The hypothesis was that supplementing with 19 g of CaPr or 4 g of Cr-Met would have similar effects in high-risk beef calves, and that combining CaPr and Cr-Met could enhance growth performance, dietary energy, and body fat reserves. However, no synergistic effects were detected; instead, similar results were observed for all variables with individual supplementation.
Although their modes of action act at different levels, both CaPr and Cr have important effects on the physiological processes and pathways related to obtaining energy for maintenance, productivity, and health. CaPr has an effect at the ruminal level, promoting greater DM digestibility and N retention and favoring more efficient ruminal fermentation processes (by increasing propionate production and reducing methane production) [
5,
30,
31,
32]. At the systemic level, CaPr increases the synthesis of GLU via gluconeogenesis in the liver by increasing the energy status [
33], whereas the effect of Cr is predominantly systemic, since it improves the response to insulin, due to its potential to assist in the cellular absorption of GLU and amino acids in tissues sensitive to insulin. In such a way, it can be stated that CaPr improves the energy status, and Cr improves the use of that energy for cell maintenance and growth [
10,
34]. In addition, Cr improves immune function, reduces serum cortisol concentration, and modulates the inflammatory response in stressed animals [
35]. Therefore, the hypothesis arises that the combination of CaPr with Cr could have a synergistic effect on optimizing the energy from food to enhance productivity and health in newly received high-risk beef calves.
4.1. Growth Performance, Dietary Energetics, Body Fat Reserves and Longissimus Muscle Area
Calves that received the combination of CaPr + Cr-Met exhibited an improved ADG and ADG/DMI ratio, similar to the individual effects of CaPr and Cr-Met supplemented separately. There are no previous reports on high-risk beef calves supplemented with CaPr + Cr-Met, and those surrounding Cr-Met are also very limited, since it is used more in the finishing stage. Previous reports regarding supplementation with 20 g of CaPr [
8,
9] have indicated improvements in ADG and ADG/DMI ratio, with no negative effects on DMI. On the other hand, supplementation for 28 d of CrPr or Cr-Yeast has been shown to significantly improve DMI, ADG, and ADG/DMI ratio [
19,
36,
37]; however, other authors did not observe effects with the inclusion of Cr-Met [
38]. The efficiency of dietary NE utilization improved with CaPr or Cr-Met supplementation by 10.2 and 11.4%, respectively, but the combination (CaPr + Cr-Met) produced very similar results, and this coincided with the observed expected dietary NE ratio; likewise, energy retention had the same behavior. Expected DMI increased by 12.5% for CaPr, and 13.8% for Cr-Met.
The presence of body fat reserves in calves upon arrival is an important indicator of their nutritional status. Therefore, it is crucial to encourage the accumulation of these reserves [
39]. In our study, the inclusion of CaPr or CaPr + Cr-Met increased RFT, but the increase was greater with Cr-Met. Previous observations in calves have shown that with CaPr, there were maximum increases of 24.9% of FAT at 42 d and 21% of RFT at 56 d [
8]. Additionally, the inclusion of CaPr, along with an increase in the concentrate level, led to a RFT increase of up to 16.6% [
9]. This is due to the fact that diets with a higher energy or protein content stimulate the deposition of body fat reserves [
40,
41], coupled with the increase in adipogenesis with the inclusion of CaPr [
42].
4.2. Enzymatic Activity and Serum Metabolites
The supplementation of CaPr, Cr-Met, or CaPr + Cr-Met in high-risk beef calves has been found to have no pathological effect or improvement in hepatic and renal metabolism. This is evidenced by the maintenance of enzymatic activity within the reference intervals (ALP, GGT, AST, and ALT) [
24]. Regarding ALP, it has been reported that CrPr in high-risk beef calves [
21], Cr-Met in finishing cattle [
43], and doses of 20 g of CaPr do not modify its activity, but when inclusion of CaPr is raised above 80 g, it causes a reduction in ALP [
8,
9]; these reductions in young growing calves are associated with a decrease in osteoblasts (reduced bone growth) [
44]. Other authors have also pointed out that GGT activity is not modified by CaPr supplementation [
8,
9], but the increase is related to greater hepatic activity [
45]. AST activity has also not been modified by CaPr supplementation [
8,
9] or with CrPr [
21], CrPic, or Cr Yeast [
14]; increases are considered a nonspecific indicator of tissue injury [
46]. Finally, ALT activity is not modified with increasing levels of concentrate and CaPr supplementation [
9] or with CrPr [
21], CrPic, or Cr Yeast [
14].
TP is the main solid component of serum and is made up of ALB and GLO, which are indicative of the nutritional status of cattle. In the present research, TP, ALB, and GLO were found to remain within the established RIs despite supplementation with CaPr, Cr-Met, or CaPr + Cr-Met [
24]. In high-risk beef calves, Rivera-Villegas et al. [
9] provided CaPr supplementation and a diet with 50% concentrate, and their results indicated that TP and ALB concentrations were reduced, although they remained within the RIs. However, following supplementation with different levels of CaPr [
8] or with CrPr [
21], CrPic, and Cr Yeast [
14], or Cr-Met in finishing cattle [
43], no effects were observed. Supplementation with doses lower than 60 g of CaPr [
8,
9] or with CrPr [
21] did not alter GLO, while an increase in its concentration was noted with 80 g of CaPr [
8], with the final value still within the RIs; this could possibly be due to chronic inflammation of the liver [
47]. Additionally, the ALB/GLO ratio decreased with the supplementation of CaPr and Cr-Met, with the lowest value observed when both products were combined (CaPr + Cr-Met). Nevertheless, these changes remained within the RIs, indicating the overall health of the cattle.
