Quantification of zinc concentrations in serum, milk and faeces of dairy cattle as a measure of effective zinc supplementation for management of facial eczema

To investigate the relationship between Zn concentrations in serum and those in milk or faeces, and to assess the ability of the Zn concentrations in milk, serum and faeces to predict intake of ZnO in dairy cattle. Seventy cows from one commercial farm in the Waikato region of New Zealand received one of seven dose rates (0, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 g/100 kg bodyweight (BW)) of ZnO given by oral drench, every morning, for 7 consecutive days. Every afternoon, milk and blood samples were collected from all cows. Free-catch faecal samples were collected during the afternoon milking on 3 days throughout the trial. Linear mixed models were used to assess the relationship between the concentration of Zn in serum and that in milk, and in faeces, respectively, and the relationship between dose rate of ZnO and concentrations of Zn in serum, faeces and milk, respectively. Receiver operating characteristic curve analysis was used to determine the ability of the Zn concentration in serum, milk and faeces to predict that a cow had been treated with a dose of ZnO ≥2.5 g/100 kg, the industry-recommended dose rate needed to protect against facial eczema. A 1-µmol/L increase in Zn concentration in milk was associated with a 0.14 (95% CI = 0.11–0.17) µmol/L increase in Zn concentration in serum. Zn concentration in faeces was scaled by its SD; a 1 SD increase was associated with a 1.83 (95% CI = 0.54–3.12) µmol/L increase in zinc concentration in serum. Zn concentrations in serum and faeces increased with increasing dose rates of ZnO. No differences in Zn concentrations in milk were noted between animals dosed with 1.5–3.5 g ZnO/100 kg BW, inclusive. At the optimal threshold of Zn concentration in serum to predict protective ZnO intake (22 µmol/L), the sensitivity was 0.76 (95% CI = 0.69–0.82) and specificity 0.85 (95% CI = 0.80–0.89). For the concentration of Zn in faeces, the optimal threshold was 17.36 mmol/kg, with a corresponding sensitivity of 0.84 (95% CI = 0.84–0.85) and specificity of 0.85 (95% CI = 0.73–0.94). At the optimal threshold for the Zn concentration in milk (76.6 µmol/L), the sensitivity was lower than the other two sample types at 0.59 (95% CI = 0.52–0.67), but with a similar specificity of 0.84 (95% CI = 0.79–0.88). The concentration of Zn in milk shows promise as an initial screening test to identify dairy farms that do not provide adequate zinc to provide protection against FE.

本研究旨在探究血清、乳汁与粪便中锌（Zn）浓度的相关性，并评估乳汁、血清及粪便锌浓度对乳牛氧化锌（ZnO）摄入量的预测能力。研究选取新西兰怀卡托地区某商业牧场的70头乳牛，每日清晨经口服灌胃给予7种剂量梯度（0、1.0、1.5、2.0、2.5、3.0、3.5 g/100 kg体重（BW））的氧化锌，连续给药7天。每日午后采集所有受试乳牛的乳汁与血液样本；试验期间共3个下午的挤奶时段，采集自由排泄的粪便样本。本研究采用线性混合模型，分别评估血清锌浓度与乳汁、粪便锌浓度的相关性，以及氧化锌给药剂量与血清、粪便、乳汁锌浓度的关联。采用受试者工作特征（Receiver Operating Characteristic, ROC）曲线分析，评估血清、乳汁及粪便锌浓度对乳牛是否接受≥2.5 g/100 kg BW氧化锌给药的预测能力——该剂量为牧场行业推荐的用于预防面部湿疹的给药剂量。乳汁锌浓度每升高1 µmol/L，血清锌浓度相应升高0.14 µmol/L（95%置信区间（Confidence Interval, CI）：0.11–0.17）。粪便锌浓度以自身标准差（Standard Deviation, SD）进行标准化；其浓度每升高1个SD，血清锌浓度升高1.83 µmol/L（95% CI：0.54–3.12）。血清与粪便锌浓度随氧化锌给药剂量升高而上升；给药剂量为1.5–3.5 g/100 kg BW（含边界值）的受试乳牛间，乳汁锌浓度无显著差异。以血清锌浓度预测保护性氧化锌摄入量的最佳临界值为22 µmol/L，此时灵敏度为0.76（95% CI：0.69–0.82），特异度为0.85（95% CI：0.80–0.89）。针对粪便锌浓度，最佳临界值为17.36 mmol/kg，对应灵敏度为0.84（95% CI：0.84–0.85），特异度为0.85（95% CI：0.73–0.94）。针对乳汁锌浓度的最佳临界值为76.6 µmol/L，此时灵敏度为0.59（95% CI：0.52–0.67），低于前两种样本类型，但特异度为0.84（95% CI：0.79–0.88），与前两者相近。乳汁锌浓度有望作为初步筛查手段，用于识别未提供充足锌以预防面部湿疹的牧场。