Background

Previous studies comparing tenderness of beef produced by steers and heifers have produced ambiguous results. Research conducted more than twenty-five years ago, suggested either that beef from steers and heifers was similar in tenderness (Gracia et al., 1970; Prost et al., 1975), or that beef from heifers was more tender than beef from steers (Kropf and Graf, 1959). However, more recent studies have suggested that heifers may produce beef that is less tender than beef from steers (Wulf et al., 1996; O’Connor et al., 1997).  

Wulf et al. (1996) suggested that the relatively high dosages of androgens used in heifer finishing implants may contribute to sex effects on tenderness. However, Choat et al. (2003) compared steers and heifers that had not received finishing implants and reported that non-implanted heifers produced less tender longissimus steaks than did non-implanted steers. Wulf et al. (1997) reported that steers had lower temperament scores (i.e., were less excitable) than heifers and that muscles from steer carcasses had lower 24-hour calpastatin activities, lower final pH values, higher a* values, and higher b* values than did muscles from heifer carcasses. Voisinet et al. (1997) also reported a difference in temperament between steers and heifers and attributed the difference to the more excitable behavior of the nulliparous female, which has been documented in various species and is thought to be associated with estrogen secretion. Other evidence suggesting that heifers tend to be more susceptible to stress-induced beef quality problems was reported by Scanga et al. (1998), who documented a significantly higher occurrence of the dark cutting condition in intact heifers than in either steers or spayed heifers.  

Collectively, these findings seem to suggest that differences in tenderness observed between steers and heifers in recent comparisons may be attributed, at least in part, to differences between the sex classes in temperament and reaction to pre-harvest stress. Therefore, the present study was conducted to determine the relationships among sex class (heifer vs. steer), temperament, reaction to pre-harvest stress, and beef tenderness. 

Methodology

Animals and Management

Crossbred (50 % British × 50 % Continental European) steer (n = 79) and heifer (n = 77) calves – contemporaries from the same herd produced by mating four Charolais sires to British crossbred cows – were selected for use in this study. The calves were born March 7 through May 17, 2004. Following immunization and weaning (on the ranch at approximately 6 months of age) calves were transported to the Colorado State University Beef Research Feedlot at the Agricultural Research, Development, and Education Center (Fort Collins, CO) for growing and finishing. Upon arrival, steers and heifers were penned separately and started on a standard “step-up” feeding program. Following a 7 to 25-d acclimation period, calves were weighed individually (initial weight) and initial implants were administered. Steers received implants containing 80 mg trenbolone acetate and 16 mg estradiol 17- β; heifers received implants containing 80 mg trenbolone acetate and 8 mg estradiol 17-β.  

The study was conducted in a 2 × 4 factorial arrangement (two sexes by four sire groups) that resulted in approximately 78 calves per sex, 39 animals per sire, and 20 calves per sex × sire subclass. Calves representing each sex × sire subclass were stratified by initial live weight and allocated to small pens (9 to 11 animals per pen) for finishing. A steam flaked corn-based finishing diet (1.11 Mcal/lb NEm, 0.67 Mcal/lb NEg, and 12.39 % CP dry matter basis) was provided once daily and consumed ad libitum. The finishing diet for heifers included melengestrol acetate (MGA) at a targeted inclusion rate of 0.50 mg/hd/d. Sixty-three d after the beginning of the finishing period, the cattle were re-implanted with the same implant products that were used at the beginning of the finishing period. Implants were chosen specifically to provide both steers and heifers with similar cumulative dosages of trenbolone acetate and estradiol 17-β during the finishing period.  

Temperament Scoring

The following behavior scores were used to assess the temperament of each animal: 1) pen score, 2) chute score, 3) load score, and 4) post-transportation score. Behavior scores were assigned using a 15-cm semi-structured line scale. The line scale was equally divided into five sections that represented the following behaviors: 1) calm (0 to 2.9 cm) - cattle that were docile, undisturbed, calm, and that had a small flight zone; 2) restless (3.0 to 5.9 cm) - cattle that were quiet, not easily disturbed, but were slightly restless; 3) nervous (6.0 to 8.9 cm) - cattle that were nervous and easily disturbed; 4) flighty (9.0 to 11.9 cm) - cattle that were very fearful, easily excited or agitated, and that had a large flight zone; and 5) aggressive (12.0 to 15.0 cm) - cattle that were very fearful, easily excited or agitated, and that exhibited aggressive behavior. 

