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14.3 Heat Load Index (HLI)

Alternative and more complex measures or indices of heat load have been developed for livestock that do incorporate effects due to air flow and in some cases solar radiation.

One example is the Heat Load Index (HLI) described in Tips and Tools[8]. The HLI requires measurement of black globe temperature, relative humidity and wind speed and has two formulae:

  • If black globe temperature is greater than or equal to 25°C:
    • HLI = 8.62 + (0.38*RH) + (1.55*BG) + exp(-WS+2.4) - (0.5*WS).
  • if black globe temperature is less than 25°C:
    • HLI = 10.66 + (0.208*RH) + (1.3*RH) - WS.
HLI=heat load index; RH= relative humidity expressed as a percentage i.e. 45 and not 0.45; BG = black globe temperature; WS = wind speed; exp = exponential 

The HLI is directly applicable to a defined reference animal which is a healthy, black, Bos taurus steer with a body condition of 4+ and no access to shade. Once a threshold HLI of 86 is reached, the reference animal will gain heat. If the HLI falls below 77 then the reference animal can lose heat. 

The adjustments in Table 14.2 can then be added to the reference animal figure. For example, a red or white coat colour will add between 1 and 3 units to the HLI, meaning that animals will not gain heat until HLI reaches 87 to 89. If the animal is sick or unacclimatised to heat load, it may gain heat once HLI reaches 81. 

For Bos taurus animals, the maximum upper threshold is 96. No matter what protective factors might be considered, if THI reaches or exceeds 96, all Bos taurus animals will gain heat.

Table 14.2: Effect of various factors on the upper HLI threshold

The HLI index can also be used to assess the cumulative effect of heat load over time through a measure called accumulated heat load units (AHLU). It is calculated by adding the number of HLI units above the threshold (i.e. 86 for the reference animal) for each hour of the day.

Table 14.3: Change in AHLU over a 14 hour period based on the reference animal

Time HLI HL Balance AHLU
8:00 85 0 0
9:00 86 0 0
10:00 88 2 2
11:00 92 6 8
12:00 94 8 16
13:00 95 9 25
14:00 97 11 36
15:00 96 10 46
16:00 89 3 49
17:00 85 0 49
18:00 79 0 49
19:00 70 -7 42
20:00 64 -13 29
21:00 62 -15 14
22:00 61 -16 0
Table 14.3 illustrates the potential use of AHLU. During a hot day the HLI exceeds the gain threshold and the animal is expected to gain heat. As the day cools in the afternoon and evening, the HLI falls below the lower threshold and the animal loses heat, eventually restoring a normal balance. If hot conditions continued well into the evening, the animal may finish that day with a positive AHLU, meaning that it has not dissipated all of the heat gained during that day. If the animal then starts the next day with a positive AHLU and heat load conditions continue then there is a risk of EHL. 

The HLI and AHLU indices were developed for feedlot situations where solar radiation is important and where wind speed can be highly variable. It is considered to be less relevant to shipboard conditions.
8. Meat and Livestock Australia, Recognising excessive heat load in feedlot cattle (2006).