The spent substrates used to grow mushrooms (Pleurotus eryngii) were used as the test feed. The original substrates used to grow mushrooms consisted of 49% sawdust, 32% rice bran, and 19% corn cob. Fresh, spent substrates from a commercial mushroom farm located at the suburb of Chung-ju City, Korea were obtained on the day of mushroom harvest and transferred to the Konkuk University Experimental Station. The SMS were screened using a 20 mm×20 mm (width×length) sieve to remove lumps and foreign materials. Four strains of bacteria (Enterobacter ludwigii, Bacillus cereus, and two strains of Bacillus subtilis) were cultured for 12 h in plate count broth, and then inoculated (1% v/v) into the SMS. These bacterial species had been previously isolated and identified from the SMS and were found to have high CMCase and xylanase activities (Kim et al., 2007b; 2008b). The inoculated SMS was packed into plastic bags inside an empty feed sack and sealed to prevent air from getting in. The material was left to undergo fermentation for more than two weeks at the ambient temperature, and then the fermented SMS (total bacteria count, 10⁸ to 10⁹ cfu/g) was fed to Hanwoo steers.

Eighteen Hanwoo steers (nine months old and 227±20 kg) were selected and randomly assigned to a control group (without SMS supplementation), a treatment group with 50% of SMS (the 1/2 of ad libitum group), and a treatment group with ad libitum access to SMS (the ad libitum group).

All of the groups were fed the same amount of formulated feed. The growing period was from nine to 13 months of age, during which the steers were fed the formulated feed at an average amount of 1.75% (as-fed basis) of the body weight. The fattening period was from 14 to 22 months of age, during which the steers were fed the formulated feed at an average amount of 1.85% of the body weight. The finishing period was from 24 to 30 months of age, during which the steers were restricted-fed the formulated feed at 9 kg/d. The formulated feed was restricted-fed during the growing and fattening period in order to prevent excessive gain and during the finishing period in order to prevent excessive back fat thickness.

During the growing and fattening periods, steers had free access to rice straw; however, during the finishing period, the steers were restricted to 1.0 kg/d (as-fed basis) of rice straw. The SMS was fed only during the growing and fattening periods by top-dressing it on the formulated feed. The SMS were not fed during the finishing period in order to meet the maximum energy requirement with high energy-containing formulated feed, which is normal feeding practice in this period. Steers had free access to fresh water during the entire feeding trial. The chemical composition of the formulated feed, rice straw and SMS is shown in Table 1. The CP and EE contents were slightly higher in SMS than in rice straw. The crude ash content of SMS was 6%, equating to about half that of the rice straw. The NDF content of SMS was similar to that of rice straw. Physically, the mean particle size of SMS was 4 mm, being much smaller than that of the rice straw.

Representative samples of the test feeds and the formulated feed that were fed to the Hanwoo steers were collected and stored in a freezer at −20°C (Lassele LOC-520F, Korea) until the analysis. The samples were thawed in the refrigerator prior to analysis. Dry matter (DM), CP, EE, crude fiber (CF), NDF, acid detergent fiber (ADF) and crude ash were analyzed according to the AOAC method (2000). Non-fibrous carbohydrate was calculated using the formula 100-(NDF+CP+EE+crude ash).

The body weight (BW) of the steers was measured monthly. The back fat thickness and the marbling degree were measured at 24 months of age using an ultrasound (Aloka SSD-500, USA). Steers were slaughtered at 31 months of age. Following a 48-h carcass chill, yield and quality grade of each carcass were measured using Korean carcass grading standards specified in Korean Livestock Enforcement Regulation (KMAF, 2007).

The General Linear Model was used for the statistical analysis (Statistix 7, 2000). To compare the mean values, orthogonal contrast was performed as follows: the control vs the 1/2 of ad libitum group and the ad libitum group; and the 1/2 of ad libitum group vs the ad libitum group.

The feed intake of the growing Hanwoo steers with SMS supplementation is shown in Table 2. Under the formulated feed and rice straw feeding system, when SMS was top-dressed on formulated feed, the daily voluntary intake of SMS was 1.6 kg in the growing period and 1.4 kg in the fattening period, which was equivalent to about 11 to 16% of the total DM intake (DMI).

When SMS was supplemented, the total DMI increased by 7 to 15%; however, rice straw intake decreased by 6 to 11% during the growing period. The Hanwoo steers showed relatively high preference for the MFSMS. The increase in total DMI was a reflection of the high SMS intake because feeding the formulated feed was equally restricted to all experimental steers during the growing period. Also, it is believed that because of the decreased intake of rice straw with long particle size and the increased intake of SMS with a relatively small particle size of 4 mm, the mean particle size of the ingested feed decreased, which probably resulted in increased DMI. Jaster and Murphy (1983) and Kato et al. (1989) reported that when steers were fed with a feed with a small particle size, total DMI increased. As the NDF content of SMS (78.8%) was 3.1% higher than that of the rice straw, the total NDF intake by the treatment groups was 5.5 kg/d, which reflects a 22.3% increase from 4.5 kg/d in the control group. Generally, if NDF intake increases, DMI decreases (Forbes, 1995; Lee et al., 2002). In the present study; however, both NDF intake and DMI increased with SMS supplementation. Although the mean particle size of SMS was 4 mm, larger than the 1.18 mm that Sniffen et al. (1992) suggested as the minimum particle size that is capable of inducing rumination, it was much smaller than that of the rice straw. It can thus be postulated that the increased DMI was due to the faster movement of the SMS to the lower intestine due to their small particle size. Supplementing with SMS increased total CP intake by 5 to 10% during the growing period and by 3 to 6% during the fattening period.

