Intrinsic determinants and predictors of superovulatory yields in sheep: Circulating concentrations of reproductive hormones, ovarian status, and antral follicular blood flow

Abstract

Hormonal ovarian superstimulation has contributed to small ruminant reproduction around the world, impacting genetic improvement and zoosanitary programs, contributing to the conservation of endangered species, and supporting other related biotechnologies. Advanced knowledge surrounding the superovulatory treatments in sheep has resulted in enhanced control of influencing factors and improved the protocols currently used. However, in spite of minimization of some adverse factors, superovulatory responses in ewes still remain variable, preventing the more widespread use of superovulation in commercial embryo transfer programs and reproductive research in this species. Recent evidence demonstrates that changes in antral follicular populations and blood supply, and circulating concentrations of certain reproductive hormones determined at the specific time points just before or during the superovulatory treatment are associated with superovulation success in ewes. This review attempts to compile the data from available literature to identify ovarian and hormonal determinants of the superovulatory outcome in ewes, which can be used to substantially improve the existing protocols and to reduce the extra cost and unnecessary stress imposed on poorly responding animals. An overview of most commonly used and some recently developed, FSH-based ovarian stimulation protocols is given at the outset to highlight variation in the frequency and timing of gonadotropin injections, estrus synchronization methods, and follicular wave synchronization and/or ovulation induction techniques during the superovulatory treatments in ewes.

Assessing the usefulness of B‐mode and colour Doppler sonography, and measurements of circulating progesterone concentrations for determining ovarian responses in superovulated ewes

“…This range of ovulatory responses was not due to the total pFSH dose used (100, 133 or 200 mg per ewe over 4 days; Figure , Table ). High variability in superovulatory outcomes is the major drawback of the MOET biotechnology in sheep; it has been attributed to several intrinsic and extrinsic factors (Bartlewski et al., ). Due to variable superovulatory responses and the fact that embryo collection in sheep is performed predominantly by a surgical technique (Fonseca et al., ), the ability to accurately and non‐invasively determine the outcome of hormonal superstimulation would be an invaluable asset.…”

“…A number of earlier studies suggested the existence of a relationship between the number of CL and circulating P 4 concentrations in cyclic ewes (Bindon, Chang, & Turner, ; Oyedipe, Pathiraja, Gyang, & Edqvist, ). There were no correlations between serum P 4 concentration and ovulatory responses in anoestrous Rideau Arcott sheep superovulated in a multiple‐dose pFSH regimen (Bartlewski et al., ; Fuerst et al., ). Similarly, Amiridis et al.…”

“…Colour Doppler sonography provides a reliable and rapid means of detecting luteal vasculature, which helps delineate individual luteal structures. Similar studies are lacking in small ruminants (Bartlewski et al., ; Oliveira et al., ).…”

“…The multiple ovulation and embryo transfer (MOET) is a reproductive biotechnology facilitating genetic improvement in livestock species, whose commercial application has increased significantly in recent years (Bartlewski et al., ; Oliveira et al., ). The high variability and unpredictability of responses to hormonal ovarian stimulation is notably the major challenge preventing further increase in MOET efficiency and its more widespread use in sheep (Menchaca, Vilariño, Crispo, Castro, & Rubianes, ; Oliveira, ).…”

Colour‐Doppler ultrasound imaging as a laparoscopy substitute to count corpora lutea in superovulated sheep

“…Despite efforts to improve superovulatory protocols (SOV) in sheep, variability in ovarian response remains the major limitation in multiple ovulation and embryo transfer (MOET) programs (Bartlewski et al., ). Thus, the CL are counted by laparoscopy to determine ovarian response and whether a surgical embryo recovery procedure is justified.…”