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The effects of cattle sex on carcass characteristics and longissimus muscle palatability^1,2

W. T. Choat^*, J. A. Paterson^*,3, B. M. Rainey^*, M. C. King^*, G. C. Smith, K. E. Belk and R. J. Lipsey

^* Department of Animal and Range Sciences, Montana State University, Bozeman 59717-2900 and Department of Animal Sciences, Colorado State University, Fort Collins 80523-1171 and American Simmental Association, Bozeman, MT 59718-9733

	Abstract

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Two experiments were conducted to determine the effects of sex on carcass traits and cooked beef steak palatability. In Exp. 1, steers (n = 99), heifers (n = 51), and intravaginally spayed heifers (n = 46) were fed a high-energy diet for 161 d. No implants were administered, and heifers were not fed melengestrol acetate to suppress estrus. In Exp. 2, 60 steers and 60 intact heifers from the same ranch source used in Exp. 1 were fed in 2 locations (sites 1 and 2). All management factors were equal across experiments except that intact heifers were fed melengestrol acetate to suppress estrus in Exp. 2. Steers in Exp. 1 were 25 kg heavier (P < 0.01) in HCW than heifers at comparable (P = 0.39) carcass fat thickness. Spayed heifers (Exp. 1) had a 5.7% smaller (P < 0.05) LM area compared with steers and intact heifers, which were similar. In Exp. 2, there was no difference (P = 0.2) in carcass weight, and heifers had greater (P < 0.01) 12th rib fat thickness compared with steers. Calculated yield grades were similar (P = 0.21) among treatments in Exp. 1 and tended (P = 0.08) to be greater for heifers compared with steers in Exp. 2. In Exp. 1, USDA quality grades and marbling scores were lower (P < 0.01) for steers compared with intact and spayed heifers, which were similar. The effects of sex on tenderness were examined at a common level of fat-thickness and marbling by covariate analysis. Steaks from steers, compared with those from nonimplanted, intact heifers, in the 2 experiments combined were: (a) superior (P < 0.05) in 2 of 9 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 6 of 9 palatability assessments when marbling score was adjusted to a common level. In Exp. 1, steaks from nonimplanted steers compared with those from nonimplanted spayed heifers were: (a) superior (P < 0.05) in 0 of 8 palatability assessments when subcutaneous fat thickness (at the 12th rib) was adjusted to a common level, and (b) superior (P < 0.05) in 3 of 8 palatability assessments when marbling score was adjusted to a common level. These findings suggest that sex should be added to the list of antemortem factors contributing to variation in cooked beef steak tenderness. However, more research is needed to precisely identify those factors contributing to the lower tenderness observed for steaks from heifer carcasses.

Key Words: heifer • palatability • steer

	INTRODUCTION

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The success of branded beef programs that guarantee tenderness has strengthened the belief that consumers are willing to pay greater prices for greater tenderness. Unfortunately, a significant portion of beef cuts are unacceptable in tenderness (Morgan et al., 1991). Several antemortem factors, such as genetics (Wulf et al., 1996; O’Connor et al., 1997), growth implants (Platter et al., 2003b), and age at slaughter (Shackelford et al., 1995), have been shown to affect beef tenderness. Moreover, many studies have evaluated different postmortem strategies for improving beef tenderness problems that developed antemortem. Strategies such as postmortem aging (Smith et al., 1978) and electrical stimulation (McKeith et al., 1981) have been shown to have positive effects on beef tenderness. By understanding the antemortem factors that detract from beef tenderness as well as the strategies available to offset some of these problems, it is possible to minimize the number of beef cuts that are unacceptable in tenderness. However, not all antemortem factors have been clearly evaluated for their effects on tenderness. One such factor is sex. Some studies have reported no difference in tenderness between steaks from steers and heifers (Gracia et al., 1970; Zinn et al., 1970; Prost et al., 1975); however, too little background information on the cattle used in each experiment was provided. Therefore, the objective of these 2 experiments was to test the hypothesis that strip-loin steaks from nonimplanted steers and heifers from similar genetic backgrounds are not different in tenderness and to define benchmark differences, similarities, or both in carcass characteristics of steers and heifers.

