Response of the laying hen to aerial moisture and ammonia concentrations at high environmental temperatures

Sixty-six 1-day-old chicks were raised to twenty weeks of age on sawdust litter in an ammonia-free, temperature-controlled environmental chamber (7 m x 2.9 m x 2.7 m). These raised hens were housed in the environmental chamber converted to individual cages and managed to keep ammonia levels below 4 ppm. The laying hens, at 61 to 74 weeks of age, were individually exposed to one of ten heat stress environments having dry bulb temperatures of 30, 32, 34, 36, or 38 C and relative humidities of either 50 or 90%. The rates of sensible and latent heat loss, respiratory rate, and rectal temperature from each hen were measured every five minutes for a maximum of five hours or until their rectal temperature reached 44.5 C. Twenty one physiological variables from these measurements were correlated with an index value calculated from various weighting combinations of the dry plus wet bulb temperatures of the corresponding heat stress environments. The weighting combination of 0.40 for the wet bulb plus 0.60 for the dry bulb temperature were selected for calculating the temperature-humidity index for laying hens. Hens from the same flock, after having a 30 day minimum recovery period from the heat exposure, were then exposed to an environment containing a 30 ppm concentration of ammonia for nine days. Rates of respiratory, cutaneous, and total latent heat loss were measured every 15 seconds for two hours in an ammonia-free calorimeter at dry bulb temperatures of either 26 or 34 C and 50% relative humidity on the day prior to exposure and on the third, sixth, and ninth days of exposure. Rates of respiratory and cutaneous latent heat loss averaged 2.3 and 2.6 watts per bird and 0.9 and 1.1 watts per bird at 26 C and 34 C, respectively. Rates of respiratory latent heat loss did not significantly vary while hens adjusted to ammonia. Average respiratory rates significantly increased at 34 C but not at 26 C. Rates of cutaneous latent heat loss did significantly vary at 34 C but not at 26 C. Using a physiological model developed from the collected data, the instantaneous rate of respiratory latent heat loss can be predicted as a function of the instantaneous respiratory rate to the 0.4 power