The impact of excess liver copper concentrations on response to a bovine respiratory disease challenge in lightweight beef-on-dairy crossbred steers

Abstract

Beef-on-dairy crossbred steers are exposed to greater amounts of copper (Cu), which may impact their resiliency to disease. To test this, 26 weaned beef-on-dairy steers (95.2 ± 7.2 kg; ~8 wk old) were blocked by weight to pens, and pens were randomly assigned to two target liver Cu statuses: adequate (ADE) and HIGH. To achieve target statuses, ADE and HIGH were fed diets containing no supplemental Cu and 20 mg Cu/kg diet DM, respectively, for 120 d before enrollment in a 13-d bovine respiratory disease challenge. Liver Cu prior to challenge averaged 279 and 608 mg Cu/kg liver DM for ADE and HIGH, respectively. Steers were infected with 104TCID50 BRSV on day 0 via aerosol inoculation. On day 5 postinfection, steers were intratracheally infected with 5 × 108 CFU Mannheimia haemolytica. A trained observer scored steers for depression, appetite, and respiration from days 0 to 14. On days 0, 5, 7, 10, and 13, thoracic ultrasound was used to score animals based on the degree of lung consolidation and lesions, and jugular blood was collected. Categorical variables (clinical and lung scores) and continuous variables were analyzed using PROC GLIMMIX and PROC MIXED of SAS 9.4 (Cary, NC), respectively. Clinical scores were affected by treatment × day (P = 0.04), where HIGH experienced a sharper increase in clinical scores in response to disease compared to ADE (P < 0.01) and remained higher throughout the remainder of the disease challenge (0.01 < P ≤ 0.08). Over the entire challenge, HIGH steers tended to have greater lung consolidation than ADE (P = 0.08). While no differences in haptoglobin were detected between treatments (P = 0.96), both treatments experienced marked increases in haptoglobin on day 7 postinfection (P < 0.01), indicating inflammation in response to disease. There was a tendency for a treatment × day interaction (P = 0.07) for plasma Cu, where HIGH steers exhibited less dramatic increases in plasma Cu than ADE. Plasma Zn was affected by treatment × day (P < 0.01) where HIGH steers did not change over time and ADE exhibited decreased plasma Zn in response to disease, characteristic of a classical nutritional immunity response. Ferric reducing antioxidant power did not differ by treatment (P = 0.33); however, both treatments decreased on day 7 postinfection (P < 0.01), indicating increased antioxidant demands. In conclusion, these results suggest excessive liver Cu concentrations result in greater disease severity and immune dysfunction in beef-on-dairy steers.