	MATERIALS AND METHODS

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Experiment 1
Animals, Experimental Treatments, and Cattle Management.
Animal procedures were approved by Montana State University’s Animal Care and Use committee under protocol number AA001. Ninety-nine steers and 97 heifers managed in a similar manner from birth to slaughter were used to evaluate the effects of sex on carcass characteristics and cooked beef steak palatability. Steers and heifers were randomly selected from a single Angus-based cowherd in northwestern Montana, which used 2 sire breeds (Angus, steers = 50, heifers = 43; Simmental, steers = 49, heifers = 54). Calves were weaned in October, 2001, backgrounded (Montana State University, 2004) for 45 d, and transported to a commercial feedlot in south-central Montana. Upon arrival at the feedyard, 19 Angus and 27 Simmental heifers were spayed (intravaginal method) by a large-animal veterinarian.

After a short growing phase on a corn silage-based starter ration, intact and spayed heifers were placed on a gradual adaptation program using 3 adaptation diets, each fed for 8 d. In contrast, steers were rapidly adapted to the finishing diet (9.92% corn silage, 64.6% corn grain, 20.8% wheat midds, and 4.7% protein supplement, DM basis) using diets similar to those used to adapt heifers, each fed for 4 d. Steers were more aggressively fed to increase the likelihood that steers and heifers would have similar levels of subcutaneous fat at comparable slaughter ages. Steers and heifers were group-fed for 161 d in 2 pens, with steers being fed separately from heifers. No implants were administered, and heifers were not fed melengestrol acetate to suppress estrus. Steers and heifers were slaughtered when the average fat thickness of the 196 cattle reached 10 mm over the LM at the 12th rib (measured using real-time ultrasound).

Carcass Data Collection and Sampling Methods.
At shipping, steers and heifers were weighed in separate groups on a platform scale, transported approximately 617 km to a commercial beef packing plant, and slaughtered using conventional procedures. Carcasses were chilled in a cooler with an air temperature of 2°C for 36 h, and sprayed intermittently (2 min on, 8 min off) with a fine mist of 2°C water for the first 8 h of the chill period. After the carcass-chilling period, a commercial carcass data collection service evaluated and recorded measurements/assessments of HCW, USDA Quality Grade, USDA Yield Grade, fat thickness, LM area, percentage of KPH, and marbling score for each carcass.

Strip loins (IMPS 180; USDA, 1988) from the right sides of each of the 196 carcasses were collected after fabrication and transported immediately to the Colorado State University Meat Laboratory. There, each strip loin was fabricated into 3 sections (cranial, middle, and caudal). The cranial, middle, and caudal sections were each placed in a vacuum-sealed bag and aged at 2°C for 7, 14, or 21 d postmortem, respectively. After reaching the appropriate length of aging time, sections were frozen and stored at –20°C. Frozen sections were then fabricated into steaks (2.54-cm thick) using an AEW-Thurne band saw (model 400 m, AEW Engineering Co., Ltd., Norwich, UK).

The cranial section (aged for 7 d) was cut to provide 2 steaks for shear force and fatty acid analysis (data not presented here), the middle section (aged for 14 d) was cut to provide 3 steaks for shear force and sensory panel analysis, and the caudal section (aged 21-d) was cut to provide 1 steak for shear force analysis. Upon completion of fabrication and cutting, each steak was identified, placed in a vacuum-sealed bag, sorted for intended use, and returned to frozen storage (–20°C). Samples used for sensory analysis were stored for approximately 135 d at frozen temperatures.

Tenderness Measurements.
For shear force determination, frozen steaks were thawed at approximately 2°C for 24 h and cooked using a belt grill (model TBG-60, Magikitch’n, Quakertown, PA) set to cook steaks to an internal temperature of 70°C (settings: top heat = 177°C, bottom heat = 177°C, preheat = disconnected, height = 1.85 cm, cook time = 6.35 min). Final internal temperatures were monitored using a handheld thermometer (model HH21, Omega Engineering Inc., Stamford, CT).

