Changes in serum and faecal zinc concentrations following supplementation with zinc sulphate heptahydrate of extensively grazed dairy cattle in New Zealand.

Aims: To examine the relationship in dairy cattle between serum and faecal Zn concentrations and daily intake of Zn supplemented with an oral drench; and whether total daily intake (TDI) of Zn in dairy cattle can be predicted from single measurements of Zn concentration in serum or faeces.

Methods: A convenience sample of 20 animals from three stock classes (lactating cows, dry cows, heifers), that had not received Zn supplementation in the previous 60 days, was enrolled in the study. From Days -7 to -1, animals received no Zn supplementation. On Day 0, 15 animals per class were assigned daily drenching with increasing doses of ZnSO₄.7H₂O while five remained controls. From Days 0-6, treatment animals received 12.5 mg/kg LWT of Zn/day; from Days 7-13, 25 mg/kg LWT Zn/day and from Days 14-20, 37.5 mg/kg LWT Zn/day. Animals co-grazed within each stock class. Pasture, serum and faecal samples were collected at the start and at weekly intervals before each increase in Zn supplementation. Mixed and non-parametric models were used to assess treatment effects and whether daily intake of Zn could be predicted from Zn concentrations in serum and faeces.

Results: Dosing with 0, 12.5, 25.0 and 37.5 mg Zn/kg LWT resulted in serum Zn concentrations of 12.1, 16.7, 27.2 and 35.8 µmol/L in heifers, 13.3, 17.1, 26.4 and 40.0 µmol/L in dry and 11.9, 12.1, 23.4 and 27.2 µmol/L in lactating cows. Dosing with the same amounts of Zn resulted in faecal Zn concentrations of 2.95, 21.72, 40.32 and 53.27 mmol/kg DM in heifers, 2.81, 23.77, 55.16 and 68.20 mmol/kg DM in dry and 3.00, 12.71, 34.86 and 57.53 mmol/kg DM in lactating cows, respectively. Treatment elevated serum and faecal Zn concentrations above controls (p < 0.001). Supplemented lactating cows had lower serum Zn concentrations than dry cows or heifers (p < 0.01). Supplemented dry cows had faecal DM Zn concentrations higher than heifers or lactating cows (p < 0.05). Analysis showed serum and faecal Zn concentrations could predict TDI of Zn (p < 0.001). Concentrations of Zn in faeces estimated TDI of Zn within a narrower predictive interval than serum Zn concentrations.

Conclusions and clinical relevance: Concentrations of Zn in serum and faeces were positively associated with TDI of Zn in dairy cattle and could predict TDI of Zn. When using serum and faecal Zn concentrations to estimate TDI Zn, stock class must be accounted for.