MLA R&D examines heat load impacts

A Meat & Livestock Australia (MLA) funded research project investigating the metabolic and inflammatory responses to high heat load in feedlot steers is returning interesting results.

Since 2013, researchers from CSIRO and the University of Queensland have conducted feedlot trials at the Queensland Animal Science Precinct at UQ’s Gatton site in south-east Queensland, and at the Mead Research Station at the University of Nebraska.

The research is being led by Dr Gene Wijffels from CSIRO and Associate Professor John Gaughan from UQ, with feedlot trials of 80 to 96 steers on finishing rations for 100 days or more.

Working in both Australia and the United States has allowed the research team to complete two summer trials a year.

“The feedlot trial conducted during the 2015 summer in Nebraska looks to be very informative,” Dr Wijffels said.

“That summer experienced a sudden and very challenging seven-day heatwave causing the loss of a reported 5,000 head during that heat event in Nebraska alone.

“The heatwave conditions were close to the Emergency Threshold, with the average maximum daily Heat Load Index (HLI) at 103 and Temperature Humidity Index (THI) at 83.6.

“In the feedlot trial during that heatwave, ADG fell close to zero and the maximum rumen temperatures rose to 41.3°C.”

Researchers are now evaluating the plasma biochemistry and inflammatory responses to this event, but as yet there is no indication of major tissue or gut damage arising from the heatwave despite clear physiological evidence of heat stress.

The next stage of the research is to replicate the conditions recorded in Nebraska, in the climate chamber facility at Gatton’s Queensland Animal Science Precinct.

“This will allow us to take a closer examination of the physiological, metabolic and inflammatory responses, and to set up for controlled feed additive trials,” Dr Wijffels said.

Key facts about heat load in feedlot cattle

• High heat load in feedlot cattle impacts productivity through reduced feed intake and decreased weight gain.
• In beef cattle, a decrease of 0.4 kg average daily gains (ADG) can be anticipated for every 1°C increase in internal body temperature.
• Increased respiration rate is used by cattle to assist in cooling but high rates of respiration and panting can lead to respiratory alkalosis.
• In taurine breeds, every 1°C increase in ambient temperature increases respiration rate by four breaths per minute. Furthermore, after severe events there is often a long interval before the affected cattle return to full feed intake. Some never do so.
• During the recovery period, there appears to be increased vulnerability to infection such as bovine respiratory disease (BRD).  Cattle close to market weight and unacclimatised cattle are particularly at risk of heat stress.