Evaluation of dietary additives to improve feedlot cattle health and performance

The study objective was to determine the effects of Bacillus subtilis PB6 and/or chromium propionate supplementation on health, performance, serum chemistry and complete blood count variables, and fecal Salmonella spp. count in high-risk beef cattle during a 56-day feedlot receiving period and the subsequent finishing period. Four truckload blocks of crossbred beef bulls (n = 300) and steers (n=84; BW = 220 ± 16.2 kg) were sourced from regional auction markets and assigned randomly to treatments arranged in a 2 × 2 factorial. The generalized complete block design consisted of 3 pen replications per treatment within block, totaling 12 pen replications per treatment (24 pen replications per main effect) with pen as the experimental unit. Treatments were: 1) placebo control (CON); 2) 13 g/animal/d of Bacillus subtilis PB6 (CST); 3) 450 ppb DM chromium propionate (CHR); and 4) 13 g/animal/d of Bacillus subtilis PB6 and 450 ppb DM chromium propionate (CST + CHR). Treatments were top dressed in feed bunks daily using 0.45 kg/animal/d ground corn carrier immediately following feed delivery. Cattle fed CON received an equivalent amount of ground corn only. Feedlot personnel were blinded to experimental treatments by assignment of color codes to treatment pens and ear tags. Blood samples were collected via jugular venipuncture on days 0, 14, 28, and 56 for analysis of serum chemistry and complete blood count (CBC) variables from a pen subset (n = 2/pen). Additional blood samples were collected for CBC analysis every subsequent 28 d following the end of the receiving period (d 56). Data were analyzed using mixed models. Repeated measures analysis was used for blood variables and salmonella counts. During the receiving period, DMI was increased (P ≤ 0.03) for CST during each interim period and overall receiving period DMI was 0.35 kg/animal/d greater for CST (P = 0.01). The CST supplemented cattle had greater (P ≤ 0.06) BW on days 14, 28, and 56. Likewise, ADG was improved for CST from day 0 to 14 (P = 0.04) and for the overall receiving period (day 0 to 56; P = 0.04). During the finishing period, CHR lowered (P = 0.02) final BW and ADG during the finishing period (day 56 to final; P = 0.01) and the entire feeding period (day 0 to final; P = 0.03). In addition, CHR fed cattle had lower (P = 0.02) DMI from d 224 to final. Main effects of both CST (P = 0.02) and CHR (P = 0.03) were observed to lower the overall percentage of calves treated for bovine respiratory disease (BRD). Morbidity was 28.0 and 39.9% for cattle with and without CST supplementation, respectively. Similarly, BRD morbidity was 28.6 and 39.3% for cattle with and without CHR supplementation, respectively. Cattle fed CHR had greater total leukocyte count (P = 0.04) and neutrophil count (P = 0.02) during the receiving period. In addition, cattle fed CHR had greater serum aspartate aminotransferase concentration (P = 0.04) and less (P = 0.02) serum calcium. Day affected (P ≤ 0.03) all serum chemistry variables evaluated except total CO2 (P = 0.34). During the finishing period, percent monocyte concentration was greatest in CST, with CON and CHR being intermediate and CST + CHR being the least (CST × CHR P = 0.03). Day affects during the finishing period were observed for hemoglobin, hematocrit, mean corpuscular volume, platelet count, neutrophil count, lymphocyte count and percent, monocyte count and percent, and neutrophil:lymphocyte (P ≤ 0.04). Hot carcass weight decreased (P = 0.01) in cattle fed CHR. An interaction of CST × CHR existed such that percentage of YG1 (P < 0.01) carcasses was greater in cattle fed CST and CHR, with CST + CHR being intermediate and CON being the least. Further, a CST × CHR interaction was observed among percent YG2 (P = 0.03) carcasses where numerically, CON and CST + CHR had a greater percentage. Interaction of CST × CHR (P = 0.05) was observed to increase percentage of YG 4 or 5 carcasses among CST and CHR, with CON being intermediate and CST + CHR being the least. Percentage of edible livers tended to increase (CST × CHR; P = 0.08) numerically in the CST treatment. Cattle fed CST had lower (P = 0.07) overall fecal Salmonella spp. count as well as during week 4 (P = 0.01). Feed intake and performance outcomes during the receiving period were improved by CST, but not CHR supplementation. However, both CST and CHR supplementation decreased BRD morbidity rate. During the finishing period, performance and HCW was reduced in cattle supplemented with CHR.