Role of trace minerals
It has become increasingly clear that there is substantial interplay between energy metabolism, immune function and oxidative metabolism in the transition cow and that these systems cannot be considered independently of one another. And trace minerals have critical roles in this.
Trace minerals are involved in the antioxidant defence system and a deficiency of any of them may depress immunity in transition cows. To date, the primary trace elements of interest in diets for dairy cattle have included zinc (Zn), copper (Cu), manganese (Mn) and selenium (Se), although data also support potentially important roles of chromium (Cr), cobalt (Co) and iron (Fe) in diets. Although the primary role of Cr appears to relate to its ability to enhance the action of insulin, studies have indicated that Cr supplementation may affect health and immune response in ruminants.
Relative to innate immunity, a study showed that supplemental Cr did not affect PMN phagocytic function of dairy cows from six weeks prepartum to six weeks postpartum. However, other studies with periparturient dairy cows indicate that Cr supplementation may affect cell-mediated and humoral immune responses. Cell-mediated immunity was assessed in vitro using antigen (ovalbumin) - and mitogen-stimulated blastogenesis of peripheral blood mononuclear cells collected from prepartum period through early lactation, with Cr supplementation leading to increased blastogenic responses of peripheral blood mononuclear cells to mitogen stimulation. This team also demonstrated in 1996 that in vitro concentrations of interferon-γ, interleukin-2, and tumour necrosis factor-α of mitogen-stimulated mononuclear cells collected during the peripartum period were decreased in cows supplemented with Cr compared with controls.
As for humoral immune response in periparturient and early lactation dairy cows, they showed that anti-ovalbumin antibody responses were increased when supplemental Cr was fed and in 2003, researchers reported that tetanus toxoid-specific antibody titres were increased in cows fed 5 mg Cr per head per day compared with non-supplemented cows. Recently, it was reported decreased circulating cortisol concentrations in cows fed Cr, which may relate to the effects on aspects of immune function described previously.
Cr during transition and early lactation
As described above, Cr has been shown to have a variety of effects on indices of immune function and metabolism. More recently, larger-scale studies have been conducted with different forms of Cr in order to determine effects on lactational performance, metabolism, and reproductive function. Results from six of these experiments are summarised in Table 1. Of these experiments, five reported increased or trends for increased milk yield during the postpartum period, five reported increased or trends toward increased DMI during at least part of the study period, and four out of six studies reported either decreased circulating NEFA concentrations or trends toward decreased NEFA concentrations, particularly during the prepartum period. Although reproductive outcomes typically were not measured in these studies they reported trends toward a greater percentage (50 vs. 39.2%) of cows pregnant during the first 28-d of the mating season and Kafilzadeh et al. (2012) reported decreased days to first ovulation in cows supplemented with Cr but no other effects on reproduction. Furthermore, although we did not find differences in milk yield in a recent study, cows fed Cr during the prepartum and postpartum periods had trends (P < 0.10) toward increased prepartum days in milk and decreased NEFA, and had decreased incidence of cytological endometritis diagnosed, as discussed previously, supporting potential effects of Cr on reproductive function that warrant further investigation.