Technical Abstract: Cattle produced in open feedlots are vulnerable to a variety of weather events; under certain conditions heat events can be especially detrimental. Shade structures are often considered as one method of reducing cattle stress. A summer study was conducted during 2007 using instrumented shade structures in conjunction with meteorological measurements to estimate relative effectiveness of various shade materials for `full sun¿ days. Polyethylene shade cloth was used in three of the comparisons and consisted of effective coverings of 100%, 60% with a silver reflective coating, and 60% black material with no reflective coating. Additionally, one of the structures was fitted with a poly snow fence with an effective shade of about 30%. Each shade structure contained a solar radiation meter and a black globe thermometer to measure radiant energy received under the shade material. Additionally, meteorological data were collected as a non-shaded treatment and included temperature, humidity, wind speed, and solar radiation. Analyses of the collected data focused on a physiological model that predicts cattle respiration rate based on relative humidity, ambient temperature, solar radiation and wind speed. An associated heat stress index was used to determine the effectiveness of the shading options. Analyses of the data revealed that time spent in the highest stress category was reduced by all shade materials. Moreover, significant differences (p<0.05) existed between all shade materials (compared to no-shade) for hourly summaries of estimated respiration rate during peak daylight hours and for `full sun¿ days.