Impact of Night-Time Cooling on Heat Load in Feedlot Cattle

Summary

​Heat stress continues to cause production losses over the summer months. And although stock losses are infrequent, extreme weather events can lead to significant cattle losses.

The Australian feedlot sector through financial input from MLA and ALFA has proactively researched the effects of high heat load on cattle well-being and productivity. Furthermore a number of recommendations and strategies have been established to help livestock managers plan for and react to heat load challenges.

The Heat Load Index (HLI) and the Risk Assessment Programme (RAP) which were established in the mid 2000’s are two key factors in managing heat load.

Feedback from users of the HLI and the RAP suggested that some modifications/adjustments were required in a couple of areas, namely:
(a) The effect of nightime cooling
(b) The effect of humidity and temperature in the early morning
(c) The effect of early summer conditions in southern Australia

The effects of (a) and (b) were determined in a study undertaken at the University of Queensland, Gatton research feedlot facility. It investigated and quantified the effects of summer conditions on the physiology (rumen temperature, blood metabolites, panting) and performance (health, growth, efficiency and carcass) of feedlot cattle.

Three breeds were used:

• Angus (n=12),
• Brahman (n=12) and
• Charolais (n=12), and

two treatments: s

• hade (3 m2/animal) or
• no shade were imposed.

The cattle entered the UQ feedlot on the 29th October 2012 and exited the feedlot on 14th April 2013.

The study confirmed the importance of shade as a method of reducing the impact of hot weather on feedlot cattle. An unexpected outcome was the use of shade by Brahman cattle (24.7% at 1400 h). Angus made the most use of shade (81.5% at 1200 h). At the same time only 33.5% of Charolais were using shade.

Angus and Charolais cattle with access to shade had lower rumen temperatures (up to 1 oC for Angus and 0.5 oC for Charolais) compared with their un-shaded counterparts. Brahman rumen temperature was not affected by shade treatment. Overall shaded and un-shaded Angus had the highest rumen temperatures, followed by Charolais, and Brahman.

Examination of panting score data also provided an unexpected outcome. Brahman steers had the greatest overall increase (from minimum to maximum) in panting score (87 and 97% respectively for un-shaded and shaded steers), albeit from a very low minimum. The increase in Angus was 40 and 47% respectively for un- shaded and shaded steers. For all breeds the increase in PS was greatest in the shaded treatment. This suggests that shade allows improved thermoregulation via respiration.

The study used the rumen and panting data to determine heat gain and heat loss from the three breeds. From these data the rate of heat loss was quantified, and adjustments to the HLI thresholds for heat loss were recommended for inclusion in the calculation of accumulated heat load. Further analysis of the data will provide additional information.
The effect of (c) was obtained by using field data collected from a feedlot located in southern NSW. The data from this work suggests there is a 40 day acclimation period for cattle to adjust to summer conditions. However, this may be somewhat skewed by high rainfall events. Shade ameliorates the effects of early summer conditions at midday, over the first 40 days. There is not sufficient data to state that southern Bos taurus cattle are physiologically different to northern Bos taurus cattle. This will need to be further investigated.

A new version of the HLI has been developed (primarily based on UQ data). The new HLI (HLI2) will be field tested at Gatton over the coming summer and outcomes reported. The new HLI will require robust examination before any recommendations on its use can be made.