Mass Generation Rates of Ammonia, Moisture, and Heat Production in Mouse Cages with Two Bedding Types, Two Mouse Strains, and Two Room Relative Humidities

Mass generation rates of ammonia, moisture, and heat production were measured for mice that were housed in standard shoebox cages at a density of five mice per cage and maintained in either a 35% or 75% room relative humidity. Two mouse strains kept on two different types of bedding materials were compared when bedding material had been in the cages for two, six, and ten days. Gas exchanges directly measured were ammonia, water vapor, oxygen, and carbon dioxide. Water balance was calculated from gain in bedding weight, drinking water consumption, partial pressure of water vapor, and metabolism (CO2 produced and O2 consumed). All measurements were taken on the second, sixth, and tenth day of each test. All weight and volume measurements were obtained in the home environmental chamber in which the mice were housed. Gas exchanges were obtained after four cages of five mice each had been placed into one of three open system calorimeters. Relative humidity was not controlled in the calorimeters because it would have led to erroneous ammonia generation measurements. Relative humidity had a dramatic influence on ammonia generation rate from the mouse cages. Generation rates at 35% relative humidity were essentially negligible across all treatments. At 75% relative humidity, it took around six days bedding age for ammonia generation to start, but it increased rapidly by ten days bedding age. Ammonia mass generation rate was 12.4 and 0.154 (mg/h)/20 mice (2.7E-5 and 3.4E-7 lb/h/20 mice) in the 75% and 35% humidity treatments, respectively. Mouse strain and bedding type had a significant effect on ammonia generation rates with CD-1 mice producing larger amounts of ammonia than BALB/c mice, and pulp bedding producing larger amounts of ammonia than chip bedding. Relative humidity significantly influenced ammonia emission interactions between strain, bedding material, and bedding age. Carbon dioxide and oxygen exchange were not affected by relative humidity. Heat production on a body mass basis was calculated to be 17.9 and 15.7 W/kg BW (27.7 and 24.3 Btu/h/lb BW) for CD-1 and BALB/c mice, respectively.