ANIMAL BREEDING AND GENETICS
Traditional Animal Breeding
Introduction
Meat production can be influenced to a large extent by animal breeding and genetics. Several breeding strategies that can affect carcass composition and meat quality will be reviewed in this article. These include (i) selection between breeds within species; (ii) crossbreeding to combine desirable characteristics from more than one breed or strain, or to exploit heterosis in crossbred progeny; and (iii) genetic selection of superior breeding stock within a breed. Genetic influences on carcass and meat quality and selection programmes designed to improve these traits will be discussed.
Differences between Breeds
When selecting between breeds, it is important to choose a breed that is able to perform well within the relevant environment, and that will meet the appropriate market demands. Differences among breeds are only relevant in the environments in which they have been measured. Most breed comparisons of carcass composition and meat quality have been performed in temperate climates, using animals on a high level of nutrition. Studies performed in more extreme environments (e.g. tropical or subtropical) or when food is less abundant have been fewer in number and have shown less convincing evidence of breed differences. It must also be remembered that breeds evolve as a result of selection, so results from breed comparisons may change over time.
Carcass Composition
Large between-breed differences exist within all farm animal species for growth and carcass composition traits. As an animal matures it undergoes an increase in the ratio of muscle to bone, followed by a decrease in muscle growth rate and an increase in the ratio offat to muscle. However, different breeds differ in their rate of maturation and average mature weight. Therefore, standardizing measurements of body composition (proportions of muscle, fat and bone) to the same stage of maturity of body weight (ratio of actual weight to expected mature weight) results in much less variation in carcass composition than standardizing to the same age or weight. One exception to this rule is the Texel breed of sheep, which shows less total body fat than expected for its mature size.
In beef cattle, late-maturing breeds, such as the Continental European breeds, are often preferred under conditions of good nutrition, producing heavier carcasses with little fat. Early-maturing beef breeds, such as the traditional British breeds (e.g.Angus, Hereford, Shorthorn), can be slaughtered at lighter weights and may be preferred when food supply is limited or for certain markets. Similarly, in lamb production systems, the use of early-maturing breeds (e.g. Southdown) will allow quick finishing of small lambs with good carcass composition.
However, the use of larger breeds that mature later (e.g.modern Suffolk strains) will result in heavier lambs with less fat. Traditionally, early-maturing pig breeds (e.g. Middle White) were used for pork production and later-maturing breeds (e.g. Large White) for bacon production. Strains and hybrids of improved pig breeds that are now used in pork and bacon production (e.g. Piétrain, Landrace, Hampshire, Large White) have better carcass composition compared to traditional British pig breeds (e.g. Tamworth,Gloucester Old Spot, Saddleback) owing to reduced fat levels and increased muscle percentage.
Breeds may partition fat and muscle differently between body depots. Dairy breeds of sheep and cattle have a higher proportion of body fat in internal depots than do meat breeds, which have higher proportions of subcutaneous fat. In general, more maternal sheep breeds, which have higher reproductive rates and higher levels of milk production, also have increased proportions of non-carcass fat.
During growth and development, intermuscular fat is deposited before subcutaneous fat, which is deposited before intramuscular fat. Therefore, relative to subcutaneous fat, large late-maturing cattle breeds have more intermuscular fat than small early-maturing breeds, which have increased levels of intramuscular fat (e.g. British beef breeds compared to ContinentalEuropean breeds). Breed comparisons in pigs have found that the Duroc, Meishan and Berkshire breeds have a high proportion of intramuscular fat compared to other improved breeds, and for some markets the level of intramuscular fat in pure Duroc pigs is too high for consumer acceptability.
Meat Quality
In addition to yield and carcass composition, meat quality is determined by traits such as colour and composition of muscle and fat, level of intramuscular fat, juiciness, tenderness and texture, flavour and aroma. Objective laboratory-based techniques, which are discussed in other articles, have been developed to quantify many of these traits. These measurements are often referred to as technological traits and some examples are listed in Table 2.
The most widely used of these techniques is ‘shear force’, which measures the force required to cut through samples of cooked meat. Trained sensory panel analysis is still considered the most relevant measure for many meat quality traits and Table 2 also gives examples of some of these sensory traits. There is substantial evidence of between breed variation in technological and sensory meat quality traits. Examples include the following.
• Paler, more watery muscle, with more exudation of fluids during storage, in ‘improved’ pig breeds compared to traditional British pig breeds.
• Yellower fat in Channel Island cattle breeds compared to other cattle breeds.
• More tender, fine-grained meat in smaller breeds of cattle, owing to smaller muscle bundles.
• More tender meat in ‘double-muscled’ Piedmontese cattle compared to other breeds.
• More tender meat in Bos taurus (domesticated) cattle breeds than in Bos indicus (humped cattle) breeds. Bos indicus cattle show increased calpastating post-mortem tenderization.
