4.1. Nutrient Intake and Apparent Digestibility
In this experiment, C:R ratios did not influence the nutrient intake of DM and OM but did affect CP, EE, NDF, and ADF intake because there is less CP and EE and more NDF and ADF contained in diets with different C:R ratios. Furthermore, the digestibility of DM, OM, EE, and NDF was not significantly affected, whereas the digestibility of CP and ADF was affected by the different C:R diets. Nevertheless, supplementation with BBC did not affect intake and digestibility. Wanapat et al. [
2] reported that increasing the proportion of concentrate from 20% to 80% in daily steers did not influence total DMI; however, an increase in concentrate has been shown to linearly increase with the digestibility of DM, OM, and CP, whereas it decreases with the digestibility of NDF and ADF. In the current study, cattle fed with the 70C:30R ratio diet showed lower (
p = 0.06) ADF digestibility than did cattle fed with the 60C:40R ratio diet. This finding could be explained by the fact that the number of cellulolytic bacteria decreased as the proportion of concentrate in the diet increased [
2,
24]. Majee et al. [
25] reported that biotin supplementation (20 mg/d) had no effect on the total tract digestibility of DM, OM, or NDF. Moreover, Shah et al. [
26] found that supplementation with betaine (15 g/day) increased the apparent digestibility of DM, OM, CP, NDF, and ADF in dairy cows. Similarly, Deka et al. [
27] reported that the apparent digestibility of DM, OM, CP, and ADF increased after supplementation with 1.5 mg Cr/kg DM in buffaloes. Productivity improvements associated with chromium supplementation have generally been observed during stress [
28]. However, Kumar et al. [
29] concluded that inorganic chromium supplementation in buffalo Murrah calves did not affect DM intake. Independent reports also support that dietary supplementation with chromium dose not impact the digestibility of DM, OM, and CP [
30].
4.2. Ruminal Fermentation and Blood Metabolites
Ruminal pH was not affected by the C:R ratio or BBC inclusion, ranging between 6.68–6.78. This result is in line with that of Suksathit et al. [
31], who found a range of 6.88–7.02 in Thai feedlot native cattle fed diets containing a 65% proportion of concentrate. In addition, Kongphaitee et al. [
32] revealed that Thai native cattle fed with TMR containing 50% rice straw have a high ruminal pH (pH 7.0). High ruminal pH was found in diets with a high proportion of roughage [
33]. Wanapat et al. [
2] suggest that a roughage-to-concentrate ratio of 40:60 has positive effects on the creation of healthy rumen (ruminal pH and ecology) in dairy steers fed with urea-treated rice straw as their source of roughage. Likewise, some researchers have reported that feeding of a diet high in concentrate (67% or 60%) has no effect on mean ruminal pH or NH
3-N concentrations [
34]. Ruminal NH
3-N (11.29–13.17 mg/dL) in the current study was in the normal range for beef cattle fed diets containing 11.15%–12.39%CP. The ruminal NH
3-N concentration should range from 10 to 25 mg/dL, which is suitable for rumen fermentation and enhanced microbial yield in cattle [
35].
In the present study, total VFA production was higher in cattle fed with the 70C:30R ratio diet than in cattle fed with the 60C:40R ratio diet but did not differ from those fed with the control diet. However, the proportion of acetic acid, propionic acid, butyric acid, and the A:P ratio did not differ. The proportion of individual VFA obtained was within the normal range when more concentrate was fed to these ruminants [
24]. Furthermore, as the roughage-to-concentrate ratio decreased, the A:P ratio also decreased [
34]. The inclusion of BBC did not affect VFA production in this study, which is in line with Zimmerly and Weiss [
4], who found that biotin supplementation at 20 mg/d did not affect the molar percentages of rumen VFA. Meanwhile, Shah et al. [
26] reported that betaine supplementation (15 g/d) in dairy cow feed increased the total VFA and acetic acid proportion but decreased the propionic acid proportion; therefore, it is possible that betaine is metabolized in the rumen and converted to acetate [
16]. Wang et al. [
17] noted that betaine in the rumen could be metabolized into acetate and increase the concentration of acetate at high-dose supplementation levels (100 g/d).
In this experiment, BUN and glucose concentrations were affected by the C:R ratio, which was lower in cattle fed with the 60C:40R ratio diet than in cattle fed with the 70C:30R ratio diet. Lower BUN concentrations in cattle fed with the 60C:40R ratio diets were due to reduced protein intake, which reflects protein degradation in the rumen. Wanapat et al. [
2] reported that increasing the concentrate proportion in the diet from 20% to 80% resulted in a linear increase in the BUN concentration (5.8 mg/dL to 7.8 mg/dL).
