How The Halothane Gene Effects Pig Performance and Pork Quality

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How The Halothane Gene Effects Pig Performance and Pork Quality

Livestock Update, September 1997

B. K. Green, A. F. Harper, C. M. Wood, J. R. Claus, and P. P. Graham, Virginia Tech Tidewater AREC, Department of Animal and Poultry Sciences, and Department of Food Science and Technology, Virginia Tech

Introduction Increased use of extremely lean, heavily muscled genotypes for terminal market hog production has resulted in increased concern about quality problems in pork products. These problems focus primarily on color, firmness, water holding capacity and marbling in pork muscle. Pale, soft, exudative (PSE) pork is a general term used to describe these pork quality problems. It has been documented that the U.S. pork supply contains approximately 16% PSE pork.

One genetic cause of PSE pork is associated with the presence of the halothane (HAL) gene, so named because pigs that carried two copies of the gene (called homozygous carriers or nn) were discovered to undergo physiological stress and die when exposed to halothane anesthesia. However, it has been shown that pigs carrying one copy of the HAL gene (called heterozygous carriers or Nn) tend to produce leaner carcasses but more PSE pork than HAL gene free pigs (called homozygous negative or NN).

In recent years DNA analysis procedures have been developed that allow determination of the 3 possible HAL genotypes using easily collected pig tissues such as blood, skin or hair. This raises important questions for the swine breeding stock industry. Should breeding stock of known HAL genotypes be used to strategically to produce certain types of market pigs or should the DNA testing be used to eliminate HAL gene carriers from swine breeding herds?

A research project was conducted to address these questions in conjunction with dietary effects and gender effects (barrows vs. gilts). Because the HAL genotype effects were similar across diet treatments and sexes, only the main effects of HAL genotype are presented in this report.

Procedures

Semen from two full-brother Hampshire boars identified to be heterozygous HAL gene carriers (Nn) was used to artificially inseminate Yorkshire X Chester White sows known to be HAL gene free (NN).
The matings were performed during two breeding periods (Nov. 1995 and Apr. 1996) at the Tidewater AREC Swine Unit to produce a genetically similar population of HAL gene free (NN) and Hal gene carrier (Nn) pigs for testing in 2 trials.
DNA analysis to determine HAL genotype was performed on tissue from the docked tail of all piglets in the Biotechnology Laboratory at Virginia Tech's Department of Animal and Poultry Sciences.
In total, 80 pigs of each genotype were identified to be similar in initial body weight, litter of origin and sex for assignment to the study.
A standard feedlot performance test was performed on the pigs from 60 lbs. to market weight.
At market weight the pigs were weighed off-test and transported to the Virginia Tech Meats Laboratory for slaughter, carcass evaluation and pork quality evaluation.

Results

HAL-carrier pigs had the same rate of growth as HAL gene free pigs but had slightly lower feed consumption. As a result, HAL-carrier pigs exhibited a tendency for slightly better feed conversion.
HAL-carrier pigs produced leaner, more heavily muscled carcasses as indicated by larger loin muscle area, less backfat depth and greater lean percentage (Table 1).
The pH of loin muscle was lower (more acidic) in HAL-carrier pigs at all time points evaluated after slaughter. Lower post-mortem pH is indicative of excess lactic acid formation and is associated with a greater occurrence of PSE pork.
Objective color instrument analysis indicated that pork loin muscle from HAL-carrier pigs was over 8% lighter (paler) in color than loin muscle from HAL gene free pigs (L· values 52.62 vs. 57.03, Table 2). In addition controlled drip loss tests indicated that HAL-carrier pork was more subject to moisture losses and lipid analysis indicated that HAL-carrier pork had less intramuscular fat (marbling) than HAL gene free pork.

Conclusion Improved feed conversion efficiency, reduced backfat depth, increased loin muscle size and greater percent carcass lean are all economically important improvements that occurred when market pigs were heterozygous carriers of the HAL gene relative to homozygous HAL gene free pigs. These traits have a direct impact on hog producer net profits when selling to packers using a carcass value based buying program. However, the negative impacts of the HAL gene on pork quality cannot be discounted. In a consumer driven market in which pork must compete with other meats and protein sources, the consistent production of a high quality end product is of major importance. Given that pork from HAL gene carrier pigs tends to have a lower post-slaughter pH, tends to be paler in color, is more prone to moisture loss during fresh storage, and contains less marbling, it seems prudent for the pork industry to use methods other than strategic breeding for the HAL gene to improve feedlot performance and carcass merit of market hogs. Such methods could include tissue testing to eliminate the HAL gene from breeding populations and strategic selection, breeding, feeding and management to produce efficient growing market hogs that consistently yield pork of high quality.

Table 1. HAL Gene Effects On Carcass Measurements
Genotype
Trait HAL-free (NN) HAL-carrier (Nn) Standard Error
Number of Pigs 80 80 ------
Number of Carcasses 80 80 ------
Hot Carcass Wt., lbs. 181.2 182.8 1.5
Loin Muscle Area, sq. in. 6.39 6.80* .08
Backfat Depth, mm 26 24* 1
Lean Percentage, % 50.9 52.5 3
Lean Gain/Day On Test, lb. .74 .78* .01
*Means for the two genotypes are significantly different (P < .02).

Table 1. HAL Gene Effects On Carcass Measurements
	Genotype
Trait	HAL-free (NN)	HAL-carrier (Nn)	Standard Error
Number of Pigs	80	80	------
Number of Carcasses	80	80	------
Hot Carcass Wt., lbs.	181.2	182.8	1.5
Loin Muscle Area, sq. in.	6.39	6.80*	.08
Backfat Depth, mm	26	24*	1
Lean Percentage, %	50.9	52.5	3
Lean Gain/Day On Test, lb.	.74	.78*	.01
*Means for the two genotypes are significantly different (P < .02).

Table 2. HAL Gene Effects On Pork Loin Quality Measures
Genotype
HAL-free (NN) HAL-carrier (Nn) Standard Error
Number of Pigs 80 80 ------
Color Instrument Measures
   L· (lightness) 52.62 57.03* .37
   a· (redness) 10.09 11.90* .17
   b· (yellowness) 7.64 9.96* .15
Muscle Moisture, % 70.21 70.40 .32
Drip Test Moisture Loss, % 3.50 5.76* .25
Intramuscular Fat, % 5.99 5.01* .28
*Means for the two genotypes are significantly different, (P < .03).

Table 2. HAL Gene Effects On Pork Loin Quality Measures
	Genotype
	HAL-free (NN)	HAL-carrier (Nn)	Standard Error
Number of Pigs	80	80	------
Color Instrument Measures L· (lightness)	52.62	57.03*	.37
a· (redness)	10.09	11.90*	.17
b· (yellowness)	7.64	9.96*	.15
Muscle Moisture, %	70.21	70.40	.32
Drip Test Moisture Loss, %	3.50	5.76*	.25
Intramuscular Fat, %	5.99	5.01*	.28
*Means for the two genotypes are significantly different, (P < .03).

Acknowledgments The financial support of the Henrietta C. Gwaltney family, founders of Gwaltney of Smithfield, is gratefully acknowledged. This support provided funding for the research and for the Masters Degree assistantship of B. K. Green.

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