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R. L. WILLHAM Genetics of Fat Content in Animal Products The synthesis of fat by animals probably evolved as a means of con- centrating available energy for deposition and secretion. The deposition of fat by animals is both an individual and a population homeostatic process that allows exploitation of available food supplies and syn- thesizes this abundance into energy reserves for future mobilization and for various protective strategies against the external environment. The secretion of fat by animals is primarily for the nurture of the young. The fat provides the energy concentration necessary for growth and development. Provision for the young is related to the "fitness" of a species and is part of the genetic complex necessary for survival. Past the protective role of deposition, the rate at which deposition occurs appears to be plastic to accommodate long-term environmental changes. Since man domesticated animals and began using animal products, changes have occurred both in the genetic composition of the species and in the agricultural environments under which the livestock have been produced. Besides this, the needs of man met by animal products have undergone change. Less need exists now for concentrated energy to accomplish manual labor than in the past. Today man is confronted with an energy shortage that may preclude the use of animal diets high enough in carbohydrates to supply an excess of energy over mainte- nance, growth, and reproduction for fat synthesis and deposition. This chapter has three purposes. The first is to describe the kind and the relative amount of genetic variation available in domestic species that can be used to change the fat content of animal products. The 85
86 R. L. WILLHAM second is to discuss the genetic problems that can result from changing the fat content. These include genetic defects and undesirable genetic correlations with other traits of economic import. The third is to define breeding programs based on available genetic knowledge-programs that, when implemented, can achieve genetic change in the fat content of animal products. DESCRIPTION Any description must be in terms of differences, since without a differ- ence the genetic process is not observable. Most genetic differences are small relative to the total amount of biological variation that exists. Thus, genetic differences are usually considered in the context of a common environmental set. Even then, animals treated as nearly alike as is physically possible differ not only in the expression of genetic differences but also by intangible environmental differences that remain uncontrolled. Coefficients of variation for numerous traits in animal populations range from 10% to 20% of the mean. The basic problem related to changing the genetic composition of animal populations is the Mendelian fact that genes have their pheno- typic expression on the diploid individual in pairs, yet the genes are transmitted singly. This fact makes the resemblance between parent and offspring the basis of selection. Since a sample half of the parental genes (one at random from each pair) is transmitted to the offspring, the degree of resemblance for a trait describes the relative amount of the phenotypic variance attributable to half of the gene effects not the gene pair effects. The correlations between relatives have provided the population geneticist with a means of estimating the relative importance of gene effects to the total variation (heritability) and, as a consequence, a means of making predictions about selection. If the resemblance is high between parent and offspring, selection of superior parents will result in above-average offspring. Even though the statistical concept of gene effects can be measured, genes have their phenotypic effect in pairs and in combinations of pairs. The average performance of the parents usually predicts offspring per- formance and is especially likely to do so when heritability is high. For many traits, especially those concerned with fitness, when the parents are of different genetic groups, the performance of the cross is superior to the parental average. This phenomenon is termed heterosis, and its converse is inbreeding depression. Heterosis is produced be- cause the dominant gene of a pair is usually more favorable than its recessive allele. Thus, when genetic groups differ in gene frequency
Genetics of Fat Content in Animal Products 87 and dominance exists, heterosis is produced. Within such genetic groups, as inbreeding proceeds, a higher frequency of gene pairs become homo- zygous recessive than under random mating, and a depression in mean performance results. The study of the response of domestic animal populations to inbreeding and crossbreeding provides the population geneticist with knowledge concerning the relative importance of gene pair effects or of nonadditive genetic variation to the total. The ability to synthesize and deposit fat, from an evolutionary point of view, is of value to the population and the individual by serving as a homeostatic mechanism Werner, 1954) to exploit environmental op- portunities and as a protective device. Probably little selection pressure exists on natural populations for fat deposition, because an ecological opportunity is soon exploited and the gain is stored as population numbers that are in equilibrium with the amount of food necessary for maintenance, growth, and reproduction only. Thus, a latent pool of genes responsible for fat deposition remains ready to buffer the popula- tion against hostile shifts of the environment or to exploit fluctuations in the food supply. The expectation is that differences in fat deposition should be reasonably heritable in current species. In contrast to fat deposition, fat secretion to nurture the young is extremely important in the "fitness" complex of a species. The nurture is critical, and the expectation would