Maximum productive output from the high producing dairy cow may be constrained in hot climates by: (i) heat load from hot ambient conditions, and/or (ii) heat arising from metabolic processes. Interactions between these two factors will produce physiological changes that can be incompatible with maximal reproduction and productive capacity. These changes can be collectively described as "excessive heat load syndrome: and is of particular importance for high producing animals. Studies of this syndrome in cows should be conducted in their normal environment so that the impact of all possible environmental and animal factors is included.
Winter and Summer studies were conducted on the interactive effects of seasonal factors, high metabolic heat and degree of cooling in high producing dairy cows in their natural environment. The higher ambient heat of summer resulted in significantly higher 1400h rectal temperatures and respiration rates, and lower daily milk yields and feed intakes when compared to winter values, regardless of dietary energy level. Dietary metabolic heat exacerbated the summer ambient heat load resulting in significantly elevated rectal temperatures and respiration rates, a reduction in total daily feed intake and a failure to convert the higher dietary energy into milk. During summer, the provision of water spray cooling alleviated the combined impact of ambient and metabolic heat and allowed the cows to maintain higher dry matter intake and milk yield compared to animals which did not have access to spray cooling. However, spray cooling did not provide sufficient heat load relief to achieve winter milk production levels.
Results of these studies showed that the ability to predict the interactive effects of environmental and metabolic thermal factors on excessive heat load syndrome and hence the production of the dairy cow would have major economic implications to the commercial producer. However, the studies highlight the complexity of such prediction for unrestrained commercial dairy cows. Combining the impact of all individual components of the ambient conditions, metabolic heat, animal behaviour and varying dairy management systems was impractical.
A thermal indicator within the animal itself would be a better predictor of the summation of metabolic and ambient factors influencing the thermal balance of cows. Change in body heat content (kJ/kg body weight) could be used as such an indicator.
A technique for monitoring temporal change in body heat content in dairy cows under their natural conditions was developed. The technique allowed for the examination of the diurnal and local fluctuations in body heat content in cows over time, and indicated that management practices, the environment and individual cow variations had an impact on changes in body heat content.
A Functional Heat Content Model was proposed which used the body heat content of dairy cows as the basic measure of thermal impact. This model could be used to better understand the excessive heat load syndrome within dairy cows and to evaluate management strategies under commercial conditions.