CASE STUDY: Feeding Ractopamine Hydrochloride to Cull Cows: Effects on Carcass Composition, Warner-Bratzler Shear Force, and Yield

INTRODUCTION

Cull cows, a byproduct of the beef cow-calf industry, can represent 10 to 20% of a producer's income (Sawyer et al., 2004). Often times cull cows are marketed due to health or structural related problems or because they did not conceive. Cull cows axe typically marketed when they are in poor body condition, resulting in less than optimal revenues at sale.

Previous research has shown that feeding cull cows a concentrate diet will increase hot carcass weights (HCW), longissimus muscle area, intramuscular fat (IMF), subcutaneous fat, and fat-free soft tissue (Wooten et al., 1979; Matulis et al., 1987; Miller et al., 1987; Faulkner et al, 1989; Cranwell et al., 1996b; Schnell et al., 1997; Gonzalez et al., 2007; Jurie et al., 2007; Stelzleni et al., 2007). Supplementation with a type I ß-adrenergic receptor agonist (ractopamine-HCl; RAC) increases protein accretion 25 to 30% at the expense of fat storage (Mersmann, 1998; Bridge et al., 1998). Also, Eisemann et al. (1988) reported that dosage level and dose duration of ß agonists can influence receptor expression.

The objective of this study was to determine whether feeding increased dietary levels of RAC for 30 d will increase lean accretion and decrease fat accretion, therefore increasing total lean yields of cull cows.

MATERIALS AND METHODS

Experimental Animals

Two truckloads (n 48 each; 10.5 ± 1.2 yr) of cull crossbred cows (predominately Beefmaster and Brangus) were transported from a commercial cow-calf operation in south Florida (Okeechobee, FL) to a designated feeding facility in Williston, FL. Upon arrival, cows were weighed and treated with a pour-on endectocide (Dectomax Pfizer Inc., New York, NY). Cull cows were randomly assigned to 1 of 4 treatments for the final 30 d of feeding consisting of control (CON; no supplemental RAC), RAC 100 (100 mg RAC/d per head), RAC 200 (200 mg RAC/d per head), and RAC 300 (300 mg RAC/d per head). Each treatment group was penned and fed together ad libitum from a self-feeder for a total of 48 d including RAC supplementation. The basal diet was comprised of the following ingredients: soybean hulls, citrus pulp, cracked corn, wheat middlings, cottonseed hulls, cottonseed meal, molasses, tallow, and urea. The diet contained approximately 87.6% DM, 14% CP (DM basis), and 79.5% TDN. On d 49, cows were transported to a commercial slaughter facility (Central Packing, Center Hill, FL) and harvested in a conventional manner.

Muscle Sample Preparation and CIE (Comission Internationale de l'Eclairage) L*, a*, and b* Color

At 24 h postmortem, carcasses were fabricated and 9 muscles [adductor; gracilis (GRA); infraspinatus (INF); longissimus thoracicus (LM); rectus femoris (REF); semimembranosus (SMB); teres major (TEM); triceps brachii, lateral and long heads (TRB, TRBlat and TRBtong); and vastus lateralis] were removed and transported to the University of Florida's Meat Processing Center (Gainesville, FL). Commodity (0.64 cm fat trim level) weights were recorded from the selected muscles, which were then trimmed to a zero (0.0 cm) fat level to determine denuded weights. During the denuding process, the TRB was further separated in lateral and long heads. The 9-10-llth-rib section was removed according to Hankins and Howe (1946) for compositional analysis. Each muscle was then wet-aged at 3.8 ± 20C for 14 d in a cryovac B2570T bag (Sealed Air Corp., Duncan, SC) and then was frozen at -4O0C. Steaks 2.54 cm thick were cut frozen from the anterior end of each muscle, with the exception of the TEM and GRA, which were utilized whole due to size limitations for Warner-Bratzler shear force (WBSF) determination. The CIE L*, a*, and b* color scores were captured with a Minolta Chroma meter (Model CR310, Minolta Corp., Ramsey, NJ) 24 h postmortem to determine if objective lean color changes occurred due to treatment.

Ether Extraction and WarnerBratzler Shear Force

Ether extraction was performed on the soft tissue of the 9-10-llth rib section to estimate compositional changes of the carcass. Ether extraction was performed on a sample of the LM to determine percent IMF of the LM. Steaks for WBSF determination were thawed for 18 h at 3.8 ± 2°C and then broiled on a Hamilton Beach HealthSmart 317.5 cm2 grill (Hamilton Beach- Proctor Silex Inc., Washington, NC) to an internal temperature of 7O0C. The steaks were turned once at 35°C during cooking. Internal temperature was monitored using a copper-constantan thermocouple placed in the geometric center of the steak, which was attached to a temperature recorder. The steaks were then chilled for 18 h at 3.8 ± 20C in preparation for WBSF core extraction. Six 1.27 cm cores were extracted from each steak parallel to the muscle fiber orientation. Each core was then sheared utilizing a WBSF device (crosshead speed = 200 mm/ min) attached to an Instron Universal Testing machine (Model 1011, Instron Corp., Canton, MA).

