NASCalf Stress Days
Stress days are defined as days when calves experience either heat stress or cold stress. The total number of these days is tallied and reported in the Stress Days report on the NASCalf home screen, shown here.
NASCalf calculates stress days using the 2021 Nutrient Requirements of Dairy Cattle framework. This approach defines two temperature thresholds: the upper critical temperature, or UCT, and the lower critical temperature, or LCT.
When the effective ambient temperature rises above the UCT, calves must expend energy to dissipate heat. When the temperature falls below the LCT, calves experience cold stress and use additional energy to maintain body temperature. This relationship between temperature and energy use was originally described in a 1981 NRC report, which showed how heat stress and cold stress relate to effective ambient temperature.
Younger calves are more susceptible to cold stress, likely because their rumen is not fully developed and they produce less metabolic heat from fermentation. As a result, the lower critical temperature depends on calf age. For calves less than 21 days old, the LCT is set at 15 degrees Celsius. For calves older than 21 days, the LCT is reduced to 5 degrees Celsius.
The upper critical temperature is the same for all calves and is set at 25 degrees Celsius.
When temperatures fall below the LCT or rise above the UCT, calves require additional energy. NASCalf applies an adjustment of 2.01 kilocalories per kilogram of empty body weight raised to the 0.75 power, for each degree above or below the critical temperature. This additional energy requirement is added to the unadjusted net energy for maintenance, resulting in an adjusted maintenance energy requirement.
The effective ambient temperature used in these calculations is set in the Farm and Calf menu, shown at the top of the screen here. After selecting the farm and calf, you can adjust the effective ambient temperature in the farm and calf information panel.
Only a single temperature can be entered for the entire simulation, so care should be taken when selecting this value. Temperatures can range from minus 35 degrees Celsius to plus 35 degrees Celsius. Temperature entry is limited to Celsius; there is no option to enter values in Fahrenheit.
As the effective ambient temperature is changed, the simulation automatically recalculates nutrient requirements using the stress adjustment equations just described.
