Effect of a pre-calving injectable trace mineral supplement on white blood cell function in seasonally calving pastoral dairy cows.

Aims: To investigate the effect of injection of trace mineral supplement (TMS) 14-28 days before calving on white blood cell count (WBCC) and function, serum antioxidant capacity (SAC) and reactive oxygen species (ROS) in pasture-fed cattle after calving.

Methods: On each of two South Island, seasonally calving, pastoral dairy farms,1 month before dry-off, a random sample of 150 multiparous cows predicted to calve within 7 days of the herd's planned start of calving (PSC) were stratified on individual somatic cell count, age, breed and expected calving date. On each farm, 14-24 days before PSC, 60 selected cows were randomly assigned for TMS (Zn, Mn, Se, Cu) injection, and 60 were controls. All 240 cows were contemporaneously injected with hydroxocobalamin, and controls with Se. Blood samples were collected pre-injection and 3, 12 and 40 days after calving. Phagocytic activity, count and proportion of neutrophils, lymphocytes and monocytes, WBCC, ROS, SAC were measured. Plasma concentrations of Se, Cu and glutathione peroxidase (GPx) were monitored from a random subset of animals. Differences attributable to TMS were estimated using mixed-multivariable Bayesian analysis, expressed as mean and highest density interval (HDI).

Results: Three and 40 days after calving, TMS-treated cows had 0.36 (90% HDI = 0.00-0.77) x 10⁹ and 0.25 (90% HDI = 0.00-0.55) x 10⁹ fewer neutrophils/L. Neutrophils comprised 6 (90% HDI = 0-11)% and 4 (90% HDI = 0-8)% less of the WBCC, and the neutrophil count was 14 (90% HDI = 0-27)% and 9 (90% HDI = 0-18)% less than controls. However, 3 days after calving, there were 7 (95% HDI = 2-12)% more cells phagocytosing and 2,900 (95% HDI = 2,600-3,200) more bacteria ingested/cell. Twelve and 40 days after calving, TMS-treated cows had 0.65 (95% HDI = 0.17-1.17) x 10⁹ and 0.28 (95% HDI = 0.00-0.59) x 10⁹ more lymphocytes/L. Lymphocytes comprised 10 (95% HDI = 3-18)% and 5 (95% HDI = 0-9)% more of the WBCC, and the lymphocyte count was 30 (95% HDI = 11-51)% and 9 (95% HDI = 0-9)% more than controls. There were no meaningful differences in ROS, SAC, ROS/SAC, other white blood cells, or WBCC. Plasma Cu, Se and GPx concentrations were above recommended thresholds.

Conclusions: Pre-calving TMS injection was associated with differences in white blood cell population and function that may reduce the risk of disease.

Abbreviations: BHOB: Beta-hydroxybutyrate; GPx: Glutathione peroxidase; HDI: Highest density interval; MESF: Molecules of equivalent soluble fluorophore; OSi: Oxidative stress index; PSC: Planned start of calving; ROPE: Region of probable equivalence; ROS: Reactive oxygen species; SAC: Serum antioxidant capacity; THI: Temperature humidity index; TMS: Trace mineral supplement; WAIC: Widely applicable information criterion; WBCC: White blood cell count.