Corncob Bedding Negatively Impacts Breeding Performance and Sexual Development in Mice

Corncob bedding is commonly used for housing rodents in research, but previous work has linked corncob to altered reproductive behavior, disrupted estrous cycling, aggression, and welfare impacts across non-murine rodents. Furthermore, corncob is used as a licensed commercial rodenticide. Corncob contains endocrine-disrupting compounds (EDCs) that interfere with aromatase activity and estrogen signaling, processes critical for normal sexual behavior and development, yet effects on reproductive outcomes in mice remain unexplored. We conducted two experiments to test whether corncob bedding influences breeding performance and male sexual development. In Experiment 1, we analyzed breeding records to compare breeding performance of NSG mice housed on corncob versus cellulose bedding across two 3-month phases (N = 488 litters). Pairs housed on corncob produced significantly fewer pups than pairs housed on cellulose. To understand this effect, in Experiment 2, hormonal and morphological effects of corncob were assessed in male mice from four genetic backgrounds (C57BL/6, BALB/c, FVB, and CD1; N = 32 cages). Mice were bred and born on aspen or corncob, with half switched at weaning and half unchanged. Corncob produced timing-dependent effects in male reproductive physiology and development. Early-life corncob exposure altered baculum morphology and reduced testosterone, estradiol, and anogenital distance. In contrast, post-weaning corncob exposure resulted in hyper-masculinization, indicated by increased anogenital distance. Alongside prior evidence that corncob contains EDCs, our results raise serious concerns about its suitability as bedding in animal research. Continued use of corncob introduces uncontrolled variation that compromises animal welfare, reproduction, experimental validity, and reproducibility.