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A sufficient supply of trace elements is essential for herd health and productivity. However, its diagnosis comprises considerable uncertainties, particularly regarding the choice of the appropriate diagnostic sample medium and the applicable reference ranges.
The aim of this study is to develop recommendations for diagnosis of trace elements supply in dairy cows.
For this purpose, samples were collected on 20 dairy farms in the states of the former East Germany between 2008 and 2010. On each farm, seven fresh (0 to 7 days p. p.), seven mid-lactation (105 to 126 days p. p.), and seven late-lactation cows (308 to 329 days p. p.) were sampled. A total of 418 clinically healthy cows were available.
The trace element concentrations of Cu, Mo, Fe, Zn, Mn, and Se in the substrates serum, plasma, whole blood, urine, liver and hair were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). Furthermore, Total Mixed Ration (TMR) and drinking water samples were analyzed.
In addition to the analyses of trace element concentrations in individual samples, pooled samples were derived from equal volumes of the seven individual samples of each lactation group, and analyzed in a similar manner. Arithmetic means were calculated from the individual sample concentrations for each lactation group in each herd.
Strong correlations (r > 0.7) between the substrate concentrations were only detected for the relationship between serum and plasma. The variability of the results of the correlation analysis can be explained by the element-specific kinetics, due to homeostatic mechanisms and status independent factors (acute-phase reaction).
Trace element concentrations were observed to differ significantly in serum and plasma (except for the Mo concentration), urine, as well as liver among the stages of lactation. The lactation dynamic of Cu, Zn and Fe in serum and plasma, as well as Zn and Fe in liver demonstrates the influence of the calving-induced acute-phase reaction on the distribution of the trace element concentrations in fresh cows. Therefore, the reliability of these substrate concentrations for Cu, Fe, and Zn to predict feed trace element supply is considered to be low in the early postpartum period and in cases of diseases which are associated with an acute-phase reaction.
Furthermore, it was shown that the concentrations of the studied trace elements vary only partly in whole blood (Cu, Mn) and insignificantly in hair during lactation.
The Bland-Altman plots between the paired serum and plasma trace element concentrations showed that, with exception of Mo, all trace element concentrations in serum are, on average, significantly lower than in plasma. Moreover the respective differences of the two values scatter to a very high extent. In contrast to serum, plasma is not artificially influenced by blood clotting and should be preferred over serum in practice.
The Bland-Altman plots between the paired group means and pool values of plasma (Cu, Mo, Fe, Zn, Mn, Se) and whole blood (Mn, Se) trace element concentrations showed that the mean concentrations of the two values do not differ significantly (except whole blood Mn). However, the two parameters agree only poorly. Therefore, factors of pre-analytic variability and the accuracy of measurement are limiting factors in trace element diagnostics.
Furthermore, reference ranges were determined from present data, from which, taking the literature into account, proposals for reference ranges were developed for practically relevant substrate concentrations. The use of different reference ranges for individual values, pool or group mean values up to a sample size of n ≤ 7 is not necessary.
For the diagnosis of the supply situation of Cu and Se clear recommendations can be given. For the determination of Cu supply liver tissue is the medium of choice, for Se it is plasma. No consistent recommendations for diagnostics of the trace element supply of Mo, Fe, Zn and Mn can be made, because there is only limited predictive ability or reliability of available diagnostic indicators. Under realistic feeding conditions the determination of feed content of these trace elements is adequate to give sufficient information about the supply situation.