Pre-Harvest Measurements

Pen scoring. Within one month of harvest, two evaluators assigned pen behavior scores (described previously) by briefly walking through each pen of cattle and independently assessing each animal’s behavior. Pen dimensions were 40 m × 6.1 m. Evaluators assigned pen behavior scores on different days. On both days, all animals were scored within a 3-hr time period. The two evaluator’s scores were averaged to obtain a single pen behavior score for each animal.

Chute measurements and observations. The cattle were scheduled for harvest on four different dates. On the day before each designated harvest date, 4 to 5 animals representing each sex × sire subclass were moved from their respective pens to a working facility where they were individually confined in a hydraulic squeeze chute and weighed to determine final live weight. During weighing, a chute behavior score (described previously) was assigned to each animal by a single evaluator who assessed behavior immediately following application of light pressure to the animal’s sides using the “squeeze” feature of the chute. Once the chute behavior score had been recorded, a blood sample was collected into a non-heparinized tube via jugular venipuncture for subsequent measurement of serum cortisol. Following blood sample collection, each animal’s heart rate (measured using a stethoscope), respiration rate (visual determination), and rectal temperature was measured and recorded. Each animal then was released from the chute. The following exit speed scores were used to characterize the speed at which each animal left the chute: 1 = walk; 2 = trot; 3 = run (moderate speed); 3 = run (high speed). 

Blood samples collected for cortisol determination were allowed to coagulate at room temperature (22 °C) for 4 to 6 h. After centrifugation at 2,500 × g for 25 min, serum was harvested and stored at - 80°C.  

Load scoring. On the morning of each harvest date, groups of four or five cattle representing each sex × sire subclass (total of 39 animals on each harvest date) were moved from their pens and loaded onto a semi-trailer for transportation to the packing plant. Two evaluators observed each group of cattle from the time they left their pen until they entered the semi-trailer and independently assigned a load score (described previously) to each animal. The evaluator’s scores were averaged to obtain a single load score for each animal. After loading was completed, the cattle were transported approximately 40 miles to Swift and Company in Greeley, CO.  

Post-transportation scoring. Upon arrival at the packing facility, the cattle were unloaded and placed into lairage pens. Within 10 min after unloading, two evaluators independently assigned a post-transportation behavior score (described previously) to each animal. Scores recorded by the two evaluators were averaged to provide a single post-transportation score for each animal. Following post-transportation scoring, the cattle were harvested using conventional, humane procedures. 

Post-Harvest Measurements

Collection of blood samples. Immediately after “sticking,” blood samples were collected from each animal and placed on ice. Samples were collected into non-heparinized tubes for quantification of cortisol and creatine kinase. Blood samples also were collected into tubes containing potassium oxalate and sodium fluoride for subsequent measurements of glucose and lactate. Blood samples were transported to the laboratory at Colorado State University, and approximately 6 to 8 h later serum and plasma were harvested following centrifugation at 2,500 × g for 25 min.  

Carcass data collection. Carcasses were chilled in a cooler with an air temperature of 2°C for 36 h. For the first 8 h of the chill period, carcasses were sprayed intermittently (2 min on, 8 min off) with a fine mist of 2°C water. Following the carcass-chilling period, a panel of two experienced evaluators (Colorado State University personnel) independently evaluated each carcass and recorded measurements/assessments of fat thickness, ribeye area, percentage of kidney, pelvic and heart fat (KPH), marbling score, lean maturity, and skeletal maturity. Values for each trait recorded by the two evaluators were averaged, resulting in a single value for each grade factor for each carcass. 

Carcass sampling. Two days postmortem, approximately 1 h following carcass ribbing, L*, a*, and b* values were measured in triplicate on the right and left ribeye of each carcass, and then averaged to obtain a single L*, a*, and b* value for each carcass (Hunter Lab Miniscan, Model 45/O-S, Reston, VI).  