Cooked steaks were allowed to equilibrate to room temperature (22 to 25°C), and 6 to 8 cores (1.27 cm in diameter) were removed from each steak parallel to the muscle fiber orientation. Each core was sheared once, perpendicular to the muscle fiber orientation, using an Instron load frame (model 4443, Instron Corporation, Canton, MA) fitted with a Warner-Bratzler shear head (settings: crosshead-speed = 200 mm/min, load cell capacity = 100 kgf). A single peak shear force was calculated using series IX software (Instron Corporation) for each core. Individual-core, peak shear force values were averaged to assign a mean peak shear force value to each steak.

Trained Sensory Evaluation.
Two steaks from each carcass, aged for 14 d postmortem, were used for sensory evaluation. Individual steaks were thawed and cooked using the same procedures used in preparing samples for shear force measurements. Warm samples (1.3 x 1.3 x 2.5 cm) from each steak were evaluated by an 8-member, trained sensory panel. Panelists were trained for 2 wk according to procedures outlined by AMSA (1995). Specifically, panelists were selected based on previous experience and desire to participate. Selected panelists were given instructions relative to the sensory attributes to be evaluated and then presented example products representing the attributes across the scale being utilized for each attribute.

During training and actual sensory analysis, panelists were seated in a room with dim red lighting, separated by booths; that was separate from the sample preparation area. Panelists used room temperature distilled water and unsalted crackers for pallet cleansing, and pallet cleansing was required between each sample. Panelists were presented samples by session; each session contained no more than 12 samples, and no more than 2 sessions were conducted per day. Once presented, panelists assigned scores to each steak sample for juiciness, muscle fiber tenderness, connective tissue amount, overall tenderness, and flavor intensity using 8-point, structured rating scales (AMSA, 1995).

Predicted Probability of Consumer Acceptance of Steaks Based on Shear Force and Marbling Score.
The predicted probability of consumer acceptance of steaks from steers and intact or spayed heifers, based on individual marbling score and shear force value, were calculated using equations developed by Platter et al. (2003a).

Statistical Analysis.
Analyses of live-animal performance traits was negated due to a lack of pen replication and due to spayed and intact heifers being fed in the same pen. Descriptive statistics of animal performance are reported in Table 1. Analysis of carcass characteristics, cooked beef steak palatability measurements, and predicted consumer acceptance were conducted using the least squares, mixed models procedure of SAS (SAS Inst. Inc., Cary, NC). Individual animal was used as the experimental unit in all analyses. The statistical model included treatment (sex) as the independent fixed effect, and sire-ID nested in sire-breed was added to the ANOVA as a random effect. Marbling score was added as a covariate in certain analysis of tenderness measurements because beef is purchased on a common quality grade basis. When F-tests were significant (P < 0.05), least squares analysis with Tukey’s adjustment was used to separate least squares means.

Carcass Data Collection and Sampling Methods.
Equal numbers of steers and heifers from each site were slaughtered using conventional procedures on 2 separate dates. Steers and heifers fed at site 1 were transported 617 km, whereas steers and heifers from site 2 were transported approximately 30 km. Carcasses were chilled in a manner comparable to that used for carcasses in Exp. 1. After the carcass-chilling period, 2 expert graders evaluated and recorded measurements/assessments of HCW, USDA quality grade, USDA yield grade, fat thickness, LM area, percentage of KPH, skeletal maturity, lean maturity, and marbling score for each carcass.

Strip loins (IMPS 180; USDA, 1988) from the right sides of each of the 120 carcasses were collected after fabrication and transported immediately to the Colorado State University Meat Laboratory. There, each strip loin was placed in a vacuum-sealed bag and aged at 2°C for 14 d. After reaching the appropriate length of aging, strip loins were frozen and stored at –20°C. From each frozen strip loin, 1 steak (2.54-cm thick) was removed from the cranial end using an AEW-Thurne band saw (AEW Engineering Co., Ltd.) for subsequent Warner-Bratzler shear force determination.

Warner-Bratzler Shear Force.
All procedures used for determination of Warner-Bratzler shear force were identical to those used in Exp. 1.

