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Equine veterinary journal2026; doi: 10.1002/evj.70160

Impact of donor mare and stallion identity on the likelihood of pregnancy after transfer of in vitro produced embryos.

Abstract: The success of in vitro embryo production (IVEP) is influenced by donor mare and stallion. Objective: To determine whether donor mare and stallion influence the pregnancy rate after transfer of in vitro produced (IVP) blastocysts and to identify factors influencing the likelihood of obtaining one or more pregnancies from a single IVEP session. Methods: Retrospective clinical study. Methods: The influence of the following variables on pregnancy outcome was assessed in 2292 transfers of IVP blastocysts: speed of in vitro embryo development, embryo grade after thawing, age of the donor mare, blastocyst rate, age and breed of the recipient mare, year of embryo production, month and year of embryo transfer. Finally, the variability in pregnancy rate between individual mares and stallions, and the effect of mare-stallion combination was investigated. Results: Overall, 85% of successful IVEP sessions resulted in at least one pregnancy. When a single blastocyst was produced, 78% of sessions yielded a pregnancy; this increased to 91% and 98% when two or at least three blastocysts were produced, respectively. Slow in vitro embryo development, poor embryo grade after thawing, advanced donor mare age, and performing embryo transfer late in the season had a negative impact on the pregnancy rate. The variation in pregnancy rates between most mares and stallions was low (~20%). For some mares, the likelihood of pregnancy varied by as much as 50% depending on the stallion used. Conclusions: Due to the limited sample size, variation in pregnancy rates between donors was assessed descriptively. Conclusions: IVEP is a very efficient way to obtain pregnancies. The likelihood of pregnancy after transfer of an IVP embryo has improved over time, and individual mare and stallion identity seems to have a low impact. However, for some underperforming mares, results differ markedly depending on the stallion used for intracytoplasmic sperm injection.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2026-03-29 PubMed ID: 41905773DOI: 10.1002/evj.70160Google Scholar: Lookup
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  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study examined how the identities of donor mares and stallions affect pregnancy success rates after transferring in vitro produced (IVP) embryos.
  • The study also explored factors influencing the likelihood of achieving one or more pregnancies from a single IVEP session.

Study Objective and Background

  • The main objective was to determine whether the donor mare and stallion influence pregnancy rates following the transfer of IVP blastocysts.
  • Additional goals included identifying variables that impact the chances of obtaining pregnancies from a single IVEP session.
  • IVEP success is important in breeding programs, especially for improving reproductive efficiency in mares.

Methods

  • Retrospective clinical study analyzing 2292 transfers of IVP blastocysts.
  • Variables analyzed included:
    • Speed of in vitro embryo development.
    • Embryo grade after thawing.
    • Donor mare age.
    • Blastocyst rate per session.
    • Age and breed of recipient mare.
    • Year of embryo production.
    • Month and year of embryo transfer.
  • Additional assessment of variability in pregnancy rates across individual donor mares and stallions.
  • Evaluation of interaction effects between mare and stallion identity on pregnancy likelihood.

Key Results

  • 85% of successful IVEP sessions resulted in at least one pregnancy.
  • Pregnancy rates increased with the number of blastocysts produced:
    • 78% pregnancy rate when one blastocyst was produced.
    • 91% when two blastocysts were produced.
    • 98% when three or more blastocysts were produced.
  • Negative factors affecting pregnancy rates included:
    • Slow embryo development during in vitro culture.
    • Poor embryo grade after thawing.
    • Advanced age of donor mare.
    • Embryo transfer performed late in the breeding season.
  • Variation in pregnancy rate between most individual mares and stallions was generally low, approximately 20%.
  • Some mares showed substantial variation depending on stallion choice, with pregnancy likelihood fluctuating by as much as 50% based on the stallion used for intracytoplasmic sperm injection (ICSI).

Conclusions

  • IVEP is an efficient technique to establish pregnancies in mares.
  • The overall likelihood of pregnancy after transfer of IVP embryos has improved over time, reflecting advancements in techniques or protocols.
  • Individual mare and stallion identities typically have a limited impact on pregnancy rates following IVEP embryo transfer.
  • However, in some underperforming donor mares, stallion identity plays a critical role, with specific mare-stallion combinations influencing pregnancy success substantially.
  • Due to limited sample sizes, differences between donor animals were mostly described rather than statistically analyzed.

Implications for Practice

  • Breeders should consider producing multiple blastocysts per session to maximize pregnancy success rates.
  • Monitoring embryo development speed and quality is important, with efforts to improve these aspects likely to enhance pregnancy outcomes.
  • Donor mare age is a factor in pregnancy success, highlighting the importance of managing breeding stock accordingly.
  • Careful selection of stallion for specific donor mares, especially those that historically perform poorly, can significantly improve chances of pregnancy.
  • Timing of embryo transfer within the breeding season should be optimized to reduce negative impacts on pregnancy rates.

Cite This Article

APA
Papas M, Stout TAE, Vernooij JCM, Leemans B, Arnold L, Beitsma MM, Wijnands LI, de Ruijter-Villani M, Colleoni S, Lazzari G, Galli C, Claes A. (2026). Impact of donor mare and stallion identity on the likelihood of pregnancy after transfer of in vitro produced embryos. Equine Vet J. https://doi.org/10.1002/evj.70160

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Papas, M
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Stout, T A E
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
  • Gluck Equine Research Center, Department of Veterinary Sciences, University of Kentucky, Lexington, Kentucky, USA.
Vernooij, J C M
  • Department of Population Health Sciences, Section of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Leemans, B
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Arnold, L
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Beitsma, M M
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Wijnands, L I
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
de Ruijter-Villani, M
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
Colleoni, S
  • Avantea, Cremona, Italy.
Lazzari, G
  • Avantea, Cremona, Italy.
Galli, C
  • Avantea, Cremona, Italy.
Claes, A
  • Department of Clinical Sciences, Section of Reproduction, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.

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