Striploins (IMPS 180; USDA, 1996) were removed from the right side of each carcass and transported immediately (under refrigeration) to the Colorado State University Meat Laboratory. At the Meat Laboratory, each striploin was assigned to a sampling scheme that randomly identified sequential (anterior to posterior) 5.08-cm sections of the longissimus that would subsequently be assigned to each of five postmortem aging periods (3, 7, 14, 21, and 28 d). Each striploin was “faced” and appropriately sized longissimus sections were sequentially removed in the order that had been specified using the sampling scheme described above. Each section was then packaged in a vacuum-sealed bag and stored at 2°C. Following completion of the appropriate aging time, longissimus sections were frozen and stored at -20°C. Subsamples of striploins were fabricated (in the frozen state) into 2.54-cm thick steaks with a band saw (model 5700, Hobart, Troy, OH) and stored (-20°C) for subsequent Warner-Bratzler shear force determination. 

Longissimus muscle samples removed from each striploin by “facing” were used for pH determination. Three days postmortem, 3.0 g of each tissue sample was added to 30 ml of deionized water, homogenized thoroughly, and used to determine ultimate pH (Model 401A, Orion Research, Boston, MA).  

Warner-Bratzler shear force measurements. Frozen steaks were tempered for 36 to 40 h at 2°C (precooking internal steak temperatures were monitored to ensure that steak temperatures were between 1 and 5°C) and cooked on an electric conveyor grill (model TBG-60 MagiGrill, MagiKitch’n, Inc., Quakertown, PA) for a constant time of 6 min, 5 s at a setting of 163°C for the top and bottom heating platens to achieve a targeted peak internal temperature of 71°C. Peak internal temperature measurements were obtained by inserting a Type K thermocouple (model 39658-K, Atkins Technical, Gainesville, FL) in the geometric center of each steak.  

After cooking, steaks were allowed to equilibrate to room temperature (22°C) and 6 to 10 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 universal testing machine (Instron Corp., Canton, MA) fitted with a Warner-Bratzler shear head (cross head speed: 200 mm/min); peak shear force measurements of each core were recorded and averaged to obtain a single Warner-Bratzler shear force (WBSF) value for each steak.  

Laboratory assays. Serum samples were shipped to the Department of Animal Sciences at New Mexico State University for cortisol quantification using a commercial RIA kit (Diagnostic Products Corp., Los Angeles, CA) that was adapted for bovine serum (Kiyma et al, 2004). Creatine kinase was analyzed using a commercial kinetic assay kit (Stanbio Laboratory, Boerne, TX). Glucose and lactate were also quantified using commercial assay kits (Stanbio Laboratory, Boerne, TX , and Trinity Biotech, Wicklow, Ireland, respectively).  

Statistical Methods  

Data for WBSF were analyzed using a restricted maximum likelihood-based, mixed-effects model, repeated measures analysis (PROC MIXED; SAS Inst. Inc., Cary, NC). The statistical model included sex, sire, harvest group (group), and postmortem aging period (aging) as independent fixed effects. All relevant four-, three-, and two-way interactions of fixed effects were included and subsequently removed from the model if not significant (P > 0.05). The final ANOVA model for WBSF included sex, sire, aging, group, sex × sire × group, sex × sire, sex × group, and sire × group as fixed effects. Aging was treated as a repeated measurement and a spatial power covariance structure was used.  

Analyses of average daily gain, marbling score, behavior scores, physiological parameters, and muscle quality traits (excluding WBSF) were conducted using the least squares, general linear models procedure of SAS (PROC GLM: SAS Inst. Inc., Cary, NC). The statistical model included sex, sire, and group as independent fixed effects. All relevant three- and two-way interactions of fixed effects were included and subsequently removed from the model if not significant (P > 0.05). 

Several analyses revealed three- and two-way interactions (P < 0.05) between group and sex and/or sire (group × sex × sire, group × sex, and group × sire). Interactions involving group remained in the statistical model; however, only main effect means for sex and sire or sex × sire are reported.  

To further investigate the effects of behavior on physiological and muscle quality traits, subsequent analyses were conducted using behavior as a fixed, independent categorical variable. Based on the structure of the continuous scale used to assess behavior, animals within each behavior score (pen behavior, chute, load, and post-transport score) were categorized as calm, restless, nervous, flighty, or aggressive. In this study no animals were categorized as aggressive, and few animals were determined to be flighty (≤ 9 of 156 animals within each behavior score). All cattle receiving behavior scores that would classify as flighty were combined with the nervous category for these analyses. Likewise, the few animals (≤ 9 of 156 cattle) that received exit speed scores classified as run, high speed (score 4) were classified as run, moderate speed (score 3). Subsequent analyses of physiological parameters and muscle quality traits also were conducted using the least squares, general linear models procedure of SAS (PROC GLM: SAS Inst. Inc., Cary, NC). The ANOVA model for all variables, excluding WBSF, included behavior category (pen, chute, load, post-transportation, or exit speed), sex, sire, and kill as fixed independent effects. The statistical model for analyses of WBSF was conducted using PROC MIXED (SAS Inst. Inc., Cary, NC) and included behavior category, sex, sire, kill and aging as independent fixed effects; aging was treated as a repeated measurement, and a spatial power covariance structure was used. 

