Background

Market cows comprise a significant portion of U.S. beef production. In 2008, 18.3% of all cattle harvested in the U.S. were cows, totaling over six million head. The 2007 National Market Cow and Bull Beef Quality Audit indicated that 100% of audited cow plants were producing and marketing middle-meat products for potential use as whole-muscle steak and roast items. It has been well documented that significant variation exists in the eating qualities of beef produced from carcasses resulting from mature cows. Given that an increased number of consumers are likely to consume cow beef in the form of steaks and roasts, it is imperative that carcasses producing higher quality, more desirable products be identified to ensure a positive beef eating experience.

The objectives for this project were to:

1. Compare stress responses of heifers and steers;
   
2. Evaluate the efficacy of short-term dietary Mg supplementation for attenuating effects of pre-harvest stress on beef quality characteristics.
   

Methodology

Key industry personnel with an interest in innovative fabrication strategies were identified by Crossbred heifer (n = 72) and steer (n = 72) calves (herd contemporaries) were weaned at five to seven months of age, placed in a research feedlot and finished on conventional, corn-based finishing diets. When the cattle were 12 to 14 months old, animals representing each sex class were sorted into sire groups, blocked by weight (light, medium, heavy), and assigned randomly to four treatment groups: A) Control, 0% added dietary Mg, B) 0.25% added dietary Mg, C) 0.50% added dietary Mg, and D) 0.75% added dietary Mg. Supplemental Mg was provided in the form of magnesium oxide (MgO) for the final 14 days of the finishing period. The basal diet contained 0.2% Mg. Cattle comprising the three blocks were humanely harvested using conventional, commercial procedures on three different dates within a two-week period. On the day before each harvest date, animals comprising each block (steers and heifers from all four treatment groups) were commingled in a large pen to induce a stress response associated with agonistic behavior. Data recorded for each animal included: 1) behavior scores (pen behavior, chute behavior, chute exit speed), 2) physiological stress indicators (heart rate, respiration rate, rectal temperature, plasma epinephrine and norepinephrine concentrations, blood lactate concentration, muscle glucose concentration, muscle lactate concentration, glycolytic potential, 48-hour longissimus pH), 3) carcass quality indicators (carcass maturity, marbling score, longissimus L*, a*, b* color measurements, and dark cutter incidence), and 4) longissimus muscle (LM) Warner-Bratzler shear force (WBSF) measured at three, seven, 14, 21 and 28 days postmortem.

Findings

Heifers and steers exhibited distinctly different stress responses. Heifers were more excitable than steers during pre-harvest handling and exhibited a short-term (acute) stress response that resulted in increased meat toughness without a concomitant increase in muscle pH. Steers, on the other hand, exhibited greater physical activity (sustained stress) during the mixing period and, therefore, produced carcasses with lower muscle glycogen concentrations and higher 48-hour muscle pH values compared with heifers. Steers also produced tougher strip steaks than did heifers. Within the range of muscle pH values observed in this study (5.3 to 6.1), increased 48-h muscle pH was associated with increased beef toughness in both steers and heifers. Effects of pH on LM shear force, which were most pronounced at three and seven days postmortem, diminished during postmortem aging and were no longer significant once LM steaks had been aged for 21days (see Figure). Results suggested that stress-induced toughness of strip steaks from beef carcasses with final LM pH values greater than 5.65 could be problematic unless strip loins are aged for about 18 days or longer. Supplementation of cattle with Mg increased serum Mg concentration, but had no effect on any of the physiological stress indicators or meat quality characteristics measured in this study. There was no evidence to support the premise that Mg supplementation of cattle lessens the effects of pre-harvest stress on beef quality characteristics.

Implications  

Identification of cattle management systems to facilitate production of beef that consistently delivers a quality eating experience would assist in attaining industry goals of building consumer demand and adding value to cattle. This study identified distinctly different stress responses for heifers vs. steers, both of which reduced tenderness. Additionally, stress-induced increases in muscle pH significantly reduced early postmortem tenderization, thereby influencing aging requirements for beef strip loins. These results emphasize the importance of stress avoidance during pre-harvest handling and shipment of cattle.