Comparative phenotypic analyses of healthy Holstein calves and calves with large offspring syndrome derived from in vitro-produced embryos.

In vitro embryo production (IVP) is a reproductive technology commonly used in the dairy cattle industry for various purposes, such as maximizing the genetic contributions of elite breeding animals. However, IVP has been often associated with increased frequency of Large Offspring Syndrome (LOS) cases, resulting in high calf mortality, welfare concerns, and economic losses. LOS is a developmental disorder in calves characterized by abnormally large birth weight and body size, often accompanied by organ enlargement and other abnormalities. Among the imprinted genes frequently dysregulated in LOS, IGF2R (insulin-like growth factor 2 receptor) is one of the most consistently affected genes. Therefore, the main objectives of this study were to perform a comprehensive characterization of LOS in calves, including an assessment of differences in organ development, physiological parameters, and histopathological examinations between healthy and LOS-affected calves, and evaluate factors influencing its expression in affected Chinese Holstein calves. Data on birth weight, health status, and gestation length were collected from 4,584 IVP-derived Holstein calves and used for the analyses. Subsequently, 17 LOS-affected calves were identified based on birth weight. The data from these calves were statistically compared with that from 6 healthy calves in terms of organ weight, blood biochemical parameters, and pathological changes using general linear models and t-tests. The results showed that the average birth weight of IVP-derived calves was 40.79 ± 6.69 kg. The proportion of healthy calves was highest (86%) among calves with birth weight ranging from 33 to 47 kg, whereas it decreased to 18.4% when birth weight exceeded 61kg. In addition, birth weight was significantly affected by calving season, embryo cryopreservation method, oocyte retrieval season, calf sex, farm, and sire. The comparative analyses showed that LOS-affected calves had significantly greater body weight accompanied by bigger hearts, livers, spleens, lungs, kidneys, and tongues than healthy calves. The ALB of LOS group is significantly higher than the control group, while the glutathione of LOS group was significantly lower than the control. Histopathological examination revealed interstitial pneumonia in the lungs and inflammatory infiltration in the portal area of the liver in LOS-affected calves. To our knowledge, this is the first study establishing a quantitative phenotypic database for LOS diagnosis in Chinese IVP-derived Holstein calves. This study systematically reveals the coordinated characteristics of multi-organ hypertrophy, functional abnormalities, and pathological alterations in LOS calves. These findings provide detailed background information for LOS diagnosis in calves, management practices, and genetic selection for reduced incidence of LOS cases in Holstein calves.