**Background:** Heat stress is a multifactorial problem that negatively impacts feedlot cattle health, performance, and survival, with increasing heat waves creating challenging environments. The temperature-humidity index (THI) has been widely used for nearly 40 years to assess heat stress risk, but it has been criticized for not accounting for solar radiation and wind speed, two critical factors affecting cattle thermal balance. This study aimed to assess the relationship among four thermal comfort indices—THI, adjusted THI (THI_adj), heat load index (HLI), and THI based on pen surface temperature (THI_PST)—and their ability to predict heat stress in feedlot cattle.
**Methods:** Data were collected from three experiments conducted during summer seasons at two University of Nebraska feedlots: Concord (2007 and 2008) and Scottsbluff (2008). Micrometeorological data including ambient temperature, black globe temperature, relative humidity, wind speed, solar radiation, soil temperature at 10 cm depth, and pen surface temperature were collected using weather stations. Physiological data—tympanic temperature (TT), respiration rate (RR), and panting scores (PS)—were collected from 34 steers (2007 Concord), 35 steers (2008 Concord), and 10 steers (2008 Scottsbluff). TT was recorded using iButton loggers inserted into the ear canal, programmed to collect readings every minute and compiled into hourly readings. RR and PS were recorded between 14:30 and 15:30 h daily by visual observation. Four thermal comfort indices were calculated using standard equations: THI = 0.8 × Ta + [(RH/100) × (Ta − 14.4)] + 46.4; THI_adj = THI + 4.51 − (1.992 × WS) + (0.0068 × SWin); HLI (calculated separately for BG>25 and BG<25); and THI_PST using pen surface temperature instead of air temperature. Statistical analyses included ANOVA for comparisons between years and locations, correlation analysis (PROC CORR and PROC MEANS in SAS), and regression analysis (JMP), with significance set at 5%.
**Key Results:** During 2007, mean values of soil temperature, PST, outgoing shortwave radiation, and TT were greater than in 2008 (p < 0.011). HLI, relative humidity, and incoming and outgoing long-wave radiation were greater during 2008 (p < 0.012). TT was positively correlated with THI_PST and THI_adj (r = 0.75 and 0.70, respectively). RR had moderate correlation with THI, THI_adj, and HLI (r = 0.32, 0.27, and 0.34, respectively; p < 0.001). In Concord, TT was highly correlated with estimated respiration rate (RRe), THI_PST, and THI_adj (r = 0.94, 0.80, and 0.70, respectively), while a weak relationship was observed with HLI (r = 0.22). Simple linear regression showed THI explained 44% and 87% of the variability of TT and RRe, respectively; THI_adj explained 46% and 92%; THI_PST explained 53% and 72%; and HLI showed lower values (0.38 and 0.10 for RRe and TT, respectively). The lag of maximum TT relative to maximum thermal comfort indices was greater in Concord than Scottsbluff (4 vs. 1 hour). Eigenberg's equation underestimated observed RR (111.75 ± 0.82 vs. 104.37 ± 0.76; p < 0.0001). Panting score explained 82% of observed RR, while RRe explained only 5%.
**Clinical Implications:** The THI_PST and THI_adj indices are practical tools for predicting heat stress in feedlot cattle, with THI_PST being the best predictor of tympanic temperature across years and locations. These indices can help producers implement timely mitigation strategies during heat waves. The weak relationship between thermal indices and observed RR confirms that TT and RR serve different physiological roles—RR is a thermoregulatory mechanism while TT reflects thermal equilibrium—and both should be considered when assessing heat stress. The finding that Eigenberg's equation underestimates actual respiration rates suggests caution when using estimated values. Location-specific factors such as soil type, wind exposure, and microrelief influence the relationship between thermal indices and physiological responses, indicating that a single universal index may not be optimal across all feedlot environments. Further research on pen surface temperature-based indices is warranted.