• More tender meat in Duroc pigs compared to most other breeds, owing to an increased amount of red fibres in the muscle.
• Increased flavour, juiciness and tenderness in pigs with an increased percentage of Duroc, Berkshire or Meishan genes.
Breeds can also react differently to pre-slaughter, slaughter and processing methods. For example, leaner and lighter animals are more likely to suffer from cold-shortening in the carcass post mortem (see Conversion of muscle to meat: Cold and heat shortening).
Crossbreeding
Crossbreeding can be used for several reasons, which can be exploited simultaneously. One reason may be to combine desirable characteristics from more than one breed or strain. This is termed ‘complementarity’. In the pig and poultry industries, different breeds or specialized lines, selected for different characteristics, are commonly crossed to produce commercial hybrids. Sire lines are often selected to be heavier and faster-growing, while female lines may be selected for reproductive traits, low maintenance requirements or other economically important factors. An example of such a system for pigs is given in Figure 1. The breeds or strains chosen for crossing will depend on their suitability for specific environments as well as market requirements.
Crosses between different breeds of sheep and cattle are also used to combine desired characteristics. In the United Kingdom, hill ewes (e.g.Scottish Blackface, Swaledale) are often crossed to rams from upland breeds (e.g. Blue-faced Leicester, Border Leicester) with high maternal performance. The crossbred female progeny are then mated to terminal sire breeds (e.g. Texel, Suffolk, Down breeds) that have been selected for improved carcass and growth traits, to produce a high number of better-quality lambs.
Another reason for crossbreeding is to exploit heterosis, or hybrid vigour (advantage over the average performance of the individual parent breeds). Heterosis is most widely exploited in the pig and poultry industries. The ‘combining ability’ of different strains is tested to increase production. Some strains combine well only when used as the male or as the female parent. The purebred lines that are used for crossing can either be selected to improve specific traits within line (e.g. meat yield, fertility) or can be selected on the performance of their crossbred progeny (reciprocal recurrent selection). Heterosis is usually greater for traits with low heritability (see below), such as those affecting overall fitness, reproduction or survival, and less for production or carcass traits.
Some evidence suggests that crossbred cattle mature more quickly and so are heavier at a given age, with more marbling fat, total fat and muscle. However, after adjusting for weight, heterosis for carcass composition traits tends to be low. Effects of heterosis on most technological and sensory traits have not been studied widely, but available estimates tend to be low for juiciness, tenderness, flavour, cooked colour and overall desirability. Results for the effect of heterosis on shear force in beef are more numerous and range from slightly unfavourable to moderately favourable.
Differences within Breeds
Genetic improvement can be made within breeds for desired characteristics. This method relies on the fact that the traits to be improved are heritable, and that more animals are produced than need to be kept for replacement stock, so allowing selection among progeny for preferred breeding animals.
Genetic Parameters for Carcass Composition and Meat Quality Traits
Heritability is the proportion of the total phenotypic (observed) variation in a trait that is explained by genetic variation. It is therefore a measure of how much a trait is controlled by genes (or, more precisely, genes that act additively), as opposed to environmental influences. Heritability is expressed on a scale of 0 to 1, where a value of 1 suggests that the trait is completely controlled by an animal’s genes, while management, feeding and other environmental factors play no part in determining the expression of the trait. Other things being equal, a higher rate of genetic improvement will result by selecting on a trait with high heritability. Estimates of heritabilities for the main carcass and meat quality traits differ between studies but tend to fall in the ranges identified in Tables 1 and 2.
There is good agreement across species for heritability estimates of these traits. Relatively few studies have been conducted on genetic parameters of eating quality traits compared to carcass composition. However, in general, the heritability of most carcass composition traits is moderately high, and objective technological measures of determined by sensory panel analysis. Selection responses in sensory meat quality traits are expected to be lower, since they are less heritable and more difficult to evaluate.
It is important to note that the magnitude of the environmental variation and, therefore, the heritability, depends very much on our ability to standardize measurements. The better we are able to measure meat quality traits, the higher the heritability and the higher the selection success. A low heritability does not necessarily mean that there is too little genetic variance for selection; rather it may reflect our difficulties in measuring the traits in a standardized, repeatable manner. Genetic parameters may also differ according to which muscle is tested within an animal, or owing to differences in preslaughter
conditions (e.g. stress during transport or at the abattoir), or processing methods (e.g. electrical stimulation, conditioning). For example, shear force has been reported to be more variable in carcasses that have not undergone electrical stimulation. In particular, genetic parameters for meat tenderness vary widely, probably owing to the fact that this is a very complex trait, depending on many factors (pH and temperature changes post mortem, glycolysis, processing) and the interactions between them.
