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

## TL;DR

Excess copper in the liver of beef-on-dairy steers worsens respiratory disease symptoms and immune response.

## Contribution

This study shows that high liver copper levels in crossbred steers increase disease severity and impair immune function during infection.

## Key findings

- Steers with high liver copper had sharper increases in clinical disease scores compared to those with adequate copper.
- High liver copper was associated with greater lung consolidation and impaired nutritional immunity response.
- Plasma zinc levels in high copper steers did not decrease during infection, unlike in adequate copper 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.

Beef-on-dairy crossbred calves commonly enter the feedlot with greater liver Cu concentrations than calves raised in conventional beef production systems, which may contribute to inflammation and immune dysfunction. In this experiment, steers with excessive liver Cu concentrations displayed heightened severity of disease, increased mucus buildup in the lungs, and an impaired nutritional immunity response to infection.