Onset Of Oestrous Activity Research Articles

Oestrous Activity and Ovulation Rate in Rasa Aragonesa Ewes Maintained at two Different Body Condition Levels Implanted or Reimplanted in the Seasonal Anoestrus with Exogenous Melatonin

The effects of melatonin implants in advancing the breeding season and enhancing both ovulation rate (OR) and litter size in the ewe are well documented (Haresign et al., 1990; López and Inskeep, 1991). Likewise, oestrous activity and OR in the early breeding season can be stimulated by a moderately and constant body condition (BC) in Mediterranean sheep breeds (Forcada et al., 1992). However, little information is available in relation to the effect of nutritional status in ewes receiving exogenous melatonin on reproductive parameters. The present study was carried out to determine the effects of a constant BC throughout the year, and implant or reimplant of exogenous melatonin on onset of oestrous activity and OR in a reduced seasonality sheep breed.The experiment was designed as a 2x2 factorial. Factors were two constant BC levels (≤2.50; L, and ≥2.75; H) and two forms of melatonin treatment (one implant -Melovine™- placed the 8th April; M, and a second implant 49 days later; 2M).

The effect of daylength and nutrition on the oestrous and ovulatory activity of Greyface ewes

SUMMARYThe onset and duration of oestrous and ovulatory activity in 26 Greyface ewes receiving natural daylength changes were determined. Precise ovulation data were obtained by endoscopy and repeated observations of the ovaries of all ewes were made on 10 occasions from 15 July 1971 to 27 June 1972. The natural breeding season for these Greyface ewes was, on average, 22 September 1971 to 18 February 1972. There were significant seasonal changes in the mean ovulation rate of the ewes which appeared to be inversely related to seasonal changes in natural daylength.An abrupt decrease in artificial daylength of 11 -43 hr applied on 22 June 1971 to 28 similar Greyface ewes advanced their mean onset of oestrous activity by 23 days. The decrease in artificial daylength also advanced the occurrence of ovulation in these ewes. The highest mean ovulation rate of the ewes receiving the decrease in artificial daylength was at least as high as the peak mean ovulation rate measured in the control ewes.At most stages of the experiment, the plane of nutrition had no significant effect on either oestrous or ovulatory activity in the ewes.

Photoperiodism in the ewe: 4. A note on the effect on onset of oestrus in Clun Forest ewes of applying the same decrease in daylength at two different times of the year

SUMMARYExperiments were carried out with Clun Forest ewes to determine the effect on the onset of oestrus of applying the same abrupt decrease in daylength at two different times of the year. An abrupt decrease in daylength of 3·75 hr was applied to the ewes on 1 July 1968 and on 24 April 1969. The mean number of days from the commencement of the light treatment to the onset of oestrous activity was 59·5 and 87·0 days, respectively. It is concluded that the time of year at which the light treatments were applied, or the previous light treatment of the Clun Forest ewes, or both, significantly (P<0·001) affected the date of onset of oestrus.

The effect of constant long daily photoperiod upon the onset of puberty in ewes

The effect upon the onset of reproductive activity of exposure of ewe lambs of three British breeds (Border Leicester, Southdown, Suffolk) to constant long daylight hours (16 h light/8 h dark) was studied.Oestrus, ovulation and conception were depressed but not inhibited; however, only the incidence of conception was significantly less than in the control group. Athough the treatment retarded the onset of oestrous activity, the age at first oestrus was still within the normal age range for puberty.

Effect of Time of Shearing on the Onset of Breeding Activity in the Ewe

IN the Clun Forest flock on the College Farm at Aberystwyth, a significant association has been noted1 between the mean ambient temperature in late summer and the onset of oestrous activity in the ewe. The greater the mean temperature from mid-July to early August, the later the start of the breeding season. The period of susceptibility has appeared to be quite restricted and would seem to coincide with the period of transition from anoestrum into oestrous activity. A fifth year's results are in very close agreement with the data already reported (my unpublished results).

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

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.