INTRODUCTION

History has a habit of repeating itself and certainly this is the case with the Halothane gene. It is over 25 years since this gene was first identified and numerous studies have been carried out subsequently to quantify its effects, both positive and negative. However, interest in this gene has recently been rekindled in the USA, largely as a result of the beneficial effect of the gene on carcass lean content and the emergence of packer premiums for leaner carcasses. The gene has other positive effects principally in terms of improved carcass yields which result in an increase in the return to the producer when animals are sold on a dead weight basis. However, the Halothane gene is also associated with increased stress susceptibility and stress related deaths and also negative effects on reproductive performance and meat quality, and particularly a high incidence of Pale Soft Exudative (PSE) pig meat. Because of the increasing importance of meat quality, a number of individuals and organization have advocated that the gene should be eliminated from all swine population. On the other hand, there are a number of breeding organizations that are offering reactor and carrier sire lines to their customers and a number of producers, both large and small scale, that are producing carrier slaughter pigs.

Historically, the vast majority of studies have used the Halothane gas challenge test to distinguish between reactor (homozygous positive, nn) and non-reactor animals [homozygous negative (NN) or heterozygous carriers (Nn)]. However, this test could not distinguish between carrier and negative animals. Studies that attempted to compare the performance of carriers with the other two genotypes generally crossed reactor and negative lines that were often of different genetic backgrounds. More recently, a DNA test has been developed that is based on the point mutation in the gene that causes the defect and this accurately identifies all three halothane genotypes. This test has allowed studies to be carried out comparing halothane genotypes within the same genetic line. In fact, if carrier male and females are mated then all three halothane genotypes can be produced within the same litter, allowing the effects of the gene to be studied within the same genetic background.

IMPACT OF THE HALOTHANE GENE - COMPARISON OF CARRIER AND NEGATIVE PIGS

The generally held view is that the halothane gene reduces litter size in the sow, although data in support of this is limited. Some studies have shown little difference between carrier and negative females for sow productivity. Never-the-less, maintaining halothane negative female lines eliminates the chance of producing reactor progeny and the consequent increases in stress-related mortality. For commercial slaughter pigs, therefore, the choice is between producing carrier or negative animals and the decision will largely be dictated by the relative economic merits of these two genotypes. In terms of carcass and meat quality characteristics, there is evidence in the literature that the halothane gene may be recessive (Webb et al., 1982) or additive (Jensen and Barton-Gade, 1985; Simpson and Webb, 1989). A possible explanation for these differences in gene expression with respect to carcass composition and meat quality was proposed by Sather et al. (1991a,b) who found a slaughter weight by halothane genotype interaction for carcass lean content and meat quality traits. At lighter weights, halothane carriers performed more like negative animals whereas at heavier weights the carriers had carcass and meat characteristics that were similar to reactors.

To investigate this potential interaction further, a study was carried out at the University of Illinois to investigate the relative performance of carrier and negative animals when slaughtered at 110, 125 and 140 kg live weight (Leach et al., 1996). The halothane genotypes were produced within the same litter by mating a carrier sire line to a negative dam line. The results of this study showed no interaction between halothane genotype and slaughter weight suggesting that the relative advantages and disadvantages of carriers and negatives are similar across the range in slaughter weights currently used in the US industry. The growth and carcass characteristics of the halothane carrier and negative animals in this study are summarized in Table 1. Carriers grew at a similar rate to the negative pigs but had improved feed efficiency and a higher carcass yield and lean content compared to negative pigs. A follow-up study that we have recently completed has also shown a feed efficiency advantage for carriers over negatives of a similar magnitude to that found by Leach et al. (1996). Obviously, the financial return to producers from carriers would be greater than from negative pigs. However, the meat quality of the carriers was generally poorer in the carriers (Table 2) and the incidence of PSE, defined as samples with a color score of 1 and drip loss greater than 6%, was 7 and 0% for carriers and negatives, respectively. This would obviously reduce the value of a carrier carcass to the slaughterer and processor.

