1. Introduction
Feeding appropriate crude protein (CP) levels is very important in beef cattle production, considering its effects on productivity. A deficiency in dietary protein in beef cattle may result in negative effects on weight gain, feed intake, and carcass results [
1].
Meanwhile, an excessive CP supply increases the energy expenditure of removing excessive nitrogen as urea and also causes a detrimental impact on the environment by increasing nitrogen excretion [
2,
3].
Studies have been conducted to investigate the effects of various CP levels on the production and body metabolism, including ruminal fermentation and blood metabolites, in beef cattle. Gleghorn et al. [
4] reported that increasing dietary CP from 11.5 to 14.5% dry matter (DM) quadratically increased the average daily gain (ADG) and hot carcass weight in finishing beef steers. In another study with young Holestein bulls, increasing CP from 10.2 to 14.2% DM linearly increased the ADG and feed efficiency [
5]. Increasing CP modified ruminal fermentation by increasing ammonia (NH
3-N), acetate, and propionate concentrations and increased the blood urea nitrogen concentration in the same study [
5]. Additionally, dietary CP levels were reported to affect enteric methane (CH
4) production in ruminants, although the results were inconsistent between studies [
6,
7].
The effects of dietary CP levels were also investigated in Hanwoo cattle, which is a native Korean breed. Jeong et al. [
8] found that higher CP increased the ADG in late-fattening (23 to 30 months) Hanwoo steers fed an iso-energetic diet. However, other studies reported no effect of increasing CP on weight gain in growing and finishing steers [
9,
10]. The carcass weight of Hanwoo beef cattle has been continuously improved by the genetic selection program run by the Korean government [
11]. According to a study reported by the Korea Institute for Animal Products Quality Evaluation, the national average carcass weight in Hanwoo steers has improved by 11.0% over the past ten years [
12]. It is certain that studies are needed to investigate the effects of dietary CP levels on productivity and body metabolism in Hanwoo steers because of genetic improvement. However, there are limited studies about the effects of dietary CP levels in Hanwoo steers in the fattening stage. Therefore, the objective of this study was to investigate the effects of different levels of dietary CP on the growth performance, rumen characteristics, blood metabolites, and CH
4 emissions of Hanwoo fattening steers. We hypothesized that an increase in the CP level of the concentrate mix would enhance productivity and have no impact on CH
4 emissions.
3. Results and Discussion
The animal performance data are shown in
Table 3. The initial and final BW, dry matter intake (DMI) of the concentrate and forage, forage/concentrate ratio, and feed conversion ratio were similar between the treatments. However, we found a trend of linear increase (
p = 0.066) in the ADG. As expected, the CP intake was linearly increased (
p < 0.001) by the treatments, while the net energy for growth intake was similar. There was no difference in the pH between the treatments (
Table 4), but the ruminal NH
3-N concentration significantly increased (
p < 0.001) with increasing dietary CP levels. The total VFA concentration and molar proportion of acetate were not different between the treatments, but the propionate proportion was linearly decreased (
p = 0.004) with increased CP. This resulted in a linear decrease (
p = 0.030) in the acetate/propionate ratio. The molar proportions of butyrate and valerate linearly increased (
p ≤ 0.003) with the treatments. The blood concentrations of total protein, glucose, albumin, creatinine, triglyceride, glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, calcium, and phosphorus were not affected by the treatments (
Table 5). The blood urea concentrations linearly increased (
p ≤ 0.003), and the non-esterified fatty acid (NEFA) and cholesterol concentrations linearly decreased (
p ≤ 0.003) in the blood with increasing CP levels. We found a trend of linear decrease (
p = 0.063) in the CH
4 concentration of the exhaled gas from eructation (
Table 6). The CH
4 concentration during eructation per DMI, forage neutral detergent fiber (NDF) intake, NDF intake, and ADG were linearly decreased (
p ≤ 0.014) by the treatments. There was a quadratic increase (
p = 0.049) in the CH
4 concentration of the exhaled gas from respiration. The treatments tended to linearly decrease (
p ≤ 0.086) the exhaled CH
4 emissions per forage NDF intake and NDF intake.