The effect of SMS supplementation on the growth performance of Hanwoo steers is shown in Table 3. During the growing period, supplementing SMS did not affect weight gain (p>0.05). During the fattening period, the 1/2 of ad libitum group gained more than the ad libitum group (p<0.05). During the finishing period, all the SMS-fed groups tended (p = 0.061) to have approximately 24 to 40 kg more weight gain compared to the control group. As a result, the BW of the animals over the whole period was 8 to 12% (approximately 36 to 52 kg/head) more in the SMS-supplemented groups than in the control group. These results were attributed to the increased DM and CP intakes and possibly to the favorable effect of the microbes fed. Huck et al. (2000) reported that the weight gain of feedlot cattle with added direct-fed microbes (DFM) increased by 2.5 to 3.0%. Galyean et al. (2000) also reported that dietary addition of DFM increased the body weight of feedlot cattle. It is believed that feeding MFSMS favorably affected the growth of Hanwoo steers because it functions as sources of roughage as well as DFM.

An ultrasound scanning test was conducted when the Hanwoo steers were 24 months of age. The back fat thickness and marbling score are shown in Table 4. Supplementing SMS did not affect the back fat thickness (p>0.05) with the fat thickness of all steers being in the range of 7.1 to 9.8 mm. Kim et al. (2003) reported a similar level (9.3±0.4 mm) at 24 months of age. Supplementing MFSMS did not affect marbling score at 24 months of age (p>0.05).

All the experimental steers were slaughtered at 31 months of age; the carcass characteristics are shown in Table 5. Supplementing SMS did not affect the cold carcass weight, yield index and yield grade (p>0.05). However, the ribeye area increased (p<0.05) by 9.5 to 10.3 cm² by supplementing MFSMS. The growth of ribeye area has been known to be affected by age, body weight and feed. The active growth of Hanwoo ribeye area reaches a peak at around 11 months of age and its growth continues until 18 months of age (HFMC, 2009). Okumura et al. (2007) reported that the ribeye area of Japanese Black steers (twins) was not different between 24 and 30 months of age. Thus, muscle growth seems complete before maturity. The feeding management during the growing and early fattening period seems to be important to maximize ribeye area. Ribeye area has been found to be correlated positively with weight gain (Kang et al., 1995), slaughter weight up to 24 months of age of Hanwoo steers (Moon et al., 2003) and cold carcass weight of Hanwoo steers (Park et al., 2002) and goats (Palmieri et al., 2012). Dietary protein levels may or may not affect ribeye area depending upon experimental conditions. Feeding high levels of protein in the early fattening stage did not affect ribeye area of Japanese Black heifers (Okumura et al., 2008). Feeding 12 to 18% dietary CP from 10 to 20 months old did not affect ribeye area of Japanese Black steers (Iwamoto et al., 2010). Although feeding 11.5 to 14.5% dietary CP levels did not affect ribeye area, CP source (urea or cottonseed meal) affected ribeye area of finishing beef steers (Gleghorn et al., 2004). However, feeding more concentrate feed with high CP during the growing period has been shown to increase ribeye area of British breed beef steers (Sainz et al., 1995). In the present study, the SMS-fed steers had a higher dietary CP intake, heavier weight and greater ribeye area (p<0.05). The increased ribeye area of SMS-fed steers was attributed to the increased CP intake under the low dietary CP (10.4%) and restricted concentrates feeding situation in the growing and early fattening stage of Hanwoo steers.

The back fat thickness of the ad libitum group steers was 6 mm less than the 1/2 of ad libitum group steers, although the difference was not significant (p>0.05) due to the large variations between animals. When compared with the ultrasound results (Table 4), the increase in the back fat thickness at 31 months of age (seven months after the body weight measurement using the ultrasound test) differed considerably between individuals. The mean increase in the back fat thickness was 6.66 mm. During the same period, the marbling score increased by an average of 2.9 grades although the difference was not significant among treatments.

Meat color, fat color, texture and maturity were not affected by supplementing SMS (p>0.05). These scores of both the control and the treatment groups were in the normal range. Ware et al. (1988) also reported that although supplementation with DFM increased the BW of steers, it did not have any influence on the marbling score.

The effect of supplementation with SMS on the economic efficiency of Hanwoo steers is shown in Table 6. The mean gross income from selling a Hanwoo steer in the treatment groups was 4.1 to 4.8% higher than that in the control group. The feed cost per 1 kg weight gain of Hanwoo steers was 13.9% lower for the 1/2 ad libitum group and 9.6% lower for the ad libitum group than that for the control group (p<0.05). The mean net income per Hanwoo steer in the treament groups was 84 to 65% higher than that of the control group. These results were attributed to both the feed cost savings and the increased BW by SMS supplementation.