For all analyses individual animal served as the experimental unit, the containment approximation was used to calculate denominator degrees of freedom, and means were separated using the PDIFF option at a significance level of P < 0.05. Simple correlations among traits were calculated using PROC CORR (SAS Inst. Inc., Cary, NC). 

Findings 

The study design controlled several known sources of biological variation in animal performance, behavior, and meat tenderness in an effort to more precisely quantify the effects of sex (heifer vs. steer) on specific traits of interest. Steers and heifers used for the study were contemporaries from a single herd, exposed to nearly identical environmental conditions and management practices from birth to slaughter. In addition, similar numbers of half-sibs representing each sex class, sired by each of four Charolais bulls, were included in the study. As a result, the two sex classes also were genetically similar.  

Primary statistics for selected traits, characterizing the experimental sample, are displayed in Table 1. Despite the similarities, detailed above, cattle used for the study exhibited considerable variation in observed behavior and physiological responses to handling and transport. Individual animal scores used to characterize behavior (pen behavior score, chute score, exit speed score, load score, and post-transportation score) and blood concentrations of cortisol, glucose, and creatine kinase (used as physiological indicators of each animal’s reactivity to various pre-harvest events) were among the most variable traits (Table 1). Three animals had serum concentrations of creatine kinase more than four times higher than the mean concentration, resulting in an extremely high coefficient of variation (CV) for that trait. Creatine kinase is released into the blood when there is muscle damage, as occurs with physical exertion or bruising (Broom et al., 2002). In the present study, the five blood samples with the highest creatine kinase concentrations all were collected from cattle with carcass bruises that involved damage to muscle tissue. Heart rate, respiration rate, blood lactate, WBSF, and marbling all were moderately variable, whereas the least variable traits were rectal temperature, ADG, and measurements of longissimus pH and color (L*, a*, b*). Page et al. (2001), surveyed industry values for beef longissimus pH and reported a range of 5.2 to 6.9, with a mean of 5.5. In their study, most (> 80%) of the carcasses measured, had longissimus pH values between 5.4 and 5.59. In addition, most of the carcasses classified as “dark cutters” in the survey had pH values of 5.87 or greater (Page et al., 2001). Longissimus pH values for cattle in the present study ranged from about 5.2 to 5.7, with a mean of 5.4. No carcasses in the present study were classified as “dark cutters” either visually or based on longissimus pH.  

Simple statistics describing the environmental temperature (°C) during events at which behavior and physiological measurements were recorded, as well as, the duration (min) of loading, transport, and lairage are listed in Table 2 for each of 4 harvest groups. Environmental temperatures recorded during routine processing and transportation ranged from 6.9 to 26.4 °C and 10.1 to 17.2 °C, respectively. Time required to load cattle for transportation to the packing facility was approximately 6 min or less for each harvest group. All groups loaded without difficulty and required minimal coaxing or prodding. Maximum time for transportation was 90 min, minimum 64 min. Duration of lairage at the packing facility was approximately 121 to 135 min. 

Analysis of Sex and Sire Effects  

Behavior. The semi-structured scale used to assess a) behavior in the pen, b) during confinement in a chute, c) during loading, and d) following transport, categorized animals as: calm (score of 0 to 2.9), restless (score of 3.0 to 5.9), nervous (score of 6.0 to 8.9), flighty (score of 9.0 to 11.9), or aggressive (score of 12.0 to 15.0). Results showing the frequencies of cattle classified into the various behavior categories (calm, restless, nervous, flighty, aggressive) during each scoring event (a through d, above) are displayed in Figure 1. In this study, most cattle (about 80% or more) exhibited behaviors characterized as either calm or restless, some (between 14 and 20%) were nervous or flighty, and no cattle were categorized as aggressive (Figure 1).  