It should be noted that terms referring to growth stages are different between studies because the production systems for beef cattle differ depending on feed resource availability, consumers’ needs in the market (e.g., high marbled beef), climate, geographical reason, characteristics of breed (e.g., body frame), and genetics [
20]. For example, in Korea, native Hanwoo steers are typically slaughtered at around 30 months of age to produce high-marbled beef. In this context, the growth stage from 7 to 14 months of age is commonly referred to as the ‘growing phase’, 15 to 22 mo. as the ‘fattening phase’, and 23 to 30 mo. as the ‘finishing phase’. In the present study, steers with an average BW of 504 kg in the fattening phase are comparable with those in the finishing phase of feedlot systems in other countries, including North and South America and Europe.
Studies have shown that increasing the CP level did not affect the feed intake in beef cattle. Boonsaen et al. [
21] reported no difference in the DMI when feedlot steers were fed total mixed ratio (TMR) with 12 or 14% CP in a 120 d feeding trial, which is consistent with the current study. Gleghorn et al. [
4] also found that the DMI was not affected by a CP increase from 11.5 to 14.5% in concentrates in cross-bred feedlot steers. There are limited studies on Hanwoo steers. Recently, Jeon et al. [
22] reported that feeding with 22.2% CP concentrate did not affect the DMI compared with 19.5% in growing Hanwoo steers. The intake response to the CP level may differ between growth stages. According to Bailey et al. [
23], increasing the concentrate CP from 11 to 14.0% did not affect the DMI in the growing phase, but quadratically increased in the finishing phase. The authors speculated that supplying ruminally degradable protein (RDP) in an 11% concentrate CP diet in the finishing phase limited and reduced microbial efficiency, leading to intake reduction [
23]. Reduced feed intake has been reported in dairy cattle fed MP-deficient diets [
24]. However, all the treatments were formulated to meet the protein requirement in the current study, which resulted in no difference in DMI.
Data about the effects of dietary CP level on weight gain in steers varied between studies. A higher ADG was reported in finishing bulls fed 15.0% CP TMR compared with 13.5% in a 162 d feeding trial [
25]. Because the feed conversion ratio (FCR) was similar between treatments, the authors concluded that the positive result in the ADG for the higher CP group was attributed to higher DMI compared with bulls fed lower CP TMR. Similar results were found in a study with finishing steers by Archibeque et al. [
26], where the ADG and gain-to-feed ratio were improved by both the medium (11.8% CP) and high (14.9% CP) groups compared with the low (9.1% CP) group. It was not discussed how the medium and high groups increased the ADG and gain-to-feed ratio and whether the 9.1% CP diet met the MP requirement of the low group in that study. However, there was a trend of increase in the DMI for the medium and high groups, which possibly led to positive effects on the production data. These studies are partially in line with the results of the current study since we observed a trend of increase in the ADG, but not for the DMI or FCR. Similar results to the current study were found in a study by Bailey et al. [
23], who reported that increasing CP concentration (11, 12.5, and 14.0%) linearly increased the ADG without effects on the DMI in growing cattle, including steers and heifers. In the same study, however, responses to the CP levels were different in finishing cattle, that is, the ADG was quadratically increased, along with a quadratic increase in the DMI [
23]. Gleghorn et al. [
4] also found a quadratic increase in the ADG but no effect on the DMI, with increasing CP levels (11.5, 13.0, and 14.5%) in finishing steers, which is also consistent with the results of the ADG and DMI in the current study. As previously mentioned, the diets in the current study were formulated to meet the nutritional requirements according to Korean feeding standards for Hanwoo steers targeting 900 g of ADG [
14]. However, the ADG in all the treatment groups was less than 900 g in the current study.
