(1) The ovaries of 24 mares were examined rectally during a total of 52
oestrous periods in the 1949-51 breeding seasons. Only 27 of the 52 periods ended
in ovulation. In 24 of the 27 normal periods the mares were mated and conception
resulted in 20 (83•3 per cent).
(2) Follicles developed but then regressed without ovulating in 23 (44•2 per
cent). Ten mares mated during anovulatory periods all failed to conceive.
(3) It is suggested that many cases of pseudo-ovarian cysts in the mare are
not real cysts but follicles which persist for a time and then regress, or ovulate
when conditions become favourable.
(4) Two cases in which oestrus was shown continuously for 119 and 171 days
respectively are recorded. In the latter the prolonged period of heat terminated in
ovulation and conception.
(5) Another two cases were noted in which oestrus occurred without follicle
development. Attention is drawn to the possibility of extra ovarian sources of
oestrogen, and of neural control over the sexual cycle.
(6) Marked fo1licular development without symptoms of oestrus was observed
in three cases.
(7) No significant difference in the relative activity of the right and left ovaries
(8) In the great majority of cases ovulation, when present, occurred within
the last 24-48 hours of oestrus, but in two mares heat persisted for four and five
days respectively after ovulation. This is attributed to the presence of other
(9) Pregnancy resulted when mares were served up to 72 hours before ovulation.
One mare which was mated only once, at 8-12 hours after ovulation, did
not conceive. Palpation of the ovaries per rectum is considered the only satisfactory
method of determining the most suitable time for service.
(10) Two cases of oestrus with ovulation during pregnancy were noted. At
post mortem one mare with a ten week old foetus had two corpora lutea, the one
regressing and the other coming up to take over its function. Two other mares,
that had aborted during the second month of pregnancy, each only had the primary
corpus luteum of pregnancy. Failure of the ovaries in these two cases to form
fresh corpora lutea in order to maintain the necessary level of progesterone is
suspected to be the cause of the abortions.
(11) Treatment with stilboestrol dipropionate and chorionic gonadotrophin
was applied to 17 mares which showed irregularities in the sexual cycle. This
was followed by ovulation in ten, and nine of these conceived. Hormone treatment
should only be applied after proper rectal examination of the ovaries, in certain
selected cases, and dosage and time of administration should be carefully calculated.
(12) Late in the second half of the 1950-51 season 24 mares, all of which
but three were shy breeders or barren, were turned out to run free with two stallions
in two camps. Within two months 21 (87•5 per cent) had conceived. This high
conception rate is attributed to psychological factors.