Hall et al. [
15] demonstrated that supplementation with betaine did not elevate glucose levels in dairy cows, except in those subjected to heat stress while receiving high-dose betaine supplementation (114 mg/kg BW). Spears et al. [
8] found that blood glucose concentrations were higher in the chromium-supplemented groups than in the control (no chromium) groups of dairy cows. In contrast, insulin concentrations were higher in the control groups than in the chromium-supplemented groups. Therefore, it is impossible to say whether chromium plays a role in animal glucose production. Zimmerly and Weiss [
4] demonstrated that concentrations of blood glucose and insulin were not affected by biotin supplementation. Biotin is a key coenzyme involved in gluconeogenesis in the liver [
5]. Because insulin governs glucose homeostasis, increased blood glucose concentration encourages insulin synthesis, which then pushes glucose into cells. As a result, high glucose concentrations are rare.
Intuitively, an increased insulin concentration resulted in a decreased glucose concentration (r = 0.42,
p < 0.05), as also observed by Zimmerly and Weiss [
4]. High-producing dairy cows of which the diets were supplemented with biotin experienced an increase in gluconeogenesis, presumably in the liver. The glucose concentration obtained in this study was in line with that of Chen et al. [
36], who reported that there was no significant difference in the plasma concentrations of glucose after biotin supplementation at 20 mg/d and 40 mg/d. Ferreira and Weiss [
5] observed that liver pyruvate carboxylase mRNA was more abundant and its enzymatic activity was increased by biotin supplementation. In contrast, a report by Stahlhut et al. [
37] showed that chromium supplementation could reduce plasma glucose concentrations in growing and finishing steers. Insulin may regulate glucose transport by stimulating the translocation of GLUT 4 from an intracellular membrane pool to the plasma membrane in adipocytes and muscle cells [
38]. Kneeskern et al. [
13] reported that chromium supplementation (3 mg/d) in feedlot steers could increase insulin concentrations, which supports the observation that supplementing chromium to heifers increased insulin sensitivity [
9,
10]. Supporting these outcomes, previous studies have reported that supplemental chromium enhanced insulin sensitivity parameters in lactating cattle receiving excessive concentrate and net energy intake [
39]. Chromium is a critical component of the glucose tolerance factor, which facilitates the action of insulin on cells, and chromium supplementation has been shown to enhance glucose metabolism in ruminants [
10]. According to the findings described here, the inclusion of BBC in the diet of beef cattle could promote the more efficient use of glucose. It was assumed that when supplemented with biotin and chromium, the liver process of gluconeogenesis or glucose transportation, along with increasing insulin activity and insulin sensitivity, would help to improve glucose utilization by enhancing energy and cellular function. In humans, Campbell [
40] has shown that using a combination of chromium and biotin enhances glucose uptake. Recently, Turgut et al. [
41] found that supplementing diets with chromium and biotin increased serum glucose and lipid levels, as well as PPAR-, IRS-1, and NF-B protein expression levels in the liver and muscle of exercise-trained rats, with the greatest efficacy resulting from their combined administration.
4.5. Weight Gain, Feed Conversion Ratio, Feed Efficiency, and Nutrient Utilization Efficiency
In this study, the weight gained by Thai feedlot native cattle fed with the control diet was 452 g/d, which was close to the initial expectation of 500 g/d. Beef cattle that were fed with diets supplemented with BBC at a rate of 6 g/kg gained more weight (305 g/d) on average than did the beef cattle fed the control diet. Moreover, the weight gain among the cattle supplemented with BBC at 6 g/kg tended (
p = 0.10) to exceed that gained by the cattle supplemented with BBC at 3 g/kg. Kongphitee et al. [
32] reported that their Thai native cattle gained from 391 to 569 g/d when fed with fermented total mixed rations that provided 1.1–1.9-fold more ME for maintenance. However, the weight gain of Thai native cattle varies greatly according to feeding level [
44].
From the results obtained in this study, the BBC supplementation groups exhibited decreased FCR and increased feed efficiency. According to the literature, there is no conclusive evidence as to the growth rate in beef cattle that are fed with diets supplemented with one of the single ingredients in BBC. Wang et al. [
20] reported that the supplementation of betaine (0.6 g/kg DM) did not affect DMI but increased average daily gain and decreased FCR in bulls. In addition, Kneeskern et al. [
15] found that chromium supplementation (3 mg/d) did not affect ADG or FCR in feedlot steers. Biotin (20 mg/d) was given to crossbred cattle by De Silva et al. [
45]; however, there was no significant change in their growth rates.
The efficiency of energy and protein utilization was significantly lower in the control than other treatments. In relation to the nutrient requirements for the maintenance and weight gain of Thai native cattle based on the equation reported by WTSR [
19], the cattle receiving the treatment diets had energy and protein intakes that were 12% and 11% below the requirement, respectively, whereas the cattle fed with the control diet had intakes that were 16.8% and 26.0% above the requirement, respectively. Dong et al. [
46] found that the energy required for maintenance was not constant, but increased as feed intake increased. Cattle fed with treatment diets probably required less energy for maintenance than cattle fed with the control diet, resulting in more energy being available for growth and greater weight gain. Suebpang et al. [
42] reported that increasing the feed amount improves ME utilization by reducing the proportion of ME required for maintenance, thereby increasing the proportion dedicated to growth.