What impact does PSE have on the yield of saleable product from a carcass. There is surprisingly little data in the literature to estimate the yields and economic values of carcasses that vary in muscle quality. A recent study carried out at the University of Illinois set out to provide data to address this issue by comparing the moisture loss at every stage from slaughter to consumption from carcasses that were PSE, normal, or Dark, Firm and Dry (DFD) (Gusse, 1996). The results of this study are presented in Table 3. Losses from PSE carcasses were greater than for the other two muscle conditions and amounted to a reduction in the weight of product from PSE carcasses of 1.5 and 2.5kg relative to normal and DFD carcasses, respectively.

The effect of PSE on consumer acceptability of pork will depend on its impact on the attractiveness of the retail products and on eating quality characteristics. The impact of pig meat color on visual appeal will be market-place specific and pale colored pork is acceptable in most situations. An interesting consumer acceptability study that was carried out in the United Kingdom a number of years ago (Smith and Lesser, 1982) showed that consumers purchased similar amounts of PSE and normal pork chops during the first day of retail display. However, consumers discriminated against PSE chops on the second and third days of display, by which time the PSE pork had lost more moisture than the normal pork and was dry and flaccid in appearance. A number of studies have suggested that PSE pork has poorer eating quality characteristics and this is of concern relative to long term consumer buying patterns.

Therefore, in terms of the halothane gene the choice is between an increased return to the producer versus a reduction in value to the meat sector and the potential reduction in consumer acceptability of the product.

PIG MEAT QUALITY AFTER THE HALOTHANE GENE

It would be naive to think that the halothane gene is the only gene involved in pork quality and there are likely to be others that affect quality aspects. This is illustrated by a recent study that compared the progeny of three commercial sire lines mated to the same halothane negative hybrid female line. One of the sire lines (line A) was a halothane reactor line and produced halothane carrier progeny. Another line (line C) was halothane negative and produced only negative progeny. The third (line B) was a carrier line and produced both carrier and negative progeny within the same litter.

The meat quality of the progeny from these lines is summarized in Table 4. Differences between halothane carrier and negative pigs were generally in line with those of Leach et al. (1996) reported in Table 2. The exception to this was for tenderness and juiciness which were poorer for carriers than negatives in the study reported in Table 4. The interesting sire line in this study was line C which was halothane negative and produced meat which had a normal pH at 45 minutes post mortem but had the lowest curing yields of any of the lines. In addition, ultimate pH was lower and Minolta L* values were higher for line C compared to the other lines, although these differences were not statistically significant. The conclusion from this study is that even in the abscence of the Halothane gene, there is still significant genetic variation in meat quality. Further research with line C has shown that its relatively poor meat quality results from the presence of pigs with high glycolytic potential, probably because of the acid meat or Rendement Napole (RN) gene. French studies have suggested that animals that carry the mutation of this gene have higher muscle glycogen levels and a greater capacity to produce lactic acid post mortem (i.e. a greater glycolytic potential).

THE ACID MEAT GENE

We have recently completed a study investigating the effect of glycolytic potential (GP) on meat quality. Animals were characterized for glycolytic potential post mortem and the distribution of values was biomodal (Figure l) suggesting that variation in GP is the result of the action of a single gene exhibiting dominance.

The meat quality of animals with high and low GP is summarized in Table 5. Longissimus samples with high GP had lower 24h pH, higher Minolta L* values, suggesting paler color, and a higher drip loss and poorer water holding capacity. Napole yield, which gives a measure of processing yield, and cured ham yields were lower for the high GP samples, however, the treatment difference for cured ham yield was not statistically significant. However, high GP samples did have better eating quality in terms of tenderness and juiciness (Table 5).

The population used in this study included Halothane carrier and negative pigs and this allowed the interaction between Halothane and Acid Meat genotype to be investigated, albeit on limited numbers of animals (Table 6). For 24 h pH, Minolta L* values, drip, purge and cooking losses, and Napole Yield the differences between carrier and negative animals were much greater for the high GP pigs suggesting an interaction between Halothane genotype and glycolytic potential, although this interaction was statistically significant for 24 h pH and Minolta L*. On the basis of these preliminary results it would appear that the combination of a rapid decline in pH in the early post mortem period, characteristic of Halothane carriers, and the low ultimate pH due to the acid meat gene has the potential to significantly reduce water holding capacity and processing yields..