Cattle behavior differed between sexes and among sire groups (Table 3, Figures 2-6). Heifers had higher (more excitable, (P < 0.001) mean pen behavior and load scores compared to steers (4.94 vs. 3.44 ± 0.25 and 4.40 vs. 3.55 ± 0.08, respectively). Based on pen behavior and load scores (Figures 2 and 4), more than 50 % of all steers were classified as calm, whereas approximately 75% of heifers were categorized as restless, nervous or flighty. On average, both heifers and steers were classified as restless during routine processing (chute score); however, there was a tendency (P = 0.051) for heifers to be more excitable than steers while confined in the chute (Table 3). Sex did not affect (P = 0.708) post-transportation score (Table 3). The present results are in agreement with Voisenet et al. (1997) and Wulf et al. (1997) who also reported that heifers had higher temperament scores (i.e., were more excitable) than steers. 

Sire was a significant source of variation in all scores for behavior (Table 3, Figures 2-6). Cattle produced by Sire 1 were more excitable (P < 0.004) in the pen than all other sire groups (Table 3, Figure 2). Likewise, chute and post-transportation scores identified offspring of Sire 1 as being more excitable than cattle produced by Sires 2 or 4 (Table 3, Figures 3 and 6). LeNeindre et al. (1995) also reported sire effects (P < 0.01) on docility of Limousin heifers. 

Physiological measurements. Consistent with their tendency to exhibit more excitable behavior when confined in the processing chute, heifers had higher (P = 0.001) mean respiration rates than steers (45.1 vs. 40.1 ± 0.75 breaths/min) and heifers produced by Sire 2 had higher heart rates compared with their male half-sibs (Figure 7). Neither chute serum cortisol level nor rectal temperature differed between steers and heifers (Table 3). Analysis of post-transportation blood samples for plasma levels of glucose and lactate showed no differences between steers and heifers (Table 3). However, heifers from Sires 1 and 2 had greater (P < 0.05) post-transportation serum creatine kinase levels compared with steers from the same sire (Figure 7), and heifers produced by Sires 2 and 4 had a higher (P < 0.05) concentration of post-transportation serum cortisol than did same-sire steers (Figure 7).  

Analysis of blood samples collected during confinement in the processing chute revealed that the more excitable offspring of Sire 1 had higher (P = 0.007) serum cortisol concentrations than did offspring of Sires 3 or 4. Sire 1 also produced calves that had higher post-transportation plasma lactate concentrations than did calves from Sires 2 or 3. Cattle produced by Sires 2 and 4 were observed to have lower rectal temperatures than cattle from Sire 1. 

Muscle quality traits. The primary objective of this study was to determine if beef tenderness differences between heifers and steers could be attributed to differences in reaction to pre-harvest stress. Previous reports (Wulf et al., 1996; O’Connor et al., 1997; Choat et al., 2003) have suggested that beef produced by heifers is less tender than beef produced by steers. In addition, Wulf et al. (1997) presented compelling evidence suggesting that heifers may produce tougher beef than steers because they are more excitable and exhibit greater reactivity to pre-harvest stress. In the present study, a carefully controlled comparison of steers and heifers showed that, despite significant between-sex differences in behavior and some physiological stress indicators, sex did not affect (P = 0.374) WBSF (Table 3). Moreover, sex had no effect (P > 0.05) on 72 h muscle pH, L* color values, or b* color values (Table 3). Heifers in the present study produced carcasses with higher (P = 0.001) a* values compared to carcasses from steers (Table 3). These results disagree with those of Wulf et al. (1997) who reported that longissimus muscles from heifer carcasses had higher final pH values, lower a* values, lower b* values, and higher WBSF values than did longissimus muscles from steer carcasses.  

Analysis of sire effects on muscle quality traits revealed that longissimus steaks from cattle produced by Sire 1 had higher (P = 0.001) WBSF values than steaks from animals produced by Sires 2 and 4. Carcasses from progeny of Sire 1 also had the highest (P = 0.001) a* values of all carcasses. Sire did not affect (P > 0.05) 72-h muscle pH or b* color values (Table 3). 