Selection on one trait will often lead to correlated responses in other traits, which are not always monitored but could have very important economic consequences. A selection programme designed to improve carcass composition should also be concerned with the effects of these changes on sensory meat quality traits and should monitor effects on reproductive traits and functional fitness. For example, there are concerns that selection for reduced fat levels may delay puberty in cattle and be associated with reduced fertility of females. Genetic improvements in poultry production have also been related to a decrease in meat quality in terms of flavour and texture. In general, intramuscular fat percentage has a negative genetic relationship with meat yield and a positive genetic relationship with total fat. Intramuscular fat in pigs was ignored in early studies, as it was thought to be unimportant to eating quality. As pigs were selected for reduced back fat, the level of intramuscular fat was also reduced, which was later linked to a reported decline in the eating quality of pig meat.
Attempts are now under way in some countries to increase intramuscular fat in pig meat without increasing total fat. Studies in some sheep and cattle populations suggest that this may be possible, as low correlations have been found between levels of fat in different depots. These low correlations may allow selection in these species to reduce one fat depot (e.g.subcutaneous), while maintaining moderate levels of fat in other depots to fuel survival and reproduction meat quality are more heritable than sensory traits (e.g. internal depots), or to maintain marbling and meat quality (intramuscular fat).
Correlations among carcass composition, technological and sensory meat quality traits differ between studies and few strong trends have emerged. In general, the literature suggests that selecting for leanness might have slight negative effects on eating quality traits (e.g. water-holding capacity, tenderness, juiciness, pH, drip loss). Sensory quality traits such as tenderness, flavour intensity and juiciness tend to be positively correlated to one another, and genetic correlations between these traits and shear force tend to be negative (lower shear force equals more tender).
Shear force also has a low to moderate negative genetic correlation with intramuscular fat. Tenderness is widely thought to be the most important determinant of meat quality to the consumer. However, genetic parameters for this trait differ, depending on the muscle tested and the method of measurement (myofibrillar fragmentation index, calpastatin activity at 24 hours, shear force, or sensory panel assessment of tenderness). Preliminary results suggest that calpastatin activity at 24 hours is highly genetically correlated with shear force, but that the phenotypic correlation between the two measurements is only moderate. Correlations of tenderness among different muscles are moderately low and the correlation between shear force and sensory panel evaluation of tenderness also changes between muscles. As a result, selection for tenderness may be difficult and further work is needed to determine how technological and sensory measurements are related to each other, both between and within breeds.
Selection Programmes
In genetic improvement programmes, animals are selected on their own performance, on that of their relatives, or on a combination of both. First, the breeding goal or goals – i.e. the traits to be improved – must be decided. The selection criteria must then be determined. These are the measurements that will be taken and then selected for in order to improve the breeding goal.
In some cases the breeding goals and the selection criteria are the same, but often they are not, especially with carcass and meat quality traits. Traits to be used as selection criteria must be highly repeatable and be practical to measure on-farm or on-line during animal processing. Selection criteria should be heritable and show sufficient variation within the population. The design of the breeding programme should define the number of male and female animals that will be selected each year, the age at mating, the generation interval and other such factors.
Genetic selection programmes differ in complexity. A relatively simple approach is to select breeding animals based on their own phenotypic performance (e.g. ultrasound data obtained from selection candidates).However, the impossibility of collecting phenotypic carcass and meat quality data on selection candidates has restricted the use of this method of selection for many of these traits. ‘Independent culling levels’ in one or more traits are often used to improve the genetic merit of the flock or herd. This method involves choosing animals for breeding only if they reach a certain threshold in each trait of interest (e.g. over a certain weaning weight, and/or below a certain ultrasound fat depth).
A selection index is a more complicated but an effective method of selecting animals on one or more traits, based on the performance of the individual and its relatives. This method allows selection on traits that can be measured directly or indirectly (using predictor traits) on the selection candidates themselves, and also traits that can only be measured on relatives (e.g. slaughter and meat quality traits).
Accurate recording of performance data and pedigree structure is vital in these programmes. The amount of emphasis or weighting on each trait in a multi-trait index can be altered and is usually determined by economic importance. Estimated breeding values (EBVs) for each trait for each animal are produced, as well as an overall ‘index score’, based on the combined merit of the individual in all traits included in the index. Selection decisions are based on these index scores. Response to selection in any individual trait per generation using a multi-trait index is smaller than could be achieved by selecting for that trait alone.
However, index selection should lead to the highest rate of change in overall economic merit. Estimated breeding values are calculated, in most selection programmes, using a statistical procedure known as best linear unbiased prediction (BLUP). This procedure separates the genetic effects for each trait from management and environmental influences. The EBV is determined by the genetic merit of the animal itself, plus that of its relatives, for the trait of interest and reflects the merit of that animal compared to the population mean. Since, on average, each breeding animal passes half its genes to its offspring, the breeding value for each trait is often expressed as the expected progeny difference (EPD), which is half the EBV of the breeding animal.
