1. Introduction
In South Korea, the ruminant livestock population primarily consists of Hanwoo cattle, which account for over 75% of the total, while sheep and goats combined constitute approximately 12% (
https://amis.rda.go.kr, 14 September 2023). Consequently, we chose to use domestically raised Hanwoo cattle, the most extensively bred beef cattle in South Korea, as the subjects of our experiment. Increasing the high-starch content in a diet during the finishing period has been known to be an important feeding strategy to produce high-marbled beef because finishing beef cattle require higher energy from non-fiber carbohydrates (NFC) to accumulate intra-muscular fat in their muscles via lipogenesis [
1,
2]. Corn is a well-known source of NFC and has traditionally been widely used in the animal industry. Recently, the cost of imported corn has sharply increased due to environmental factors such as war and climate change, as well as an increased demand for livestock farming. Furthermore, the growing demand for bio-ethanol production as a replacement for fossil fuels to reduce greenhouse gas emissions has also contributed to the rising cost of corn. South Korea ranks 110th in global corn production and heavily relies on imports (
https://www.atlasbig.com, 5 September 2023). Consequently, Korean animal nutritionists are researching suitable energy sources to replace corn. South Korea ranks 16th in global rice grain production (
https://www.atlasbig.com, 5 September 2023). Rice grain is a valuable starch source with a nutritional value comparable to that of corn [
3]. However, the utilization of rice grains as an animal feed source has been limited in the South Korean animal industry due to their higher cost compared to corn, as well as the traditional perception that rice is primarily intended for human consumption rather than for animal feed. Meanwhile, the South Korean government has initiated projects to utilize excess stockpiled rice as a feed ingredient for livestock due to the surplus of rice in storage since 2017. The reason is that stored rice has a very low level of human palatability, and significant storage costs arise for the stored rice.
In our previous study [
3], we used a total mixed ration (TMR) containing 20% rice grain instead of corn and reported that the TMR containing rice grain did not have any negative effects on growth performances in growing Hanwoo steers
in vivo and rumen fermentation characteristics
in vitro. A further study focusing on the effects of rice grain feeding on fattening beef would be necessary to fully understand the potential of rice as a feed ingredient for all stages of beef production. Furthermore, to the best of our knowledge, most ruminant research involving rice grain as a starch source has been conducted on dairy cattle [
4,
5]. We hypothesized that rice grain could be utilized as an alternative source of starch feed during the fattening period for Hanwoo steers. Therefore, an
in vivo trial was conducted to investigate the effects of rice grain usage on growth performance, blood metabolites, and rumen parameters in fattening Hanwoo steers. In the present study, the additional amount of rice grain was increased compared to the growing beef feeding trial because the previous study used over 30% of rice grain instead of corn in dairy cattle [
4,
5].
4. Discussion
We hypothesized that rice grain could be utilized as an alternative source of starch feed during the fattening period for Hanwoo steers. The selection of the rice grain level in our study, set at 33% DM, was a carefully considered decision aimed at optimizing animal performance while minimizing potential adverse effects on rumen fermentation in fattening beef cattle. In the context of dairy cattle, previous research has shown that incorporating brown rice grain at levels of 31% and 33% in the TMR did not yield adverse effects on DMI, milk yield, and milk composition when compared to the control TMR [
4,
21]. However, when the diet included rice grain at a 40% level, it led to a reduction in both DMI and milk yield in dairy cattle [
5]. White et al. [
22] reported a decrease in both DMI and ADG when a diet for beef cattle included 60% rice grain. In our study, the two feeding groups exhibited numerical differences in initial BW, but no statistically significant difference was observed. DMI was significantly higher in the Rice TMR group compared to the Corn TMR group. Although the ADG showed a numerical increase in the Rice TMR group, no significant difference was observed. Based on these findings, the difference in final live weight is approximately 24 kg, which is higher than the initial weight difference of 17 kg. This suggests that the inclusion of 33% DM rice grain in the TMR for Hanwoo beef cattle might have positively influenced the growth performance, with these effects more likely attributed to the rice grain inclusion rather than differences in the initial BW.
