Although the BCS-difference of 0.73 was not great enough to induce significant effects on most of the parameters it was proven to be significant (p = 0.03) at the onset of the experiment. When setting up the experiment we considered the “normal” BCS range of our herd without artificial forcing for extremely low or high BCS animals through strong dietary manipulations. Therefore, we assigned cows from the given BCS range to groups with a higher and an adequate BCS. As these cow groups still differed significantly in mean BCS we hypothesized that BCS variation around a normal BCS would also influence metabolic and health traits significantly.
Cows with a higher BCS at calving are frequently considered being
post partum more susceptible to metabolic disorders and vulnerable to infectious diseases through a compromised immune-responsiveness compared to cows with an adequate BCS [
7]. As cows with higher BCS are less capable
post partum to increase intake of DM and energy to appropriate levels, they suffer more from the
post partum NEB than adequately conditioned cows. To investigate the influence of differences in BCS within an overall mean range often observable under practical feeding conditions, we grouped our herd into cows with a higher and an adequte BCS.
4.1. Hematology and Lymphocyte Subsets
The state of pregnancy is characterized by various changes in endocrine and metabolic alterations. Also, the transition from pregnancy to lactation is accompanied with an adaptive change in physiological, metabolic and immune status. The present study describes hematological changes of circulating cell populations during the transition period, reflected by a significant increase of WBC around parturition. Granulocytes as the major population of leukocytes were impacted markedly compared to lymphocytes which is in line with other studies [
12,
21]. A possible explanation for the increased GR counts towards calving is given by Burton et al. [
3], who assumed an impaired trans-capillary-migration-capacity. Glucocorticoids induce a down-regulation of the expression of adhesion molecules on the surface of neutrophils, which results in a reduced infiltration in affected tissues. In addition, glucocorticoids, such as cortisol, stimulate the release of immature neutrophil granulocytes from the bone marrow [
3]. Around calving, cows are exposed to high stress levels, due to exogenous and endogenous changes, such as the adaption from a dry period diet to a lactation diet or the transition from late gestation to lactation [
2,
21,
29]. These marked endocrine changes might explain the observed time course in granulocyte counts in particular and in white blood profile in general. Endo et al. [
4] showed a negative correlation between BCS and hair cortisol concentration. If cows with a higher BCS are characterized by a lower cortisol level an increased granulocyte count in blood would be expectable which, however, was not observed in the present experiment. Obviously, the BCS in the BCS
H groups was not high enough to induce significant differences in cortisol levels compared to the BCS
A groups.
Based on this assumption and the known influence of cortisol on release of granulocytes, it seems comprehensible that GR count increased more pronounced in BCS
A groups before calving in the present study. Burton et al. [
3] proposed that the surge of cortisol could also be fetus-derived and that it induced adaptive changes of the neutrophil system, which resulted in the favoring of tissue remodeling instead of antibacterial defense. This, in turn, could increase the susceptibility to diseases [
2]. Chapwanya et al. [
30] demonstrated endometrial infiltration of leukocytes, mainly neutrophil aggregates and elevated expression of pro-inflammatory genes in uteri of
post partum cows, indicating endometrial inflammatory processes, which explain the
post partum decrease of GR counts in the present study. The authors described these inflammations as necessary events and beneficial for normal endometrial involution and bacterial clearance.
A higher BCS
ante partum often leads to complications at calving [
7,
8] with consequences for
post partum susceptibility to metabolic disorders and infectious diseases. Even though the BCS groups did not differ concerning WBC and GR count at the start of the experiment (−42 d), the increase of both parameters was less pronounced in the BCS
H groups
ante partum in the present study without consequences for the dynamics of these cell types
post partum. Although for lymphocyte counts similar relations were observed
ante partum, this cell type fluctuated at a lower level when cows with a higher BCS were supplied with more energy (BCS
HC
60)
post partum. In contrast, Eger et al. [
31] denied a relation between body condition and lymphocytes, when grouping the experimental cows in lower BCS (mean 2.77) and higher BCS (mean 3.73). The BCS difference of the experimental groups in the present study was even smaller and might not have been large enough to trigger marked effects on lymphocytes.
In accordance with this result, the pre-calving BCS itself did not affect lymphocyte subsets, such as CD4
+ and CD8
+ in the current investigation. The percentage of CD4
+, as well as that of CD8
+ increased after calving, which is in accordance with previous reports [
21,
32].
The CD4
+:CD8
+-ratio, which was neither affected by treatment, remained within the range indicating a balanced immune homeostasis, as described in previous studies [
33,
34]. The correlation between BCS and CD4
+:CD8
+-ratio was performed to examine the relation on an individual basis. The results support the idea of the nonexistent relation between the body condition and the CD4
+:CD8
+-ratio at least within the rather small depicted BCS range of the present study.
In the present study, CD14
+ cells decreased in all groups after calving, independent of experimental treatment. Endometrial inflammatory processes and uterine bacterial infections were observed in
post partum dairy cows. These infections are associated with LPS, which translocate into the uterus [
30,
35]. Not related to treatment 33% of the cows in the BCS
H/C
60 group, 40% of the animals in both the BCS
H/C
35 and BCS
A/C
35 group and 53% of the cows in the BCS
A/C
60 group developed production diseases, such as mastitis and metritis. This results in migration of monocytes, which in turn explains the decrease of peripheral CD14
+ cells after calving in the present study. The interaction between time, BCS and C in the current study suggests that an enhanced C in combination with a higher BCS triggered a higher CD14
+ cell proportion.
4.2. Haptoglobin and Kynurenine:Tryptophan-Ratio
The results of this experiment indicate that a BCS effect on mobilization was only detectable in individual fat depots [
14]. It was not verifiable that higher conditioned cows have a higher potential to mobilize body fat in general. Moreover, compared to the hepatic production the Hpt synthesis of the subcutaneous and visceral adipose tissue constitutes only 0.02% of the overall synthesis [
11]. Based on these facts the unaltered Hpt levels in higher BCS cows of the present study might be explained.
Other studies already indicated that rations differing in C did not influence Hpt and even when drastically increasing C, circulating Hpt remained unaltered [
13,
36]. Moreover, Drong et al. [
22] demonstrated, that Hpt was even unaffected by different BCS in combination with different dietary compositions. Furthermore, the regulation of acute-phase-proteins is described as a function of several parameters, which interact in a complex manner [
37]. The individual cow factor explains 22% of the Hpt variation [
38].
The Kyn:Trp-ratio provides information about the activity of indoleamine-2,3-dioxygenase, which is activated during inflammations or infections and also suggested to regulate the implantation of the embryo. Due to its degradation via the kynurenine-pathway it leads to a decrease of Trp during gestation. The Trp degradation prevents a maternal immune reaction against the fetus [
5,
39]. Therefore, the increase of the Kyn:Trp-ratio towards calving observed in the present study is physiological necessary [
5,
40,
41]. Hüther et al. [
23] described lower Kyn:Trp-ratios in cows with higher BCS
post partum compared to cows with adequate BCS. Although we observed a three-way interaction between time, concentrate feed proportion and BCS class
post partum, a clear effect of BCS was not detected. The relation between BCS and several immune parameters is non-linear and therefore, the impacts are difficult to predict [
7,
13]. This becomes also apparent in the missing significance for the correlation between BCS and Kyn:Trp-ratio. However, in the afore mentioned study, dry cow nutrition differed concerning C, whereby different diets did not impact the Kyn:Trp-ratio. The mentioned interactions in the present study suggest that different energy levels after calving have greater influence on several parameters than varying dry cow nutrition.