If the breeding goal of a selection programme is to improve carcass composition, the selection criteria will often include predictors of composition taken on the live animal. Live weight is a very poor predictor of carcass composition. However, carcass composition can be estimated in vivo using techniques such as mechanical and optical probes, ultrasound scanning or computed tomography (CT) scanning. Ultrasound is commonly used in selection programmes for sheep, cattle and pigs to measure depths and areas of subcutaneous fat and muscle (Figure 2) and greatly improves the predictions of body composition above those estimated from live weight alone. In cattle, intramuscular fat has also been estimated using ultrasound. CT scanning increases the accuracy of predictions of total fat, muscle and bone compared to ultrasound and allows the measurement of tissues in different body depots and regions (Figure 3). Two-stage selection can be carried out in sheep and pig populations, where ultrasound scanning is used on-farm to screen large numbers of animals, then a small number of top-ranking animals are CT scanned to make final selection decisions based on conformation or composition of breeding stock.
Because there are no live-animal predictors of most technological and sensory meat quality traits, breeding programmes designed to improve meat quality use measurements taken on slaughtered relatives of selection candidates to calculate EPDs (or EBVs depending on the scale used) for these traits. For example, most cattle breed associations in the United States now produce EPDs for marbling and a few have published EPDs for tenderness measured by shear force.
A low proportion of beef cattle and sheep are performance-recorded and included in genetic improvement programmes. In these industries, there are many small-scale breeders and, although abattoirs usually provide some financial incentives to improve conformation and reduce fat levels, and in some countries to improve marbling levels, few incentives are given to improve other aspects of eating quality.
There has been some genetic progress in lamb and beef carcass composition due to selection programmes. In several countries (e.g. United States, Canada, Europe), ‘central testing’ has been used to identify sheep or cattle of superior genetic merit, where high-ranking individuals from different farms are tested together at a central station to reduce environmental variation. There is some concern over the effectiveness of this approach, especially if animals are submitted at later ages, and the use of this method is now decreasing. ‘Progeny testing’ can also identify superior breeding animals by recording data on progeny of high-ranking animals, either at a central testing station or on-farm. This method allows carcass and meat quality traits to be measured directly on progeny of breeding stock. However, cenare given to improve other aspects of eating quality.There has been some genetic progress in lamb and beef carcass composition due to selection programmes.
In several countries (e.g. United States, Canada, Europe), ‘central testing’ has been used to identify sheep or cattle of superior genetic merit, where high-ranking individuals from different farms are tested together at a central station to reduce environmental variation. There is some concern over the effectiveness of this approach, especially if animals are submitted at later ages, and the use of this method is now decreasing. ‘Progeny testing’ can also identify superior breeding animals by recording data on progeny of high-ranking animals, either at a central testing station or on-farm. This method allows carcass and meat quality traits to be measured directly on progeny of breeding stock. However, central and progeny testing are time-consuming and expensive and are only likely to be used to select sires for use in widespread artificial insemination programmes.
Group breeding and sire reference schemes are now being used by sheep and beef breeders in several countries. Group breeding schemes usually involve a nucleus breeding flock or herd of elite animals taken from different group member farms. This nucleus undergoes intensive recording and selection to produce breeding animals (usually males) of high genetic merit to be used on breeders’ farms. More popular now are sire reference schemes, in which all flocks or herds are linked by the use of common sires on a proportion of females on each farm (Figure 4). These schemes use BLUP on data from all farms to produce EPDs (or EBVs) that are comparable across all member flocks or herds.
Pig and poultry production is mainly controlled by relatively few large national or international breeding companies. In these industries, ‘production pyramids’ exist, where intensive selection takes place in the elite breeding herds or flocks. The resulting animals, of superior genetic merit, are multiplied in number and usually crossed, to produce commercial animals for meat production (Figure 1). All tiers of the industry are therefore influenced by improved genetics in the top breeding herds or flocks. High selection intensities, short generation intervals and reduced environmental influences on production maximize the output of high-quality product. As a result of this structure, there have been industry-wide improvements in growth rate, uniformity, muscle yield, feed conversion efficiency and fat levels in both pigs and poultry.
Major Genes
Most production traits are continuous in their distribution and are controlled by the action of many genes, each having a small effect. These are termed polygenic or quantitative traits. However, some traits are under the genetic control of a single major gene. Genes are considered to be ‘major genes’ when the difference in performance between two genotypes is at least one phenotypic standard deviation in the trait of interest. Some major genes are known to have large effects on carcass composition and meat quality traits in the populations in which they are found. Examples are given in Table 3.
