Modeling lactating cow respiration rates during heat stress based on dry-bulb and dew-point temperatures, daily milk production and air velocity

Temperature-humidity index (THI) values do not consider the impact of air velocity and milk production level on the level of heat stress lactating cows experience. High producing cows experience more heat stress than low producing cows (West, 2003). A thermoregulation model for lactating dairy cows described by Nelson and Janni (2016) was used to estimate lactating cow respiration rates. Dry-bulb temperatures ranged from 20°C to 40°C and dew-point temperatures from 0°C to 30°C. Respiration rates were found for cows with milk production levels from 30 to 50 kg d^-1 and air velocities from 0.6 to 5.4 m s^-1. Model respiration rates ranged from 27 breaths per minute (bpm) to greater than 123 bpm. Respiration rate tables were developed for 545 combinations of dry-bulb and dew-point temperatures, air velocities and milk production levels. The tables can be used to assess the impact of each factor on respiration rate and heat stress level. Results indicate that the respiration rate of a cow producing 45 kg d^-1 at a THI level of 75 can be reduced from 76 to 55 bpm when air velocity is increased from 0.3 to 5.4 m s^-1. At a THI level of 85 the respiration rate can be reduced from 108 to 91 bpm. Results can also be used to assess respiration rate changes when evaporative cooling is used. A cow producing 40 kg d^-1 at 36°C dry-bulb and 9°C dew-point has a respiration rate of 79 bpm which can be reduced to 66 bpm by reducing the dry-bulb temperature to 32°C and increasing the dew-point temperature to 15°C with evaporative cooling when the air velocity is 2.7 m s^-1