ObjectiveTo determine the optimal bedding depth of wood shavings and cage-changing interval for post-weaning (21-day-old) SPF C57BL/6J mice housed in open cages within a barrier environment. MethodsThree bedding groups with average depths of 3 cm, 4 cm, and 5 cm were established, forming six experimental groups (three groups each for female and male mice, with 60 mice per group and 20 mice per cage, totaling 18 cages). The mice were housed in accordance with the maximum housing density requirements specified in GB 14925—2023 Laboratory Animal—Environment and Housing Facilities. Indicators, including body weight, food intake, waste load, and bedding cleanliness, were continuously monitored in mice aged 21-54 days. ResultsAt the age of 21-54 days, the body weight of male mice in the 4 cm bedding group at 42 days was significantly higher than that in the 3 cm and 5 cm groups (P<0.01); at the age of 45-54 days, the waste load of male mice in the 4 cm group was significantly higher than that in the 3 cm group (P<0.05). There were no statistically significant differences in body weight, feed intake and waste load of female mice among each bedding height group (P>0.05). Gender comparison showed that the body weight, feed intake and waste load of male mice were significantly higher than those of female mice at multiple age groups (P<0.05). However, there was no statistically significant difference in cleanliness scores between female and male mice (P>0.05). The scores of mice in the 3 cm and 4 cm groups were close to 3 points from day 6 to day 12, and the scores of mice in the 5 cm group were close to 3 points on day 12. After 42 days of age, the cleanliness scores of each group increased rapidly, and the cage change cycle needed to be shortened to 4 days. Comprehensive recommendation: the cage change cycle for 3 cm and 4 cm bedding heights is 6 days, and it can be extended to 12 days at a height of 5 cm bedding, and shortened to 4 days after 42 days of age. ConclusionUnder the open-cage housing mode, a bedding depth of 4 cm combined with a 6-day cage-changing interval during the growth phase can maintain cage cleanliness through bedding adsorption while optimizing the use of bedding resources. This protocol successfully balances animal welfare assurance with facility operational efficiency and is suitable for the large-scale management of C57BL/6J mice and inbred strains with similar genetic backgrounds.

Based on the development history that the modern medicine and laboratory animal science have long been hindered by animal welfare groups and animal-rights activists in Western countries,and considering the relevant circumstance in China,the author appeals and recommends that the laboratory animal practitioners and biomedical researchers,the scientific community in China should recognize and identify the boundaries of our professional activities.Firstly,do not participate in the activity related to so called"World(Lab)Animal Day"that is said in some Chinese websites,to originate from NAVS,an antivivisection society Ltd.,London.Secondly,do not participate in the activity related to so called"animal memorial stone"that was originated in Japan.These movements have nothing to do with biomedicine and laboratory animal sciences,and fundamentally are antiscientific and are incompatible to traditional Chinese culture.Thirdly,distinguish the standpoints of laboratory animal science from various viewpoints of animal welfare/rights activists,promote scientific management of the institutional animal care and use for better supporting biomedical research in China.