Seasonal variation in reproductive performance of Merino ewes in northern New South Wales

Abstract

Two groups of Merino ewes were observed for five years to determine the variation within years and to make comparisons between years in the numbers of ewes exhibiting oestrus, in fertility, and in lamb birth weights. A group of 35 ewes (continuous), was run constantly with vasectomized rams and services were recorded. From the other (changing) group, a fresh sub-group of 15 ewes was joined with fertile rams every four to six weeks; services were recorded, together with lambing performance and lamb birth weights.Distinct breeding and anoestrous seasons similar to those shown by other workers were observed in both groups of ewes. However, the onset of oestrous activity was a month earlier in the changing ewes than in the continuous ewes.Fertility was low during spring and early summer and reached a maximum in autumn.The birth weight of the lambs appeared to be predominantly influenced by the ewes' nutrition during pregnancy.

Post Parturient Oestrus in Merino and Dorset Horn Sheep

“…The incidence of post parturient oestrus observed in the small number of ewes which lambed in June and July 1963 is consistent with a high level of oestrus activity in winter noted by Barrett, Reardon and Lambourne ( 1962). In the subsequent observations the greater oestrus activity post partum in the winter than the spring lambing group could be explained on the grounds of photoperiodic control of breeding activity (Yeates 1949).…”

Seasonal Changes in the Occurrence of Oestrus Following Progesterone Suppression of Ovarian Function in the Merino Ewe

“…In studies carried out on the same Research Station, Barrett et al (1962) observed that about 80 % of the possible oestrous periods, on the basis of 17-day cycles in ewes continuously with rams, were detected by the rams. Presumably some ewes did not show oestrus during a period when all ewes might be ex¬ pected to show oestrus.…”

“…Except in December 1961 and October 1962, there was a higher pro¬ portion of ewes showing ovarian cycles (denoted by the presence of a normal corpus luteum) than would be expected from the proportion of ewes observed in oestrus during the period of 18 days before ovarian examination. Even in the period March to July, which is the time of the year when the highest pro¬ portion of ewes experience regular oestrous cycles at the Research Station {see Barrett, Reardon & Lambourne, 1962) approximately 20% of ewes still had normal corpora lutea but were not marked by the ram at the assumed time of ovulation.…”

Quantitative Studies of the Interaction Between Progesterone and Pregnant Mare Serum on Ovarian Function in the Ewe

“…Commencing on the morning of Day 2, the ewes were tested for oestrus at 7.00 to 8.00 a.m., 10.00 to 11.00 a.m., 2.00 to 3.00 p.m., 5.00 to 6.00 p.m. and 10.00 to 11.00 p.m. each day. Small groups of ewes were joined with harnessed vasectomized rams for f to 1 hr, at each time (see Lamond & Bindon, 1962 (Lamond & Bindon, 1962 (Barrett, Reardon & Lambourne, 1962;Lamond, 1964). The doses of progesterone used were two to three times greater in May than in August.…”