In agreement with earlier studies (e.g. [11, 21]), we detected a significant increase in the numbers of WBC at calving. This was mainly due to an increase in the numbers of circulating neutrophils, and to a less extent, an increase in monocytes. At calving, the levels of corticosteroids are elevated [41, 11]. Corticosteroids induce neutrophilia by an increased output of neutrophils from the bone marrow, by neutrophil demargination from the blood vessel wall, or by a combination of the two [36, 24]. According to , the neutrophil expression of CD18 increases, while the expression of CD62L decreases at calving. Such changes were not observed in this study as the expression of CD62L and CD18 remained constant over time. However, we observed a depression in the proportion of CD62L+ neutrophils at calving, in accordance with . Fewer cells expressing this molecule means that the marginating pool of neutrophils, rolling along the vessel wall, will shift to the main blood flow stream contributing to the leukocytosis. As a result, fewer neutrophils are able to migrate into the tissues. In agreement with , we found that the numbers of blood lymphocytes were reduced at calving.  presented the hypothesis that lymphocytes migrate in a different manner than neutrophils, suggesting that the high levels of cortisol detected at calving do not affect the adhesion molecules of lymphocytes and therefore they can migrate into the tissues.
Our results have shown a marked decline at calving in serum concentrations of vitamins A and E, and in Zn in agreement with earlier reports [18, 9, 49, 53]. A drop in the serum concentrations of these nutrients is associated with impaired immune functions and a higher incidence of diseases, like mastitis [18, 34, 27, 43].
The drop in serum concentrations of vitamins A and E is largely due to colostrum formation , but can also be due to changes in dry matter intake and ruminal metabolism . Moreover, storage and season can have negative effects on the amount of vitamins A and E in the feedstuffs [9, 28, 33]. Dry matter intake (DMI) can drop remarkably during the week before calving [2, 10]. As a result, reduced blood concentrations of nutrients can be expected, especially as the nutrient demands to initiate milk synthesis is increasing. However,  reported less reduction in DMI before calving in Swedish dairy cows fed high quality feedstuffs. Ruminal metabolism has been implicated in the destruction of vitamin E , but others have suggested that ruminal vitamin E metabolism is essentially nil [25, 51]. Vitamin E in blood is present mainly as a component of lipoproteins. As parturition approaches, the liver secretion of lipoproteins decreases. As a consequence, its transport capacity of vitamin E is lowered . However, the ruminal destruction of vitamin A can be substantial and increases as the level of concentrates in the diet is elevated [35, 51].
The significant drop in serum Zn concentration reported at calving, is most likely a consequence of colostrum formation  and increased stress e.g. in association with an acute phase response due to inflammatory reactions in the uterus. Stress induces synthesis of metallothionein, a protein associated with Zn distribution. As a consequence, Zn is redistributed from blood to other tissues, such as the liver [44, 53]. Physiological fluctuations occur immediately before and after calving in the blood levels of Ca, P, K and Na . Blood levels of Ca and P is expected to decrease at calving due to the large demand of colostrum and milk production. In agreement with , we detected a reduced blood P concentration one month after calving. There is an inverse relationship between milk production and plasma P concentration . The K values were also depressed one month after calving, which might be related with K being the major cation secreted into the milk of cattle .
The blood Cu status undergoes several changes during the periparturient period. The lower value before calving could be due to the drainage by the fetal liver . In contrast to other reports [15, 53], an increased blood level of Cu was detected at calving in this study.  suggest that cattle undergoing stressful periods have increased blood levels of Cu and ceruloplasmin, as Cu transport protein. Ceruloplasmin is considered an acute phase protein and its concentration increase in response to injury, infections and inflammation . This might be one reason for the increased blood level of this nutrient, as calving is considered a stressful period with tissue damages for example in the uterus.
There is a relationship between the Se status of the animals around parturition and the functions of the immune system and disease resistance [12, 43]. From these studies it can be concluded that beneficial effects of Se supplementation occur only when the animals are Se deficient. Whole blood Se levels in the range of 0.1–0.2 mg/l could be considered optimal from immunological standpoint [23, 20]. In this study, whole blood Se concentrations were in the range of 0.167–0.180 mg/kg, i.e. according to recommendations.  hypothesised that the increase in the level of Se at calving may be related to the high fragility of the red blood cells detected at calving.
In conclusion, the results obtained under Swedish conditions were mainly in line with earlier reports. At calving, leukocytosis due to neutrophilia and monocytosis was detected. A lower proportion of CD62L+ neutrophils at calving suggests that fewer of these cells can migrate into the tissues with negative consequences for the defence against infections. Moreover, reduced concentrations of vitamins A and E, and the trace element Zn, were observed at this time. This can also have negative effects on the functions of the immune system resulting in increased susceptibility to diseases, such as mastitis. Despite the fact that vitamin A was fed according to recommendations, and vitamin E above recommendations, the levels of both nutrients decreased at calving. Vitamin A levels dropped below the normal reference value, while vitamin E levels remained within the normal range. Several authors [42, 27, 48] reported improvements in milk production, immune functions and mammary gland health when additional vitamins A and E were given compared with NRC recommendations. Results from the present study, in combination with aforementioned data, suggest that the NRC vitamin A and E recommendations may not be adequate, at least not around calving. However, further studies are needed to evaluate whether the low blood values reflect a true body deficiency and the importance of decreased absorption of vitamins during this period.