Effects of energy levels on growth performance, carcass characteristics, and fatty acid composition of Holstein steers at different slaughter ages

Experimental protocol was approved by the Institutional Animal Committee of Yeungnam University, Korea (approval #: YUH-12-0340-016). Forty Holstein steers with an average age of 7.78 ± 0.22 months (average weight of 234.21 ± 7.42 kg) were classified into 22-month and 24-month groups depending on their age on slaughter. The energy feeding level for each group by slaughter age was set to 70% and 72% of total digestible nutrien (TDN) for the fattening phase and 74% and 76% for the finishing phase, respectively. Considering their weight and age, the test animals were randomly allocated to four groups (10 steers in each) depending on their slaughter age and energy feeding level (Table 1). The specification measurements were taken after 449 days (shipping date for the 22-month group) and 502 days (shipping date for the 24-month group).

Concentrates of the test feeds were produced by order at a feed factory in Incheon (Korea). These were divided into two stages for the fattening (7.7 to 10-month) and finishing (11 to 24 months) phases; rice straw was used as forage. Chemical composition for the experimental diets and formula of the feed ingredients are shown in Tables 2 and 3, respectively. Concentrate and roughage feeding amounts were designed considering the nutritional level of Holstein steer by growth stage. (Table 4).

Steers were accommodated in eight pens of 5.0 m × 10.0 m in size, with five heads each, and were fed twice a day (morning and afternoon). Steers were fed fattening phase diets until they were 10 months old, and then switched to finishing phase diets until slaughter. Experimental animals were managed according to the traditional Korean farm regulation and given ad libitum access to water. Throughout the experiment, feed intake was recorded every day. Body weights (BW) of experimental animals were weighed every month.

At the end of the experimental period, animals were fasted for 24 h, weighed, and slaughtered at a commercial abattoir located in Deagu, Korea. Carcass was chilled for 24 h at 4°C followed by characterizing the carcass including carcass yield and meat quality. Carcass was given appropriate meat grades by meat judgement according to the criteria provided by Livestock Quality Assessment [8].

Contents of crude protein, crude fat, moisture, and ash of carcass was analyzed based on the protocol provided by AOAC [9]. First, 2 g of muscle samples were homogenized and dried at 105°C and moisture content (%) was determined by weight loss upon drying in an oven. Next, a muffle furnace was used to determine ash content according to the AOAC method. Total fat content was determined using the Soxhlet extraction method after completing the measurement of moisture content. Finally, crude protein content was measured using the Kjeldahl nitrogen analysis. Briefly, longissimus dorsi muscle samples (0.5 g) were digested at 450°C for 5 h, distilled, and neutralized by the addition of 50% sodium hydroxide (NaOH) and titrated with hydrochloric acid (HCl). Content of crude protein was determined by the equation as follows [10]: Crude protein (CP) = % nitrogen (N) × 6.25.

Meat colors were displayed as CIE L*, a*, and b* using a Chroma Meter (CR-10, Minolta, Osaka, Japan). The standard color used in this study was set to Y = 94.5, x = 0.3132, and y = 0.3203 according to the manual, and three parts per sample were measured and expressed as an average value.

Fat extraction for fatty acid composition analysis of longissimus dorsi muscle fat was performed by extracting fat according to Folch et al. [11], followed by methylation of fatty acids in keeping with the method of Lepage and Roy [12], and analyzed by gas chromatography [13]. Briefly, we took approximately 3 g of the sample pulverized at −80°C (in liquid nitrogen), thawed it slightly, placed it in a glass tube, and added 5 ml of chloroform:methanol (2:1). Then, we ground it for 2–3 min at 11,000 rpm using a homogenizer (Polytron PT-MR-2100, Kinematica AG, Malters, Switzerland) and filtered it using an aspirator (Tokyo Rikaikai, Tokyo, Japan) after 30 min. Subsequently, 8 mL of 0.74% KCl was added, and the sample was left in a cold chamber for approximately 2 h. Next, the supernatant was separated, lower layer was transferred to a scintillating vial, and solvent was volatilized for approximately 2 h using nitrogen in a 70°C water bath. Extracted FAMEs were mixed with 2 mL methanol:benzene (4:1, v/v), 200 μL acetyl chloride, 1 mL isooctane, and 8 mL 6% potassium carbonate (K₂CO₃), followed by centrifuged at 1,200×g for 10 min. The supernatant was analyzed by a gas chromatograph (Clarus 500, Perkin Elmer, Waltham, MA, USA) equipped with a fused silica capillary column (Supleco SP-2560, 100 m × 0.25 mm). Experimental samples (1 μL) were then injected into the vials (split ratio 100:1 at 250°C) with nitrogen as a carrier gas and flame ionization detector (FID) at 270°C. The oven temperature was set at 170°C for 5 min, increased 2°C per min up-to 220°C, and finally held for 40 min.

