Meat and eating quality : genetic opportunities for change

JSR Technical Conference, 22 September 1998

Huge improvements in lean meat production of pigs have been made over the last 20 years. For example, since 1972, backfat depths have nearly halved in 60-80 kg carcasses, according to the MLC Pig Yearbook for 1998. So improved pig genotypes and changes in pig nutrition have resulted in the supply of leaner carcasses to the processors and consumers, in line with demand. The next challenge is to shift the emphasis of genetic improvement programmes from quantity of pig meat production to incorporate both quantity and quality of pig meat production. One reason for the change of emphasis is the concern that eating quality may have suffered, as a result of the reduced carcass fat content. If the assertion is valid, then the pig breeding industry needs information on the likely changes in meat and eating quality to different selection strategies. However, before deciding to change the direction of existing, successful breeding programmes, what evidence is there that selection for increased carcass lean content has changed meat and eating quality ?

Determining the effect of selection for carcass composition on meat and eating quality is difficult, as there is no genetic baseline with which current, lean genotypes can be compared. Use of nutritional treatments to generate "lean" and "fat" animals provides a test of nutritional effects on meat and eating quality, not a test of genetic effects. Likewise, evaluation of Duroc and Large White pigs for meat and eating quality is a between-breed comparison, such that a "cause and effect" relationship between traits cannot be determined. For example, if there is a difference in meat and eating quality between Duroc and Large White pigs, is it due to differences in intramuscular fat or to other traits for which the breeds differ? One method of determining if selection for carcass lean has an effect on meat and eating quality is to compare a selected line and a control line, or two selected lines, which have been derived from the same base population. Differences in meat and eating quality between the lines can be attributed to selection (after accounting for genetic drift).

Selection on carcass composition

Different selection strategies have been comprehensively evaluated, in the Edinburgh lean growth selection experiment. Selection strategies which emphasised the rate of lean growth and the efficiency of lean growth both increased carcass lean content, but the routes taken to increase carcass lean content were different. Selection for rate of lean growth increased lean growth rate, without changing the rate of fat deposition, while selection for efficiency of lean growth reduced the rate of fat deposition, without changing lean growth rate. The different routes taken to achieve similar changes in carcass composition provide the means to assess the effect of genetic improvement in carcass composition on meat and eating quality.

So what about meat and eating quality ? Intramuscular fat content was reduced with selection for both rate or for efficiency of lean growth. Muscle colour was darker with selection for efficiency of lean growth, while muscle shear force (the force required to cut muscle) was increased with selection for rate of lean growth. There were no significant changes in muscle moisture content, muscle pH or in myofibrillar fragmentation index (a measure of post-mortem protein degradation). So as far as meat quality is concerned, it is the route taken to change carcass composition, rather than the actual change in carcass composition, that affects meat quality. As for eating quality; tenderness and flavour liking were lower with selection for efficiency of lean growth, when assessed by a trained taste panel, but not with selection for rate of lean growth. Overall, there were no changes in juiciness or acceptability with either selection strategy.

Meat and eating quality in breeding programmes ?

A conservative approach to meat and eating quality, from a breeding programme perspective, could be to primarily focus on increasing the rate of lean meat production, as given the scale of the likely changes in meat quality, incorporation of meat and eating quality traits in the selection objective may not be necessary. In the short term, such an approach may be acceptable, although it would be pertinent to periodically evaluate nucleus herd genotypes for meat quality and, to a lesser extent, for eating quality. However, from a proactive viewpoint, there is an opportunity to include meat and eating quality traits in a breeding programme, so that both quantity of product and quality of product can be increased. The question to be addressed is "How to select for meat and eating quality ?"

Genetic opportunities

A breeding objective focusing on meat and eating quality, with growth and carcass composition included as secondary traits, could have an appreciable rate of genetic improvement, as there is substantial genetic variation in meat quality, and to a lesser extent, in eating quality. It is important not "to go in too many directions at once", so the selection objectives should be focused on a few, well-defined traits. The quantitative genetics framework is in place to incorporate either predictors of meat and eating quality, using measurements on the live animal, or actual meat and eating quality, assessed on relatives of potential breeding animals. However, there are two constraints on genetic progress. A set of traits which "describe" meat and eating quality must be identified and information on the genetic parameters is required. If inappropriate parameters are used to construct the selection criterion, then there is a real danger that the predicted rate of genetic improvement will be substantially different from the realised rate of genetic improvement. Once the information is available, then there is a real genetic opportunity to simultaneously improve quantity and quality of pig meat production.