Yoo et al. [
10] observed no difference in CP digestibility between corn TMR and rice TMR, and they also found no significant difference in NH
3-N concentration, which was positively correlated with CP digestibility. Similar to the findings of Yoo et al. [
10], our study showed no significant difference in NH
3-N concentration between the groups fed with Corn TMR (7.12 mg/dL) and Rice TMR (7.33 mg/dL). Moreover, the ruminal NH
3-N concentration in our study fell within the previously suggested optimal range for proper rumen fermentation (3.3 to 8.5 mg/dL) [
23]. Regarding the VFA composition, the proportion of butyrate was higher in the Rice TMR group than in the Corn TMR group. This differed with the findings of Yoo et al. [
10], who reported no difference in butyrate proportion between the rice TMR and corn TMR. Ha et al. [
24] reported that the normal range for butyrate is 80–150 mmol/mol; therefore, the butyrate proportion in the Rice TMR (149 mmol/mol) fell within the normal range. The total VFA production in this experiment was lower than that reported in previous studies [
4,
5]. This discrepancy in the total VFA production might be attributed to the difference in rumen fluid collection time because we collected rumen fluid before the morning feeding when the total VFA concentration is typically lowest during the day, owing to unavoidable experimental environmental factors.
Blood metabolites are substances produced during the metabolic process that can be used as indicators of an animal’s health status, nutrient utilization, and overall metabolism [
25]. The concentrations of IP and Ca, mainly controlled by parathyroid hormones, are associated with bone growth [
25]. Kwon et al. [
25] reported the normal ranges for IP (6.0–8.2 mg/dL) and Ca (9.9–10.3 mg/dL), which are similar to our results. Blood cholesterol concentration may be associated with intramuscular fat deposition, and an increase in blood cholesterol can enhance the marbling score in beef meat [
26]. In our study, no significant difference existed in blood cholesterol levels between the two groups. This suggests that substituting corn with rice grains in the diet during the fattening phase of Hanwoo steers may not negatively impact marbling formation. The BUN concentration in the Rice TMR group was higher than that in the Corn TMR group. This result might be attributed to differences in the DMI between the treatment groups. In this study, steers fed the Rice TMR had higher BUN levels as the DMI increased, which is consistent with the findings of Huntington et al. [
27], who reported that an increased intake of concentrate feed can lead to higher BUN in fattening steers. The serum concentrations of AST and ALT serve as reliable indicators for hepatic diagnostics in animals [
28]. The AST in the Rice TMR was higher than that in the Corn TMR (Corn TMR, 70.7 U/L; Rice TMR, 79.5 U/L), but there was no significant difference in ALT between the two groups. Previous studies had suggested ranges for AST (60.6–88.7 U/L) and ALT (18.9–24.1 U/L) levels in the fattening phase of Hanwoo steers [
29,
30,
31]. Since both ALT and AST levels were within the ranges indicated by previous research in this study, it is presumed that substituting rice grain for corn had no adverse effect on liver function. The PLS–DA based on serum metabolites did not show a clear separation between the two groups, indicating that rice grains could potentially be used as a substitute for corn in the diet (
Figure 2b).
In this study, we investigated the absolute abundances of general bacteria, ciliate protozoa, and fungi in the rumen. Additionally, we measured the relative abundance of the main rumen carbohydrate-degrading bacteria.
S. bovis, a main starch-utilizing bacterium among bacteria that degrade nonstructural carbohydrates in the rumen, which showed a high tendency in the Rice TMR. Cotta [
32] reported that
S. bovis grows very rapidly in starch-rich environments. Therefore, the trend toward higher
S. bovis in the Rice TMR group might be linked with the higher DMI in the steers fed the Rice TMR. Among the structural carbohydrate-degrading bacteria in the rumen, we found no differences between the two groups in the relative abundance of major fibrinolytic bacteria, including
R. albus,
F. succinogenes, and
R. flavefaciens [
33]. Scheilbler et al. [
21] observed that the 33% brown rice grain level of TMR showed similar aNDF digestibility to the control TMR. Therefore, it is estimated that the level of 33% DM of rice grain in TMR did not have a negative effect on fiber digestibility. The relative abundance of
B. fibrisolvens was higher in the Corn TMR but the butyrate proportion in the rumen fluid was lower. Although the major metabolic end product of
B. fibrisolvens is butyrate [
34], the difference in butyrate proportion might not be explained by only
B. fibrisolvens abundance because the rumen is a unique habitat consisting of such diverse microbes. Overall, there is a limit to explaining nutritional and physiological parts only by analyzing some microorganisms, and it is judged that a microbiome analysis using metagenomic techniques is necessary.