(1) Duration of oestrus.
(a) The normal oestrous duration in Korean mares (having no foals) varied from 4 days to 14 days, and the mean value is 8.98±0.253 days, the standard deviation being 2.68±0.179 days.The duration is shorter in fat condition than otherwise.
(b) The duration of oestrus in mares of two stud-farms varied from 2 days to 59 days and most of them (92.40%) are in the limit from 3 days to 15 days, the total number in this limit being 1, 179 cases; the mean value is 7.53±0.050 days and the standard deviation 2.53±0.035 days.
In the normal breeding season (May-July) the total number is 705 cases, most of them (95.5%) are in the limit from 3 days to 13 days. The mean value is 7.25±0.0574 days, and the standard deviation 2.208±0.0406 days.
In mares with suckling the duration is shorter than that of mare with none.
In March the duration is at the longest and then it has the tendency of gradual shortening and in July at the shortest.
Age or breed does not correlate with duration.
(2) Oestrous cycle.
(a) The mean value of the 40 cases measured by ovulation is 22.65±0.272 days and the standard deviation 2.55±0.192 days; the mean value of the 37 cases measured by the first day of oestrus is 22.86±0.325 days and the standard deviation 2.78±0.218 days.
The interval between the heat periods of fatter mares is shorter than those of otherwise condition.
The interval of the oestrous cycle is not always the same by individuals.
(b) The oestrous cycle in mares of two stud-farms.
The total number of oestrous cycle in normal mares in the two stud-farms is 790 cases varying from 9 days to 124 days, most of them (594 of 790 cases, about 75.2%) are in the limit from 15 days to 33 days. The mean value is 22.79±0.1226 days, the standard deviation being 4.437±0.0867, days.
In the normal breeding season (May-July) the total number is 309 cases varying from 9 days to 86 days, 254 cases of them (82.2%) are in the limit from 15 days from 30 days. The mean value-is 21.40±0.146 days, and the standard deviation 3.465±0.1038 days.
(3) The interval between the parturition and the forthcoming oestrus.
The total number in this investigation is 813 cases varying from 3 days to 100 days, 744 cases of them (about 91.5%) being in the limit from 5 days to 12 days. The mean value is 8.03±0.035 days, and the standard deviation 1.40±0.024 days.
In March it is longest and then it has the tendency of gradual shortening.
The correlation between the breed and the interval is uncertain.
The more old they become, the more reduced the intervals are.

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