Contents

Overview

Heat Load Index

Accumulated Heat Load Units

Equations

Risk Analysis Program

Forecast

Overview

A weather forecasting system was developed by Katestone Environmental to assist in warning feedlot operators of impending adverse weather conditions that could lead to excessive heat loads for feedlot cattle. The development of this system was funded by Meat and Livestock Australia. The forecasting system covers locations where Bureau of Meteorology automatic weather stations (AWS) are located.

This forecasting system was originally based on the calculation of the Temperature Humidity Index (THI), previously developed as an indicator of human comfort. Later studies on cattle heat stress (Gaughan et al., 2002) indicate that a new Heat Load Index (HLI) was a better indicator of cattle heat stress than the originally used THI. These studies also found that the number of hours that the HLI was above a threshold (89) was also a good indicator of accumulated heat load in cattle. The studies also found that if the HLI fell below 77 for a number of hours then the cattle would be able to recover somewhat from the heat stress. Further studies have indicated that the Accumulated Heat Load Unit (AHLU), a parameter obtained by accumulating the number of hours the HLI exceeds a certain threshold, is indicative of the heat stress in feedlot cattle.

The ability of cattle to tolerate heat load varies depending on factors such as cattle breed, health status, coat colour, degree of finish, and pen conditions (i.e. whether the pen is shaded or unshaded and the manure management practices employed). For this reason the threshold at which heat load starts to accumulate also varies depending on these factors.

The Katestone Feedlot Cattle Heat Load Forecast Service provides a forecast for 17 sites throughout Australia, giving the expected HLI, and the AHLU at four threshold levels. A Risk Analysis Program (RAP) is provided to allow feedlot operators to calculate the AHLU threshold that they should use in reading the forecast. This RAP also calculates the risk of excessive heat load events for their operation. In addition, the HLI calculator and the AHLU calculator may be used with local data to give results for individual feedlots.

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Heat Load Index

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 minimum and maximum ambient temperature
• A high ongoing relative humidity
• An absence of cloud cover with a high solar radiation level
• Minimal air movement over an extended period (4-5 days)
• 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.

A new index – 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 in this index, rather than ambient temperature, takes into account radiation effects as well as air temperature. The new 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. Instructions for using the calculator are included.

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Accumulated Heat Load Units (AHLU)

The concept of Accumulated Heat Load Units has been developed to give some indication of the amount of heat that is accumulated by an animal when it is exposed to environmental conditions that are above its ability to maintain thermoneutral conditions, (i.e. for every hour that an animal is above its threshold HLI value, it will gain heat) and this additional heat load accumulates over time and is reflected as an increase in body temperature. While it is a normal physiological response for animals to gain heat during the day and dissipate this accumulated heat to the environment at night, if this heat loss is not possible at night the animal carries forward a heat load to the following day and is more susceptible to the effects of subsequent heat load.

There are three aspects in determining the potential for heat stress in feedlot cattle:

• Firstly, there is a time effect, (i.e. the number of consecutive days that cattle are exposed to some accumulated heat load in excess of 25 units, even where they receive night time relief). This appears to be a major determinant in terms of susceptibility to the effects of subsequent excessive heat load events.
• Secondly, there is an intensity effect. Feedlot cattle are highly vulnerable where there is a rapid onset (over 4 – 7 hours) of an extreme heat event where the HLI exceeds 100 units. Under these conditions cattle are highly vulnerable even where the Accumulated Heat Load Units are not excessive (e.g. 25 - 50 units).
• Thirdly, feedlot cattle are very susceptible where they don't have the opportunity to dissipate their accumulated heat load during the night or have the opportunity to recover sufficiently, and enter the next day with a pre-existing heat load. This is reflected as a ramping up of accumulated heat load units over subsequent days.

The AHLU calculator allows users to input the HLI or climate data for their site to determine the AHLU values. Instructions for using the calculator are included.

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Equations

Details of the equations used in the calculation of HLI and AHLU are given here for information. They are also included in the instructions for using the calculators.

