175 Effects of Random Metaphylaxis Administered at Feedlot Arrival to 0, 33, 66, Or 100% of High-Risk Beef Cattle on Health Outcomes, Complete Blood Count, and Growth Performance

Abstract

Abstract Our objective was to evaluate the effects of random metaphylaxis administered on arrival to 0, 33, 66, or 100% of high-risk beef cattle on clinical health, growth performance, and complete blood cell counts during a 35-d receiving period. Crossbred beef steers (n = 216; arrival BW = 197 ± 6.4 kg) were used in a generalized complete block design consisting of 2 source blocks and 4 treatments. Experimental treatments and steers were randomly assigned to pen within source block: 1) negative control, subcutaneous injection with sterile saline (NCON); 2) 33% of steers/pen given metaphylaxis (33M); 3) 66% of steers/pen given metaphylaxis (66M); and 4) positive control; conventional metaphylaxis given to 100% of steers (CONV). Body weight and blood samples for quantification of complete blood count were collected on d 0, 14, and 35. Pen was considered the experimental unit for all analyses. Continuous data were analyzed using PROC MIXED, and categorical data were analyzed as binomial proportions using PROC GLIMMIX. Treatment was included in the model as a fixed effect and source block was a random effect. Complete blood counts were analyzed as repeated measures, where day was the repeated effect and pen within treatment was the subject. The percentage of steers treated with an antibiotic for bovine respiratory disease (BRD) once was greater (P < 0.01) for NCON and 33M than for 66M and CONV. Neither BW on d 35 nor average daily gain from d 0 to 35 differed among treatments (P > 0.65), but dry matter intake (DMI) and DMI as a percentage of BW from d 0 to 35 was greater (P < 0.03) for 66M and CONV than for NCON and 33M. No differences in hematological variables were detected among treatments (P > 0.19), whereas all complete blood count variables were affected by day (P < 0.01), except for mean corpuscular hemoglobin concentration (P = 0.81). Most notably, the neutrophil:lymphocyte ratio was greater (P < 0.01) on d 0 than on d 14 and 35. These data suggest that arrival metaphylaxis can be administered to 66% of cattle at random without increasing BRD morbidity while simultaneously increasing DMI similar to conventional metaphylaxis. Random metaphylaxis may be a viable method to decrease antibiotic use in the feedlot industry, while maintaining health outcomes and growth performance of high-risk beef cattle.