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By FK Siebriets, CM Potgieter and PH Heinze, ARC Animal Nutrition and Animal Products Institute, Irene

In order to decide whether the pork industry of South Africa should continue to invest in royalties payable for the MH gene analysis, the question of what the financial implications of the MH gene for the various sectors of the industry are, needs to be answered. To this end the ARC-ANPI was commissioned to undertake an extensive literature search.

In the quest to breed a pig that is lean enough to satisfy the consumer and grows fast enough to satisfy the producer, breeders inadvertently indirectly selected for the Malignant hyperthermia (MH) gene.

This gene is situated near other qualitative genes on the chromosome and therefore became part of the "gene package" of lean fast growing pigs. A pig gets half of it’s genetic matter from the sow and half from the boar and is called alleles on the chromosomes. Alleles are denoted by small or capital letters, the small letters meaning recessive. The MH gene is recessive which means that it only expresses in the homozygous situation. An individual could therefore be a nn or a NN or a Nn, where the nn denotes a recessive homozygote (positive MH pig), the NN is a dominant homozygote (normal pig) and the Nn is a heterozygote (carrier). In Table 1 the various possible combinations are shown where the highlights denote pigs with MH. The NN pigs are normal and the heterozygotes (Nn) are carriers of the gene while they also have qualitative production genes and therefore perform better than normal (NN) pigs.

Malignant hyperthermia is a condition where "the fire of life" runs out of control and the animal literally dies of over-heating when triggered under conditions of stress. MH is also triggered when nn animals are exposed to halothane, an anaesthetic. Halothane was formerly used to identify MH positive (nn) pigs, but could not identify carriers of the condition (Nn). This problem has been overcome by the use of a genetic probe for the gene. Breeders use the gene in the Nn condition to utilise the positive effects of the other qualitative genes associated with the MH gene.

The literature survey undertaken by the ARC consisted of some 375 scientific articles of which the effects were tabulated. A substantial number reported either non-significant differences, or the results were not statistically analysed per allele configuration. It was therefore decided to omit those references for the purpose of this article. Furthermore, in order to estimate some indication of the cost of the MH gene, it was decided to look only at some of the production parameters and to some of the meat quality parameters; mainly those that impact directly on losses. For example, meat colour or proportion of certain cuts was ignored although they also contribute to the value of the product or to consumer preferences. The financial effect of the MH gene is calculated in Table 2 (assuming normal slaughtering conditions).

The following can be summarised when looking only at variables that differ significantly statistically:

Bone content differed significantly between genotypes, NN having 8,6% more than Nn and 11,4% more than nn. This represents 11,5% bone, 10,52% and 10,2% respectively. In terms of waste, it means that Nn pigs have 1% less than NN pigs and nn pigs 1,3% less (i.e. 13 kg per tonne = 13 x R8 = R104).

Furthermore, dressing percentage differed significantly, NN being 75,9, Nn 77,4, and nn 78,3. This was the average difference in 4 studies and 1100 pigs and represents a difference in product between the NN and nn genotypes of a further R192 per tonne in yield. The corresponding difference between NN and Nn amounts to R120 per tonne.

Eye muscle areas in 3 studies (286 pigs) were respectively 38,9, 40,2, and 40,9 cm2 for NN, Nn and nn. Likewise eye muscle diameters were 42,7, 45,1, 48,7 mm at 90kg in 262 pigs. This will probably manifest in higher lean contents of the carcasses.

In 8 studies comprising 4 132 pigs, the average fat thickness measurements were 20,4 mm, 19,1 mm and 19,2 mm respectively for NN, Nn and nn pigs.

In 10 studies with 1 947 pigs the average lean contents were respectively 66,15, 67,67 and 69,25 percent for NN, Nn and nn pigs. Higher lean content will be to the advantage of the processor. It is difficult to ascribe a cost benefit to lean content as such.

In 290 pigs the ham weights were respectively 13,5 kg, 13,5 kg and 14.2 kg for NN, Nn and nn pigs. Again, this would mean a higher ham yield (more expensive cut) from the same carcass weight.

In 10 studies with 2 052 pigs, the average of the average daily gains (ADGs) obtained were respectively 668,8 g/d, 651g/d and 625,5 g/d for NN, Nn and nn pigs respectively.

In 5 studies with 965 pigs the average feed conversion ratios were 2,88, 2,83 and 2,76 for NN, Nn and nn pigs. To produce one tonne of carcass weight would need 62kg less feed for Nn pigs (R62) and 150kg less for nn pigs (R150).

In a study with 60 pigs the cooking losses were respectively 25,6%, 26,4% and 28,2%. At R8.00 per kg, this represents an extra loss of R64 for Nn and R208 for a tonne of nn pork The cooking loss obtained from 418 pigs in a further 7 studies were respectively 15,35%, 16,69% and 18,25% translating to an additional loss of R107 for Nn and R232 per tonne pork for nn pigs.

In 8 studies with 386 pigs the drip losses found were on average 3,9%, 3,5% and 5,2%. This translates to R32 less loss for Nn but an extra loss of R104 per tonne of nn pork.

In 16 studies comprising 1 855 pigs, the average pH1 values were 6,31, 6,09 and 5,36 respectively.

In 11 studies comprising 1 077 pigs the average pHu values were 5,6, 5,6 and 5,5 respectively.

The water binding capacity of 891 pigs in 6 studies were on average 17,09, 21,68, and 32,56 where a higher number means more loss.

The further question is who gains from the gene and who loses? Table 3 shows the financial implications for the producer and the processor.

The total mortalities (including on farm, on road and according to season) in a study with 228 pigs were 1,35%, 2,65% and 13,70% respectively for pigs with NN, Nn and nn genotypes. Assuming a live weight of 90kg, one tonne of pig meat is made up by 11,1 pigs. Assuming a dressing percentage of 80% (carcass includes head and trotters) and a carcass price of R8,00 per kg, the value of one pig carcass is R576. 1,3 more pigs per 100 die in the Nn genotype, i.e. 0.144 pigs per tonne = R83. The corresponding extra loss on the nn pigs is R790 per tonne pigs.

Furthermore, research has shown that nn pigs will produce pale, soft and exudative (PSE) meat when slaughtered under normal conditions, whereas Nn pigs are extremely sensitive to stress before and during slaughter. Therefore, the more stressful the slaughtering conditions, the higher the incidence of PSE amongst Nn pigs, and the smaller the difference between Nn and nn pigs after slaughter, and especially for the meat processor.

When the figures above are multiplied by the number of tonnes of pork produced and by the incidence of the alleles in the population, an estimation can be made of the cost or the worth of the gene to the pig industry in South Africa. The resulting figures should also give an indication (particularly the nn-values) of the cost of suspending the analysis of the MH gene. It can furthermore also be concluded that in the event of two carriers mating, half of the offspring will be positive (nn) with a resultant R680 per tonne loss suggesting that great care needs to be taken when using the gene. Some countries, such as Denmark, have decided to eliminate the gene from the population — shouldn’t we?

Left: Loins showing PSE meat. Right: Lions without signs of PSE meat