Hanwoo is the most consumed beef in Korea due to its superior meat quality. Holstein is mainly bred for milk production. Comparing the chemical composition of Hanwoo and Holstein beef, the crude fat content was not significantly different, but Holstein breed had a greater content of crude protein [1,2]. Although Holstein is less popular with consumers than Hanwoo, the number of Holstein being fattened is increasing because of its greater mature body weight (BW), more consistent and predictable daily gain as well as feed efficiency [1,3]. Compared to Hanwoo of the same age, Holstein has a greater BW including skeletal muscle and bone showing 1.3–1.5 kg more daily gain, but shows lower rate of intramuscular fat accumulation (marbling) in carcasses . There is an increasing demand for male Holstein calves because they are cheaper and have shorter production cycle than Hanwoo steers. In 2020, 62,382 Holstein steers were sent to slaughterhouses, which is approximately 15.3% of the total number of Hanwoo steers slaughtered (405,785) . However, the Holstein fattening system in Korea was recently modified with extended fattening period without considering the type of cattle or feed supplies, resulting in decreased feed efficiency and increased production costs . Feeding and fattening strategies of Holstein steers then better be optimized to achieve improved growth performance and economic returns. Therefore, this study was performed to establish the optimal slaughter age by evaluating the growth performance, carcass characteristics, and profitability of Holstein steers slaughtered at different fattening periods.
MATERIALS AND METHODS
All experimental protocols were approved by the Institutional Animal Committee of Yeungnam University, Korea (approval #: YUH-12-0340-016). Each treatment group was allotted based on animal’s BW and age and the feeding period was from 387 days to 533 days at cattle farm located in Gyeongbuk province, Korea (Gunwi Chuk-Hyup). A total of 60 Holstein steers (8 months old and an average weight of 231.88 ± 2.61 kg) were randomly allotted into 5 groups with different slaughter ages of 20, 21, 22, 23, and 24 months (12 steers per group).
Experimental diets were formulated by an animal feed manufacturing company located in Incheon, Korea and divided into 2 stages including fattening (–10 month) and finishing (11–24 month) periods. Rice straw was used as the forage source. The chemical composition of the experimental diets is shown in Table 1, and the formula of the feed ingredients is shown in Table 2. Feeding program including the amount of concentrate and roughage used in the experimental diets was determined by growth stage and nutrient requirements of the steers (Table 3).
1) Supplied per kg concentrate feed: 13,000 U vitamin A,2500 U vitamin D3, 15 mg vitamin E, 1 mg vitamin B1, 0.56 mg vitamin B2,0.5 mg vitamin B6, 0.01 mg vitamin B12, 12.5 mg vitamin niacin, 1.9mg pantothenic acid, 0.15 mg folic acid.
Steers in each treatment group were placed in 5.0 m × 10.0 m pens (three steers per pen), and assigned diets were administered twice per day. Steers were fed early fattening diets until they were 11 months old, and then switched to late fattening diets until slaughter. All steers had ad libitum access to water. Feed intake was recorded every day, and the steers were weighed every month throughout the experiment. Steers were cared for and managed according to traditional Korean farm regulations.
At the end of the experimental period, steers were fasted for 24 h, and weighed and slaughtered at a commercial abattoir located in Daegu, Korea. Carcass characteristics were obtained after chilling for 24 h at 4°C. Carcass yield and quality were graded by meat graders using criteria provided by Livestock Quality Assessment .
Musculus longissimus dorsi (LD) muscles were obtained from the 12th and 13th rib sections and cooled at 0°C–5°C for laboratory analysis. Samples were trimmed and then minced using a Hanil Mini Cooking Cutter (HMC-150T, Hanil Electronics, Seoul, Korea), and stored at −80°C for cholesterol, melting point, and fatty acid composition analysis.
The chemical composition of meat samples, including moisture, ash, crude protein, and fat content, were analyzed according to the AOAC methodology . Moisture content (%) of loin muscle samples (2 g) was measured by homogenizing and drying the samples at 105°C in an oven and measuring the weight loss during drying. Ash content was determined according to AOAC method using muffle furnace. Total lipids were analyzed using the Soxhlet extraction method. Crude protein content was measured using the Kjeldahl method. Briefly, loin samples (0.5 g) were digested at 450°C for 5 h, distilled by addition of 50% sodium hydroxide (NaOH), titrated with hydrochloric acid (HCl), and the amount of protein was calculated by multiplying the % nitrogen (N) by 6.25 .
Meat color of loin sample slices including Hunter L, a*, and b*, was determined using a Chroma Meter (CR-200, Minolta, ToKyo, 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.
Cooking loss was measured by calculating difference between raw sample weight and cooked sample weight. LD muscle samples that were approximately 0.5 mm thick and weighed approximately 25 g were wrapped in film and roasted in a water bath at 70°C (center temperature) for 30 min. Samples were then cooled for 1 h, and cooking loss was determined using the amount of liquid removed.
Fatty acids composition was measured according to previous methods with modifications [9–11]. Briefly, approximately 3 g of each sample was put in liquid nitrogen, homogenized (Polytron PT-MR-2100, Kinematica AG, Lucerne, Switzerland) with chloroform:methanol (2:1, v/v), and filtered. Extracted FAMEs were then mixed with 2 mL methanol:benzene (4:1, v/v), 200 μL acetyl chloride, 1 mL isooctane, and 8 mL 6% potassium carbonate (K2CO3), and centrifuged at 1,200×g for 10 min. The supernatant was analyzed by gas chromatography (Clarus 500, Perkin Elmer, waltham, MA, USA) equipped with a fused silica capillary column (Supleco SP-2560, 100 m × 0.25 mm). One microliter of sample was injected at the split ratio of 100:1 at 250°C, N was used as a carrier gas, and a flame ionization detector (FID) was used to detect the signal at 270°C. The oven temperature was set at 170°C for 5 min, increased to 220°C (2°C/min), and held for 40 min.
