Trenbolone

Abstract

Three feedlot experiments examined the effects of vitamin A (VA) on marbling and serum retinal (ROL) in Angus × Simmental cattle. Diets contained 2300 [low VA (LVA)] or 7250 IU VA/kg [high VA (HVA)]. In Exp. 1, 48 early weaned (51.0 ± 2.2 d) heifers (309.3 ± 7.0 kg; 12 pens) were fed ad libitum or were limit-fed 70 or 85% of ad libitum intake for 183 d then were fed for ad libitum intake for 149.3 ± 4.1 d. Vitamin A treatments were fed for 163.3 ±4.1 d, and heifers were harvested at compositional endpoints. Serum was collected three times during the experiment. Limit-fed heifers had improved (P

(Key Words: Beef Cattle, Vitamin A, Restricted Intake, Carcass Quality, Serum Retinol.)

Introduction

Understanding the mechanisms regulating adipocyte differentiation may enable producers to better manage cattle to maximize marbling deposition (Kawada et al., 1996). Kawada et al. (1990) reported vitamin A (VA) affected terminal differentiation of adipocytes. Japanese researchers established that serum retinol (ROL) concentration was negatively correlated with beef marbling (Nakai et al., 1992; Oka et al., 1992; Torii et al., 1996). Steers consuming low vitamin A (LVA) diets had less ROL concentrations (4 to 6 mo preharvest) and produced more desirable marbling scores than did steers consuming greater VA diets (Oka et al., 1998; Adachi et al., 1999; Nade et al., 2003). Researchers hypothesize excessive circulating ROL (5 to 50 m) may contribute to lesser quality grade (QG) via the inhibition of adipocyte differentiation. Restricting intake has reduced feed cost, manure output, visceral mass, and maintenance requirement and has improved digestibility and feed efficiency (Galyean, 1999). However, restriction of 5 to 15% has generally resulted in reduced ADG and carcass quality, increased carcass leanness, and/or increased days on feed to attain similar fat composition as counterparts fed ad libitum (Galyean, 1999). Previous research has linked increased days on feed and total starch intake to improved intramuscular fat deposition (May et al., 1992; Dolezal et al., 1993; Van Koevering et al., 1995). Rate of gain to achieve maximal protein accretion varies among cattle types (NRC, 1996); therefore, level of restriction must be optimized to initiate lipogenesis. Nutrition and management strategies to enhance marbling, minimize subcutaneous fat, and maintain production efficiency may have a dramatic affect on carcass value and profitability. The objective of these experiments were 1) to determine the effects of dietary level of VA on performance, rate of marbling deposition, ROL status, and carcass characteristics in feedlot cattle; 2) to examine the relationship between ROL status and carcass quality; and 3) to evaluate the influence of limit feeding or programmed intake on performance, rate of marbling deposition, carcass characteristics, and days on feed to a constant backfat endpoint. We hypothesized that maintaining lesser dietary levels of VA may enhance marbling deposition and aid in producing high quality beef.

Materials and Methods

Three experiments were conducted at the University of Illinois Campus Beef Research Unit to examine the effects of dietary VA level on performance, carcass, and ROL traits in feedlot cattle. All animals were fed similar finishing diets, consisting primarily of whole shelled corn and corn silage and similar treatment levels of dietary VA (calculated values: 2300 or 7250 IU/kg). All animals were managed according to the guidelines recommended in the Guide for the Care and Use of Agriculture Animals in Agriculture Research and Teaching (Consortium, 1988). Experimental protocols were submitted and approved by the Institutional Animal Care and Use Committee.