Predicted Probability of Consumer Acceptance of Steaks Based on Shear Force and Marbling Score.
The predicted probability of consumer acceptance of steaks from steers and intact heifers, based on individual marbling score and shear force value, was calculated using equations developed by Platter et al. (2003a).

Statistical Analysis.
Analyses of live-animal performance traits was negated because of a lack of pen replication and because of spayed and intact heifers being fed in common pens. Descriptive statistics of animal performance are reported in Table 2. Analysis of carcass characteristics, cooked beef steak shear force values, and predicted consumer acceptance were conducted using the least squares, mixed model procedure of SAS. In all analyses, individual animal was used as the experimental unit. For analysis of carcass traits, slaughter date x sex was added along with sire-ID (sire-breed) as random effects because not all animals were slaughtered on the same day.

	RESULTS

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Experiment 1
Carcass Characteristics.
Steers, compared with intact heifers, had (P < 0.05) heavier carcasses, with smaller LM areas per carcass weight, and lower marbling scores and quality grades (Table 3). Steers, compared with spayed heifers, had (P < 0.05) larger actual LM surface areas and lower marbling scores and quality grades. Intact heifers, compared with spayed heifers, (a) had larger (P < 0.01) actual and carcass-weight adjusted LM surface areas, and (b) had more desirable (P = 0.02) yield grades.

Shear Force and Trained Sensory Panel Ratings at a Common Level of Carcass Fat Thickness (Table 4).

Shear Force and Trained Sensory Panel Ratings at a Common Marbling Score (Table 5).

Predicted Probability of Consumer Acceptance of Steaks Based on Shear Force and Marbling Score (Table 6).

Carcass Characteristics and Warner-Bratzler Shear Force (Table 7).

Predicted Probability of Consumer Acceptance of Steaks Based on Shear Force and Marbling Score (Table 8).

	DISCUSSION

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Carcass fatness measured at the 12th rib was similar between steers and heifers in Exp. 1; however, heifers had greater 12th rib fat thickness compared with steers in Exp. 2, which may be a result of the failure to properly adapt steers to the finishing diet in Exp. 2. The KPH responded similarly in both experiments; carcasses from steers and heifers had comparable percentages of KPH as was previously reported by Marchello et al. (1970). Marchello et al. (1970) also reported that spaying reduced LM area, which is consistent with the findings in Exp. 1 and may coincide with reduced production of estrogen-related compounds. The results of sex on quality grade in Exp. 1 contradicted the findings of Zinn et al. (1970) and Marchello et al. (1970); heifers produced carcasses with greater quality grades compared with carcasses from steers.

Tenderness and Consumer Acceptability
The current findings relative to the effects of sex on cooked beef steak tenderness are inconsistent with the previously reported results of Gracia et al. (1970) and Prost et al. (1975) who concluded that sex had no effect on cooked beef steak tenderness. However, the current findings would agree with those of Wulf et al. (1996) and O’Connor et al. (1997) who did observe differences in tenderness between steers and heifers. However, Wulf et al. (1996) and O’Connor et al. (1997) confounded sex by implant regimen. The tenderness advantages observed for steaks from steer carcasses compared with steaks from heifer carcasses in the current study are difficult to explain. The differences in tenderness might be due to the ovarian function of intact heifers, and the fact that no differences were observed between intact and spayed heifers might be attributed to the chronological age at which the ovaries were removed from the spayed heifers. However, the latter hypothesis would not be supported by the findings of Field et al. (1996) who also observed no difference in tenderness of steaks from spayed vs. intact heifers. Initially, it was hypothesized that the differences in tenderness observed in Exp. 1 were due to differences in carcass maturity. However, this hypothesis was not supported by results of Exp. 2 inasmuch as there were no differences in bone maturity, lean maturity, or overall maturity between steer and heifer carcasses.

The equations developed by Platter et al. (2003a) were used to predict the possible differences in consumer acceptability of steaks from steer and heifer carcasses. However, in this particular case, the low shear force values observed in steaks from steer and heifer carcasses resulted in similar estimates of consumer acceptability, and the ultimate effect of sex on consumer acceptability of beef based on results of the current experiments would be expected to be minimal.