Relationships among behavior, physiological measurements, and tenderness  

Another focus of this study was to determine relationships among temperament, physiological indicators of reactivity to pre-harvest handling and transport stress, and beef tenderness. Simple correlations characterizing relationships among behavior scores, physiological measurements, and meat quality characteristics are displayed in Table 4. Pen behavior score, chute score, exit speed score, and post-transportation score all were moderately correlated (P < 0.05), while load score was correlated (P < 0.05) only with chute score. Positive correlations among the various scores used to characterize animal behavior reflected a tendency for animals to exhibit relatively consistent behavior (i.e. calm, restless, nervous, etc.) during different handling events. The highest correlation (r = .62) was observed between pen score and exit speed score (both non-restrained scores, where cattle were allowed to move freely in an open area). 

Higher values for pen behavior scores, chute scores, and exit speed scores (indicative of greater reactivity to stress associated with handling events) were associated with higher serum cortisol concentrations, elevated heart rates, and increased rectal temperatures measured during confinement in a processing chute (Table 4). Plasma lactate and serum creatine kinase levels determined from post-transportation blood samples were positively correlated (P < 0.05) with pen behavior, chute, exit speed, and post-transportation scores (Table 4).  

Warner-Bratzler shear force was positively correlated (P < 0.05) with pen behavior (r = 0.23), chute (r = 0.23), exit speed (r = 0.23), and post-transportation scores (r = 0.37), as well as, post-transportation plasma lactate levels (r = 0.27), heart rate (r = 0.18), and rectal temperatures (r = 0.23), suggesting that more reactive cattle produced less tender steaks. Pen behavior, chute, and exit speed scores were positively correlated (P < 0.05) with a* color values. In addition, higher post-transportation plasma lactate levels (r = 0.21), heart rates (r = 0.31), respiration rates (r = 0.23) and rectal temperatures (r = 0.26) were associated (P < 0.05) with higher a* values. Measurements of b* color were positively correlated with chute score, load score, chute cortisol, and post-transportation lactate concentration (Table 4). Previous research (Wulf et al., 1997) has documented relationships among muscle pH, muscle color (L*, a*, b*) and beef tenderness. In this study, neither pH nor any of the measurements of longissimus color (L*, a*, or b*) were correlated with WBSF. Moreover, marbling score was not significantly correlated with longissimus WBSF (Table 4). 

Significant correlations among behavior scores and various physiological indicators of stress (Table 4) confirmed that, in this study, evaluators’ observations of differences in animal behavior during handling and transport reflected measurable differences in reaction to stress. Moreover, several of the behavior scores and some of the physiological measurements exhibited significant correlations with WBSF (Table 4).  

To further investigate the effects of behavior on physiological parameters and meat quality traits, additional analyses were conducted using each behavior category (i.e., pen, chute, load and post-transportation behavior classified as: Calm, Restless, Nervous or exit speed score classified as: Walk, Trot, Run) as a fixed, independent categorical variable in a least squares model. Results of these analyses are shown in Tables 5 through 7.  

Physiological Stress Indicators. The series of subjective scores for observed behaviors, together with corresponding physiological indicators of stress (Tables 5 and 6), were used to assess and characterize each animal’s reaction to pre-harvest handling and transport stress as follows. Approximately one month prior to harvest, a pen behavior score was recorded to assess and document individual reaction to human interaction without physical restraint or handling. On the day before harvest, each animal was scored a second time while he/she was restrained in a squeeze chute (chute score). Also at this time, physiological measurements associated with fear/arousal (rectal temperature, heart rate, respiration rate, serum cortisol concentration) were recorded. As animals were released from the restraining chute, an exit speed score was recorded. Finally, on the day of harvest, behavior was assessed during loading for transport (load score) and again after the cattle had been unloaded and penned at the packing plant (post-transport score), and physiological measurements associated with fear/arousal (serum cortisol concentration) and physical exertion (serum creatine kinase and plasma lactate concentrations) were measured at harvest. 