Experiment 1. A 3 × 2 factorial design was used to evaluate the effects of limit feeding and VA level on performance, carcass, and serum traits of growing and finishing cattle. Forty-eight early weaned Angus × Simmental heifers (117.9 ± 0.4 kg) were randomly allotted to one of 12 pens (four head per pen) on June 5, 2001 so that mean initial pen BW was similar. Calves were weaned at 51.0 ± 2.2 d of age and vaccinated for infectious bovine rhinotracheitis (IBR), bovine viral diarrhea (BVD), parainfluenza-3 (PI3), bovine synctical respiratory virus (BSRV), leptospirosis, vibriosis (Cattlemaster 4 + VL5; Pfizer, Exton, PA), Pasteurella multodda (One Shot; Pfizer), and 7-way Clostridlum and Haemophilus somnus (Vision 7/Somnus; Bayer, Kansas City, MO). Prior to allotment, heifers were adapted to the experimental diet (Table 1). Limit-feeding treatments were 1) ad libitum, 2) 85% of ad libitum intake (LF-85), or 3) 70% of ad libitum intake (LF-70). Feed allotment for limit-fed heifers was calculated daily based on average ad libitum (treatment) intake from the previous day. Restricted intake was maintained for 183 d, at which time consecutive day BW were taken, and limit-fed heifers were adjusted to ad libitum intake over a 2-wk period. Dietary levels of VA were imposed on two pens of each limit-fed treatment on November 21, 2001 (d 169; 309.3 ± 7.0 kg). Previously, VA content of the experimental diet (d O to 169; 980 IU/kg) was formulated to minimize variation in ROL and liver retinol stores. Despite marginal levels of dietary VA, no animals exhibited deficiency symptoms during this time period. Dietary supplements (Table 2) were calculated to contain either 8250 (LVA) or 33,000 IU VA/kg (high VA; HVA). Total mixed rations containing LVA (2300 lU/kg) were formulated to meet VA requirements for growing and finishing beef cattle, whereas, HVA diets (7250 IU/kg) contained 3.3 times recommended levels (NRC, 1996). Feed samples were not analyzed to confirm formulated values; serum samples were utilized as a biological indicator of VA intake. Vitamin A treatments were maintained for 163.3 ± 4.1 d until harvest. Heifers were implanted with Component E-C with Tylan® (100 mg progesterone, 10 mg estradiol benzoate, and 29 mg tylosin tartrate; Vetlife, Overland Park, KS) (d 84) and Component T-H with Tylan® (200 mg trenbolone acetate and 29 mg tylosin tartrate; Vetlife) (d 248), successively. Serum samples were collected at the beginning (d 169), middle (d 248), and completion of dietary VA treatments for subsequent retinol analysis. Aloka® 500V (Corometrics Medical Systems, Inc., Wallingford, CT) ultrasound equipment with an Aloka® UST-5049-3.5-MHz transducer was used to monitor rate of marbling deposition among treatments. Images were interpreted using the CVI Scan Session Reporting software (Version 6.2b) in combination with Rib-O-Matic® software (Version 3.5; Critical Vision, Inc., Atlanta, GA). Serial ultrasound scans were collected by a trained technician at d 182, 248, and prior to harvest. Individual heifers were harvested at a common compositional 12th rib fat endpoint (1.07 ± 0.04 cm) as determined by real-time ultrasound. One heifer was removed from trial as a result of illness unrelated to treatment.

Experiment 2. Forty-two Angus × Simmental yearling steers (371.8 ± 0.8 kg) were utilized in a completely randomized design to evaluate the effects of dietary level of VA on performance, carcass traits, and ROL in feedlot steers. Calves were weaned (191.2 ± 3.7 d of age) in April 2001 and grazed on mixed endophyte-infected tall fescue and clover pastures through December at Dixon Springs Agriculture Center (Simpson, IL). Steers were transported to the University of Illinois Campus Beef Research unit in January 2002 and were fed a dry hay maintenance diet for 58 d. Steers were vaccinated for IBR, BVD, PI3, BSRV, leptospirosis, vibriosis (Cattlemaster 4 + VL5), Pasteurella multocida (One Shot), and 7-way Clostridium and Haemophilus somnus (Vision 7/ Somnus) prior to removal from pastures and were revaccinated at the initiation of the experiment. Animals were allotted by weight to six pens (seven head per pen; three pens per treatment) on March 6, 2002 so that initial pen weights were similar. Treatment diet (Table 3) was randomly assigned to pens, and dietary supplements (Table 4) were calculated to contain either 8250 (LVA) or 33,000 IU VA/kg (HVA), similar to Experiment 1. Feed samples were not analyzed to confirm formulated values; serum samples were utilized as a biological indicator of VA intake. Yearlings were adapted to the experimental diet over a 3-wk period at the initiation of the trial. All steers were implanted (d 0) with Component TES with Tylan® (120 mg trenbolone acetate, 24 mg estradiol, and 29 mg tylosin tartrate; VetLife), were fed experimental diets for ad libitum intake, and were harvested at 105 d. Serum samples were collected at the initiation (d 0) and completion (d 104) of the experiment. Ultrasound scans (Corometrics Medical Systems, Inc.) were collected by a trained technician on d 62 and 104 to monitor rate of marbling deposition between treatments.

