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Title: Studies on conception, embryo survival and the early growth of foetuses in prolific ewes
Author: Dingwall, William Scott
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1982
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The literature on the physiological processes involved in the establishment of pregnancy in the ewe was reviewed with particular reference to the prolific and nursing ewe. Experiments were designed to provide information on the early growth of foetuses in prolific ewes and on conception and embryo survival in the nursing ewe. Experiment 1 provided data on foetal growth during early pregnancy in Finnish Landrace x Dorset Horn ewes. In experiment 2, the consequence on the within-litter variability in foetal size of standardizing the number of embryos in each uterine horn was examined. Two embryos were transplanted into each uterine horn of 15 recipient ewes. The ewes were slaughtered on the 60th day of gestation and their gravid uteri dissected to ascertain litter size and the distribution of the foetuses within the uterus. Foetal weights and placental sizes were recorded. The implications of the findings of experiments 1 and 2 that within-litter differences in foetal weight are established in early pregnancy and are associated with within-litter differences in placental size are discussed in relation to the viability of the newborn lamb. Experiment 3 provided information on the endocrine changes in the ewes in experiment 1 following progestagen withdrawal. The timing and magnitude of the preovulatory surges of oestradiol-17 and LH and subsequent luteal secretion of progesterone were considered in relation to the ovulation rate of the ewes. In experiment 4, the effects of introducing a restricted-suckling regime after 5 weeks of lactation on the conception and lambing rates following matings to an induced oestrus in the ninth week of lactation were investigated, both outwith and during the breeding season. Pregnancy failure in free-suckling ewes was associated with both premature and delayed luteal regression. In experiment 5, detailed observations were made on the endocrine status of ewes subjected to different suckling frequencies after 4 weeks of lactation both outwith and during the breeding season. The resumption of ovarian activity in the post-partum period was delayed in nursing ewes when basal prolactin concentrations were low, but compared to the early-weaned ewe the nursing ewe remained anovular for an extended period post-partum when basal prolactin concentrations were high. Differences in the interval from parturition to the resumption of ovarian activity were not associated with differences in tonic LH secretion. Prolactin release at a suckling episode was greater during anoestrus than in the breeding season. During the breeding season basal prolactin concentrations in lactating ewes at the time of remating had reached the same low level observed in ewes weaned at 4 weeks. The transition from anoestrus to cyclic activity was marked by an increase in the frequency of the pulsatile release of LH and a decrease in the amplitude of each LH pulse. Following progestagen withdrawal and PMSG injection the pre-ovulatory rises in oestradiol-17, LH and prolactin were not influenced by lactation or the frequency of suckling but showed marked variations with season. At the induced oestrus in December, oestradiol-17 concentrations rose higher and reached maximum concentrations earlier than in May. The interval from progestagen withdrawal to the LH surge was shorter and the magnitude of the surge lower in December than in May. In December when basal concentrations of prolactin were very low, the pre-ovulatory surge of prolactin was more pronounced than in May.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available