Summary
This project aims to review the relevant literature and industry experience, and provide factual information on the impact and interrelationship of:
Elements of the physical environment,
Animal related factors
Nutrition related factors on heat load in feedlot cattle.
It summarises the results of FLOT.307, 308 and 309. Brief Methodology The study reviews the available literature and industry experience, and provides an understanding of: the relative importance of the factors associated with heat load in cattle, their predicability, and the techniques available to possibly modify their impact. Proactive counter- measures for minimising heat load effects in the Australian Feedlot Industry are discussed in the context of strategic and tactical management programs and practices. Main Results and Conclusions 1. Basis of Heat Load Heat load in cattle may be largely explained in terms of the practical application of the principles of thermodynamics. Cattle are homeotherms and try to maintain their body core temperature within reasonably narrow temperature ranges so that their body cells and tissue can function optimally. Excessive heat load (EHL) occurs where a combination of local environmental conditions and animal factors lead to an increase in body heat content beyond the animal's normal physiological range and the animal's ability to cope. EHL in cattle is the result of a number of complex interacting factors: -Physical (conduction, convection, radiation, evaporation); -animal (breed, adaptation and acclimatisation, behaviour, coat colour/type, body condition, health status); -nutrition (metabolic heat of nutrients, ingredients, diet, OMI, time of feeding, water availability); and -management practices (resource use and maintenance, livestock care, personnel). Cattle are normally able to maintain their preferred level of body heat and thus body temperature by behavioural and physiological thermoregulatory mechanisms. This may however be difficult during very hot adverse weather.
The principal overall source of heat gain for a typical feedlot steer during hot, humid and low wind conditions, is metabolic heat, and the principal route of heat loss is via evaporative cooling. Other components (conduction, convection, radiation), while not large, are additives which at times may be sufficient to contribute critically to the accumulation or dissipation of body heat. If the normal thermodynamic processes are compromised, body heat content can build up and hyperthermia can develop. EHL occurs in the Australian feedlot industry, as it does in the USA industry and elsewhere. Australia experiences ongoing periodic instances when EHL is associated with mortality and production loss. There have been occasions when these losses have been most significant. It is reasonable to assume additional unnoticed and/or unreported losses have occurred, and will continue to occur. Overall, the economic costs associated with morbidity and mortality during EHL events are high. Commonly, it is the most finished, longest fed, and hence most valuable stock that suffer the highest mortality.
Furthermore, it has been demonstrated that there are long-term financial losses associated with hot weather induced reduced feed intake (OMI) and subsequent reduced production, which can exceed the financial loss from cattle mortality. 2. Strategies There are strategies recommended to reduce the incidence and/or minimise the effect of EHL in feedlot cattle. Whilst there is no single definitive action or structure able to fully eliminate its occurrence in the current feedlot industry, there are aspects of the feedlot operation which if properly managed can combine to reduce or eliminate EHL significance. These address site selection, infrastructure and management practice. The important elements in managing EHL are planning, recognition, and action. It is more effective to implement a program (that has been carefully prepared in advance of the event), to minimise the risk of EHL and/or reduce its effect, than to respond to the event after it strikes. Feedlot management can benefit from advance notice of weather conditions likely to be associated with an EHL event.
It appears probable a tailor-made EHL weather alert forecasting service can be developed in Australia. Such a service, meaningfully forecasting a possible EHL event 3-6 days in advance, would enable pro-active environmental management counter- measures, planned prior to the onset of hot weather, to be implemented most efficiently. Recommendations It is recommended that the existing knowledge on infrastructure design and optimum management practice be developed, and extended to industry. There is a need to determine the optimum effective shade structure for the industry, the development of effective weather alert forecasting services and an EHL Environment Index (THI- hours), the validation of developing nutritional concepts under commercial feedlot conditions, and the further development of heat load management programs.
