LENGTH OF OESTROUS CYCLE AND DURATION OF PREGNANCY IN EWES.

New Zealand Journal of Agriculture, Volume 47, Issue 5, 20 November 1933, Page 305

LENGTH OF OESTROUS CYCLE AND DURATION OF PREGNANCY IN EWES.

SOME OBSERVATIONS UNDER NEW ZEALAND CONDITIONS. D. A. Gill, M.R.C.V.S., D.V.S.M., Assistant Officer in Charge, Wallaceville Veterinary Laboratory. The writer is not aware of any observations on the length of oestrous cycle and duration of pregnancy in ewes having been recorded in New Zealand, and, while work was undertaken among the ewes at the Wallaceville Laboratory farm for another purpose, it seems worth while to put on record the information that was gathered there. The flock comprises some fifty ewes of varying ages. They were in excellent fettle when the ram was put out to them, having been flushed for two or three weeks on a specially rich young rye-grass and clover pasture. The ewes are Romneys, and a ram of that breed was mated with them.

The procedure was as follows : The ram was smeared over the brisket daily with a preparation of vaseline containing a red dye, and it was found quite easy in consequence to pick out the ewes he had served. The flock was inspected daily, and the ewes that had been served since the previous day were drafted out. These ewes were later run with another ram that was similarly smeared with a green raddle and the daily inspection of these ewes soon made it clear that any which returned to service could easily be detected by the plentiful smearing of green colour on the rump. It was found necessary to keep ewes that had been served by the flock ram away from the mob containing the second ram for about a week. This was not done at first, and the consequence was that one ewe was still in oestrum and took the second ram two days after being served by the first one (from her lambing date it seems probable that she had not actually been served by the first ram and was probably just coming into oestrum when removed from him), and another took the second ram after an interval of five days (subsequently lambing to the first service).

Length of Oestrous Cycle.

Quite a number of the ewes came back to a second, and four to a third service, and thus the number of days from the commencement of one period of oestrum to the commencement of another could be determined. This is shown in the following summary : —

It is quite apparent from this that the normal interval between periods of oestrum was from 15 to 18 days in the small number observed. . Four of the ewes each . returned to service twice, and the intervals they gave were 17 and 18 days, . 16 and 18 days, 16 and 16 days, 17 and 16 days.

Duration of Pregnancy

The ewes being ear-tagged and the dates of service known, it was a simple matter to calculate the duration of pregnancy by noting the date of lambing. That the gestation period was remarkably constant will be gathered from the following summary :—

Discussion and Conclusions.

It was found in Missouri/ 1 ) that among Hampshire, Shropshire, and Southdown ewes the interval between successive manifestations of oestrum varied from 14 to 16 days in 92 out of 116 ewes that were observed. The duration of pregnancy varied from 143-7 days in Southdowns to 144-6 days in Hampshires and Shropshires. These latter are average figures, however.

Quinlan and Mare( 2 ), studying Merino ewes under South African conditions, found that the oestrous cycle was remarkably constant at 16 to 19 days. Out of 234 ewes observed 200 showed cycles within those limits. They found that the duration of pregnancy among 338 Merino ewes varied from 142 to 156 days, but that the average duration might be considered as from 146 to 152 days, with the majority at 149 days.

Marshall( 3 ) states that Scotch Blackface sheep in the Highlands have two periods of eostrum separated from each other by 21 days, whereas similar sheep in the Lowland country have at least three and often more periods with intervals of 13 to 18 days.

Lydekker( 4 ) states that the duration of pregnancy is 144 days in Southdowns and 150 days in Merinos, while the hybrid is intermediate.

Both as regards the oestrous cycle and duration of pregnancy, it will be noticed how closely the Romney in New Zealand compares with the Merino in South Africa. References. (1) Annual Report, Missouri Agric. Expt. Station, 1931. (2) Quinlan and Mare, 17th Report, Director of Veterinary Services, South Africa, 1931. (3) Quoted by Marshall, Physiology of Reproduction, Longman, Green, and Co., 1922, page 40. (4) Quoted by Marshall, Physiology of Reproduction, Longman, Green, and Co., 1922, ' page 68.

Number of . Returns. . Interval in Days. Remarks. I 14 Ewe- proved empty. A ■ Ewe- proved empty. 3 15 One of these second services very doubtful, and the ewe lambed to the first. 6 16 6 ■ ■. . . * 1 7 I'/ 2 18 I 22 Second service very doubtful; lambed to first. I 35 First service doubtful; lambed to. second. I 39 Both services quite definite ; lambed to second. I 5 Unusually prolonged oestrum; lambed to first service.

Number of Ewes. Duration of Pregnancy in ? Days. Remarks. 4 Ewes proved empty. I . . T 53 Twin. 2 . . • 152 Twin and single. 3 • • 15T 2 twins and 1 single. 7 150 3 twins, 4 singles. ' ' 12 . . 149 4 twins, 8 singles. 21 148 10 twins, 11 singles. 2 147 1 twin (dystobia ; died), 1 single. 3 146 3 twins (1 lamb died at birth). i . . 145 Single. Total ewes in flock . . .. .. 56 (excluding 3 deaths) Total lambs • ■ 77 Lambing percentage •• 137-5 Ewes pregnant • • 52 Average duration of pregnancy . . 148-75 days.

Breed of Sheep. Locality. . Duration in Days of Oestrous Cycle. Hampshire, Shropshire, and Southdown Missouri, U.S.A. 14-16 Merino South Africa 16-19 Scotch Blackface .. Highlands . . 21 Scotch Blackface .... Lowlands . . I 4-18 Romney Missouri, U.S.A. South Africa Highlands Lowlands . . New Zealand , 14-16 16-19 21 13-18 15-18 Duration of Pregnancy. Southdown Missouri 1437 (average). Southdown Not stated .. 144 Hampshire and Shropshire Missouri Not stated Missouri 1 437 (average). T 44 144 '6 (average). Merino . . . . ... Not stated .. 150 Merino South Africa 149 (146-152). Romney . . . . Not stated .. South Africa New Zealand 150 149 (146-152). 148 (145-153).

Summarizing the observations mentioned above and the conclusions that can be drawn from the Wallaceville flock records we have the following:—