Experiment 3. A 3 × 2 factorial design was used to evaluate the effects of programmed rate of BW gain and dietary VA level on performance, carcass, and serum parameters of growing and finishing cattle. One hundred forty-four early weaned Angus × Simmental steers (184.8 ± 1.6 kg) were randomly allotted to one of 18 pens (eight head per pen) and fed treatments for 280.1 ± 3.9 d. Intake treatments were: 1) ad libitum intake 2) programmed to gain 1.27 kg/d (PF-H) throughout; or 3) programmed to gain 0.91 kg/d for 140 d then fed ad libitum (PF-L). The amount of feed offered to program-fed steers was regulated according to NRC net energy equations for medium-frame steer calves (NRC, 1996). Program-fed intakes were adjusted every 14 d to meet the increasing energy needs for maintenance and growth (NRC, 1996). Intake adjustments made on weight days were based on actual growth, whereas adjustments between weigh days were made on assumed growth changes. Vitamin A treatment diets and supplements (LVA or HVA) were similar to Experiment 1; however, cattle were switched to diets and supplements fed in Experiment 2 at approximately 392 kg. Feed samples were not analyzed to confirm formulated values; serum samples were utilized as a biological indicator of VA intake. Calves were vaccinated for IBR, BVD, PB, BSRV, leptospirosis, vibriosis (Cattlemaster 4 + VL5), Pasteurella multocida (One Shot), and 7-way Clostridium and Haemophilus somnus (Vision 7/ Somnus) at Dixon Springs Agriculture Center and were weaned at 69.8 ±1.9 d of age. Calves were transported to the University of Illinois Campus Beef Research unit in January 2002. Prior to allotment, steers were fed a high grain diet for 58 d. Calves were implanted with Ralgro® (36 mg zeranol; Schering-Mallinckrodt, Union, NJ) on d 0 and Component TE-S with Tylan® (VetLife) at similar BW on d 140, 168, and 196 for the ad libitum, PF-H, and PF-L treatments, respectively. Serum samples were collected at the initiation, middle (186.7 ± 3.2 d), and completion of the experiment. Serial ultrasound scans (Corometrics Medical Systems, Inc.) were collected at 56-d intervals to monitor rate of marbling deposition among treatments. Individual steers were harvested at a common compositional 12th rib fat endpoint (1.05 ± 0.02 cm) as determined by real-time ultrasound. Two steers were removed from the trial as a result of illness unrelated to treatment, and one steer was harvested early because of injury.

Performance Data Collection. Two weights were collected on consecutive days at the initiation of each experiment and averaged to establish initial BW. Cattle were weighed (full weights; prior to feeding) at 28-d intervals during the feeding period. Dry matter intakes and orts were recorded on a daily basis. Pen BW gain and feed efficiency (gain:feed; G:F) data were calculated based on adjusted final BW and feed intake data. Adjusted final BW was calculated by dividing hot carcass weight (HCW) by the average dressing percentage.

Carcass Data Collection. All experimental cattle were harvested at a commercial packing facility, and carcass data were collected by trained university personnel. Animals were stunned via captive bolt pistol and exanguinated. Hot carcass weight was recorded on the day of harvest, while 12th-13th rib fat thickness, longissimus area, marbling score, and kidney, pelvic, and heart fat estimates were collected after a 24-h chill at -4°C. An image of the longissimus was made using chromatography paper, and grid measurements of the image were used to measure longissimus area. Yield grade (YG) was calculated according to the method of Taylor (1994). Quality grade was established based on subjective marbling score.

ROL Analysis. Blood samples (12 mL) were collected via jugular venipuncture into non-heparinized tubes. Samples were immediately removed from direct light and allowed to clot prior to chilling on ice. Within 6 h after collection, samples were centrifuged at 3000 × g for 20 min at 4°C. Serum was transferred into duplicate vials and stored at -20°C.

All analytical procedures were carried out under yellow lights, and argon was used for storage to minimize the breakdown and isomerization of ROL caused by light and oxygen. All solvents were HPLC quality. Solvents were filtered and degassed by sonification and vacuum before use. Butylated hydroxytoluene (0.1%) was used as an antioxidant for all extractions. Crystalline retinol was purchased from Fisher Scientific (Hanover Park, IL) and was used to create external standard curves to quantify retinol in samples.

