Random metaphylaxis effects on health outcomes, complete blood count, antimicrobial use, and growth performance of high-risk beef steers

Objective: Our objectives were to evaluate the effects of random metaphylaxis administered at feedlot arrival to 0, 33, 66, or 100% of high-risk beef cattle on clinical health, growth performance, complete blood cell counts, and antimicrobial use during a 35-d receiving period. Materials and Methods: Crossbred beef steers (n = 216; arrival BW = 197 ± 6.4 kg) were used in a general- ized complete block design consisting of 2 source blocks, each with 4 BW blocks, and 4 treatments. Experimen- tal treatments and steers were randomly assigned to pen within source block: (1) negative control, s.c. injection with sterile saline (0M); (2) 33% of steers per pen giv- en metaphylaxis at random with tildipirosin (33M); (3) 66% of steers per pen given metaphylaxis at random with tildipirosin (66M); and (4) positive control; conventional metaphylaxis given to 100% of steers with tildipirosin (100M). Body weight and blood samples for quantification of complete blood count 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: The percentage of steers treated with an antimicrobial for bovine respiratory dis- ease once was greater (P < 0.01) for 0M and 33M than for 66M and 100M. Neither BW on d 35 nor ADG from d 0 to 35 differed among treatments (P ≥ 0.65), but DMI and DMI as a percentage of BW from d 0 to 35 was greater (P ≤ 0.03) for 66M and 100M than for 0M and 33M. No dif- ferences in hematological variables were detected among treatments (P ≥ 0.19), whereas most complete blood count variables were affected by day (P < 0.01). Most notably, the neutrophil:lymphocyte ratio was greater (P < 0.01) on d 0 than on d 14 and 35. Implications and Applications: These data suggest that arrival metaphylaxis can be randomly administered to 66% of cattle without increasing bovine respiratory dis- ease morbidity, while simultaneously increasing DMI simi- lar to conventional metaphylaxis. Random metaphylaxis to 66% of steers at arrival decreased antimicrobial use for metaphylaxis while simultaneously maintaining health outcomes and growth performance of high-risk beef cattle.