Vie CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. I

AN APPRAISAL OF THE EFFICIENCY OF THE OFFICIAL MONTHS OF THE THOROUGH-
BRED AND STANDARDBRED STUD SEASON IN AUSTRALIA

VIRGINIA OSBORNE
Faculty of Veterinary Science
University of Sydney, Australia,

Administrative bodies controlling racing of the Thoroughbred
and Standardbred in Australia age the horse officially from August 1st
of each year, Consequently, active stud procedures begin on September
8th and continue for approximately four months through to the middle
or end of December. These dates place the official season to begin
several weeks before the spring equinox and to close approximately
coincidental with the summer solstice.

Problems of management arise in the early phase of this
season., Hesitant and uncertain ovarian activity follows a winter
anoestrus with irregular oestrous-dioestrous rhythm, persistent
anoestrus, anovular oestrus, unpredictable follicle progression ete..
It was considered advisable, therefore, to review the exact nature of
the annual reproductive cycle in these latitudes in order to pinpoint
the naturally most fertile part of the year, and then to be in a
position to appraise the wisdom and efficacy of the official ruling
which places the stud season in the months of the year where it now
stands.

Consequently, a week-by-week postmortem survey of the ovaries
of a total of 6,763 mares was undertaken. A constant supply of
abattoir material was available from the eastern Australian States
of Queensland and New South Wales between the latitudes of —25O 5¢
and -350 5'. The survey period ran over seven whole or part years
and included five successive stud seasons. By elimination of pregnant
abortive, immature, senile and pathological specimens, 5,198 paired
ovaries were acceptable to the sample. Ovulation was assumed by the
presence of a recent corpus luteum within the ovulation fossa.

Figures were recorded on a weekly basis and a month-by-
month table of ovulation percentages was drawn up. A frequency
distribution curve was plotted showing the position of the official
stud season within the annual reproductive cycle.

As expected, a classic recurring annual cycle was confirmed,
Relative ovarian quiescence occurred in the two months succeeding the
winter solstice, i.e. in July and August, with the peak period of
activity and therefore, the naturally selective, most fertile period in
the two months following the summer solstice, i.e. in January and
February.

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