Duplicate 1-mL samples of serum were pipetted into 12-mL test tubes. Ethanol (1 mL; containing 0.1% butylated hydroxytoluene) was added to precipitate protein in the samples. The mixture was extracted three times with hexane (1 mL). The combined extracts were transferred to an additional test tube, evaporated to dryness, flushed with argon, and stored at -20°C. Serum samples were reconstituted in 1.5 mL of methanol before an HPLC injection of 50 µL. All HPLC injections were performed with a Spectra Physics (Franklin, MA) AS3500 autosampler. All samples were analyzed using a Dionex (Sunnyvale, CA) DX300 HPLC system. Quantitative analysis of ROL was accomplished using a Vydac 201TP54 25-cm × 4.6-mm C-18 reverse phase column (5-µ pore) with matching guard (column). The mobile phase gradient of methanohacetonitrile was 60:40 at a flow rate of 1.5 mL/min, ramping to 70:30 by 5 min. A UV detector gradient pump propelled solvent through a detector at 325 nm. Detector output was integrated by Dionex Peaknet chromatography software version 5.2.

Statistical Analysis. Data were analyzed as a factorial arrangement of treatments within a completely randomized design (Experiment 1 and 3) or completely randomized design (Experiment 2) using the MIXED procedure (SAS Inst., Inc., Cary, NC). Pen served as the experimental unit for treatment analysis of feedlot performance, and individual animal served as the experimental unit for ROL and carcass parameters. Serum and ultrasound parameters were analyzed as repeated measures. All data variables were determined to be normally distributed, and differences in treatments were separated using t-tests. Restricted intake treatment, VA treatment, and their interaction were included in the model for both Experiments 1 and 3. No interaction was observed among intake and VA treatment; therefore, only main effects were presented. Linear and quadratic contrasts were evaluated for restricted intake treatments (Experiments 1 and 3). Differences in frequency distribution of QG that resulted from treatments were separated using chisquare of GENMOD (SAS Inst., Inc.). Serum retinol and marbling score were standardized to a common grand mean across experiments. Observations were combined to evaluate a correlation between ROL and marbling score using PROC CORR® (SAS Inst, Inc.).

Results and Discussion

Performance Traits. Dietary VA effects on performance traits of growing and finishing heifers from Experiment 1 are exhibited in Table 5. No differences (P>0.05) were observed between VA treatments for initial or final BW, gain, DMI, or G:F. Heifers fed LVA tended to require fewer days on feed (154.3 vs 169.8 d; P=0.09) to reach the desired back fat endpoint when compared with heifers fed HVA.

Limit-feeding effects on performance traits of growing and finishing heifers from Experiment 1 are displayed in Table 6. Initial BW did not differ (P>0.05) among intake treatments. However, BW at d 183 tended (quadratic; P=0.10) to decrease with severity of restriction. Final BW of heifers fed ad libitum was intermediate (quadratic; P0.05) days on feed to reach the desired back fat endpoint. Daily BW gain during the growing period (d 0 to 183) decreased linearly (P

Loerch (1990) found no differences in ADG, but improved feed efficiency in cattle that received a high-moisture corn-based diet at 72 or 87% of ad libitum intake (restricted diet). However, steer BW gain was compared with a corn-silage based control, and cattle were restricted for only 85 d (Loerch, 1990). Hicks et al. (1990) noted decreased ADG (7.4%), but improved feed conversion (8.4%) in yearling steers restricted to 85% of ad libitum intake throughout the finishing period. Heifers (Experiment 1) restricted to 70 or 85% of ad libitum intake had reduced growing-period ADG (22.8 and 8.7%, respectively), but improved growing-period G:F (6.6 and 4.9%, respectively). The LF-70 and LF-85 heifers exhibited compensatory growth during the finishing period and finished with superior cumulative G:F (7.7 and 8.2%, respectively) when compared with heifers fed for ad libitum intake. These data agree with previous research evaluating restricted-growth programs and subsequent finishing performance (Loerch, 1990; Murphy and Loerch, 1994; Sainz et al., 1995). Conversely, Wertz et al. (2001) reported limit-feeding heifers at 80% of ad libitum intake for ≥84 d negatively affected finishing-period performance.

