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Extreme summertime conditions can have negative impacts on growth, performance, and can cause death in vulnerable animals. A study was conducted over a three-year period to evaluate the characteristics that contribute to heat stress. Three hundred eighty four feedlot heifers of four distinct breeds/composites were selected based on their coat color and included: Angus (black), MARC III composite (dark red), MARC I composite (tan), and Charolais (white). Animals were assigned to one of 16 pens, eight shaded and eight non-shaded, based on breed, weight, and previous cases of pneumonia (two heifers of each breed/composite per pen). Respiration rates were collected twice a day, 5 days a week, on alternating groups of 64 animals. Linear regression equations correlating respiration rate with ambient temperature were developed for each individual animal. The slopes were used as a measure of responsiveness to heat stress. This measure of responsiveness was used to characterize factors affecting heat stress. It was determined that color/breed plays an important role in heat stress, but there was still a distribution of slopes in all colors/breeds. The animals with lower responsiveness gained more weight than those with a higher responsiveness. Shade preferentially lowered responsiveness in the higher responsive animals, and had a greater impact in the darker colored breeds (Angus, MARC III), and lowered responsiveness in MARC I composite animals but did not impact the responsiveness in the Charolais heifers.