TIME OF ONSET AND POTENTIAL LENGTH OF THE BREEDING-SEASON OF ICELANDIC SHEEP - LUTEAL ACTIVITY

A total of 40 mature Icelandic ewes were studied during four breeding seasons. Blood samples were collected at regular intervals and the progesterone concentrations assayed. The progesterone profiles were evaluated and the time of onset, cessation and duration of the breeding season as well as the pattern of the luteal function in non-pregnant and pregnant ewes was established. On average, the onset of the breeding season occurred on 15 November and its cessation on 20 February. The length of the breeding season was 97 +/- 24 days. The earliest onset was in October and ovarian cyclicity ceased in April in the ewes with the longest breeding period. The onset of cyclicity spanned 53 days and its cessation 90 days. In 90% of the ewes the onset spanned 28 days and the cessation 70 days. In relation to daylength, all the ewes were cycling during the period when the centre of the sun was 5 degrees above the horizon at noon with decreasing daylength until it was 5 degrees above the horizon with increasing daylength (sunrise-sunset 5.5 h). The length of the ovarian cycle was 16 +/- 2 days. The length of the luteal phase was 11 +/- 2 days and was not significantly different between cycles but the mean progesterone concentration of the luteal phase of the first ovarian cycle was lower and the area under the progesterone curve was smaller (P<0.01) than in successive cycles. A transient rise in progesterone (3-5 nmol l(-1) for 5 days or less) preceding a full length ovarian cycle was seen in 30% of the ewes. The average number of luteal phases during the breeding period was 5.5 +/- 1.5. The length of the interluteal or follicular phase was 5 +/- 1 days and did not vary significantly between ovarian cycles. The gestation length was 138-148 days and during the last 50-70 days of gestation progesterone increased four- to six-fold owing to placental production.