The feed costs for both concentrate and roughage used in this study were applied as the actual purchase price of the farm where the experiment was performed. The carcass selling price was computed as the average meat price based on the carcass grade during the slaughter period. Profits from by-products were also considered as economic values. Costs for purchasing the calves, expenses of bedding, medicine, utilities (water and heating), and castration were averaged based on the number of steers used in this experiment.
The Data was analyzed using the generalized linear model (GLM) procedure in SAS . The differences between individual means were evaluated using Duncan’s multiple-range test. The significance was considered at p ≤ 0.05.
RESULTS AND DISCUSSION
The growth performance of steers at different fattening periods is shown in Table 4. There was no difference in average daily gain (ADG) between groups, and BW during the early fattening period ranged from 370.82 to 381.24 kg. Final BW was highest (p < 0.05) in the 24-month fattening period group among other groups (812.84 kg). The ADG of the steers in the 20- and 21-month treatment groups were greater than that of steers in the 22-month treatment group during the late fattening period. The total weight gain increased (p < 0.05) as the fattening period increased. The average ADG of steers in the 22-month treatment group was lower than that of steers in the 20- and 21-month treatment groups, and average ADG values of steers in the 20-, 21-, 22-, 23-, and 24-month treatment groups were 1.36, 1.33, 1.22, 1.21, and 1.14 kg/day, respectively. During the fattening period, the feed conversion ratio (FCR) increased as the fattening period increased (Table 5). Steers in 20-month treatment group had a 12.88% lower FCR than those in the 24-month treatment group. Maintaining the maximum growth rate is important for increasing the feed efficiency of Holstein steers  because ADG begins to decrease after 20 months of age . The final BWs of Holstein steers are similar with previous study . Taken together, these results indicate that Holstein steers in the 20- and 21-month treatment groups had better growth performance, including greater ADG and FCR.
Carcass weight and back fat thickness were lowest in steers in the 20-month treatment group, and highest in steers in the 24-month group (Table 6; p < 0.05). There was no difference in loin-eye area, marbling score, meat color, fat color, nor texture between the treatment groups. In general, compared to Japanese black cattle (Wagyu), Holstein steers had greater carcass weight, and lower back fat thickness and marbling scores [14,15]. Cho et al.  demonstrated that 24 Holstein steers raised from 5 to 22 months had an average carcass weight of 463.6 kg, and an average marbling score of 2.1–3.6. The percent of quality grade over 1 was 16.33%, and the percent of quality grade over 2 was 74.97%–83.33%.
Carcass moisture and crude protein content did not differ among the treatment groups (Table 7). The content of crude protein in LD muscle ranged from 10.78% to 11.76%, and there were no statistically significant differences between treatments. There were no significant differences in the meat color and cooking loss among the experimental groups. Holstein steers generally have a lower carcass fat content than other breeds . Meat quality grade increases with higher fat content and marbling score. In contrast, moister and protein contents decreases with increasing quality grade [17,18]. Similarly, cooking loss decreases as crude fat content increases  because thermal degradation of fat protects moisture from evaporation .
Major fatty acid (C14:0, C16:0, and C18:0) content in beef did not differ significantly among the treatment groups (Table 8). There were no significant differences in one of the unsaturated fatty acids, C18:1, but C18:2 content was significantly greater (p < 0.05) in beef from steers in the 24-month treatment group than in beef from steers in the 20- and 21-month treatment groups. There were no significant differences in saturated fatty acid (SFA), unsaturated fatty acid (UFA), and polyunsaturated fatty acid (PUFA) contents among the treatment groups that agreed with previous studies showing no significant differences in C18:0, SFA, UFA, and monounsaturated fatty acid (MUFA) in loin muscle samples of Holstein steers slaughtered at 18, 21, or 24 months of age [20,21]. However, other studies reported different results, showing an increase in saturated fatty acids, including C16:0 and C18:0, as the fattening period increased , and decreases in UFAs such as C18:1 as the quality grade increased [23,24]. This discrepancy might stem from the small effects of the fattening period on carcass quality grade found in the present study. The marbling score of Holstein steers was not affected by an increase in the fattening period from 20 to 24 months.
The carcass sale price ranged from 3,844 to 3,996 thousand won (Table 9), and feed costs for concentrate and forage increased as the fattening period increased. Total operating expenses, including calf purchase expenses, feed costs, slaughter costs, and other expenses increased, but gross income per head decreased as the fattening period increased. The gross income for steers in the 24-month treatment group was 35.8% and 23.5% lower than that of the steers in the 20-and 21-month treatment groups, respectively. This decrease in total revenue for steers in the 24-month treatment group is attributed to decreased ADG compared to steers in the 20- and 21-month treatment groups.
1) Carcass price (won/kg): A1+, 12,005; B1+, 11,300; C1+, 10,820; A1, 10,930; B1, 9,975; C1, 9,200; A2, 9,350; B2, 9,072; C2, 8,182; A3, 7,930; B3, 7,775; C3, 7,122.
In conclusion, slaughtering Holstein steers at the age of 20–21 months is most profitable in terms of ADG, FCR, and gross income.