BEHAVIOR VARIABLES OF FEEDLOT CATTLE AS AN EARLY INDICATOR OF BOVINE RESPIRATORY DISEASE
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Advancement in technology used to monitor activity in cattle could improve the ability to diagnosis bovine respiratory disease (BRD) in the feedlot as current methods are based on subjective visual assessment of clinical signs that result in low sensitivity and specificity. We hypothesized that cattle affected by BRD express detectable alterations in behavior that can be used to diagnose the disease earlier. Our primary objective was to analyze the association between changes in cattle activity in response to clinical identification of BRD in calves maintained in a commercial feedlot setting. Four arrival blocks of high risk, crossbred beef calves (109 steers, 27.1%, and 293 bulls, 72.9%; initial BW = 176.3 ± 18.8 kg) were received and housed for a 56 d evaluation period at a commercial feedlot (OT Feedyard and Research Center; Easter, TX). The cattle were processed according to standard feedlot protocol and affixed with an accelerometer device (IceQube, IceRobotics, Ltd., Midlothian, Scotland), around the metatarsus of the right rear leg, and continuous activity variables were recorded (standing time, number of steps, number of lying bouts and motion index) relative to clinical BRD diagnosis. Activity data were summarized as daily mean ± standard error for d -6 to -1 relative to the d of clinical BRD diagnosis (d 0). Data were continuously recorded and reported in 15 min intervals and pooled by d for 56 d. Castration status, body weight, and arrival date were recorded at the time of arrival and their effects on clinical BRD morbidity and mortality rate were analyzed using the Chi-square test. Cases and controls were based on our BRD definition that utilized a combined clinical illness and depression score system. Each case calf’s activity was compared with control pen mates (calves that were never BRD-diagnosed during the entire study period) at the same time relative to the disease event. Least squares means of activity variables were calculated and compared between cases and controls within d relative to BRD diagnosis of cases. Additionally, activity variables for d -5 to -3 prior to diagnosis were calculated and compared to the activity on d -1 relative to diagnosis. The percentage of calves diagnosed with BRD at least one time was 49, 23, 62, and 71.6% for block 1, 2, 3 and 4, respectively. Overall, 51.5% of the calves were diagnosed at least once with BRD, while 15.2 and 4.5% had a second and a third BRD diagnosis, respectively, during the 56-d study period. The BRD-associated mortality was 4, 2, 11, and 6.9% for block 1, 2, 3 and 4, respectively, resulting in an overall mortality of 6.0% for the total study period. A trend (P = 0.10) was observed for increased BRD morbidity in cattle arriving as bulls (53.9%) compared to those that were steers at the time of arrival (44.9%). Pertinent to arrival BW, a trend for a difference was noted between BW quartiles (P = 0.06); cattle in the lower <25% BW quartile averaged a BRD incidence rate of 50.7%, the intermediate 26 to 75% BW quartile averaged 44.8%, and the upper >75% BW quartile averaged 38.7% BRD morbidity rate. Average ± SE standing time on the d prior to diagnosis (d -1) was 559 ± 1.94 min for cases compared to 613 ± 0.32 min in controls. The difference between d -1 and d -5 to -3 for standing time in cases was -26.6 ± 1.5 min compared to 2.33 ± 0.57 min for controls. Similarly, the number of steps on d -1 for cases and controls were 843 ± 7.8 and 1,472 ± 2.2 steps, respectively. The difference in steps between d -1 and d -5 to -3 for BRD cases was -123.1 ± 4.2 steps compared to 50 ± 2.3 steps in controls. The number of lying bouts for cases and controls was 11.4 and 14.5 on d -1, respectively. The difference between d -1 and d -5 to -3 for lying bouts in cases was -0.58 ± 0.04 min compared to 0.71 ± 0.03 min in controls. These data suggest that cattle that arrive as bulls and cattle in the lighter BW quartiles upon arrival are more prone to be diagnosed with clinical BRD compared to cattle that are steers and those with heavier BW. Furthermore, it was determined that cattle clinically diagnosed with BRD have decreased activity (as expressed by standing duration, steps taken, motion index, and lying bouts) when compared to healthy cohorts and the reduction in these variables was observed several d prior to clinical diagnosis. Accelerometer use as an objective method for diagnosing cattle may assist in the management of clinically ill cattle and provide a framework for understanding the efficacy of cattle behavior as it pertains to clinical BRD diagnoses, potentially improving the sensitivity and specificity of BRD diagnostic methods in the clinical setting.