Genotype Affects Response to Stress

Though relatively equal in muscle growth ability, different genotypes can respond differently to early postmortem stress and may contain inherently different capacities to develop quality pork.

This means progressive seedstock suppliers must consider the quality characteristics of a genotype along with rate of gain, feed efficiency and cutability in their selection programs, say researchers at Purdue University.

They conducted a study for the National Pork Producers Council to determine the effects of stress (simulated by early postmortem electrical stimulation) and genotype on pork quality traits in halothane-negative pigs.

In the experiment, 150 pork carcasses from three genetic lines virtually identical in muscling and composition were electrically stimulated at various times postmortem to investigate how muscle of various genotypes responds to varying stress. (See Table 14.)

The first genetic line consisted of pigs from terminal cross sires mated to Landrace females. The second line was from Duroc-Large White sires mated to Yorkshire-Landrace females, and the third genotype consisted of pigs from U.S. Duroc sires mated to Yorkshire by Duroc-Landrace dams.

Treatments consisted of one of four postmortem treatments of electrical stimulation (13 pulses): 100 volts at 15 minutes, 100 volts at 25 minutes, 200 volts at 15 minutes or 200 volts at 25 minutes or controls (no stimulation).

Pork chops were taken from the 10th rib at 24-hours postmortem and used to evaluate drip loss, color (Hunter and NPPC), firmness and marbling scores.

Genotypes responded differently to electrical stimulation, which altered muscle pH decline and meat quality characteristics, the researchers report. Differences were observed in pH at 20, 30, 40, 50 and 60 minutes postmortem between carcasses stimulated at 15 minutes and those stimulated at 25 minutes.

Animals stimulated with greater voltages tended to have lower pH values. In addition to affecting pH, electrical stimulation also affected muscle temperature.

Researchers say the differences in pH and temperature declines of these three lines of superior muscled animals indicate that some genotypes respond more dramatically (metabolize energy and dissipate heat) to an insult or stress. They conclude that some lines of heavy muscled animals have inherent muscle differences that make them more susceptible to adverse meat development. Other lines, they say, are less vulnerable to stress and trepidation, such as aggressive live animal or carcass handling procedures.

In addition, researchers say this finding shows that during the conversion of muscle to meat, some genotypes are more sensitive to electrical stimulation, and that calls into question the widely used electrical stunning methods in most U.S. pork slaughtering facilities.

More research is needed to pinpoint exactly what is causing these differences across highly selected genotypes.

Researcher: David E. Gerrard, Purdue Research Foundation. Phone Gerrard at (765) 494-8280 or e-mail dgerrard @purdue.edu

CLA Supplementation Improves Feed Efficiency

Conjugate linoleic acid (CLA) supplementation improves feed efficiency, and in some cases average daily gain, in normal, stress-free genotype pigs.

In a study by Iowa State University (ISU) for the National Pork Producers Council, researchers conducted two experiments to assess the efficacy of feeding CLA to market pigs.

Their objective was to determine the length of time necessary to feed CLA to optimize production efficiencies, the effects of CLA on body and carcass composition, and loin, ham and belly quality characteristics.

In the first experiment, 92 pigs were fed either a control diet with no CLA or 0.75% CLA in the diet at varying starting points in the growing phase (62 lb., 125 lb. and 189 lb.) until slaughter at 253 lb. of body weight.

CLA supplementation did not affect feed intake and average daily gain, but feed efficiency increased with increased time on CLA, ISU researchers say. Pigs started on CLA from 64 to 128 lb. of body weight had the highest gain-to-feed ratios.

Backfat thickness decreased, and loineye area increased in response to increasing days of feeding CLA, but carcass weight was not affected.

In response to CLA feeding, subjective quality scores for marbling and firmness improved significantly. Researchers say marbling and loin muscle firmness scores were highly correlated, suggesting that fat from pigs fed CLA is firmer than fat from control pigs.

No differences were observed for subjective color scores between the control and CLA loin samples, but the Hunter color system determined some differences in objective color scores. CLA loin samples tended to have a higher redness value and were significantly higher for yellowness values than the controls.

Bellies tended to be firmer with CLA, and at one day of retail shelf storage, CLA loin samples showed lower lipid oxidation values. This difference was not carried into longer storage times of seven, 14 and 28 days.

CLA supplementation did not appear to affect pH values, water-holding capacity or sensory characteristics. All loins were acceptable for juiciness, tenderness, flavor intensity and overall acceptability traits, researchers say.

Fresh and processed hams also were evaluated for various quality characteristics, and researchers observed improvements in fresh ham color, color uniformity and marbling in the CLA-supplemented pigs.

The second experiment included 64 pigs from three different genotypes of pigs u stress gene free (normal), heterozygous stress (carrier) or homozygous stress (stress). They were fed 0.75% CLA or a control diet.

Normal pigs on CLA had significantly greater feed-to-gain ratios and average daily gains than normal pigs on the control diet. However, carrier and stress pigs did not exhibit the same response, researchers say.

CLA supplementation did not affect 10th rib fat, but last rib fat was higher for normal pigs on CLA than normal pigs fed the control diet. Stress-positive pigs exhibited lower 10th rib fat, but this difference was insignificant because of extreme variation within each genotype of pigs.

Except for the stress-positive pigs, which were unacceptable for marbling regardless of diet, all other pigs were acceptable for quality attributes. CLA supplementation did not seem to affect color, marbling, firmness or sensory characteristics of any genotype.

In this experiment, pig and carcass losses adversely affected the total number of pigs in each treatment. So, researchers repeated this experiment with pigs of like genetics to get a more accurate account. They say that experiment provides more evidence confirming the value of CLA supplementation to improve feed efficiency, decrease backfat and increase pork quality attributes of loin marbling and firmness.

Researchers: Frederick Parrish, Jr., Bryon Wiegand, Justin Swan, J.C. Sparks, Steve Larsen and Tom Bass, Iowa State University. Phone Parrish at (515) 294-3280 or e-mail fparrish @iastate.edu. or e-meil fparrish @iastate.edu.