Rendement Napole Gene and Pork Quality
Table of Contents
Rendement Napole (RN) gene is a swine gene found to cause low ultimate pH and water holding capacity (WHC) in pork. The gene is also commonly called the "acid meat gene" or "Hampshire effect" due to the low ultimate meat pH it causes and because this gene has primarily been observed in purebred or crossbred Hampshire populations. The negative effects of the RN gene on pork quality result in economic losses in the pork industry. Pork producers can reduce the impact of the RN gene by adopting RN gene-free policies for their herds.
A research study in 1985 identified the relationship between the RN gene and low pH pork. Unlike the porcine stress syndrome (PSS) or Halothane gene, which has incomplete recessive effects on stress incidence and meat quality, the RN gene appears completely dominant. This dominance implies that a copy of the RN gene inherited from only one parent can cause poor meat quality.
In genetic terms, inheritance of the RN gene is at a single locus and there are 2 alleles: one dominant mutant allele (RN-) and one recessive normal allele (rn+). The dominant RN- allele causes low ultimate pH of all muscles, especially the loin and ham muscles.
Each parent provides one allele to his or her progeny, or offspring. Therefore, there are 3 possible genotypes: homozygous normal (rn+/rn+), heterozygous or carrier (RN-/rn+) and homozygous mutant (RN-/RN-). For example, if a homozygous mutant boar is mated to a homozygous normal sow, 100% of their offspring will be RN gene carriers. The mating of a RN gene carrier boar with a homozygous normal sow will result in 50% of the piglets being carriers. The only way to avoid the effects of the RN gene is to breed a homozygous normal sow to a homozygous normal boar. Table 1 lists possible mating combinations and resulting genotypes.
Table 1. Possible Mating Combinations and Resulting Rendement Napole (RN) Genotypes.
Extensive research on the effects of the RN gene on pork production, especially on pork quality, has been carried out worldwide since it was discovered. Studies on the impact of the RN gene on fresh and processed pork quality have consistently shown that the RN gene has negative effects on meat pH, WHC, colour, drip loss, cooking loss and processing yield.
The negative effects of the RN gene are a result of a dramatic increase in glycogen levels in the muscle of live pigs that have the gene. Glycogen is the form in which sugars are stored, particularly in liver and muscle. After slaughter, muscle glycogen is converted to lactic acid, which lowers the muscle pH. Therefore, the more glycogen the muscle contains, the more lactic acid will be produced and the lower the ultimate pH of the muscle will be. The increased lactic acid levels may result in muscle pH dropping below 5.5 within 24 hours after slaughter. This low and dramatic drop in meat pH causes a breakdown in protein, which results in pale muscle colour and poor WHC in the meat.
The "acid meat" condition is very similar in characteristics to the pale, soft and exudative (PSE) pork condition caused by the PSS gene. In fact, when the PSS gene is present, it intensifies the effect of the RN gene on meat quality.
Figure 1 illustrates the relationship between meat quality and post-mortem changes in muscle pH in normal pork, acid pork, PSE pork, and dark, firm and dry (DFD) pork.
Figure 1. The relationship between post-mortem changes in muscle pH and pork quality. (Source: Austin Murray, Lacombe Research Centre, Agriculture and Agri-Food Canada) (Text version of graphic)
One study reported that the RN gene reduces the ultimate pH of all muscles, especially the loin and ham muscles. Drip and cooking losses of meat from heterozygous RN pigs were 21% and 12% greater, respectively, while the ham yield was 7% lower than that from RN gene-free animals. A study completed in the U.S. indicated that loin ultimate pH, colour scores and WHC were all significantly poorer for RN gene carriers than for those of normal pigs. In addition, loins from heterozygous RN pigs had significantly lower marbling scores and intramuscular fat content.
On the other hand, there are also reports indicating that the RN gene appears to have some positive effects on eating quality. Pork from the RN gene carriers has increased tenderness, a stronger taste and improved juiciness. For example, the National Pork Producers Council's (NPPC) Terminal Line Study showed that Hampshires had more tender and juicier meat after cooking than a variety of genotypes. However, another study found no differences for mechanical and sensory tenderness between the genotypes measures. It also found that pork from RN gene carriers had poor flavour and taste in comparison with pork from RN gene free animals. In addition, this study found no differences for growth rate and carcass characteristics, such as 10th-rib backfat thickness and loin muscle area, between the RN carriers and RN gene-free pigs.
The economic losses associated with the RN gene can be considerable. High drip and cooking losses mean losses of meat yield or weight, which directly translate to losses of profit to processors. Poor meat colour, either pale or dark, does not appeal to consumers and, therefore, the meat is less marketable. Meat with extremely low ultimate pH has poor curing and processing characteristics. The Canadian Centre for Swine Improvement estimated that the average cost of the RN gene to the pork industry was approximately $12 per carcass.
Prior to the discovery of the causative mutation and availability of DNA test techniques, the RN gene was determined or predicted by using an indirect indicator termed "glycolytic potential" (GP). The GP is the sum of glycogen and the intermediate compounds (glucose-6-phospate and glucose) that can all be converted into lactic acid or lactate. The GP-based classification of RN genotypes is not accurate and sometimes can produce misleading results. With this method, it is also not possible to distinguish between homozygous mutant (RN- /RN-) and heterozygote (RN-/rn+) without backcrossing. Currently, there is a DNA diagnostic test commercially available to the pork industry for RN gene detection. The DNA test is accurate and reliable, and takes less time to conduct than the GP test. Unlike the GP test, the DNA tests can accurately distinguish all three genotypes without backcrossing.
The RN gene is present in Canada, primarily in the Hampshire and Hampshire-crossed populations. A study conducted at the Lacombe Research Centre of Agriculture and Agri-Food Canada (AAFC) in Alberta showed that, based on GP measurements, 25% of the pork chops purchased from retail outlets in Alberta had very high glycolytic potential and were probably from RN gene carriers. The researchers suggested that this high RN gene frequency in Alberta might be due to the controlled or uncontrolled introduction or use of Hampshire in market hog production in the province.
DNA test results from a recent joint study carried out in Canada indicated that three major Canadian breeds (Duroc, Yorkshire and Landrace) appear to be free from the RN gene. The study also found that there was high RN gene frequency in the Canadian Hampshire population, with the frequency varying among the herds studied (see Table 2, next page). This DNA study was done on a sample of 404 pigs: 305 purebred boars (Duroc, Yorkshire and Landrace) from artificial insemination (AI) centres located in four regions - Atlantic, Quebec, Ontario and West, 85 Hampshire male and female pigs, 4 AI Hampshire boars and 10 others.
With evidence of the detrimental effects of the RN gene on pork quality and industry profitability, the RN gene should be completely removed from Ontario pigs and breeding stocks. Attention should be paid to the Hampshire breed and its crossbreds because the RN gene frequency in this population is high.
Eradication of the RN gene from the Ontario commercial pig population would require commitment and coordinated efforts from all parties involved in the pork chain. Eradication of the gene can be achieved by taking the following measures:
Table 2. Distribution of RN Genotypes by Pig Breeds in Canada.
Source: Houde et al, 2002
Numerous studies support the conclusion that the RN gene has significant negative effects on pork quality and financial returns of the pork industry. Although there were reported benefits of the RN gene on pork eating quality, these benefits are neither significant nor conclusive. The RN gene's detrimental effects on pork quality and financial returns outweigh its positive effects, if any. To further improve pork quality and economic returns, the pork industry should take steps to eliminate the RN gene from the entire commercial swine population.
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