Validation of the Heat Load Index for use in the feedlot industry

Summary

A number of new indicators for assessing the impacts of extreme heat events on feedlot cattle were developed in a previous MLA funded research project (FLOT.327). The new indicators were developed to assist feedlot operators to improve their heat load management plans and thereby reduce the impact of extreme heat load events. A new biologically based heat index for feedlot cattle (Heat Load Index; HLI) was developed by assessing the relationship between some key biological responses (respiration rate, panting score and body temperature) of a reference animal to key climatic variables that had been identified in previous research projects. The new HLI uses black globe temperature (°C), relative humidity (%) and wind speed (m/s) to calculate an index that reflects the impact of the surrounding climate on feedlot cattle. The reference animal is a black steer, body condition score 4+, 100 + days on feed, healthy and with no access to shade.

The index took the form:

For Black Globe Temperatures less than 25°C:

H​LI = 10.66 + 0.28 x RH + 1.3 x BGT - WS
For Black Globe Temperatures above 25°C:

H​LI = 8.62 + 0.38 x RH + 1.55 x BGT – 0.5 x WS + EXP(-WS +2.4)
The Accumulated Heat Load Units (AHLU) indicator was developed in an attempt to include both the intensity of exposure and the duration of that exposure to extreme heat. The AHLU is based on the THI-Hrs concept (Hahn and Mader 1997). The AHLU is a two dimensional function incorporating time and heat balance. The AHLU is calculated by determining the difference between the HLI at a given time and an upper and lower threshold HLI. The thresholds have been developed largely from climate room studies, but also from feedlot studies. When the HLI is above the upper threshold (≥86), cattle will not be able to effectively dissipate body heat, which means that there is likely to be an increase in core body temperature. When the HLI is below the lower threshold (≤79) then cattle are likely to dissipate body heat back to the environment. A transition zone exists when HLI is between 79.1 and 85.9. When the HLI is between the two thresholds it is not clear if cattle will be gaining or losing heat, therefore a zero value is recorded for heat balance which means that the animals are not gaining or losing body heat.

A new risk assessment program (RAP) was also developed in an attempt to assist feedlot operators in quantifying the risk of extreme heat events occurring at specific feedlots and specific feedlot pens. This new RAP software estimated the effect of certain key feedlot variables (presence of shade, dominant animal genotype, animal coat colour, etc.) on the upper HLI threshold mentioned above. By doing this, the probability of extreme heat load events occurring at specific feedlots and feedlot pens can be quantified. However, the assumptions used to quantify the risk of extreme heat load events occurring need to be validated using independent datasets.

The datasets required to independently validate the risk assessment tools produced in the FLOT.327 projects were collected from 13 Eastern Australian feedlots from 24 January 2005 to 11 March 2005. The majority of the data collection was undertaken by feedlot staff, with the animal observation data collected three times daily and the climatic data collected from the feedlot weather station. Once all of the data had been received, data collation and analysis was undertaken by the project team.

The data analysis undertaken suggests that the new HLI and AHLU adequately reflect the impact of the feedlot micro-climate on cattle. Further, some of the assumptions made in the RAP software were also tested. For example, it was shown that there is a genotypic difference in animal response to elevated heat load. As well, there were differences noted in the response of animals with varying coat colour, however, the actual impact of coat colour is difficult to separate from genotypic effects and needs further research.​