be that natural selection pressure over eons of time has reached a stable equilibrium of nutrient amount and content that would best encourage growth and development of the young and promote the survival of both the maternal parent and the offspring. This reasoning suggests that fat secretion by animals for the nurture of their young may be a part of the complex of traits sur- rounding "fitness" and thus have a low heritability but exhibit some hybrid vigor indicative of nonadditive genetic variance. Domestication brought control of reproduction and change in diet, and these affected both deposition and control of fat. For the first time in the course of evolution, reproductive curbs and maximum availability of feed occurred together. The result was that the feed consumed exceeded maintenance, growth, and reproductive needs, and the ability to deposit fat or secrete more of it was allowed to be expressed. To the extent that these differences were heritable, groups within each domesticated species began to be molded genetically by man. The population structure of most domestic species consists of pedigree isolates called breeds. Breeds have existed for about 200 years in many species. Breeds are genetically distinct. They result from differ- ent selection goals and by chance from Mendelian segregation in small populations. When breeds overlap in a definite management system, the
88 R. L. WILLHAM breed differences are mostly genetic. As such, any shift in the percentage breed composition of a species used in livestock production represents a genetic change. A consequence of industrialization and the growth of commercial agriculture has been the development of red-meat animals in which the rate of maturity has been increased. In these animals, fat deposition occurs early in life, and early deposition of fat shortens the period required for making the animals ready for market. Specialized markets prompted the development of breeds that differed in the amount or quality of the products for which they were grown. Milk-producing breeds of cattle were gradually separated from breeds used for produc- ing beef which hints at physiological limits, at least within cattle. In poultry production, layer breeds and broiler breeds were developed. Some breeds of sheep were developed for wool production and some for mutton production. Both in fat deposition and in secretion, vast differences exist between breeds within domestic species. Current research that most clearly il- lustrates this point for fat deposition is that from the Meat Animal Research Center, U.S. Department of Agriculture, where many newly introduced breeds of cattle belonging to several biological types are being evaluated for the economical traits of beef production (Agri- cultural Research Service, 19741. Results indicate large differences in rate of maturity and fat deposition among the several breeds (Table 1~. A difference of more than 1~/4 inches in carcass fat exists between the TABLE 1 Phase 1 Results of Top Crosses Used in the Germ Plasm Evaluation Project at the U.S. Meat Animal Research Center, Agn- cultural Research Service, U.S. Department of Agriculture (ARs-Nc-13, March 1974) Fat Thick- Fat Cut ness Kidney Trim ability Marbling Breed of Sire a No. (inches) ( To ) ( % ) ( % ) Score Hereford end Angus 154 0.61 2.9 21.9 53.0 11.6 Hereford crossed with Angus 211 0.67 2.9 23.0 52.2 1 1.8 Jersey 134 0.47 4.8 22.6 52.0 13.7 South Devon 94 0.51 3.5 21.5 53.1 11.7 Limousin 175 0.42 3.O 17.5 56.7 9.2 Simmental 177 0.42 3.1 18.2 55.3 10.3 Charolais 178 0.40 3.0 17.8 56.1 10.9 a Dams were commercial Hereford and Angus.
Genetics of Fat Content in Animal Products 89 Hereford-by-Angus cross and the Charolais cross. The large percentage of kidney or internal fat shown in the Jersey cross is characteristic of the dairy breeds. Even in marbling score (fat in the lean), large differences exist. The percentage of curability reflects not only less fat deposition but also some real differences in muscling among the breeds. Swine breeds differ in their fat content at a given carcass weight. Sheep of the mutton breeds differ also. The fat secretion of the dairy breeds in the United States when expressed in amount or in percentage of total production, differs by breeds (Wilcox et al., 1971) (see Table 21. The Holstein breed pro- duces the most total fat, even though it has the lowest percentage test. Large differences exist in pounds of milk produced by the breeds. Changing the fraction of the several breeds that contribute to com- mercial production can bring about a large genetic change. The in- crease in the frequency of the Holstein breed in the United States has helped milk production remain relatively constant while the number of dairy cows has decreased. Within breeds, consideration of fat deposition leads to an accumula- tion of data in most of the domestic species, suggesting that fat differ- ences among animals treated alike are highly heritable. The range for a highly heritable trait is 40%-60%. That is, on the average, 50% of the differences among animals treated alike would be due to genetic differences that can be utilized by selection. Two sources of evidence exist on how heritable fat deposition is. The first source is the numerous estimates of heritability derived from calculating the correlation between relatives for the trait. The second is a comparison of the response to actual selection with the amount of selection pressure actually applied. This is termed realized heritability. The latter evidence is more positive because it demonstrates that selection response is possible. More than genetic likeness often exists between related groups of domestic ani TABLE 2 Five Dairy-Breed Comparisons a Yields (305 day 2x, M.E. lactation) Milk Fat Fat Breed (lb) (lb) ( % ) Ayrshire 11,567 466 3.99 Guernsey 10,601 521 4.87 Holstein 15,594 583 3.70 Jersey 9,798 507 5.13 Brown Swiss 12,814 539 4.16 a SOURCE: Wilcox et al. ( 1971 ) .
