Evaluation of reticulorumen temperature boluses for the diagnosis of subclinical cases of bovine respiratory disease in feedlot cattle.

Bovine respiratory disease (BRD) is the most important and costly health issue of the feedlot industry worldwide. Remote monitoring of reticulorumen temperature has been suggested as a potential tool to improve the diagnostic accuracy of BRD. The present study aimed to evaluate 1) the difference and degree of reticulorumen hyperthermia episodes between healthy and subclinical BRD feedlot steers, and 2) determine the correlation between reticulorumen hyperthermia and lung pathology, performance, and carcass traits. Mixed-breed feedlot steers (n = 148) with a mean arrival weight of 321 ± 3.34 kg were administered a reticulorumen bolus at feedlot entry and monitored for visual and audible signs of BRD until slaughter when lungs were examined and scored for lesions indicative of BRD. Post-slaughter animals with no record of BRD treatment were assigned to one of three case definitions. Healthy steers had no visual or audible signs of BRD (i.e., CIS=1), and total lung consolidation score < 5% or pleurisy score < 3 at slaughter. Subclinical BRD cases had a CIS of 1, and a lung consolidation score ≥ 5% or a pleurisy score of 3 at slaughter. Mild CIS cases had at least one CIS of 2, and a lung consolidation score < 5% and a pleurisy score < 3 at slaughter. Subclinical BRD and mild CIS cases had longer total duration of reticulorumen hyperthermia, more episodes and longer average episode duration above 40.0 °C compared to healthy steers (P < 0.05). A moderate positive correlation was found between lung consolidation and total duration (r = 0.27, P < 0.001), episode duration (r = 0.29, P < 0.001), and number of episodes (r = 0.20, P < 0.05). Pleurisy score was also found to be moderately and positively correlated with total duration (r = 0.23, P < 0.01), episode duration (r = 0.37, P < 0.001), and number of episodes (r = 0.26, P < 0.01). Moderate negative correlations were found between reticulorumen hyperthermia and carcass traits including hot standard carcass weight (HSCW) (-0.22 ≤ r ≤ -0.23, P < 0.05) and P8-fat depth (-0.18 ≤ r ≤ -0.32, P < 0.05). Subclinical BRD reduced carcass weight by 22 kg and average daily gain (ADG) by 0.44 kg/day compared to healthy steers (P < 0.05), but mild CIS cases had no effect on performance (P > 0.05). The reticulorumen bolus technology appears promising for detection of subclinical BRD cases in feedlot cattle as defined by lung pathology at slaughter.