There are a number of climatic conditions that may predispose feedlot cattle to high body heat loads, including:

  • A recent rain event
  • A high ongoing relative humidity
  • Minimal air movement overnight or for an extended period (2-3 days)
  • High overnight temperatures
  • High ambient temperatures with little or no cloud cover
  • A sudden change to adverse climatic conditions

It is usually a combination of some or all of these conditions that leads to an excessive heat load event, which may result in cattle deaths if conditions persist for a number of days. The Heat Load Index (HLI) has been developed as an indicator of the environmental heat load placed on cattle. The use of Black Globe Temperature (BGT) in this index, rather than ambient temperature, takes into account radiation effects as well as air temperature. The HLI has been tested and proves to be a good indicator of physiological stress. The HLI calculator allows users to input site specific meteorological data to determine an HLI value.

HLI is calculated as a composite of HLILOW and HLIHIGH, with a weighting factor determined as a function of the difference in the calculated BGT and a threshold of 25 °C. In equation form, HLILOW and HLIHIGH are calculated as follows:

  • HLILOW = 1.3 x BGT + 0.28 x RelHum – WSpeed + 10.66
  • HLIHIGH = 1.55 x BGT + 0.38 x RelHum – 0.5 x WSpeed + exp (2.4 – WSpeed) + 8.62

The weighting factor is calculated and used as:

  • FRACTIONHIGH = 1.0 / (1.0 + exp(-((BGT-25.0)/2.25)))
  • HLI = (FRACTIONHIGH * HLIHIGH)+ ((1 – FRACTIONHIGH) * HLILOW)

Forecast HLI values are based on forecast meteorological data, while HLI observations are based on AWS data.