The formula used to calculate the Black Globe Temperature (BGT) is:

The formula used to calculate the HLI is as follows:

if BGT < 25° C

HLI = 10.66 + 0.28 * RH + 1.3 * BGT - WS

if BGT > 25° C

HLI = 8.62 + 0.38 * RH + 1.55 * BGT - 0.5 * WS + exp ( 2.4 - WS )

Where T = temperature (°C), SR = solar radiation (W/m²), RH = relative humidity (%), WS = wind speed (m/s), exp(...) is the exponentiation function.

In order to calculate the Accumulated Heat Load, several parameters are required:

• The Heat Load Index (HLI)
• Upper limit of the Thermoneutral Zone (UL)
• Lower limit of the Thermoneutral Zone (LL)
• Interval (in hours) between successive HLI estimates (T)

The Lower Limit is fixed at a value of 77. The Upper Limit is variable and depends on factors such as:

• Stock characteristics
• Pen management practices
• Mitigation measures

and must be determined for a specific feedlot and stock characteristics. The formula used to calculate the AHLU is as follows:

    if  HLI > UL
    
      Excess = HLI - UL

    if  HLI < LL

            Excess = HLI - LL

    else

            Excess = 0

    if Excess > 0

            New_AHLU = Old_AHLU + Excess * T

    if Excess < 0

            New_AHLU = Old_AHLU + Excess * T / 2.0

    else

            New_AHLU = Old_AHLU

The upper limit of the thermoneutral zone can be found using the HLI calculator.

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Risk Analysis Program (RAP)

The ability of cattle to tolerate heat load varies depending on factors such as cattle breed, health status, coat colour, degree of finish, and pen conditions (i.e. whether the pen is shaded or unshaded and the manure management practices employed). For this reason the threshold at which heat load starts to accumulate also varies depending on these factors.

For the purposes of forecasting, various heat load thresholds have been incorporated to account for these factors. The RAP is used to calculate the HLI threshold to use for your particular operation. If the threshold you calculate for your operation falls between the values utilised in the forecasts, you will have to estimate the results for your situation by interpolating between the forecast values.

The RAP also gives an assessment of the risk of heat load events based on the site specific data entered and historical climatic data.

Instructions for using the RAP are available here.

Understanding the Forecast

Forecasts are available for 17 locations where there are automatic weather stations. These have been used in past seasons and have given reliable data. It is intended to increase the number of locations in future seasons, and at present data is being collected and evaluated from a large number of additional weather stations.

Users may select a site close to their location from the lists on the Heat Load Forecasts page. Chosen site forecast pages can be bookmarked so that they can be revisited easily without going through the selection process every time.

The forecast lists the HLI maximum and minimum values for out to 6 days ahead.

The AHLU corresponds to the predicted amount of heat load accumulated in the cattle that has developed during the day. When looking at these values first select the relevant heat load threshold for your operation, as calculated using the RAP. The AHLU reported in the forecasting is based on an understanding of the previous few days calculated AHLU along with the forecast. The AHLU reported in the forecast is a level of risk which is based on the following categories:

AHLU	Heat stress indicator	Cattle indications
0-20	Low risk	No stress or panting score 1
20-50	Medium risk	Panting score 1-2
50-100	High risk	Panting score 2-4
Over 100	Extreme risk	Panting score 4

The rainfall gives an indication of the rainfall that is likely to be received in the region. It is very difficult to forecast rainfall distribution (particularly summer storm rain) accurately for a particular location. For this reason, it should be noted that the rainfall predictions are Bureau of Meteorology regional forecasts, and may not necessarily relate specifically to your site.

If you have received rain, the forecast HLI for your site may be underestimated (due to the increased relative humidity at your site) and therefore particular care should be taken and cattle monitored regularly.

Additional Information

Further information on recognising and managing heat stress in feedlot cattle is available in the MLA Tips and Tools publication ‘Heat load in feedlot cattle 2002’ available through the MLA Producer Hotline – 1800 155 900.

Further information or assistance with the forecast service is available from Des Rinehart, MLA Feedlot R&D Project Manager. Phone (07) 5464 2277 or email drinehart@mla.com.au.

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