There were no differences (P>0.05) in initial BW, final BW, BW gain, DMI, or G:F between VA treatments during Experiment 2 (Table 7). These data agree with the results of Experiment 1.

Dietary VA effects on performance traits of growing and finishing steers from Experiment 3 are displayed in Table 8. No differences (P>0.05) were observed between VA treatments for initial or final BW, days on feed, or DMI. These data agree with the results of Experiments 1 and 2. Steers consuming LVA had greater ADG (P

These data agree with previous research that did not observe differences in animal performance with graded levels of supplemental VA (Perry et al., 1966; Matsuzaki et al., 1998; Bryant et al., 2004). Perry et al. (1966) did not observe differences in rate of BW gain between controls and yearling steers supplemented with 4400 IU VA/kg DM or 30,000 or 60,000 IU A/d, nor did those researchers observe differences between heifers supplemented with 4400, 8800, or 13,200 IU VA/d while summer-grazing mixed pastures. Bryant et al. (2004) reported no differences (P>0.10) in final BW, ADG, DMI, or feed efficiency among yearling steers fed a 91% concentrate diet supplemented with 0, 1103, 2205, 4410, or 8820 IU VA/kg DM. Similarly, Matsuzaki et al. (1998) observed no differences in final BW, DMI, or feed efficiency among VA treatments. However, those researchers reported ADG was numerically greater (13%; P= 0.14) for steers supplemented with VA when compared with non-supplemented controls. Similarly, previous research has reported that cattle fed supplemental VA had greater ADG when control diets did not meet NRC requirements (Oka et al., 1998; Nade et al., 2003). In Experiment 3, all steers were fed adequate VA; however, a growth response to LVA was unexpected.

Programmed rate of BW gain effects on performance traits of growing and finishing steers from Experiment 3 are exhibited in Table 9. Initial BW did not differ (P>0.05) among intake treatments. By design, BW at d 140 decreased linearly (P

Daily feed intake during the growing period (d O to 140) was quadratic (P

Carcass Characteristics. Dietary VA effects on carcass traits of growing and finishing heifers from Experiment 1 are given in Table 10. No treatment differences (P>0.05) existed for HCW; backfat; longissimus area; kidney, pelvic, and heart fat; YG; marbling score; or percentages of carcasses grading low or average Choice or greater. Heifers were harvested at similar compositional endpoints. Heifers fed LVA displayed a numeric advantage in marbling score (P=0.18) and percentage grading premium Choice or greater (P=0.16).

Limit-feeding effects on carcass parameters of growing and finishing heifers from Experiment 1 are exhibited in Table 11. Carcass weight was quadratic (P0.05) were observed among treatments for kidney, pelvic, and heart fat. Yield grade was linear (P

Dietary VA effects on carcass parameters of yearling steers from Experiment 2 are exhibited in Table 12. Yearling steers were harvested at similar (P>0.05) backfat compositional endpoints. No treatment differences (P>0.05) existed for HCW; longissimus area; kidney, pelvic, and heart fat; YG; marbling score; or percentages of carcasses grading low or premium Choice or greater. The percentage of carcasses grading low Choice or better was numerically larger (14.3%) for steers fed LVA; however, this trend was not noticed for carcasses grading premium Choice or greater. Dietary VA effects on carcass parameters of growing and finishing steers from Experiment 3 are presented in Table 13. No treatment differences (P>0.05) existed for HCW; kidney, pelvic, and heart fat; marbling score; or percentages of carcasses grading low or premium Choice or greater. Steers fed LVA tended (P=O. 10) to be harvested at leaner backfat endpoints than cattle fed HVA. Similarly, LMA tended (P= 0.07) to be greater for LVA treatments. Significant or numerical trends for greater HCW and longissimus area, and lesser backfat and kidney, pelvi, and heart fat measurements resulted in less (P