The ruminal pHs in the current study were comparable with the ones in a similar study with Hanwoo steers [
27]. A linear increase in the ruminal NH
3-N concentration in the current study is in agreement with other studies in beef cattle [
5,
28,
29]. This indicates that increasing the CP in concentrates up to 19% as-is in Hanwoo steers could result in excessive ruminal NH
3-N and increase the conversion into urea in the liver and excretion of urea in urine. The effects of increasing the CP on ruminal VFA concentrations are not consistent between studies. Some studies in beef and dairy cattle reported no effects of increasing CP on ruminal VFA concentrations [
28,
30]. Chanthakhoun et al. [
29] found a linear increase in the propionate concentration with a concentrate CP range of 9.2 to 21.9% in buffaloes, which is inconsistent with the current study. Changes in the VFA concentrations were probably due to content differences in carbohydrates in diets. The NFC content numerically decreased with increasing CP in the current study, which likely caused the linear decrease in the propionate concentration in the rumen. The linear increase in butyrate in the present study was not in agreement with other studies. Oh et al. [
27] and Chen et al. [
28] observed no effect of increasing CP on ruminal butyrate concentration. Brandao and Faciola [
31] also reported no relationship between dietary CP and ruminal butyrate in a meta-analysis study using a dual-flow continuous culture system. It is likely that the nutrient composition of diets in the current study might increase the butyrate concentration in the rumen. Butyrate-producing bacteria, such as
Butyrivibrio Spp., could be increased by the fiber amount, and the NDF amount was numerically higher in higher-CP diets in the current study [
32]. Valerate is one of the branched chain fatty acids, which are by-products of the deamination of amino acids in the rumen [
33]. It is possibly thought that valerate was produced more in higher-CP diets, which is consistent with the NH
3-N results in the current study.
It has been reported that increasing CP levels enhanced the blood urea nitrogen concentration in studies with beef cattle [
5,
29]. This is in agreement with the current study, and it could be speculated that excessive ruminal NH
3-N resulted in increased urea concentration in blood. The concentrations of blood NEFA in the current study were comparable with those in a study with Hanwoo steers [
34]. It is unclear how increasing CP decreased the blood NEFA in the current study. Bharanidharan et al. [
35] found no difference in the blood NEFA between two different dietary CP levels in Hanwoo steers. Decreases in the blood cholesterol concentration with increasing dietary CP were also found in other studies with growing calves and beef steers [
9,
36]. Park [
36] demonstrated that a higher protein diet increased lecithin cholesterol acyltransferase activity compared with a lower protein diet, which relates to the cholesterol lipid distribution in the liver.
The effects of increasing (or decreasing) the CP concentration in the diet on enteric CH
4 production in ruminants have been inconsistent between studies. Hynes et al. [
7] found no effect of increasing CP (14.1 to 18.1% DM) in diets on CH
4 emissions in dairy cattle. Kidane et al. [
37] also reported that increasing dietary CP (13.0 to 17.5% DM) did not affect CH
4 emissions in dairy cattle. However, Arndt et al. [
6] observed quadratic increases in the CH
4 emissions (g/d) and emission yield (g/kg of DMI) with increasing dietary CP (16.6 to 18.0% DM) in different ratios of alfalfa silage to corn silage. In contrast, in a meta-analysis study, dietary CP concentration had a negative relationship with the CH
4 emission yield (g/kg of DMI) in dairy cows [
38]. This is in agreement with the current study’s findings showing that increasing the CP content linearly decreased the CH
4 concentration and CH
4 concentration/kg DMI. One should carefully interpret these results because ruminal VFA concentrations (decrease in the propionate and increase in the butyrate) in the current study were not supportive of the CH
4 data. It is unclear how the ruminal CH
4 concentration was negatively affected by the CP content in the present study. Possibly, higher inclusion of distiller’s grains with solubles (DDGS) might decrease the CH
4 concentration, similar to other studies in dairy and beef cattle [
39,
40,
41]. It is known that DDGS contains a relatively high PUFA concentration [
42], and unsaturated fatty acids have a negative effect on CH
4 formation by eliminating hydrogens in the rumen. However, the fatty acid composition in diets was not analyzed in the current study.