served fewer times than the ewes in the other sub-flocks and that it was the low number of services received by ewes in sub-flocks 1 and 2 that resulted in the low pregnancy rates. The two factors likely to influence the number of times a ewe is served during oestrus are the duration of that oestrus and the service activity of rams. Ewes in sub-flocks 1 and 2 were nonparous 1.5-year-old ewes and these ewes have a significantly shorter duration of oestrus than parous 2.5-year-old ewes (Lambourne 1956; Blockey and Cumming 1970; Blockey 1971), and are served fewer times during oestrus (Lambourne 1956). Of particular relevance to this discussion is the fact that in the study of Blockey (1971), 63% of the parous 2.5-year-old ewes were still in oestrus 15 hours before ovulation compared to only 31% of the maiden 1.5-yearold ewes (p < 0.05). These data would suggest that in all flocks non-parous 1.5-year-old ewes would have a lower first service pregnancy rate than the older, parous ewes. Yet in the present study sub-flocks 3 and 4 of maiden 1.5-year-old ewes had pregnancy rates (80 and 9 1 % ) comparable with those of the parous 2.5-year-old ewes (71 to 8 6 % ) . Also in 8 of the 11 Merino flocks studied by Connors and Giles (1970) and Dawe et al (1974), the proportion of maiden and mature ewes returning to service in each flock differed by no more than 10%. However, in the other 3 flocks, the young ewes had return to service rates from 19 to 49% higher than the older ewes. Thus in some flocks but not in others, non-parous 1.5-year-old ewes suffer a greater wastage of ova than parous, older ewes. The sporadic occurrence of this higher wastage of ova in young ewes might be explained in terms of service activity of rams. This is highly variable both within ram flocks and between ram Australian Veterinary Journal, Vol. 51. June, 1975
flocks (Mattner et al 1971a, b; Knights and Lindsay 1973). It is probable that the rams of high service activity are capable of at least 3 services to ewes with either a short or long oestrus whilst rams of low service activity are incapable of even 3 services to ewes with even a long oestrus. It is logical to assume that there are also rams of intermediate service activity, that is, capable of 3 or more services to parous, 2.5year-old and older ewes with an oestrus length of 2 0 . 5 2 1.2 (S.E.) hours but capable of only 1 or 2 services to non-parous 1.5-year-old ewes with an oestrus length of 12.6+ 1.1 (S.E.) hours (Blockey and Gumming 1970). Fertile rams with this level of service activity would be expected to impregnate a high proportion of mature ewes but a lower proportion of young ewes. In the present study no detailed observations were made on the service activity of rams. HOWever, there are indications that the rams mated to sub-flocks 1 and 2 did have low service activity. The Merino mated to sub-flock 1 frequently showed weakness in the hindquarters which was associated with poor mating dexterity and the low and progressively declining pregnancy rate in sub-flock 2 suggests that the ram mated to this group of ewes initially had poor service activity which decreased as mating progressed. Whether these 2 rams were of low or intermediate service activity must remain unknown. There is no published information on the stage at which embryonic mortality occurs in young Merino ewes and the only studies on the time of embryonic death in mature Merino ewes gives conflicting results. Mattner and Braden (1967) found that virtually all of the 20% loss of fertilised ova in their sheep occurred prior to day 12 post coitum whilst in 3 separate experiments Entwhistle (1970) showed that embryonic death varied from 16 to 21% and occurred after day 12 post coitum. In the present study, all the 27% and 6% embryonic mortality suffered by the 1.5- and 2.5-year-old ewes respectively, appears to have occurred after day 12 post coiturn. Eleven of the 24, non-parous 1.5-yearold ewes losing embryos had extended cycle lengths whilst the remaining 13 ewes had not returned to service by 24 to 29 days post coitum. There are 2 possible explanations for the occurrence of the latter. After embryonic death and resorption these ewes ovulated but either did not display oestrus or exhibited oestrus with insufficient intensity for the 8 vasectomised rams to detect it. Alternatively despite the lack of embryonic remnants the corpus luteum of pregnancy may have failed to regress by 24 to 29 days post 301