Previous researchers have suggested that cattle consuming diets with lesser VA had greater intramuscular fat deposition compared with animals consuming high VA diets (Oka et al., 1998; Adachi et al., 1999; Nade et al., 2003). Oka et al. (1992) concluded that VA level negatively influenced adipocyte hyperplasia early in the fattening stage, indicating that some period of maturation is necessary to detect adipocytes as marbling. The authors concluded that the low dietary level of VA and carotenoids stimulate bovine marbling development. In the current experiments, LVA did not lead to greater (P>0.05) rate of marbling deposition or QG outcome. We recognize, based on the limited number of observations from Lxperiments 1 and 2, that these data are not conclusive to disqualify the influence of dietary VA on marbling score. These data do suggest that feeding VA at three times NRC recommendations does not decrease the rate of marbling deposition. However, these data agree with others, who have demonstrated no differences in QG between supplemented and non-supplemented VA groups (Kohlmeier and Burroughs, 1970; Matsuzaki et al., 1998; Bryant et al., 2004). Supplemental levels of VA (7500 IU/kg fresh matter or 8571 IU/kg DM) reported by Matsuzaki et al. (1998) were comparable with the levels formulated in our HVA diets and yielded similar results. Similarly, Bryant et al. (2004) reported that dietary supplemental VA concentrations between O and 8820 IU/kg DM did not induce differences (P>0.10) in marbling score or percentage of carcasses grading Choice, Select, or Standard.

These data agree with previous research that did not show differences in other carcass traits among graded levels of supplemental VA (Matsuzaki et al., 1998; Oka et al., 1998; Bryant et al., 2004). Matsuzaki et al. (1998) and Bryant et al. (2004) reported no differences (P>0.10) in HCW, longissimus area, or backfat among treatment levels of VA. In Experiment 3, the improved YG in response to LVA was unexpected; however, Matsuzaki et al. (1998) noted a numerical increase in carcass leanness (7%; P= 0.12) in control steers when harvested at a constant age.

Programmed rate of BW gain effects on carcass parameters of growing and finishing steers from Experiment 3 are given in Table 14. Carcass weight was linear (P0.05) were observed among treatments for backfat, longissimus area, or YG. These data agree with previous research reporting similar carcass characteristics among programfeeding strategies (Eoerch and Fluharty, 1998; Rossi et al., 2001). Cattle were harvested at similar compositional endpoints. Program-fed cattle tended (P=0.08) to have greater kidney, pelvic, and heart fat compared with the carcasses of cattle fed ad libitum. Marbling score was linear (P

ROL Concentration. For Experiment 1, dietary VA treatment effects on serum parameters are exhibited in Table 10. Mean ROL concentrations for LVA and HVA groups were not different (P>0.05) at the initiation of the experiment. The ROL concentrations increased in both dietary treatment groups; however, intermediate (d 79) and final serum samples were greater (P

For Experiment 2, dietary VA treatment effects on serum parameters are exhibited in Table 12. No treatment differences (P>0.05) existed for initial ROL concentration. The ROL concentrations increased in both dietary treatment groups during the finishing period; however, no differences (P>0.05) were observed for final ROL concentrations among treatments.

For Experiment 3, dietary VA treatment effects on ROL are presented in Table 13. Mean ROL concentrations for LVA and HVA groups were not different (P>0.05) at the initiation of the experiment. Intermediate (d 186.7 ± 1.1) and final ROL values were greater (P

Past research has shown that steers consuming low VA diets exhibited proportionally lesser blood VA levels (Matsuzaki et al., 1998; Oka et al., 1998; Nade et al., 2003). Matsuzaki et al. (1998) noted plasma retinol concentration decreased with time in steers consuming low VA diets but remained unchanged in cattle consuming high VA diets, creating differences (P

Relationship Between ROL and Marbling Score. Correlation coefficients between ROL samples and final marbling score are exhibited in Table 15. No relationship (P>0.05) between ROL status (initial, intermediate, or final concentrations) and carcass quality was observed. Initial ROL concentration was moderately correlated with intermediate and final ROL levels. Similarly, intermediate ROL concentration was moderately correlated with final ROL level.

These data agree with Matsuzaki et al. (1998), who reported neither mean plasma retinol level during the whole experimental period (15 to 26 mo of age) nor the end of the trial (25 to 26 mo of age) were related to lipid content of the longissimus thoracis, carcass composition, or carcass dissectible fat distribution. Conversely, previous research illustrated ROL concentration was inversely correlated (range; r = -0.20 to -0.71) with beef marbling index (Nakai et al., 1992; Torii et al., 1996; Chae et al., 2003). Based on previous research, we hypothesized that a threshold level of ROL may be necessary to inhibit adipocyte differentiation and suppress marbling expression. Researchers noted a depression in marbling score of cattle consuming high levels of VA when ROL exceeds 300 ng/mL in the months preceding harvest (Oka et al., 1998; Nade et al., 2003). Oka et al. (1998), Adachi et al. (1999), and Nade et al. (2003) observed differences in quality outcome when VA-supplemented or high marbling score cattle had ROL >300 ng/ mL. However, Oka et al. (1998) found no response in QG when cattle consuming high VA diets had ROL concentrations below this threshold. Serum retinol levels in the current experiments were

