What weather conditions can cause problems?

Intensity event

Feedlot cattle are highly vulnerable when 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 AHLUs are not excessive (e.g. 25 – 50 units).

These types of events sometimes follow significant rain events such as cyclones when poor weather conditions are followed by hot, humid and still weather.

Accumulated event

Feedlot cattle are very susceptible when 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 AHLU over subsequent days.

Repeated event

There is also a time effect, (i.e. the number of consecutive days that cattle are exposed to AHLU in excess of 25, even where they receive nighttime relief). This appears to be a major determinant in terms of susceptibility to the effects of subsequent excessive heat load events.