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This article was published in 1967
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Control of Breeding Performances in Sheep
J. N. SHELTON, B.V.Sc., Ph.D., Department of Animal Husbandry, University of Sydney
Introduction
Statistics show that low lamb marking percentages are the major problem of the Australian sheep industry. Three of the more obvious aspects of reproduction where losses occur are the number of ewes served, the number of lambs born per ewe lambing, and the proportion of lambs that survive to marking. Precise control of the oestrous cycle could help to reduce wastage in these areas.
Probably it always will be desirable and profitable to mate crossbred ewes in the spring and early summer when seasonal anoestrus is common. Such joinings would be much more productive if fertile oestrus and ovulation could be induced at this time. Control of the oestrous cycle during the breeding season (autumn) could help in at least two ways. First, it would allow treatment of large numbers of animals with gonadotrophins in order to induce multiple ovulations. Without such control it is impracticable to treat numbers of ewes at the appropriate stage of the oestrous cycle (Day 11 to Day 13). Numerous researchers and producers have shown how losses of young lambs can be reduced by more enlightened husbandry methods at and shortly after lambing. Implementation of these methods becomes more feasible if the time of lambing can be predicted accurately and concentrated into one or more brief periods. Thus there clearly is benefit to be derived from techniques that will induce oestrus and ovulation in anoestrus ewes and will control precisely the time of oestrus and ovulation in cycling ewes.
HISTORICAL BACKGROUND
Dutt and Casida (1948) showed that a series of progesterone injections would inhibit oestrus and ovulation in cycling ewes. Cessation of treatment allowed oestrus and ovulation in three to four days.
Cole and Miller (1933) reported that a single injection of PMS in anoestrus ewes regularly induced ovulation but this was not usually accompanied by oestrus. Not until 1952 was it shown conclusively by Dauzier and Wintenberger in France, Dutt in America, and Robinson in Australia, that oestrus accompanied ovulation if the PMS was preceded by a course of progesterone treatment.
These findings offered physiological methods of inducing oestrus in anoestrus sheep and of regulating the cycles of cycling sheep. However, field use of the technique revealed several disabilities. First, it was tedious and required much time and labour for daily injection of sheep for 16 days. Second, fertility was reduced at the induced or controlled oestrus.
Between 1950 and 1960, there was much research effort aimed at simplifying the technique. Initially, researchers looked at formulations of progesterone that would last 16 days with one or at most only a small number of injections. Later, as new analogues of progesterone became available, many of these were examined in the hope of finding one that would be suitable for a one-injection technique. Clearly, such compounds are usually long-acting and this characteristic is not compatible with the need for a rapid withdrawal of progestational influence after the termination of treatment. The feeding of progestogens has also been explored but this method is subject to variability of animal intake in addition to the draw-backs associated with the use of long-acting compounds.
RECENT ADVANCES
It became evident that the requirement was a method of easily terminated continuous administration of a suitable compound. Continuous injection and subcutaneous implantation of progesterone were not successful. Robinson (1960) suggested the use of an intravaginal pessary loaded with a compound of high activity but of short duration of activity. Progesterone was not considered suitable because it seemed unlikely that the required dose (10 mg./day) could be absorbed from a mucosal surface.
Shelton (1965), using a spayed ewe bioassay, tested a range of progestogens and showed that one of these ("Cronolone", G. D. Searle) was 25 times as potent as progesterone and had a short duration of activity. Subsequently, Robinson (1965) used Cronolone-impregnated intravaginal sponges to control ovulation in ewes. In the early experiments, results were remarkably good (Table 1). The pessaries inhibited oestrus and ovulation; the timing of oestrus and ovulation after withdrawal was quite precise; and fertility to natural service and artificial insemination was normal. However, in large field trials (Robinson, Salamon, Moore and Smith, in the press) it became clear that fertility was depressed at the first post-treatment oestrus. Despite a relatively normal time relationship between ovulation and oestrus (Robinson and Smith, in the press), fertilisation rate was depressed (Moore, Quinlivan, Robinson and Smith, in the press). There is no evidence (Shelton and Moore, 1966) of any increase in post-fertilisation losses. It is now apparent (Quinlivan, in the press) that the cause of fertilisation failure is an abnormality of sperm transport. Twenty-four hours after insemination, there is a much smaller number of sperm in the fallopian tubes of treated ewes than in untreated ewes. There is some evidence that this situation can be overcome by massive doses of sperm as deposited by highly fertile rams at natural service.
