An observational study on the relationship between zinc concentrations in bulk tank milk and in serum and farmer-reported zinc supplementation of dairy cattle for facial eczema prophylaxis.

AIMS: To describe the concentration of Zn in bulk tank milk (BTM) in a sample of New Zealand dairy farms, investigate the association between the method of Zn administration for facial eczema prophylaxis and Zn concentrations in BTM and investigate the relationship between the concentration of Zn in serum and that in BTM. METHODS: Multiple BTM samples (n = 3,330) collected during milk pick-up by the milk tanker driver were stored and tested for 121 farms, in Northland (n = 50), Waikato (n = 51) and Southland (n = 20) from February to May 2017. Enrolled farms provided retrospective information on the type of Zn supplementation (if any) used for the prevention of facial eczema and the timeframe over which supplementation occurred. In addition, the concentration of Zn in serum was measured in blood samples collected from ≥15 cattle per farm for 22 farms from Northland (n = 11) and Waikato (n = 11), and compared against the concentrations of Zn in BTM on the day of blood sampling. A linear mixed model was used to model log Zn concentrations in BTM using method of Zn supplementation, region, milk fat and protein percentage, volume of milk, and frequency of milk pick-up as risk factors. A mixed logistic regression model was used to assess the relationship between Zn concentrations in BTM and the presence of cows with a concentration of Zn in serum of ≥20 µmol/L. RESULTS: The median Zn concentration in BTM was 67.9 (min 38.9, max 146.6) µmol/L. The median range of Zn concentrations for repeated samples of BTM within farm was 22.6 µmol/L. In comparison to farms that did not use any form of Zn supplementation, farms that supplemented Zn through a slow-release capsule, oral drench, in feed or a combination of in-feed and water were associated with increased concentrations of Zn in BTM (p < 0.001). There was no difference in Zn concentrations in BTM between farms that administered Zn through the water only and farms that did not administer Zn (p = 0.22). Every 15.3 µmol/L increase in Zn concentration in BTM was associated with 2.2 times (95% CI=1.7-2.9) the odds of a cow having Zn concentration in serum ≥20 µmol/L. CONCLUSION AND CLINICAL RELEVANCE: Zn concentration in BTM is highly variable between farms, days and Zn administration method. Zn concentration in BTM content has modest potential as a way to signal whether a herd has achieved the high Zn status considered to be protective against FE.