Heat load alleviation in beef cattle: water application during continuous high temperature exposure

Abstract

The live export industry is an integral part of the Australian cattle industry and makes a significant contribution to the Australian economy. Heat stroke has been a major cause of cattle death on long haul sea voyages and has the potential to threaten the viability and sustainability of cattle exports. To mitigate high heat loads on these voyages, it is necessary to implement cooling strategies to lower the temperature of the microclimate surrounding the cattle. The main cooling method used on live export voyages is high pressure hosing but no research has been undertaken to evaluate the efficacy of this or other water application methods. The major focus of this thesis was therefore, to explore heat load mitigation of confined beef cattle. The initial hypothesis was that the application of water onto cattle exposed to a continuous high heat load will reduce the severity of heat stress. As, different breeds of cattle and composites are exported together, the second hypothesis was that different cattle breeds and composites respond differently to heat exposure and to water cooling. Climatic conditions for all three studies were based on the conditions encountered on board a live export vessel travelling from southern Australia to the Middle East (during the northern hemisphere summer). The first study obtained data on changes in the microclimate and responses of Bos taurus cattle (Angus) when water was applied via a hose to cattle maintained in single pens under continuous hot conditions. The results showed that the water application had a positive impact on cattle comfort (respiratory rate reduced by 50 - 80 bpm; rectal temperature reduced within an hour after wetting of cattle). The dry-bulb temperature was also reduced following longer duration water applications. The impact of water application on physiological parameters, such as rate of respiration and rectal temperature, was a function of water volume applied and ambient conditions. For all steers mean daily rectal temperatures and respiratory rates were lower (P < 0.01) during thermoneutral conditions than during the 11 days of hot conditions. Four water application methods (hosing, overhead sprinklers, leg sprinklers and misters) were evaluated in the second experiment with the same Bos taurus breed. Volume and frequency of water application was assessed with the cattle were housed in groups (n = 9) on sawdust bedding. Of the four methods evaluated, hosing was the most effective in mitigating the heat load on the cattle. The leg wetting and misting treatments had little to no effect. While not as effective as hosing, overhead sprinklers did afford heat relief and may have advantages for cooling large numbers of cattle at the same time. Bedding had to be removed after it became wet due to increased concentrations of atmospheric ammonia that posed a risk to cattle and human health. As live export ships carry both Bos taurus and Bos indicus cattle, the third experiment investigated the effects of high heat load conditions on three breeds of cattle and the effect of the three water application treatments. Duration of exposure to high heat load conditions caused a decrease in the ability of the Angus cattle to cope. Respiration rates and rectal temperatures of the Angus were greater (P < 0.05) than the Brahman and Brangus cattle, with the Brahmans having the greatest heat tolerance. The most effective method for lowering heat load (in terms of rectal temperature) was hosing, followed by overhead sprinkler and misting. It was found that breed differences in body surface temperature, in conjunction with respiration rate, may be a good predictor of heat stress The present studies have highlighted the complexities of investigating beef cattle exposed to continuous high heat loads and have demonstrated the diversity of data required to assess heat stress mitigation strategies. Only when comprehensive data is available, following a sustained research effort, will it be possible to improve the wellbeing of intensively maintained beef cattle, whether in feedlots or on the high seas.