Applied Scientific Evaluation of Feedlot Shade Design

Summary

Hot weather conditions such as those experienced over an Australian summer can cause a loss of production and in extreme cases, catastrophic losses amongst lot fed cattle. These losses in production are due to levels of heat stress in lot fed cattle that results when the thermal loads on an animal are greater than the animals ability to lose heat by normal metabolic means. These levels of heat stress can vary from causing discomfort to the animal that results in panting, through to death. Rises in thermal loads leading to heat stress events may result from energy derived through the ingestion and digestion of feed and water, incident radiation from the atmosphere and the ground, and direct heating by surrounding air mass. Ambient temperatures in excess of body temperature typically induce behavioral and physiological changes in cattle that are intended to reduce the animal's heat load by adjusting its radiative, convective, conductive and evaporative exchanges with its environment.

In hot climates, shade can be utilised by cattle to provide relief from radiant energies by intercepting direct beam, shortwave solar radiation that may otherwise induce excess body temperatures. The lack of shade in some feedlots has been implicated as restricting the ability of cattle to reduce their radiative energy load. Although shade is a useful tool for reducing heat loads and stress in livestock, if incorrectly designed and poorly maintained, shade can at times lead to increased levels of stress and cattle discomfort. This can occur due to a reduction in localised wind speeds and an increase in localised levels of humidity. Wind speeds may be reduced by shade structures that in turn results in a reduction in the capacity of the animal to be cooled through convective and evaporative means. An increase in localised levels of humidity will result when the capacity of the feed yard pad to dry out is reduced. This in turn can also result in a reduced capacity to lose heat via evaporative means and can lead to other issues such as a greater build up of ammonia gas and increases in odour generation

The effectiveness of shade is largely dependant on the following factors: The thermal properties of the shade material; The height of the shade structure; Size of shadow; The slope of the shade; Location of shadow; Shadow orientation; and The level of ventilation. Current feedlot shade designs have evolved over time. Most are of simple designs to minimise capital and ongoing maintenance costs. However, even though the structures are agricultural, structures of such size should be engineer-designed and certified. This includes the structural connection details, especially where tensioned cables are involved, and the fixing details for the corrugated iron sheeting. This report has completed an investigation of current shade designs and has made recommendations to the feedlot industry based on these observations and available literature. A summary of this report suitable for industry is attached in Appendix F.