CONCLUSIONS

This paper has provided an overview of recent research studies carried out at the University of Illinois investigating genetic effects on meat quality. Studies comparing Halothane carrier and negative pigs have confirmed the benefits and problems associated with this gene and suggest that these are relatively constant across the range of slaughter weights used in the US industry. In addition, muscle with high glycolytic potential, most likely resulting from the acid meat gene, is pale with low water holding capacity but has improved tenderness and juiciness. Thus, both of these genes have advantages and disadvantages and the decision on whether to exploit or eliminate them is likely to be situation specific.

REFERENCES

Jensen, P. and P.A. Barton-Gade. 1985. Performance and carcass characteristics of pigs with known genotypes for halothane susceptibilty. In: Stress susceptibility and meat quality in pigs. Proceedings of the Commission on Animal Management and Health and Commission of Pig Production. EAAP publ. No. 33. P80. Pudoc, Wageningen, The Netherlands.

Leach, L.M., M. Ellis, D.S. Sutton, F.K. McKeith and E.R. Wilson. 1996. The growth performance, carcass characteristics, and meat quality of halothane carrier and negative pigs. Journal of Animal Science 74:934-943.

Sather, A.P., S.D.M. Jones and A.K.W. Tong. 1991a. Halothane genotype by weight interactions on lean yield from pork carcasses. Canadian Journal of Animal Science 71:633.

Sather, A.P., S.D.M. Jones, A.K.W. Tong and A.C. Murray. 1991b. Halothane genotype by weight interactions on pig meat quality. Canadian Journal of Animal Science 71:645.

Simpson, S.P. and A.J. Webb. 1989. Growth and carcass performance of British Landrace pigs heterozygous at the halothane locus. Animal Production. 49:503.

Smith, W.C. and D. Lesser. 1982. An economic assessment of pale, soft exudative musculature in the fresh and cured pig carcass. Animal Production 34:291.

Webb, A.J., A.E. Carden, C. Smith and P. Imlah. 1982. Porcine stress syndrome in pig breeding. Proc. 2^nd World Congr. Gen. Appl. Livest. Prod., Madrid, Spain 5:558.

Table 1. Comparison of Halothane carrier and negative pigs for growth and carcass characteristics
Halothane genotype	Carrier	Negative	Avg. SE	Sig.
Average daily gain, g	974	964	16.9	ns
Gain:feed	.36	.33	.005	**
Dressing percentage	75.3	74.4	.29	***
Carcass length, cm	83.9	84.6	.43	ns
Tenth rib fat depth, cm	2.4	2.6	.12	ns
Loin eye depth, cm	6.1	5.8	.22	ns
Loin eye area, cm2	42.9	41.5	1.20	ns
Side trimmed boneless cuts Weight, kg Percentage	19.8 44.1	19.2 43.1	.24 .49	** *
Side fat-free lean Weight, kg Percentage	24.7 55.1	23.9 53.8	.35 .70	* ns

(Leach et al., 1996)
In this and subsequent tables, ns, *, **, *** = not statistically significant, P < .05, P < .01, P < .001, respectively.

Table 2. Comparison of Halothane carrier and negative pigs for meat quality and curing Yields
Halothane genotype	Carrier	Negative	Avg SE	Sig
45 min pH	6.4	6.6	.05	***
24 h pH	5.6	5.7	.03	**
Colora	2.2	2.7	.12	***
Firmnessa	2.2	2.9	.12	***
Marblinga	1.2	1.7	.12	***
Minolta L*	45.7	42.0	1.03	***
Drip loss, %	5.2	3.4	.43	***
Shear force, (kg)	3.4	3.4	.17	ns
Cooking loss, (%)	27.1	25.6	.89	ns
Juicinessb	7.3	7.6	.27	ns
Tendernessb	9.1	9.2	.30	ns
Cured belly yield, %	100.4	100.6	.52	ns
Cured ham yield, %	100.8	102.9	.62	**

(Leach et al., 1996)
^aSubjective score from 1 = extremely pale, soft, and devoid of marbling to 5 = extremely dark, firm and moderately abundant or greater marbling.
^bSubjective score from 0 = extremely dry and tough to 15 = extremely moist, and tender.

