REPRODUCTION IN THE EWE
Authors: ROBINSON T. J. 
Published: January 2008

1. The relative low prolificacy of the ewe is a factor severely limiting the sheep's ability to compete with other forms of livestock production in intensive agriculture. This article is an attempt to examine critically all the available information on the reproductive physiology of the ewe, with particular emphasis on the factors controlling the various phenomena and the possibility of increasing prolificacy.

2. All breeds exhibit, to a greater or less extent, a breeding season which is under light control. Available data on the extent of this season are tabulated.

3. The ‘height’ of the breeding season corresponds roughly to the shortest day of the year, and the ‘depth’ of anoestrum to the longest day.

4. This and other data suggest a seasonal fluctuation in pituitary gonadotrophic activity controlled by, and roughly inversely related to, the period of daylight relative to dark. It reaches a maximum when the days are shortest and a minimum when they are longest.

The ability of some breeds to come into oestrus and conceive shortly after parturition depends on the breeding season being sufficiently long to permit ewes mated early in the season to lamb well before its end. Provided then that the lactation anoestrum, the length of which appears to vary according to the stage of the breeding season, does not run into the true anoestrum the ewes can mate and conceive again while lactating.

6. The onset of the breeding season is preceded by an ovulation without heat–‘silent heat’. The evocation of heat with ovulation appears dependent on the presence of a waning corpus luteum.

7. Seasonal variation in the reproductive capacity of the ram is evident but is not as marked as in the ewe.

8. The literature on the dioestrous cycle, ovulation, fertilization, attachment and subsequent development of the embryo and foetus is reviewed. Very loose attachment to the cotyledons occurs at about 11 days after ovulation, which attachment remains loose up to the 30th day. Full placentation is not completed until the middle of pregnancy.

9. Embryonic mortality within the first 21 days accounts for an appreciable number of the ova fertilized. While the number of ova shed is probably the main factor controlling the number of lambs born, this mortality must be considered.

10. The use of gonadotropins in anoestrum to obtain early pregnancies is fully discussed and the literature reviewed. While, in general, results have been disappointing there appears to be a definite pattern of behaviour depending on the state of the ovaries at the time of injection.

11. An interesting report on the use of pregnant mare's serum (PMS) in inducing fertile matings shortly after parturition within the breeding season is discussed. If this can be confirmed, it may be of value in obtaining two pregnancies within a year.

12. The subcutaneous injection of 500 i.u. of PMS 12 days after the preceding oestrus will result in multiple ovulations at the ensuing heat period 4 or 5 days later. However, the individual response to any given level of PMS is extremely variable, from two to nine ovulations having been observed following 500 i.u. Mating at this oestrus results in a high proportion of fertilizations and attachments. Subsequent embryonic mortality within the first 21 days following ovulation results in a reduction in the number of embryos surviving to two or three, occasionally four and rarely five or six. Up to four live young have been obtained at Cambridge and five in Russia. The number of lambs born in a small group of Suffolk ewes treated at Cambridge was greater than in the control group. However, some were born dead and the number weaned was similar in both groups.

13. There appear to be breed differences in the number of embryos surviving the first 3 weeks of pregnancy following such PMS treatment, probably due to differences in uterine environment.

14. Overstimulation with gonadotropins results in accelerated tubal transport of the ova, with lack of fertilization and complete failure to attach.

15. Taking a long‐term view, increase in prolificacy can probably be best obtained by selection for a long breeding season and for multiple births. In most breeds there is sufficient variation in these characteristics to offer considerable hope of success.