Brogan, E., Kirchain, S. 2021. When to wean? A visual guide to Black, White, and Agouti mouse pup growth P14 – P28. Laboratory Animal Science Professional 9(3) (May/June), 46-48.

Laboratory mouse pups are commonly weaned between postnatal day 21–28. Utilization of an exact wean date postpartum is a concrete metric for mouse colony management, but variation can result such as: developmental differences among genetically modified strains, personnel differences in assessing pup developmental stage, different breeding schemes which can lead to early weaning due to overcrowding, and lack of precision estimating birthdate in non-timed pregnancies. Clinical and operational outcomes of weaning pups too young may include failure to thrive, unsuitability for study assignment, undue distress on the pups, and extensive time and resource commitment to monitoring and supportive care by veterinary staff. We previously published2 a pup growth chart for black mice (C57BL/6J) ages 14 -28 d as a point-of-use visual aid for developmentally appropriate weaning time. The tool proved to be a highly beneficial tool for many institutions with mouse breeding colonies, and augmented previously available tools that covered up to 14 d.4 We recently expanded our original poster (v1.0) with a new poster (v2.0) to include three coat colors: black (C57BL/6J), white (BALB/c) and agouti (129 strain background) mouse pups from age P14 to P28, providing a more comprehensive point of use weaning visualization that includes three of the most commonly used mouse coat colors (poster provided in article). Overall, the program saw a 50% decrease in total pup mortality with the roll-out of the first poster in 2018 despite stable to increasing rodent populations. This decrease in mortality was attributed to several factors including the black pup poster along with a program-wide education and re-training initiative, concerted efforts to decrease environmental perturbations (such as not entering during the dark cycle, gentle handling of cages, etc.), new rodent separation guidelines and operating procedures. While data analysis following roll-out of the Black, White and Agouti Pup poster (v2.0) in July 2020 has been complicated by COVID-19 pandemic shutdowns, we have continued o see positive weaning effects in the mouse colonies. In summary, this point-of-use visual representation of mouse pups provides a universal standard to help vivarium personnel and researchers identify mouse pup age and developmental stage through the critical weaning age period, even when English is not the first language.

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