Table 3. Effect of muscle condition on moisture loss from pig carcasses (percentage of carcass weight)
Muscle condition	PSE	Normal	DFD	Avg SE
Carcass shrink	2.08	2.37	2.49	.26
Curing/smoking loss	.97a	.43b	.58b	.08
Vacuum package loss	2.13a	1.32b	.33c	.20
Retail display loss	1.76	1.30	1.33	.23
Cooking loss	3.65a	3.50a	2.85b	.09
Total loss	10.59a	8.95b	7.58c	.36

(Gusse, 1996)
^abcMean in the same row with different superscripts differ (P < .05)

Table 4. Comparison of sire lines for meat quality and curing yields
Sire line	A	B		C	Avg SE	Sig
Halothane genotype	Carrier	Carrier	Negative	Negative
45 min pH	6.15ab	6.03a	6.28b	6.23b	.06	*
24 h pH	5.68	5.68	5.70	5.47	.07	ns
Minolta L*	42.3	43.5	42.7	45.1	1.40	ns
Shear force, kg	4.3	3.6	3.2	3.3	.17	ns
Juiciness	7.6ab	7.2a	8.8b	8.8b	.43	**
Tenderness	7.5a	7.8a	9.1ab	9.8b	.79	**
Cured belly yield, %	95.7b	94.8ab	95.5ab	94.0a	.81	*
Cured ham yield, %	96.6a	96.3a	98.1b	95.5a	.9	ns

Miller, 1996 (unpublished)
^abcMeans in the same row with different superscripts differ (P < .05)

Table 5. Effect of glycolytic potential on meat quality
	Glycolytic potential		Avg SE	Sig
	High	Low
Glycolytic potential, Fmoles/g	189.4	111.1	3.14	***
24 h pH	5.47	5.64	.02	***
Minolta L*	44.2	40.9	.60	***
48 h drip loss, %	7.50	4.97	.43	***
Napole yield, %	91.7	95.3	.31	**
Longissiumus composition, %:
Moisture	75.6	75.3	.14	ns
Fat	1.77	1.47	.13	ns
Protein	21.1	21.9	.18	***
Purge loss, %	4.47	3.55	.30	*
Excess water holding capacity, %	41.2	46.0	1.0	***
Excess water binding capacity, %	96.7	111.8	1.95	***
Cured ham yield, %	108.2	109.4	.57	ns
Shear force, kg	2.06	2.33	.063	**
Tendernessa	9.4	8.6	.16	***
Juicinessa	8.6	8.0	.16	**

Sutton, 1996 (unpublished)
^aSubjective score from 0 = extremely dry and tough to 15 = extremely moist, and tender.

Table 6. Interaction between Halothane genotype and glycolytic potential
	High glycolytic potential		Low glycolytic potential		Sig. of interaction
Halothane genotype	Carrier	Negative	Carrier	Negative
No.of animals	18	32	19	39	-
24h pH	5.4	5.52	5.65	5.63	*
Minolta L*	46.9	41.4	41.9	39.8	*
Drip loss (%)	7.80	7.19	4.84	5.11	ns
Cooking loss (%)	25.4	22.8	21.0	20.1	ns
Purge loss (%)	5.50	3.44	3.99	3.12	ns
Napole yield (%)	90.6	92.7	95.2	95.3	ns
Shear force, kg	2.04	2.08	2.39	2.25	ns
Tenderness	9.5	9.3	8.1	9.2	**
Juiciness	8.7	8.6	7.7	8.4	ns

Sutton, 1996 (unpublished)