1. Introduction
The transition period is one of the most critical phases in the life of dairy cows, as it is characterized by several metabolic, physiologic, endocrine, and immune adaptations [
1,
2]. This period includes the last three weeks before and the first three after calving. With the onset of lactation, voluntary feed intake decreases while energy requirement increases, resulting in a negative energy balance [
1]. The magnitude of the negative energy balance during the transition period can vary, and it is strongly related to susceptibility to diseases. Moreover, almost all periparturient cows, even the healthy ones, experience inflammation in this phase [
3]. The severity of the inflammatory response, which mainly arises from the uterus, the mammary glands, or the gastrointestinal tract, determines the outcomes (i.e., risk of clinical disorders or diseases). Additionally, the drastic change from the fibrous diet of the dry period to the more energy-dense diet of lactation requires structural and functional adaptations in the rumen [
4]. The rumen itself plays an important role in the inflammatory and oxidative stress responses, producing proinflammatory mediators through interaction with lymphoid tissues along the gastrointestinal tract [
5].
The use of nutraceuticals might represent a practical solution to mitigate the inflammatory response during the transition period, reducing disease incidence in turn [
5]. In this context, the use of yeast-based products is receiving growing interest, particularly as a means to reduce antibiotic and antimicrobial use. The most widely used yeast species is
Saccharomyces cerevisiae, which can be administrated to cattle in live form [
6]. Although the exact mode of action of
S. cerevisiae has yet to be fully elucidated, multiple mechanisms have been proposed. Live
S. cerevisiae can use ruminal oxygen [
7] and promote the action of either cellulolytic or lactate-utilizing bacteria [
8], stabilizing rumen pH. Moreover,
S. cerevisiae contains a mixture of different components (e.g., polysaccharides, oligosaccharides, and oligopeptides) that can influence metabolism and the inflammatory response. In particular, some components of its cell wall, in particular β-glucans, can interact with the host immune cells and prevent the binding of bacteria to the intestinal wall, resulting in an immunomodulatory effect [
9]. Several studies demonstrated that live
S. cerevisiae supplementation can have positive effects on the performance and health of dairy cows [
10,
11], mainly because of its stabilizing effect on rumen pH. Some authors proposed that the positive effects on rumen pH could be associated with an increased meal frequency with yeast supplementation [
12]. Other positive effects of yeast supplementation have been observed on milk yield, dry matter intake, and organic matter (in particular, fiber) digestibility [
13]. This could be a consequence of the stabilization of the ruminal environment, which, in turn, protects the animal from diseases occurring at a lower rumen pH as consequence of the increase in concentrates in the diet during the lactation period.
We hypothesized that inclusion of live S. cerevisiae in the diet of periparturient dairy cows could improve feed intake, rumination time, milk yield, and metabolic profiles because of the improvements in rumen function. Thus, the aim of this study was to evaluate the effects of live S. cerevisiae yeast supplementation in multiparous Holstein cows during the transition period on feed intake and digestibility, milk yield and composition, rumen activity, and metabolic profiles.
4. Discussion
Adaptation to the metabolic imbalances that take place during the transition period plays a fundamental role in determining the success of the future lactation and, overall, in the productive career of dairy cows. The administration of live yeast in this critical phase can result in improved performances, supported by improved rumen functionality, as a consequence of the modulation of rumen fermentation [
10,
11]. In fact,
S. cerevisiae activity promotes fiber degradation by cellulolytic bacteria, stimulating their adhesion to cellulose [
21]. Moreover, it has been shown that, in vitro,
S. cerevisiae can compete with
Streptococcus bovis, a lactate-producing bacteria, for the available sugars, causing a decline in the lactate concentration and the stabilization of ruminal pH [
22]. At the same time, it promotes the activity of lactate-utilizing bacteria [
23]. These actions could, in turn, improve fiber degradation and feed intake. If DMI is increased, positive effects on milk production are expected [
24].
