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Relationships of behavioral and physiological symptoms of pre-slaughter stress to beef LM tenderness

S. L. Gruber^*, J. D. Tatum^*, T. E. Engle^*, P. L. Chapman, K. E. Belk^* and G. C. Smith^*

^* Department of Animal Sciences, Colorado State University, Fort Collins 80523-1171 Department of Statistics, Colorado State University, Fort Collins 80523-1877

J.Daryl.Tatum{at}Colostate.edu

Abstract

Relationships between behavioral and physiological symptoms of pre-slaughter stress and LM Warner-Bratzler shear force (WBSF) were investigated using Bos taurus steers (n = 79) and heifers (n = 77). Measurements of heart rate, respiration rate, rectal temperature, and concentrations of serum cortisol and plasma epinephrine were used as indicators of stress associated with physical handling and chute restraint, while concentrations of cortisol, glucose, lactate, and creatine kinase in blood samples obtained at exsanguination were measured to reflect physiological reactions of animals to transportation stress. Increased plasma epinephrine concentration, indicative of acute handling stress, was associated with elevated heart rate (r = 0.42, P < 0.001) and rectal temperature (r = 0.34, P < 0.001) during restraint, increased plasma lactate (r = 0.22, P = 0.006) and serum creatine kinase (r = 0.28, P < 0.001) concentrations at slaughter, and greater LM WBSF (r = 0.22, P = 0.006). Plasma lactate concentration at slaughter, which reflected an adrenergic stress response to transportation, was associated with lower final LM pH (r = -0.30, P < 0.001) and higher LM WBSF (r = 0.26, P = 0.002). Categorical analyses of chute and post-transportation behavior scores (calm vs. restless vs. nervous) showed that cattle exhibiting adverse behavioral reactions to handling and chute restraint had increased (P < 0.05) values for plasma epinephrine concentration, heart rate, and rectal temperature during chute restraint, elevated (P < 0.05) plasma lactate concentration at slaughter, and increased (P < 0.05) LM WBSF. In addition, cattle showing behavioral symptoms of stress following transportation had higher (P < 0.05) plasma glucose and lactate concentrations at slaughter and produced LM steaks that were 0.34 kg tougher (P < 0.05) when compared with calm cattle. No carcasses were identified as dark cutters, and LM pH did not differ (P > 0.05) among behavior categories. Grouping cattle according to differences in plasma lactate concentration categorized them according to mean differences in LM WBSF. Moreover, steaks from cattle with the highest plasma lactate concentrations at slaughter (91^st to 100^th percentile) had a delayed response to aging that persisted until 14 d postmortem. Stress-induced differences in LM tenderness observed in this study were independent of differences in muscle pH.

Key Words: beef • cortisol • epinephrine • sex • stress • tenderness