(1) Observations were carried out on the oestrous cycle of 30 mares from
January, 1944, to June, 1946.
(2) The mares were stabled at night and ran out in a small paddock during
the day. They were never let out to grazing but received a uniform ration throughout
the year with the exception that green feed was given during the summer
months but not during winter.
(3) They were tested for oestrus every day and mares that showed symptoms
of oestrus were mated.
(4) The results revealed that the oestrous cycle in these mares was not
seasonal, but that it continued throughout the year in the majority of animals.
(5) There was, however, a noteworthy decline in the percentage of mares that
showed oestrus in April and May.
(6) The cycles were very irregular and varied in length from 6 days to 113 days.
The irregularities occurred throughout the year, but were most pronounced during
the months of February to July. The average length of the cycles for this period
was 35•9 days as against 23•8 days for the remaining six months of the year. The
average length of the oestrous cycles for the whole year was 29•9 days.
(7) The increase in the average length of the cycles for February to July is
largely due to the fact that a number of mares went into varying periods of
anoestrus during these months. This also accounts for the abnormal length of the
average for the cycle throughout the year.
(8) There were 17 periods of anoestrus varying in duration from 46 to 113
days. With one exception all these occurred during the first seven months of the
(9) The average duration of oestrus was found to be 6•93 days. Oestrus was
shorter (average of 5•7 days) in the first half of the year than in the second half when
the mean was 8•03 days. In the latter it was highest during the early months of
breeding (July to October).
(10) The oestrous period was found to be as variable as the oestrous cycle.
The shortest period was 1 day and the longest 55 days.
(11) The months of April, May, June and July were found to be barren in so far
as none of the mares that were mated then became pregnant. It is concluded that
the 50 oestrous periods that were shown during these months were not accompanied
(12) Fecundity was low during the first three months of the year. Then
ceased for four months from April to July and commenced again in August.
The highest percentage of pregnancies (55•6) was obtained from service in November.
(13) Nutritional factors are considered to be responsible for irregularities in
the oestrous cycle and for the low fertility.
(14) Most of the pregnancies resulted from mating during oestrous lasting
from 3 to 8 days.
(15) It is doubtful whether the duration of oestrus influences fertility. The
latter depends on service at the time of ovulation. This is also variable and may
even occur 19 or 20 days after the commencement of oestrus.