Serum BUN levels help to estimate the amount of N excreted and the efficiency with which it is utilized [
48]. This is influenced by the level of crude protein intake, rumen degradability, and liver and kidney function [
49]. In the current study, N utilization efficiency was not affected by the treatments. Similarly, in high-risk beef calves, studies using low doses of CaPr [
8,
9] or CrPr [
21], CrPic and Cr Yeast [
14], or Cr-Met in finishing cattle [
43] did not show significant changes. However, with high doses of 80 g of CaPr, increases were observed without exceeding RIs [
8,
24]. Waggoner et al. [
50] noted that calves with immunological problems have lower nitrogen retention due to increased muscle catabolism to obtain proteins and improve the immune response. Protein catabolism is higher (indicated by high BUN levels) at the beginning of the reception phase (first 20 d), but it decreases as the animal becomes more dependent on dietary protein rather than muscle catabolism [
51].
Supplementation with CaPr, Cr-Met, or CaPr + Cr-Met did not affect the CRE concentration, which remained within the RIs [
24]; these results coincide with those reported by other authors in high-risk beef calves who were administered low doses of CaPr [
8,
9] or with CrPr [
21], as well as with Cr-Met supplementation in finishing cattle [
43], indicating the maintenance of an appropriate renal glomerular filtration rate and the absence of treatment interference. However, it has been observed that escalating the inclusion of CaPr from 20 to 80 g results in elevated CRE levels [
8].
TBIL is an important indicator of liver function, exhibiting an increase during severe lipidosis and a decrease in the presence of a healthy liver [
52,
53]. In this study, TBIL concentrations remained unaffected by the treatments, falling within the normal range (0.20–1.30; [
24]), signifying the absence of adverse effects of the treatments on liver function. This aligns with the findings of other studies involving increased concentrate levels and CaPr supplementation [
9] or CrPr [
21].
Lipids primarily consist of TCHO and TG, reflecting the liver’s energy metabolism [
54]. Increases in lipid levels are associated with a rise in propionic acid production in the rumen, leading to increased TCHO production in the liver, while decreases indicate an energy deficit [
55]. The supplementation of Cr-Met or CaPr + Cr-Met has been found to boost the energy of high-risk beef calves, observed with the increase in TCHO concentration, which did not exceed the RIs [
24]. Similar effects were observed with 80 g CaPr supplementation [
8]. However, no significant effects were noted with 20 g CaPr supplementation [
8,
9], CrPic and Cr Yeast [
14], or Cr-Met in finishing cattle [
43].
The supplementation of CaPr, Cr-Met, or CaPr + Cr-Met did not alter the Ca concentration beyond the RIs [
24]. This finding is consistent with various studies on high-risk beef calves that also received CaPr supplementation [
8,
9,
56,
57] or CrPr [
21], as well as Cr-Met in finishing cattle [
43]. However, other authors have mentioned that as the level of CaPr supplementation increases, it leads to an increase in Ca concentration [
8,
41].
Blood GLU concentration is often used as a metabolic indicator of nutrient intake in beef cattle [
58]. CaPr and Cr-Met supplementation increased GLU levels, with the highest levels observed with CaPr + Cr-Met, while still remaining within the RIs [
24]. Ermita et al. [
59] also noted increases with CaPr administration, whereas Bernhard et al. [
19] observed reductions with CrPr. The increase may be linked to adequate DMI, since circulating GLU is influenced by nutrient availability and utilization [
60,
61]. However, different authors have pointed out that in high-risk beef calves supplemented with CaPr [
8,
57,
62], CrPr [
21], CrPic, and Cr Yeast [
14], or Cr-Met in finishing cattle [
43] exhibited no change in glycemia.
Supplementation with CaPr, Cr-Met, or CaPr + Cr-Met did not alter the values of electrolytes (Na
+, K
+, and Cl
−), and they remained within the established RIs [
24]. This finding is consistent with other reports on the effects of CaPr supplementation [
8,
9] or CrPr [
21] in high-risk beef calves. All of the above indicates that the basal diets adequately met the nutritional electrolyte requirements of the calves [
63]. Low Na
+ and Cl
− values are often due to diarrhea [
46], and K
+ deficiency is commonly associated with stressed cattle due to dehydration and loss of K
+ in the tissues [
64].
4.3. Hematological Responses
Supplementation with CaPr, Cr-Met, or CaPr + Cr-Met helped maintain WBC, PLT, and RBC (hematological response) values within the established RIs [
24]. This indicates that high-risk calves have healthy immunological status [
65].
WBC values were not affected by CaPr, Cr-Met, or CaPr + Cr-Met supplementation, which aligns with findings from Rodríguez-Cordero et al. [
8] and Rivera-Villegas et al. [
9] with 20 g CaPr, and Smock et al. [
21] with CrPr supplementation. However, increasing CaPr inclusion from 40 to 80 g reduced LYM%, and increased GRA and GRA% [
8].
Regarding PLT, MCHC, RBC, RDW%, HGB, HCT, and MCV, no discernible effects of CaPr, Cr-Met, or CaPr + Cr-Met were noted, which coincides with what was observed by Rodríguez-Cordero et al. [
8] and Rivera-Villegas et al. [
9] with CaPr supplementation. Additionally, Smock et al. [
21] also found no significant impact on most of these variables with CrPr supplementation, although they did observe reductions in HGB, HCT, and MCV. Only CaPr supplementation had an effect on MPV and RBC, resulting in elevated values. However, other authors have observed no effect with CaPr [
8,
9] or CrPr [
21] supplementation. Likewise, CaPr + Cr-Met supplementation increased MCH values, which coincides with Rodríguez-Cordero et al. [
8], who also observed an increase when CaPr inclusion was raised from 40 to 80 g; conversely, Smock et al. [
21] reported that CrPr supplementation led to a decrease in MCH values.