Phenotypic records from relatives can be monitored to detect the presence of major genes and identify individuals and families with the desired genotypes. The use of molecular techniques to identify animals with different genotypes (see Animal breedbreeding and genetics: DNA markers and marker-assisted selection) will allow much greater exploitation of these major genes or other genes with smaller, but important, effects. More advanced molecular techniques, such as cloning and genetic modification of livestock species, may also play important roles in the meat industry in future (see Biotechnology in meat animal production: Cloning; Transgenic and genetically modified organisms).
Future Considerations
Traditionally, the aim of selection was to increase production efficiency and lean yield in farm animals raised for meat production. However, recent consumer preferences for healthy, convenient meat products, produced in welfare-friendly systems call for different selection objectives and breeding goals. Future selection objectives are likely to incorporate more meat quality issues. Genetic variation has been blamed for an inconsistent product. However, genetic variation provides the opportunity to increase meat quality within livestock populations. The potential to improve meat quality by traditional breeding methods would be greatly increased by the development of tools to measure or predict meat quality in vivo. The incorporation of such measures into large-scale organized breeding programmes would allow direct selection for meat quality traits.
By N Lambe and G Simm, SAC (The Scottish Agricultural College), Edinburgh, UK in the book 'Encyclopedia of Meat Sciences',vol. 1 Elsevier Ltd. UK, 2004,Editors Carrick Devine and Michael Dikeman, Editor-in-chief Werner Linth Jensen, p.11-18- Edited to be posted by Leopoldo Costa
Introduction
Meat production can be influenced to a large extent by animal breeding and genetics. Several breeding strategies that can affect carcass composition and meat quality will be reviewed in this article. These include (i) selection between breeds within species; (ii) crossbreeding to combine desirable characteristics from more than one breed or strain, or to exploit heterosis in crossbred progeny; and (iii) genetic selection of superior breeding stock within a breed. Genetic influences on carcass and meat quality and selection programmes designed to improve these traits will be discussed.
Differences between Breeds
When selecting between breeds, it is important to choose a breed that is able to perform well within the relevant environment, and that will meet the appropriate market demands. Differences among breeds are only relevant in the environments in which they have been measured. Most breed comparisons of carcass composition and meat quality have been performed in temperate climates, using animals on a high level of nutrition. Studies performed in more extreme environments (e.g. tropical or subtropical) or when food is less abundant have been fewer in number and have shown less convincing evidence of breed differences. It must also be remembered that breeds evolve as a result of selection, so results from breed comparisons may change over time.
Carcass Composition
Large between-breed differences exist within all farm animal species for growth and carcass composition traits. As an animal matures it undergoes an increase in the ratio of muscle to bone, followed by a decrease in muscle growth rate and an increase in the ratio offat to muscle. However, different breeds differ in their rate of maturation and average mature weight. Therefore, standardizing measurements of body composition (proportions of muscle, fat and bone) to the same stage of maturity of body weight (ratio of actual weight to expected mature weight) results in much less variation in carcass composition than standardizing to the same age or weight. One exception to this rule is the Texel breed of sheep, which shows less total body fat than expected for its mature size.
In beef cattle, late-maturing breeds, such as the Continental European breeds, are often preferred under conditions of good nutrition, producing heavier carcasses with little fat. Early-maturing beef breeds, such as the traditional British breeds (e.g.Angus, Hereford, Shorthorn), can be slaughtered at lighter weights and may be preferred when food supply is limited or for certain markets. Similarly, in lamb production systems, the use of early-maturing breeds (e.g. Southdown) will allow quick finishing of small lambs with good carcass composition.
However, the use of larger breeds that mature later (e.g.modern Suffolk strains) will result in heavier lambs with less fat. Traditionally, early-maturing pig breeds (e.g. Middle White) were used for pork production and later-maturing breeds (e.g. Large White) for bacon production. Strains and hybrids of improved pig breeds that are now used in pork and bacon production (e.g. Piétrain, Landrace, Hampshire, Large White) have better carcass composition compared to traditional British pig breeds (e.g. Tamworth,Gloucester Old Spot, Saddleback) owing to reduced fat levels and increased muscle percentage.
Breeds may partition fat and muscle differently between body depots. Dairy breeds of sheep and cattle have a higher proportion of body fat in internal depots than do meat breeds, which have higher proportions of subcutaneous fat. In general, more maternal sheep breeds, which have higher reproductive rates and higher levels of milk production, also have increased proportions of non-carcass fat.
During growth and development, intermuscular fat is deposited before subcutaneous fat, which is deposited before intramuscular fat. Therefore, relative to subcutaneous fat, large late-maturing cattle breeds have more intermuscular fat than small early-maturing breeds, which have increased levels of intramuscular fat (e.g. British beef breeds compared to ContinentalEuropean breeds). Breed comparisons in pigs have found that the Duroc, Meishan and Berkshire breeds have a high proportion of intramuscular fat compared to other improved breeds, and for some markets the level of intramuscular fat in pure Duroc pigs is too high for consumer acceptability.
