Extremes in summer heat have significant negative impact on the production of cattle in Midwest feedlots. This challenge to feedlot producers has created the need for accurate predictors or models that identify how the typical animal will respond to these climatic changes. Many studies have been conducted within our laboratory utilizing controlled thermal challenges of cattle, to evaluate the stress response and the resultant effect on animal production and health. While these studies have provided useful data for modeling cattle responses to thermal challenge, they do not utilize the realistic conditions that feedlot cattle are exposed to during a typical Midwest summer. The present study was a continuation of the controlled experiments conducted in Brody Climatology Laboratory chambers at the University of Missouri-Columbia where cattle were exposed to progressively increasing ambient temperatures. The objective of this study was to expand the model established using the data from controlled studies with the addition of the data collected under field conditions. Angus x Simmental steers (n=24) were housed at the University of Missouri’s beef farm and placed in unshaded feedlots or completely shaded pens. The cattle were fed a typical finishing diet once daily at 0800 and water was available ad libitum. Determination of thermal status was accomplished by using telemetric temperature transmitters (CowTemp, Model BV-010) implanted into the peritoneal cavity and/or intragastrically into the rumen. This allowed for not only a determination of core body temperature but a comparison of temperature differences between rumen and peritoneal regions. Measured ambient conditions included determinations of air temperature, percent relative humidity, and black globe temperature (for assessment of radiant heat load) using standard data loggers (Onset “Hobo”). Data was collected to provide hourly records of both animal and ambient conditions. The study was conducted during the traditionally hottest portion of a Midwest summer (early July through early August).The results were analyzed for comparison with previous summer studies to identify the most reliable ambient predictors of thermal status and critical ambient conditions which result in heat strain. In addition, there was an evaluation of adaptation to summer conditions and a determination of the benefit of shade protection.