Information from full-sibs or progeny

Meat and eating quality measurements on full-sibs or progeny can provide information on the genetic merit of animals available for selection. Intramuscular fat content is measured on the full-sibs of potential breeding animals in Switzerland, while muscle colour and pH are measured on progeny in Finland. The different systems depend on the genetic parameters, as given the same number of relatives, the higher accuracy of information from progeny, compared to full-sibs, is offset by the longer time taken to obtain measurements. The benefit of including measurements on progeny or full-sibs will depend on the genetic parameters, which are currently sparse other than those for muscle moisture and fat content and muscle pH. Unfortunately, there are only a few, reliable estimates of genetic correlations between meat quality and eating quality or between performance test traits with meat and eating quality. Therefore, until more information is available, recommendations on the traits to measure on relatives and on the number of relatives may have to be taken with caution.

Muscle fibre type and muscle metabolism

The average moisture content of muscle is 75%, so any changes in the remaining 25% of muscle may have important consequences for meat and eating quality. Muscle fibres are the building blocks of muscle and muscle size can be increased with different combinations of the number of muscle fibres or the size of each fibre. Information on muscle fibres, obtained from muscle biopsy, could be used as genetic predictors of meat and eating quality. Broadly, muscle fibres of the longissimus dorsi (LD) muscle can be classified as large, fast reacting, white fibres and small, slow reacting, red fibres. The frequency of white fibres in the LD muscle is positively correlated with juiciness, but negatively correlated with tenderness. Traits involved in muscle metabolism, such as glycolytic potential, could also be used for selection purposes, but genetic information on these traits is scant. Information on muscle fibre characteristics, derived from muscle biopsy, can be incorporated in breeding programmes, but the impact of muscle fibre traits on genetic improvement in meat and eating quality may be limited, due to the low heritabilities of fibre area and frequency.

Genetic markers

There is evidence of two genes associated with meat quality (RN and intramuscular fat), but the genes have not been identified, so direct selection is not yet possible. However, if close to the gene, there is variation in a section of DNA, called a genetic marker, and animals can be genotyped for the marker, then animals could be selected on the basis of the marker genotype, a process called marker assisted selection (MAS). A region of DNA with an effect on the trait is called a quantitative trait locus (QTL), but there is essentially no difference between MAS used to predicted genetic merit for a trait associated with one gene or with a QTL. A simulation study of a pig breeding programme suggested that MAS could increase short term rates of genetic improvement by 10-20%, particularly when animals have to be selected before the trait can be measured, as is the case with meat and eating quality. However, the greater short-term response with MAS, compared to phenotypic selection, is not sustained in the long term.

Genotype and nutrition

Feeding regime and diet have been shown to have significant effects on meat and eating quality. Ad-libitum fed animals had higher tenderness, flavour and overall acceptability scores than restricted fed animals. Likewise, animals fed diets with low lysine : energy ratios had high eating quality. Therefore, to enhance (or remove nutritional constraints) a genotype's eating quality with nutrition, one scenario would be to feed a diet with moderate lysine : energy ratio to retain efficient lean growth and then, prior to slaughter, fed a diet with low lysine : energy ratio to enhance eating quality.

However, if several genotypes are evaluated for meat and eating quality on one nutritional regime and if a genotype with nutrition interaction exists, then the nutritional regime will impose constraints on the performance of some genotypes and not others. Therefore, the ranking of genotypes for meat and eating quality traits may be dependent on the feeding regime or diet on which animals were evaluated. The genotype with nutrition interaction for carcass composition, meat and eating quality was estimated using animals from the Edinburgh lean growth selection experiment. Litter mates were fed three diets differing in lysine : energy ratio on ad-libitum or restricted feeding. In general, there were no genotype with nutrition interactions for meat and eating quality, such that genotype specific nutritional inputs are not required to identify animals of high genetic merit. Therefore, combinations of nutritional treatments can be used to reduce nutritional constraints on the genetic merit of animals for meat and eating quality.

Short and long-term

In the short term, the combination of genotypes and nutritional treatments will be used to achieve both efficient lean growth and meat of high eating quality. However, in the longer term, an assessment of meat and eating quality, either on the live animal or based on relatives' information, will be used to effectively improve meat and eating quality, through use of specialised genotypes.

Projects from the Edinburgh lean growth pig experiment, contributing information to the paper, were funded by the Meat and Livestock Commission and the Ministry of Agriculture, Fisheries and Food.

Return to Publications

Return to Results Page

Return to Home Page