Meat Quality
In addition to yield and carcass composition, meat quality is determined by traits such as colour and composition of muscle and fat, level of intramuscular fat, juiciness, tenderness and texture, flavour and aroma. Objective laboratory-based techniques, which are discussed in other articles, have been developed to quantify many of these traits. These measurements are often referred to as technological traits and some examples are listed in Table 2.
The most widely used of these techniques is ‘shear force’, which measures the force required to cut through samples of cooked meat. Trained sensory panel analysis is still considered the most relevant measure for many meat quality traits and Table 2 also gives examples of some of these sensory traits. There is substantial evidence of between breed variation in technological and sensory meat quality traits. Examples include the following.
• Paler, more watery muscle, with more exudation of fluids during storage, in ‘improved’ pig breeds compared to traditional British pig breeds.
• Yellower fat in Channel Island cattle breeds compared to other cattle breeds.
• More tender, fine-grained meat in smaller breeds of cattle, owing to smaller muscle bundles.
• More tender meat in ‘double-muscled’ Piedmontese cattle compared to other breeds.
• More tender meat in Bos taurus (domesticated) cattle breeds than in Bos indicus (humped cattle) breeds. Bos indicus cattle show increased calpastating post-mortem tenderization.
• More tender meat in Duroc pigs compared to most other breeds, owing to an increased amount of red fibres in the muscle.
• Increased flavour, juiciness and tenderness in pigs with an increased percentage of Duroc, Berkshire or Meishan genes.
Breeds can also react differently to pre-slaughter, slaughter and processing methods. For example, leaner and lighter animals are more likely to suffer from cold-shortening in the carcass post mortem (see Conversion of muscle to meat: Cold and heat shortening).
Crossbreeding
Crossbreeding can be used for several reasons, which can be exploited simultaneously. One reason may be to combine desirable characteristics from more than one breed or strain. This is termed ‘complementarity’. In the pig and poultry industries, different breeds or specialized lines, selected for different characteristics, are commonly crossed to produce commercial hybrids. Sire lines are often selected to be heavier and faster-growing, while female lines may be selected for reproductive traits, low maintenance requirements or other economically important factors. An example of such a system for pigs is given in Figure 1. The breeds or strains chosen for crossing will depend on their suitability for specific environments as well as market requirements.
Crosses between different breeds of sheep and cattle are also used to combine desired characteristics. In the United Kingdom, hill ewes (e.g.Scottish Blackface, Swaledale) are often crossed to rams from upland breeds (e.g. Blue-faced Leicester, Border Leicester) with high maternal performance. The crossbred female progeny are then mated to terminal sire breeds (e.g. Texel, Suffolk, Down breeds) that have been selected for improved carcass and growth traits, to produce a high number of better-quality lambs.
Another reason for crossbreeding is to exploit heterosis, or hybrid vigour (advantage over the average performance of the individual parent breeds). Heterosis is most widely exploited in the pig and poultry industries. The ‘combining ability’ of different strains is tested to increase production. Some strains combine well only when used as the male or as the female parent. The purebred lines that are used for crossing can either be selected to improve specific traits within line (e.g. meat yield, fertility) or can be selected on the performance of their crossbred progeny (reciprocal recurrent selection). Heterosis is usually greater for traits with low heritability (see below), such as those affecting overall fitness, reproduction or survival, and less for production or carcass traits.
Some evidence suggests that crossbred cattle mature more quickly and so are heavier at a given age, with more marbling fat, total fat and muscle. However, after adjusting for weight, heterosis for carcass composition traits tends to be low. Effects of heterosis on most technological and sensory traits have not been studied widely, but available estimates tend to be low for juiciness, tenderness, flavour, cooked colour and overall desirability. Results for the effect of heterosis on shear force in beef are more numerous and range from slightly unfavourable to moderately favourable.
Differences within Breeds
Genetic improvement can be made within breeds for desired characteristics. This method relies on the fact that the traits to be improved are heritable, and that more animals are produced than need to be kept for replacement stock, so allowing selection among progeny for preferred breeding animals.
Heritability is the proportion of the total phenotypic (observed) variation in a trait that is explained by genetic variation. It is therefore a measure of how much a trait is controlled by genes (or, more precisely, genes that act additively), as opposed to environmental influences. Heritability is expressed on a scale of 0 to 1, where a value of 1 suggests that the trait is completely controlled by an animal’s genes, while management, feeding and other environmental factors play no part in determining the expression of the trait. Other things being equal, a higher rate of genetic improvement will result by selecting on a trait with high heritability. Estimates of heritabilities for the main carcass and meat quality traits differ between studies but tend to fall in the ranges identified in Tables 1 and 2.