Plasma glucose concentration also was measured at harvest as stress indicator. Blood glucose concentrations increase rapidly (within about 1 h) after an animal is stressed as a result of increased levels of glucocorticoids and catecholamines (Apple et al., 2005).  Least squares means for physiological parameters stratified by behavior category (for each behavioral trait) are displayed in Tables 5 and 6. Pen behavior scores, chute scores, and exit speed scores were correlated (Table 4) and seemed to reflect similar animal responses to handling and transport. Differences in pen behavior, chute behavior, and exit speed all were associated (P < 0.05) with differences in rectal temperature, heart rate, plasma lactate concentration, and serum creatine kinase concentration (Tables 5 and 6). In addition, chute behavior and exit speed were associated with differences (P < 0.05) in chute cortisol concentration (Table 5). Cattle categorized as Nervous in the pen or while restrained in the chute, and those that ran as the left the chute, had the highest heart rates and rectal temperatures (measured in the chute) and the highest post-transport lactate and creatine kinase concentrations (Tables 5 and 6). Moreover, cattle that were Nervous while restrained in the chute and those that ran when released from the chute had the highest chute cortisol levels (Table 5). Collectively, these differences confirm relationships between observed behavior and the stress response and indicate that cattle categorized as excitable, in this study, were more reactive to pre-harvest handling and exhibited a greater response to stress during shipment for harvest. 

Both load behavior and post-transportation behavior were associated with differences (P < 0.05) in post-transport plasma glucose concentrations. Cattle classified as Calm during loading and after arrival at the plant had the lowest blood glucose levels at harvest (Table 5). Post-transportation behavior category also was related to differences (P = 0.001) in post-transport plasma lactate concentrations (Nervous > Restless > Calm). These differences in blood lactate levels appeared to reflect an acute response associated with physical stress during transport to the packing plant. 

Meat quality traits. Least squares means for meat quality traits corresponding to the various behavior categories are presented in Table 7. Data in Table 7 show a general tendency for excitable cattle to produce tougher beef. Cattle that were categorized as Nervous while confined in the processing chute produced longissimus steaks with higher (P < 0.05) WBSF values compared with cattle having either Calm or Restless chute behaviors (Table 7). These results are consistent with those reported by Voisinet et al. (1997) who reported that temperament score (chute score) was associated with tenderness differences in Bos indicus crossbred cattle. In the present study, differences in post-transport behavior reflected the most pronounced differences (P = 0.001) in longissimus WBSF (Table 7). Cattle exhibiting Calm behavior immediately post-transport, produced more tender (P < 0.05) longissimus steaks than did cattle that were Restless or Nervous following transport. Differences in WBSF (Table 7) approached significance for cattle differing in pen behavior (P = 0.084) and load behavior (P = 0.051). In contrast to other reports (Burrow, 2002; Falkenberg, et al., 2005), exit speed had no effect on tenderness in the present investigation (Table 7). Cattle that exhibited Nervous behavior in the chute produced carcasses with higher a* and b* color values, compared with Calm or Restless cattle (Table 7). No other meaningful relationships between behavior assessments and meat quality characteristics were observed. 

Data in Table 4 revealed potentially important relationships among post-transportation behavior, plasma lactate concentration, and WBSF that warranted further examination. Cattle differing in post-transportation behavior showed very pronounced differences in plasma lactate concentrations at harvest (Figure 8). Previous studies have documented increased levels of blood lactate in cattle in response to handling and transportation, which are thought to be indicative of muscle glycogenolysis (Apple et al., 2005). Additionally, in the present study, plasma lactate levels were significantly correlated with WBSF (Table 4). A regression of mean WBSF on mean plasma lactate concentration for cattle classified as Calm, Restless, or Nervous with respect to their post-transportation behaviors (Figure 8) revealed a relatively strong linear relationship (r2 = 0.85) between lactate level and longissimus tenderness. The distinct differences in lactate concentrations observed among Calm, Restless, and Nervous cattle immediately following transport, suggest that plasma lactate concentration may be an effective physiological indicator of acute transport stress and the apparent relationship between blood lactate and shear force (Figure 8) may be an important factor in explaining the link between pre-harvest stress and beef tenderness. 

Implications 

The primary objective of this study was to determine relationships among sex class (heifer vs. steer), temperament, reaction to pre-harvest stress, and beef tenderness. Our results showed that when genetic, environmental, and management differences were minimized, beef produced by steers and heifers did not differ in tenderness, even though heifers were more reactive to pre-harvest stress than were steers. The majority of cattle included in the study exhibited relatively calm temperaments. Even so, differences in observed behavior were associated with significant differences in physiological responses to pre-harvest handling and transport stress and to beef tenderness. Results of this study underscore the importance of differences in animal temperament and avoidance of stress during pre-harvest shipment in the application of best management practices for enhancing beef tenderness.