Relationships between ambient conditions, thermal status, and feed intake of cattle during summer heat stress with access to shade.

Heat stress in feedlot cattle is known to reduce their performance. The challenge comes in determining reliable predictors of current and near-future changes in thermal status and performance. A 42-d study, using crossbred (Bos taurus) steers was conducted during summer months (July through August) to identify best environmental determinants of rumen temperature (Trumen) and feed intake (FI) in feedlot cattle with access to shade. A further goal was to define the relationship between Trumen and FI. Shade coverage was approximately 50%, and all animals were provided standard feedlot diets and water ad libitum. Intraruminal telemetric boluses recorded Trumen several times each hour. Ear tags, telemetrically connected to a feed monitoring system, provided FI data using RFID technology. Data loggers recorded ambient conditions in sun and shade, along with black globe temperature. Regression analyses identified daylight black globe and air temperatures in shade, with one hour delays, as the best predictors of Trumen. Prediction of FI was much less reliable. Unexpectedly, Trumen was not superior to ambient variables in predicting FI. Maximum daily temperature humidity index, calculated using BG in sun with a 5-d lag, was the best significant predictor of FI. These results indicate for feedlot cattle that although air temperature alone in the shade may be the best predictor of Trumen in the heat, black globe temperature in the sun may be a better determinant of feed intake over time. Additional studies are needed to verify the delayed FI response which seems unusually long.