It is important to note that the magnitude of the environmental variation and, therefore, the heritability, depends very much on our ability to standardize measurements. The better we are able to measure meat quality traits, the higher the heritability and the higher the selection success. A low heritability does not necessarily mean that there is too little genetic variance for selection; rather it may reflect our difficulties in measuring the traits in a standardized, repeatable manner. Genetic parameters may also differ according to which muscle is tested within an animal, or owing to differences in preslaughter
conditions (e.g. stress during transport or at the abattoir), or processing methods (e.g. electrical stimulation, conditioning). For example, shear force has been reported to be more variable in carcasses that have not undergone electrical stimulation. In particular, genetic parameters for meat tenderness vary widely, probably owing to the fact that this is a very complex trait, depending on many factors (pH and temperature changes post mortem, glycolysis, processing) and the interactions between them.
Selection on one trait will often lead to correlated responses in other traits, which are not always monitored but could have very important economic consequences. A selection programme designed to improve carcass composition should also be concerned with the effects of these changes on sensory meat quality traits and should monitor effects on reproductive traits and functional fitness. For example, there are concerns that selection for reduced fat levels may delay puberty in cattle and be associated with reduced fertility of females. Genetic improvements in poultry production have also been related to a decrease in meat quality in terms of flavour and texture. In general, intramuscular fat percentage has a negative genetic relationship with meat yield and a positive genetic relationship with total fat. Intramuscular fat in pigs was ignored in early studies, as it was thought to be unimportant to eating quality. As pigs were selected for reduced back fat, the level of intramuscular fat was also reduced, which was later linked to a reported decline in the eating quality of pig meat.
Attempts are now under way in some countries to increase intramuscular fat in pig meat without increasing total fat. Studies in some sheep and cattle populations suggest that this may be possible, as low correlations have been found between levels of fat in different depots. These low correlations may allow selection in these species to reduce one fat depot (e.g.subcutaneous), while maintaining moderate levels of fat in other depots to fuel survival and reproduction meat quality are more heritable than sensory traits (e.g. internal depots), or to maintain marbling and meat quality (intramuscular fat).
Correlations among carcass composition, technological and sensory meat quality traits differ between studies and few strong trends have emerged. In general, the literature suggests that selecting for leanness might have slight negative effects on eating quality traits (e.g. water-holding capacity, tenderness, juiciness, pH, drip loss). Sensory quality traits such as tenderness, flavour intensity and juiciness tend to be positively correlated to one another, and genetic correlations between these traits and shear force tend to be negative (lower shear force equals more tender).
Shear force also has a low to moderate negative genetic correlation with intramuscular fat. Tenderness is widely thought to be the most important determinant of meat quality to the consumer. However, genetic parameters for this trait differ, depending on the muscle tested and the method of measurement (myofibrillar fragmentation index, calpastatin activity at 24 hours, shear force, or sensory panel assessment of tenderness). Preliminary results suggest that calpastatin activity at 24 hours is highly genetically correlated with shear force, but that the phenotypic correlation between the two measurements is only moderate. Correlations of tenderness among different muscles are moderately low and the correlation between shear force and sensory panel evaluation of tenderness also changes between muscles. As a result, selection for tenderness may be difficult and further work is needed to determine how technological and sensory measurements are related to each other, both between and within breeds.
Selection Programmes
In some cases the breeding goals and the selection criteria are the same, but often they are not, especially with carcass and meat quality traits. Traits to be used as selection criteria must be highly repeatable and be practical to measure on-farm or on-line during animal processing. Selection criteria should be heritable and show sufficient variation within the population. The design of the breeding programme should define the number of male and female animals that will be selected each year, the age at mating, the generation interval and other such factors.
Genetic selection programmes differ in complexity. A relatively simple approach is to select breeding animals based on their own phenotypic performance (e.g. ultrasound data obtained from selection candidates).However, the impossibility of collecting phenotypic carcass and meat quality data on selection candidates has restricted the use of this method of selection for many of these traits. ‘Independent culling levels’ in one or more traits are often used to improve the genetic merit of the flock or herd. This method involves choosing animals for breeding only if they reach a certain threshold in each trait of interest (e.g. over a certain weaning weight, and/or below a certain ultrasound fat depth).
A selection index is a more complicated but an effective method of selecting animals on one or more traits, based on the performance of the individual and its relatives. This method allows selection on traits that can be measured directly or indirectly (using predictor traits) on the selection candidates themselves, and also traits that can only be measured on relatives (e.g. slaughter and meat quality traits).
