347. Effects of Trace Mineral Source and Dietary Starch Level on Feedlot Performance, Carcass Characteristics, and Tissue Mineral Concentrations of Nellore Bulls Finished in Brazil.

Abstract The objective of this study was to investigate the impact of trace mineral (TM) source and dietary starch level on growth parameters, carcass characteristics, and mineral status in Nellore bulls finished in a feedlot system under tropical conditions in Brazil. Ninety-six Nellore bulls (358.05 ± 8.26 kg BW; approximately 24 months of age) were blocked by BW, and randomly assigned to one of four pens with a 2 × 2 factorial arrangement of treatments with factors being: 1) TM source [sulfate forms of Cu, Mn, and Zn (STM) and hydroxy TM forms of Cu, Mn, and Zn (HTM)] and 2) dietary starch level (15 or 35% DM). Each pen was equipped with three electronic feeding systems to record individual animal daily feed intake. Bulls were fed twice daily to allow access to feed over a 24 h period. On day 74, all animals were transported to a commercial abattoir, fasted for 16 h with ad libitum access to water, and harvested on day 75. Following harvest, carcass data, liver, rumen fluid, blood, and testicle samples were collected. Data were analyzed using linear models for a randomized block design. Initial BW, overall DM intake, feed efficiency, and hot carcass weights were similar (P > 0.10) across treatments. Final BW and average daily gain tended (P < 0.10) to be greater for bulls receiving HTM compared to bulls receiving STM. A tendency (P = 0.08) for a TM source × starch interaction was detected for dressing percentage. Dressing percentage was greater for HTM-supplemented bulls consuming 35% starch and STM-supplemented bulls consuming 15% starch compared to all other treatments. Bulls receiving 35% starch had greater (P = 0.02) rumen fluid Zn concentrations and tended (P = 0.06) to have greater Mn concentrations compared to bulls receiving 15% dietary starch. A TM source × starch interaction (P = 0.04) was detected for testicle Cu concentrations. Bulls supplemented with HTM receiving 35% dietary starch and STM receiving 15% dietary starch had greater testicle Cu concentrations compared to all other treatments. Bulls receiving 35% dietary starch had greater (P = 0.02) testicle Mn concentrations compared to bulls receiving 15% dietary starch. Additionally, a tendency (P = 0.07) for a TM source × starch interaction was detected for plasma Zn concentrations. Bulls receiving HTM and 15% dietary starch or STM and 35% dietary starch had greater plasma Zn concentrations compared to all other treatments. Overall, these results suggest that TM source may influence growth performance in Nellore finishing bulls and that dietary starch level impacts tissue mineral concentrations.