DAIRY COW THERMAL BALANCE MODEL DURING HEAT STRESS: PART 2. MODEL ASSESSMENT

A steady-state process-based lactating cow thermal balance spreadsheet model developed by Nelson and Janni (2023) was compared to mean measured body temperatures, respiration rates, and skin temperatures from two published studies (Gebremedhin et al., 2010; Chen et al., 2015). Model body temperatures were also compared with reticular temperatures from cows standing in unshaded paddocks that were part of a solar shade study (Sharpe et al., 2021). Gebremedhin et al. (2010) reported measured mean rectal temperatures, 39.4 ± 0.5 C and 40.6 ± 0.4 C for hot and dry conditions with and without a solar load; model body temperatures for similar hot and dry conditions were 39.7 C and 40.6 C with and without a solar load, respectively. Model respiration rates were within one standard deviation of measured mean respiration rates (Gebremedhin et al., 2010). The model body temperature for a baseline condition was 39.1°C, which was within 0.1°C of the mean baseline temperature of 39.2 ± 0.6°C (Chen et al., 2015). The model respiration rate was 63 breaths per minute (bpm); much lower than the reported baseline respiration rate of 88 bpm (Chen et al., 2015). Model body temperatures were 0.1°C to 0.7°C lower than the measured mean reticular temperatures of standing cows in non-shaded paddocks with solar loads when ambient temperatures ranged from 24.4°C to 26.5°C. Model results compared well with mean measured parameters from three studies. The model can be used to assess the impact of factors affecting heat exchange (e.g., body mass, milk yield, solar load, air dry-bulb temperature, dew-point temperature, and air velocity) on heat exchange flux, cow respiration rate, and body temperature.