Accurate recording of performance data and pedigree structure is vital in these programmes. The amount of emphasis or weighting on each trait in a multi-trait index can be altered and is usually determined by economic importance. Estimated breeding values (EBVs) for each trait for each animal are produced, as well as an overall ‘index score’, based on the combined merit of the individual in all traits included in the index. Selection decisions are based on these index scores. Response to selection in any individual trait per generation using a multi-trait index is smaller than could be achieved by selecting for that trait alone.
However, index selection should lead to the highest rate of change in overall economic merit. Estimated breeding values are calculated, in most selection programmes, using a statistical procedure known as best linear unbiased prediction (BLUP). This procedure separates the genetic effects for each trait from management and environmental influences. The EBV is determined by the genetic merit of the animal itself, plus that of its relatives, for the trait of interest and reflects the merit of that animal compared to the population mean. Since, on average, each breeding animal passes half its genes to its offspring, the breeding value for each trait is often expressed as the expected progeny difference (EPD), which is half the EBV of the breeding animal.
Because there are no live-animal predictors of most technological and sensory meat quality traits, breeding programmes designed to improve meat quality use measurements taken on slaughtered relatives of selection candidates to calculate EPDs (or EBVs depending on the scale used) for these traits. For example, most cattle breed associations in the United States now produce EPDs for marbling and a few have published EPDs for tenderness measured by shear force.
A low proportion of beef cattle and sheep are performance-recorded and included in genetic improvement programmes. In these industries, there are many small-scale breeders and, although abattoirs usually provide some financial incentives to improve conformation and reduce fat levels, and in some countries to improve marbling levels, few incentives are given to improve other aspects of eating quality.
There has been some genetic progress in lamb and beef carcass composition due to selection programmes. In several countries (e.g. United States, Canada, Europe), ‘central testing’ has been used to identify sheep or cattle of superior genetic merit, where high-ranking individuals from different farms are tested together at a central station to reduce environmental variation. There is some concern over the effectiveness of this approach, especially if animals are submitted at later ages, and the use of this method is now decreasing. ‘Progeny testing’ can also identify superior breeding animals by recording data on progeny of high-ranking animals, either at a central testing station or on-farm. This method allows carcass and meat quality traits to be measured directly on progeny of breeding stock. However, cenare given to improve other aspects of eating quality.There has been some genetic progress in lamb and beef carcass composition due to selection programmes.
Group breeding and sire reference schemes are now being used by sheep and beef breeders in several countries. Group breeding schemes usually involve a nucleus breeding flock or herd of elite animals taken from different group member farms. This nucleus undergoes intensive recording and selection to produce breeding animals (usually males) of high genetic merit to be used on breeders’ farms. More popular now are sire reference schemes, in which all flocks or herds are linked by the use of common sires on a proportion of females on each farm (Figure 4). These schemes use BLUP on data from all farms to produce EPDs (or EBVs) that are comparable across all member flocks or herds.
Major Genes
Most production traits are continuous in their distribution and are controlled by the action of many genes, each having a small effect. These are termed polygenic or quantitative traits. However, some traits are under the genetic control of a single major gene. Genes are considered to be ‘major genes’ when the difference in performance between two genotypes is at least one phenotypic standard deviation in the trait of interest. Some major genes are known to have large effects on carcass composition and meat quality traits in the populations in which they are found. Examples are given in Table 3.
Phenotypic records from relatives can be monitored to detect the presence of major genes and identify individuals and families with the desired genotypes. The use of molecular techniques to identify animals with different genotypes (see Animal breedbreeding and genetics: DNA markers and marker-assisted selection) will allow much greater exploitation of these major genes or other genes with smaller, but important, effects. More advanced molecular techniques, such as cloning and genetic modification of livestock species, may also play important roles in the meat industry in future (see Biotechnology in meat animal production: Cloning; Transgenic and genetically modified organisms).
Future Considerations
Traditionally, the aim of selection was to increase production efficiency and lean yield in farm animals raised for meat production. However, recent consumer preferences for healthy, convenient meat products, produced in welfare-friendly systems call for different selection objectives and breeding goals. Future selection objectives are likely to incorporate more meat quality issues. Genetic variation has been blamed for an inconsistent product. However, genetic variation provides the opportunity to increase meat quality within livestock populations. The potential to improve meat quality by traditional breeding methods would be greatly increased by the development of tools to measure or predict meat quality in vivo. The incorporation of such measures into large-scale organized breeding programmes would allow direct selection for meat quality traits.
By N Lambe and G Simm, SAC (The Scottish Agricultural College), Edinburgh, UK in the book 'Encyclopedia of Meat Sciences',vol. 1 Elsevier Ltd. UK, 2004,Editors Carrick Devine and Michael Dikeman, Editor-in-chief Werner Linth Jensen, p.11-18- Edited to be posted by Leopoldo Costa
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