	IMPLICATIONS

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The findings of the current experiment suggest that sex be added to the list of antemortem factors contributing to variation in cooked beef steak tenderness. However, more research is needed to precisely identify those factors contributing to the lower tenderness observed of steaks from heifer carcasses.

	Footnotes

 
¹ This project was funded in part by the Montana Beef Network, Bozeman, and by beef and veal producers and importers through their $1-per-head checkoff.

² The authors express appreciation to the Bair Foundation, Martinsdale, MT, for their contributions to this project.

³ Corresponding author: johnp{at}montana.edu

Received for publication August 9, 2004. Accepted for publication February 16, 2006.

	LITERATURE CITED

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AMSA. 1995. Research guidelines for cookery, sensory evaluation and tenderness measurements of fresh meat. Am. Meat Sci. Assoc., Chicago, IL.

Field, R., R. McCormick, V. Balasubramanian, D. Sanson, J. Wise, D. Hixon, M. Riley, and W. Russell. 1996. Growth, carcass and tenderness characteristics of virgin, spayed, and single-calf heifers. J. Anim. Sci. 74:2178–2186.[Abstract]

Gracia, E., J. D. Sink, L. L. Wilson, and J. H. Ziegler. 1970. Sex, sire and physiological factors affecting muscle protein solubility and other characteristics. J. Anim. Sci. 31:42–46.[Abstract/Free Full Text]

Marchello, J. A., D. E. Ray, and W. H. Hale. 1970. Carcass characteristics of beef cattle as influenced by season, sex, and hormonal growth stimulants. J. Anim. Sci. 31:690–696.[Abstract/Free Full Text]

McKeith, F. M., J. W. Savell, and G. C. Smith. 1981. Tenderness improvement of the major muscles of the beef carcass by electrical stimulation. J. Food Sci. 46:1774–1776.

Montana State University, Department of Animal and Range Science. 2004. Montana Beef Quality Assurance Manual. Bozeman, MT.

Morgan, J. B., J. W. Savell, D. S. Hale, R. K. Miller, D. B. Griffin, H. R. Cross, and S. D. Shackelford. 1991. National Beef Tenderness Survey. J. Anim. Sci. 69:3274–3283.[Abstract]

O’Connor, S. F., J. D. Tatum, D. M. Wulf, R. D. Green, and G. C. Smith. 1997. Genetic effects on beef tenderness in Bos indicus composite and Bos taurus cattle. J. Anim. Sci. 75:1822–1830.[Abstract/Free Full Text]

Platter, W. J., J. D. Tatum, K. E. Belk, P. L. Chapman, J. A. Scanga, and G. C. Smith. 2003a. Relationships of consumer sensory ratings, marbling score, and shear force value to consumer acceptance of beef strip loin steaks. J. Anim. Sci. 81:2741–2750.[Abstract/Free Full Text]

Platter, W. J., J. D. Tatum, K. E. Belk, J. A. Scanga, and G. C. Smith. 2003b. Effects of repetitive use of hormonal implants on beef carcass quality, tenderness, and consumer ratings of beef palatability. J. Anim. Sci. 81:984–996.[Abstract/Free Full Text]

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Smith, G. C., G. R. Culp, and Z. L. Carpenter. 1978. Postmortem aging of beef carcasses. J. Food Sci. 48:823–826.

USDA. 1988. Institutional Meat Purchase Specifications for Fresh Beef. Agric. Marketing Serv. USDA, Washington, DC.

Wulf, D. M., J. D. Tatum, R. D. Green, J. B. Morgan, B. L. Golden, and G. C. Smith. 1996. Genetic influences on beef longissimus palatability in Charolais- and Limousin- sired steers and heifers. J. Anim. Sci. 74:2394–2405.[Abstract]

Zinn, D. W., R. M. Durham, and H. B. Hedrick. 1970. Feedlot and carcass grade characteristics of steers and heifers as influenced by days on feed. J. Anim. Sci. 31:302–306.[Abstract/Free Full Text]

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