Recent research examining the relationship between marbling and ROL has been primarily evaluated in Korean Hanwoo and Japanese Black cattle, specifically Wagyu and Tajima strains. To date, little consideration has been given to response differences based on cattle breed or type. Torii et al. (1996) observed the strongest relationship between ROL and carcass quality in Wagyu or Wagyu × Holstein steers and heifers (r = -0.71), while Chae et al. (2003) reported the weakest correlation in Hanwoo steers (r = -0.20). In the US, Bryant et al. (2004) observed no treatment differences in performance or carcass traits utilizing single-source black, yearling steers. In the current experiments, no relationship was observed in Angus × Simmental steers and heifers. Differences in lipid metabolism between breed types likely exist and may contribute to variation in VA treatment response. Further evaluation is necessary to quantify the relationship between beef quality and retinol status among beef breeds utilized in US production systems.

Furthermore, previous nutrition and management would likely have an effect on the relationship between ROL and liver retinol concentrations and marbling outcome. Japanese production systems utilize roughagebased, low energy diets over a longterm feeding period, often harvesting cattle at a minimum of 24 to 36 mo of age. In the current experiments, cattle were fed grain-based, high energy diets for 105 to 332 d and harvested at 14 to 22 mo of age. Dietary energy substrates may override the effect of retinol on adipocyte differentiation. Additionally, no differences were found in the current studies between early weaning and yearling management systems common in the US. Still, Kohlmeier and Burroughs (1970) evaluated plasma VA levels of cattle coming off of wheat pasture or grass-legume hay diets. Many lush forages and pastures have 100,000 to 300,000 IU/kg VA activity (NRC, 1989). Surplus levels of VA can be stored in liver reserves for later distribution. Cattle grazing wheat pasture had greater plasma VA concentrations (340 vs 230 ng/mL) at placement, requiring 84 d to diminish levels to placement concentration of hay-fed steers (Kohlmeier and Burroughs, 1970). These data are comparable with the results of Experiment 2, where yearling steers consuming a dry-hay diet for 58 d prior to allotment had low initial ROL (275.5 ± 13.4 ng/mL). Differences in plasma VA concentration existed between treatments until d 140 in the feedlot (Kohlmeier and Burroughs, 1970). Those reseachers noted depletion rates of 1.5 ng/mL plasma per d. The rate at which liver VA reserves were depleted (50% every 28 d) suggested that yearling cattle entering the feedlot with medium reserves (20 to 40 µg VA/g liver), either from natural feeds consumed previously or by injection, need little or no VA for periods of 90 to 120 d (Kohlmeier and Burroughs, 1970; Kirk et al., 1971). Nonetheless, those researchers did not observe differences in final QG related to previous nutritional treatment. Further research is necessary to elucidate the timing, intake level of VA and derivatives, and circulating level of ROL necessary to elicit an effect on beef quality.

Implications

Understanding the mechanisms regulating adipocyte differentiation may enable producers to better manage cattle to maximize marbling deposition. Dietary intakes of VA at 3.3 times NRC recommendations during the feedlot-finishing period increased circulating retinol, but did not affect carcass quality in Angus × Simmental feedlot cattle. Rate of BW gain to achieve maximal protein accretion varies among cattle types; therefore, level of restriction must be optimized to initiate lipogenesis. Intake restriction improved feed efficiency, but tended to reduce marbling and YG. Program-feeding steers slightly improved growing-period feed efficiency, but reduced cumulative BW gain and increased days on feed to reach the compositional endpoint. Program-feeding rate of BW gain did not alter rate of marbling deposition, but added days on feed improved carcass quality outcome. Further investigation is needed to examine manipulation of dietary VA levels and the potential of restricted-intake strategies to improve production efficiency and beef quality.

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N. A. PYATT, L. L. BERGER1, PAS, and T. G. NASH

Department of Animal Sciences, University of Illinois, Urbana 61801

1 To whom correspondence should be addressed: llberger@uiuc.edu

Copyright American Registry of Professional Animal Scientists Aug 2005
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