coitum. Of greater relevance is the fact that some of these ewes may not have re-mated before the end of a 6 to 8 week mating period. In addition those ewes that did return to service after a long cycle would have a worse lambing performance than ewes returning to service after a normal cycle length (Edey 1970). It is concluded that a high incidence of late embryonic mortality, associated with failure to re-mate or depressed fertility in those that do re-mate, could be a major contributing factor to the low lambing percentages experienced by some flocks of non-parous 1.5-year-old Merino ewes. Summary
Eight sub-flocks each of 48 to 51 Merino ewes were mated for 18 days with 1 ram to each subflock. Four of the sub-flocks were of non-parous 1.5-year-old ewes and the other 4 were of parous 2.5-year-old ewes. Fertilisation rates for the subflocks of 1.5-year-old ewes varied from 83 to 94%, and for the sub-flocks of 2.5-year-old ewes ranged from 81 to 94%. By day 29 post coitum 42% and 56% of 1.5-year-old ewes in sub-flocks 1 and 2 respectively were no longer pregnant. Embryonic mortality was low in other sub-flocks of 1.5- and 2.5-year-old ewes. Virtually all embryonic death occurred after day 12 post coitum. The sporadic occurrence of high ova wastage in maiden 1.5-year-old ewes in this experiment and in others is discussed in relation to the short duration of oestrus in young ewes and the great variation in service activity of rams. Acknowledgments
The authors wish to thank Dr R. H. Watson for advice and encouragement, Dr D. B. Galloway for the breeding soundness examinations of the rams, Mr P. Langdon and Mrs M. Perry for skilled and willing technical assistance throughout the experiment, and Messrs L. P. Cahill, A. W. Makin, A. H. Williams, R. W. Baxter, R. Kearins
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and P. Rozea for assistance with the hysterotomies. This work was financed by the Wool Research Trust Fund. Ref erenm Blockey, M. A. de B. (1971)-''Studies on Infertility in Young Ewes", M.V.Sc. Thesis, University of Melbourne, Melbourne, Victoria. Blockey, M. A. de B. and Cumrning, I. A. (1970)Proc. Aust. SOC. Anim. Prod. 8: 344. Cahill, L. F. and Blockey, M. A. de B. (1974)-Proc. Aust. Soc. Anim. Prod. 10: 258. Connors, R. W. and Giles, J. R. (1970)-Proc. Aust. SOC. Anim. Prod. 8: 331. Dawe, S. T., Archer, W. R., Bennett, N. W., Brunskill, A.. Cahill. J . R.. Donnellv. F. B.. Roberts. B. C. and Trimmer, B. I. (1974)-&oc. A z h . Soc. Anim. Prod. 10: 274. de Haas, H. J. and Dunlop, A. A. (1969)-Ausr. J . agric. RFS.20: 549. Dziuk, P. J. (1970)-J. Reprod. Fert. 22: 277. Edey, T. N. (19671-4. Reprod. Fert. 13: 437. Edey, T. N. (1970)-J. ugric. Sci., Camb. 74: 199. Edgar. D. G. (1962)--3. Reprod. Fcrt. 3: 50. Entwhistle, K. W. (1970)-Proc. Aust. Soc. Anim. Prod. 8: 3 5 3 . Galloway, D. B. (1966)-Aust. vet. J . 42: 7 9 . Galloway, D. B. (1972)-A.M.R.C. Review N o . 10. Giles. J. R. (1968)-Ausr. -3. exp. Agric. Anim. Husb. 8: 149. Kelley, R. B. (1939b-Aust. vet. J . 15: 184. Knights, T. W. and Lindsay, D. R. (1973)--Aust. J . agric. Res. 24: 579. Lambourne, L. J. (1956)-Proc. Ruakura Fmrs' C o n f . Week. p. 16. Mattner, P. E. and Braden, A. W. H. (1967)-Aust. J . exp. Agric. Anim. Husb. 7 : 110. Mattner, P . E., Braden, A. W. H. and George, J. M. (1971a)-J. Reprod. Fert. 2 4 149. Mattner, P. E., Braden, A. W. H. and George, J. M. (197lb)-Aust. J . exp. Agric. Anim. Husb. 11: 473. Quinlivan, T. D., Martin, C . A., Taylor, W. B. and Cairney, I. M. (1966)--3. Reprod. Fert. 11: 379. Radford, H. M., Watson, R. H. and Wood, G. F. ( 1 9 6 0 ) -Aust. vet. J . 36: 57. Turner, H. G. and Dolling, C. H. S. (1965)--Ausr. J . agric. Res. 1 6 699. (Received for publication I October 1974)
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