Calving outcomes after ipsilateral versus contralateral supplementary embryo transfer in repeat-breeder Holstein cows

Supplementary embryo transfer (ET) following artificial insemination (AI) is used in repeat-breeder cows; however, this dual-embryo approach inherently increases the risk of twin outcomes. This study evaluated whether transferring an embryo to the uterine horn contralateral to the corpus luteum (CL) could be associated with different calving outcomes from ipsilateral transfer. Holstein cows (n = 187; 79 nulliparous and 108 multiparous) received ipsilateral or contralateral ET 7-8 days after AI using frozen-thawed, in vitro-produced embryos. Repeat-breeder cows were clinically defined as cows failing to conceive after ≥3 artificial inseminations without evident reproductive abnormalities on routine examination. Data were analyzed using a generalized linear mixed model accounting for parity and technician. The calving rate was 34.8% (32/92) in the contralateral group and 46.3% (44/95) in the ipsilateral group, with no significant difference between groups (aOR = 0.62, 95% CI 0.33-1.15; P = 0.137). Twin calving incidence was numerically lower after contralateral ET (4.8%, 1/21) compared with ipsilateral ET (21.2%, 7/33; P = 0.131). Because this study was based on calving records and involved a limited number of animals, the findings should be interpreted cautiously. Contralateral supplementary ET may represent a practical option for further investigation.