The data obtained from this study were analyzed using the generalized linear model (GLM) method in SAS [14]. In addition, the significance of each experimental group was tested using Duncan’s multiple-range test; significance was considered at p < 0.05.

The growth performance of Holstein steers considering the energy feeding level is shown in Table 5. During the fattening phase, there were no significant differences in ADG and BW among the experimental groups. However, the final BW of T4 experimental group at the end of the finishing phase was higher than that of T1 experimental group (820.31 and 745.57 kg, respectively; p < 0.05). The ADG of the finishing phase was significantly higher in T2 experimental group than in T1experimental group (1.27 and 1.11 kg, respectively; p < 0.05) in the 22-month group. However, there was no difference among the experimental groups in the 24-month group. The ADG of the overall period was also higher in T2 experimental group than in T1 experimental group (1.26 and 1.12 kg, respectively; p < 0.05).

No difference was observed in feed intake (Table 6) and TDN intake (Table 7) between the experimental groups. However, compared to T1 experimental group, T2 experimental group showed decreases in the FCR and TDN conversion ratio by 10.23% and 7.73%, respectively, in the finishing phase. Holstein cattle generally show higher ADG when fed high-energy feeds during all fattening periods except when they were calves [15]. In particular, a higher energy feeding level during the finishing phase is reported to significantly increase the BW gain [16]. Studies have reported that raising the energy feeding level of beef cattle increases the feed efficiency rather than the feed intake [17,18], and our study showed similar results. The results of our study showed that 22-month group (T2) with higher energy feeding levels showed a significant improvement in the ADG and FCR over the 24-month group (T4). This could be attributed to the feeding program used in this study (Table 4) which maximized feed intake during the finishing phase to induce rapid growth. In addition, the ADG of the 24-month slaughter age group decreased as the finishing phase got relatively longer. In addition, it has been reported that the ADG and feed efficiency of Holstein steers considerably decreases after the 22-month slaughter age [2].

The cold carcass weight of T4 experimental group was higher than that of T1 experimental group (458.72 and 417.13 kg, respectively; p < 0.05), as shown in Table 8. However, there was no significant differences in the back fat thickness and longissimus area between the experimental groups. Regarding the marbling score, T2 and T4 experimental groups, which were fed at a higher energy feeding level, showed increases of 13.40% and 7.11% compared to T1 and T3 experimental groups, respectively. Difference, however, was not statistically significant. The experimental groups did not differ in other meat qualities such as meat color, fat color, or texture. Holstein steers generally have thinner back fat and lower marbling score than other beef cattle breeds, such as Hanwoo and Wagyu [7,19]. A study has reported that 24 Holstein steers slaughtered at 22 months of age had an average marbling score of 2.10–3.60, similar to the present study [20]. However, as shown in the results of our study, there was no significant difference in the back fat thickness and marbling score in the experimental groups with high-energy feeding levels. These results are because Holstein steers accumulate body fat at a lower rate than other beef cattle breeds.

Among the physicochemical characteristics of the carcass, the moisture and crude protein content did not differ by experimental groups (Table 9). The crude fat content increased to 12.74% in T2 experimental group compared to T1 experimental group, but there was no significant difference. The meat CIE values also showed no difference among the experimental groups. In general, the physicochemical characteristics of the carcass are reported to be significantly affected by the marbling score, and Holstein steers, which do not have a high marbling score, are known to have a slight variance in the physicochemical characteristics [21]. Studies have reported that among the physicochemical characteristics of the beef carcass, the crude fat content increases, and the moisture and crude protein content decrease as the meat quality grade increases [22]. In the recent specification test results of Holstein steers between the slaughter age of 20 to 24 months, it was reported that there was no significant difference in the physicochemical characteristics of carcass between the experimental groups [2], which is similar to the results of our study.

Among the fatty acid compositions of longissimus lumborum muscle, the contents of saturated fatty acids (SFA) C14:0 and C16:0 did not differ between the experimental groups. However, the C18:0 content was significantly lower in the T4 experimental group than in the T1 experimental group (p < 0.05) (Table 10). Similarly, the content of C18:1, the major unsaturated fatty acid (UFA) of beef, did not differ by experimental groups, but the content of C18:2 for T4 experimental group was higher than those of the experimental groups that had lower energy feeding levels (T1 and T3; p < 0.05). However, the contents of SFA, UFA, and monounsaturated fatty acid (MUFA) did not differ between the experimental groups. In general, it has been reported that in the fatty acid composition of beef cattle, the contents of C16:0 and C18:0 decrease, and the contents of C18:1 and UFA increase as the meat quality grade increases [23–25]. Similarly, in line with our study, studies have reported that there were no significant differences in the contents of SFA, UFA, and MUFA between the carcasses of different experimental groups of Holstein steers slaughtered at 18, 21, and 24 months of age [26,27].