In the present study, however, effects of yeast on DMI were lacking. On the contrary, rumination time was surprisingly reduced in LSC cows after calving. With live yeast administration, positive effects on DMI and rumination time would be expected [
25] because of the effects of
S. cerevisiae activity in the rumen described above. However, in agreement with our study, other authors did not find effects on DMI [
12,
26]. Nevertheless, a possible explanation for the lower rumination time recorded after calving (with a similar DMI) might be related to faster degradability of fiber, usually promoted by yeast activity [
27], which could have resulted in a reduced retention time and increased ruminal passage rate [
28]. Unfortunately, in the current experiment, rumen fluid samples were not collected to characterize the rumen fermentation pattern; thus information, to confirm this speculation is lacking. Moreover, no difference was noted in our study in fecal ADL or VFA proportions, used as markers of feed digestibility and hindgut fermentation, respectively. Therefore, the reason for the difference observed in rumination time remains to be elucidated.
Despite the lack of effects on DMI, a different pattern in the lactation curve was observed, with increasing milk yield in LSC cows at the end of the last phase of the experimental period (second month of lactation) compared with a rather constant yield in CTR. Despite the greater concentration of blood BHB observed in LSC in the whole lactation period considered, this outcome could not be explained by an increased mobilization of the body fat reserves. Body weight and BCS variations during the period investigated were similar between groups and unaltered by yeast supplementation, and the same was true of plasma NEFA concentrations (also considering the lack of differences in liver function). Additionally, no cases of clinical ketosis were observed in either group. Thus, the reason for the greater plasma concentration of BHB, which always fluctuated within physiological ranges (0.4–0.7 mmol/L), is not clear. Yuan et al. [
29] found similar results when supplementing the diet of cows with a yeast product during transition. The increase in plasma BHB might be explained by greater synthesis of ruminal butyrate, which is transformed into BHB during absorption from the ruminal epithelium [
30]. Nevertheless, it is not clear whether the activity of live
S. cerevisiae can increase the ruminal butyrate concentration. The greater blood BHB observed at 42 DRC could also have been associated with the numerically greater milk yield recorded in this period. In this stage of lactation (i.e., after the first month of lactation), considering the unaltered DMI and plasma glucose (as a marker of energy availability), BHB concentrations in those ranges are not pathological and can, indeed, support lactation [
31].
The postpartum inflammatory response was comparable between the two groups, although LSC cows showed a faster decrease of the concentration of ROMs after calving. This could suggest better modulation of the inflammatory response [
21]. Components of yeast cell walls (specifically, β-glucans) can have an immunomodulatory effect [
32], likely mediated by the rumen epithelium, and supplementation with
S. cerevisiae improved the inflammatory conditions in transition-period cows in another study [
33]. Cows involved in the trial did not experience any disease or metabolic disorder, as also supported by the body temperature values, which never reached the critical thresholds suggesting potential issues. Indicators of the innate immune response (acute phase proteins and myeloperoxidase) showed similar trends in both groups. Indeed, at the same time, cows treated with yeast showed a lower oxidative stress response, i.e., a lower level of ROMs paired with unchanged levels of available antioxidants (i.e., FRAP) [
21]. The greater Ca level observed in the plasma of LSC cows before calving might be related to the numerically different DMI [
34], whereas the differences in plasma P are unclear. However, the magnitude of these changes was very limited and unlikely to cause relevant biological effects, as supported by the similar blood concentrations of these minerals after calving and the lack of milk fever cases.
Overall, the effects of live
S. cerevisiae supplementation in the present study were limited compared with those observed in the literature [
6]. There could be two possible reasons for this difference. First, the dose of live yeast used in our study (6 × 10
10 CFU/d) was smaller compared with those used in other studies (for instance, 10 × 10
10 CFU/d in the study by Cattaneo et al. [
33]). The dose and strain of the yeast used and the productivity, physiological condition, and diet of the cows can affect the animals’ responses to supplementation [
35]. Second, the experiment was carried out on an experimental farm, where cows are constantly monitored and raised with high welfare standards. This is supported by the concentrations of the biomarkers related to the inflammatory response and oxidative stress, which were within the reference ranges proposed for the transition period [
19] and indicate overall limited peripartum inflammation. The intensity of the inflammatory response was related to the physiological adaptations occurring during the transition period [
36]. Considering that the benefits of yeasts might be greater in stressful conditions [
37,
38], the mild periparturient stress experienced by these cows might have mitigated the observed results. In stressful conditions, the effects of yeasts are usually greater, as cows consistently reduce DMI, and animals supplemented with live yeast can better cope with this condition thanks to the yeast-related improvements in feed digestibility and rumen function [
38], as well as the attenuation of immunometabolic pressure. Based on the latter, further studies on this product carried out in different settings could show different outcomes.