Statistical Analysis

The study was designed as a randomized complete block design with individual animal as the experimental unit. Data was analyzed using PROC MIXED, Least Squares Means procedure of SAS (SAS Inst. Inc., Cary, NC) with a significance level of P < 0.05 and trends indicated with an equal sign. Random effects in the model included animal and muscle within animal. Fixed effects in the model were treatment group and muscle within treatment group. There was a significant treatment ? muscle interaction for WBSF; therefore, means will be presented by muscle across each treatment group for this variable.

RESULTS AND DISCUSSION

Carcass Traits

No differences (P > 0.05) were observed for HCW or dressing percent (Table 1). Kutzler et al. (2006) reported no differences in either HCW or BW between controls and RAC fed cull cows (200 mg/d per head), which agrees with this study. In contrast, Talton et al. (2006) reported RAC (200 mg/d per head) feeding tended to increase HCW in fed heifers; these findings suggest that mature animals may not be as responsive to a ß-agonist compared with younger animals. Adjusted preliminary YG, a measure of subcutaneous fat deposition (Table 1), was similar (P > 0.05) across all treatment groups. Significant differences (P < 0.05) were observed for commodity weights in the clod, SMB, sirloin tip, and the top round, but when expressed on a percentage of HCW, no significant (P > 0.05) differences were observed (data not shown). Talton et al. (2006) reported similar results in RAC fed heifers. Feeding RAC did not (P > 0.05) alter the HCW percentages for any of the individual denuded muscles evaluated in this study.

Ribeye area (REA; Table 1) was similar (P > 0.05) for all treatment groups. These findings agree with Holmer et al. (2006), but are in contrast with Schroeder et al., (2003b) who reported increased (P < 0.05) REA in steers and heifers fed 300 mg RAC/d per head. When REA is expressed per 100 kilograms of HCW (indicator of muscling), all treatment groups were similar (Table 1).

9-10-llth Rib Composition

Differences (P < 0.05) were observed in percent fatfree lean of the 9-10-11th rib section (Table 2), an indicator cut used to relate body composition changes between treatments. The RAC 300 treatment was 6.3% higher in fat-free lean compared with the CON treatment, and the RAC 100 and RAC 200 treatments were intermediate of the CON and RAC 300 treatments. With higher levels of RAC, lean accretion is generated at a faster rate than fat deposition. This would be expected considering RAC is a ß-adrenergic agonist that redirects nutrients away from fat production and applies nutrients toward lean accretion (Eisemann et al, 1988; Miller et al., 1988; Mersmann, 1998).

Carcass Quality

Percent IMF, marbling, color, texture, and firmness score was not significantly different (P > 0.05) among the treatment groups (Table 3). These results agreed with previous studies conducted on heifers (Schroeder et al, 2003b; Talton et al., 2006), with the exception of the Schroeder et al. (2003b) study which reported improvements (P < 0.06) in muscle color based on a subjective acceptability muscle color scale for heifers treated at all (100, 200, or 300 mg/d per head) RAC dose levels. The CIE L*, a*, and b* color scores (Table 3) were not (P > 0.05) different when CON treatment was compared with RAC treatments.

Warner- Bratzler Shear Force

Empirical data on WBSF reveals that RAC affects individual muscles (Table 4) rather than specific muscle groups such as muscles of locomotion or muscles of support. The INF, REF, SMB, and TRB^sub lat^ all had significant (P < 0.05) differences for WBSF across treatment groups. WarnerBratzler shear force values for adductor, GRA, LM, TEM, TRB^sub long^ and vastus lateralis were not significantly (P > 0.05) affected by RAC treatment. The INF decreased (P < 0.05) in tenderness with RAC treatment compared with CON. In contrast, with RAC supplementation the SMB was more (P < 0.05) tender for the RAC 100 treatment level compared with CON treatment. The REF and TRBlat experienced interactions between RAC treatments. The REF in the RAC 100 group had less (P < 0.05) tender WBSF values compared with the RAC 200 group, which had the most tender WBSF values of all treatment groups. The TRBlat was similar, indicating that the RAC 100 group produced less (P < 0.05) tender WBSF values in comparison to the RAC 300 group. Schroeder et al. (2003a) reported WBSF values increased in the LM from steers and heifers as RAC dosage increased. The data for the REF and TRB^sub lat^, do not support the Schroeder et al. (2003a) findings. Shear values are independent of fat, which would eliminate cold shortening as a potential explanation. Possibly protein turnover diluted the existing crosslinked collagen content in the RAC 200 and RAC 300 treatment groups compared with the RAC 100, which would have more crosslinked collagen causing the RAC 100 treatment to be tougher in muscles of locomotion. Cranwell et al. (1996a,b) reported implanted fed cull cows with higher lean gains had more soluble collagen compared with control cows. Those findings do support those of the RAC 300 group due to the 6% increase in fat-free lean compared with the CON group; however, this is only speculation because the present study did not investigate collagen content.

IMPLICATIONS

Feeding RAC at the 100, 200, or 300 mg/d per head to cull beef cows for the last 30 d on feed had little effect on carcass characteristics compared with CON treatment and had inconsistent effects on WBSF values, but RAC did increase total fat-free lean percentage. Because some of the results found from this study were insignificant, it would not be advantageous for producers to feed RAC to cull cows at the levels and time period used in this study. However, the 6% increase of fat-free lean in the RAC 300 group would prove to be a plus for the packer producing lean trimmings intended for further processing.

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