Random metaphylactic effects of tulathromycin on health outcomes, complete blood count, antimicrobial use, growth performance, and Salmonella enterica shedding in high-risk beef steers

Abstract

Objective

We evaluated the effects of random metaphylaxis administered with tulathromycin at feedlot arrival to 0%, 33%, 66%, and 100% of high-risk beef steers on health outcomes, growth performance, complete blood cell count, Salmonella shedding, and antimicrobial use during a 35-d receiving period.

Materials and Methods

Crossbred beef steers (n = 232; arrival BW = 213 ± 6.3 kg) were used in a generalized complete block design consisting of 2 source blocks, each with 4 BW blocks and 4 treatments. Steers were randomly assigned to pen within source block, and experimental treatments were (1) negative control, s.c. injection with sterile saline (0M); (2) tulathromycin administered at random to 33% of steers within a pen (33M); (3) tulathromycin administered at random to 66% of steers within a pen (66M); or (4) conventional metaphylaxis with tulathromycin administered to 100% of steers within a pen (100M). Body weight, whole blood, and fecal samples were collected on d 0, 14, and 35. Metaphylactic treatment was included in the model as a fixed effect, and BW block within source block was included as a random effect.

Results and Discussion

Initial bovine respiratory disease treatments were 108% and 62.2% greater for 0M and 33M, respectively, compared with the average treatment prevalence for 66M and 100M steers. No differences were detected between treatments for BW, ADG, DMI, or G:F from d 0 to 35 with dead cattle excluded or included. Hematology metrics were mostly affected by day, though reticulocyte count and eosinophil percentage differed among treatments. Fecal Salmonella concentration and prevalence did not differ among treatments but increased from d 0 to 14 before equalizing through d 35.

Conclusions and Applications

These data suggest that metaphylaxis can be administered to 66% of cattle without increasing bovine respiratory disease morbidity or decreasing growth performance. Moreover, random metaphylaxis to 66% of steers at arrival decreased the amount of active drug compound administered compared with 0M and the number of antimicrobial injections administered in each pen compared with 100M. Salmonella shedding was not affected by random metaphylaxis with tulathromycin in high-risk beef steers.