One may postulate a number of reasons why sperm transport is adversely affected but to date these have not been investigated thoroughly. When the cause is known it should be possible to undertake a rational approach to amelioration of the problem.
When Cronolene-impregnated pessaries are used in anoestrus sheep with PMS, ovulation is accompanied by oestrus in a high proportion of animals. Here again fertility is below normal at the first oestrus. However, if treatment is initiated six to eight weeks prior to the time of onset of the breeding season, cyclic ovarian activity will continue with normal fertility at subsequent cycles. When the technique is used in deep anoestrus only one cycle is obtained and the ewes relapse into anoestrus.
SUMMARY OF THE PRESENT TECHNIQUE
1. Following investigation of several dose rates, it is evident that 30 mg. Cronolone is adequate when dispersed throughout a polyurethane pessary.
2. The time of mating in cycling ewes can be controlled in most animals to within a 24-hour period.
3. Fertility at the first oestrus, particularly to A.I., is reduced to about one-half the normal level. Frequently natural mating at the first oestrus results in reasonable fertility.
4. A high degree of synchronisation persists for several cycles, at which fertility is normal.
5. Artificial insemination at the second and third oestrous periods results in high fertility.
6. Lambing is greatly concentrated and most lambs are dropped over a period of a week. There is no overlap between the lambings to first, second or third oestrus.
7. The predictable and brief period of lambing can greatly assist lamb survival by making practicable techniques for preventing losses.
TABLE 1. Ovulation/oestrus blocking and release and subsequent fertility in young cyclic ewes treated with Progesterone or Cronolone impregnated sponges inserted intravaginally.
| Progesterone 500 mg. | Cronolone 30 mg. | ||||||||
| No. of ewes | |||||||||
| Total | Sponges intact | Blocked oestrus | Released oestrus | Lambed (A.I.) | Percentage of ewes lambed of ewes inseminated | ||||
|---|---|---|---|---|---|---|---|---|---|
| 2-4 days1 | 5-6 days2 | Total | |||||||
| Progesterone | 54 | 46 | 46 | 25 | 5 | 30 | 17 | 68 | |
| Cronolone | 52 | 51 | 51 | 42 | 9 | 51 | 37 | 88 | |
| Totals | 106 | 97 | 97 | 67 | 14 | 81 | 54 | 81 | |
1 Artificially inseminated (A.I.)
2 Served naturally: lambing data not presented
From Robinson (1965)
8. In the anoestrus ewe, treatment is effective but if given too early only one cycle is induced and fertility may be low. When the technique is used only six to eight weeks before commencement of natural breeding cycles, a series of cycles is initiated.
9. If used in November December in cross-bred ewes, PMS is necessary. If used in January, PMS may not be necessary.
10. In the pure British breeds, the times of use are later depending on the normal time of onset of cyclic activity in the breed used.
References
1. Cole, H. H. and Miller, R. F. (1933) — Amer. J. Physiol. 104:165
2. Daurier, L. and Wintenberger, S. (1952) — Ann. Zootech. (Paris) No. 4, 49
3. Dutt, R. H. (1952) — 1. Anim. Sci. 11:792
4. Dutt, R. H. and Casida, L. E. (1948) — Endocrinology 43:208
5. Moore, N. W., Quinliyan, T. D., Robinson, T. J. and Smith, J. F. — (In the press) in The Control of the Ovarian Cycle in the Sheep, ed. Robinson, T.) Syd. Uni. Press
6. Quinlivan, T. D. — (In the press) in The Control of the Ovarian Cycle in the Sheep, ed. Robinson, 1. J., Syd. Uni. Press
7. Robinson, T. J. (1952) — Nature (Lond.) 170:373
8. Robinson, T.J. (1960) — Proc. N.Z. Soc. Anim. Prod. 20:42
9. Robinson, T. J. (1965) — Nature (Lond.) 206:39
10. Robinson, T.J., Salamon, S., Moore, N. W. and Smith, J. F. (In the press) in The Control of the Ovarian Cycle in the Sheep, ed. Robinson, T.J. Syd. Uni. Press
11. Robinson, T. J. and Smith, J. F. — (In the press) in The Control of the Ovarian Cycle in the Sheep, ed. Robinson, T.J., Syd. Uni. Press
12. Shelton, J. N. (1965) — Nature (Lond.) 206:156
13. Shelton, J. N. and Moore, N. W. (1966) — J. Reprod. Fertil. 11:149