go R. L. WILLHAM mars. This increases the correlation and tends to reduce the selection response from that predicted by using the inflated estimates. Because of size, individual value, time, and biological parameters concerned with reproductive rate, few selection studies using domestic species have been conducted. The most recent traditional high-low selection study reported in domestic animals is for backfat thickness in Duroc and Yorkshire breeds of swine (Hetzer and Harvey, 19651. Table 3 gives realized heritability estimates from selection responses expressed as deviations from the controls. The authors concluded that selection was highly effec- tive in both the upward and downward directions and that the heritabil- ity of backfat thickness is about the same for both breeds. The realized heritability was similar to estimates in the literature that were obtained by using correlations among relatives. Heritability estimates for various measures of fat deposition are high in the red-meat domestic species. Values for heritabilities are sum- marized in the following caners and books: ~ A ~ Beef (reviews of evidence): Warwick (1958), Cundiff and Gregory 1 968 ), Lasley ~ 1 972 ~ Swine (summaries of estimates): Craft (1958), Omtvedt (1968), Lasley(1972) Sheep: Terrill (1951,1958), Lasley (1972) The actual point estimate of heritability is not important. The breeder needs it classified as high, moderate, or low. Heritability estimates are of use in the design of breeding programs, since the heritability of a trait is the criterion of what selection method will make the most rapid genetic change. Since fat deposition is highly heritable, simple selec TABLE 3 Realized Heritability Estimates from Selection Responses Expressed as Deviations from Controls ~ Line Generation Breed of Selection High Fat Low Fat Duroc 0-5 48 73 5-10 29 30 0-10 47 48 Yorkshire 0-4 13 64 4-8 60 46 ~8 38 43 a SOURCE: Hetzer and Harvey (1965).
Genetics of Fat Content in A nimal Products 91 lion of parents on the basis of their own phenotypic values maximizes selection progress per unit of time. The problem is that unless fat deposition can be measured on the live animal, selection must be based on slaughtered sibs or progeny. This usually reduces selection progress per unit of time. Review of many breed-and-line cross studies with beef, sheep, and swine provide evidence that very little heterosis or inbreeding depres- sion is present in most measures of fat deposition. Studies reviewing the subject are as follows: Beef: Warwick ( 1958 ), Cundiff and Gregory ~ 1968 ), Cundiff ( 1970) Swine: Craft ( 1958 ), Lasley ( 1972) Sheep: Terrill (1958) The crossbreds are a bit fatter than the average of the parents, which at present is not desirable. Willham and Anderson (1974) found about 4% heterosis for marbling score but negative heterosis for retail product, indicating that the crossbreds, which included beef-dairy crosses, were fatter than the parental average. Nothing like the hybrid vigor of 5%- 10% found in the lowly heritable reproductive complex exists for the carcass traits of domestic species. Such evidence leads to the conclusion that the kind of genetic variation available for making genetic change in fat deposition is additive primarily and can be used in a selection program both among breeds that differ in fat deposition and within breeds. Further, the additive genetic variance in measures of fat deposi- tion among animals treated alike accounts for about 50% of the phenotypic variance. No evidence exists that suggests that selection will be ineffective if a change in fat deposition is desired. In fact, breeds in beef cattle and sheep and lines within breeds of swine already exist that could be used to raise or lower the amount of fat deposited at any age or weight when slaughtered. Primarily, the fat deposited relates to differences in rate of maturity among these genetic groups or in the rate at which fat deposition occurs. Fat secretion by the mammary tissue of cattle and the fat secreted for the development of the egg yolk in poultry appear to be less highly heritable when amount is considered than when percentage is con- sidered. Wilcox et al. (1971) gave heritability figures of 20%-30% for amount of fat and other milk constituents, and Nordskog et al. (1974) gave values of 20%-30% for egg weight in laying poultry. The heritabilities for milk constituents as percentages are high between 40 % and 50% (Wilcox et al., 1 97 1) . The basic problem of altering fat content in milk and probably yolk
92 R. L. WILEHAM is the very high positive genetic correlations that exist within the amount traits and the percentage traits. Selection to decrease the percentage of fat in milk or the pounds of fat would appear to be possible. How- ever, the accompanying decrease in solids-not-fat, total solids, and protein would be disastrous to the product in dairy production. This suggests that natural selection has set up the composition of milk and yolk within the limits of optimum nutrients for nurture of the young. This is suggested by Lerner (1951) in domestic poultry. He demon- strated that the highest reproductive fitness was found in birds with genotypes for intermediate egg size and that the optimum in populations subjected to artificial selection for large egg size fell below the mean. To increase protein and decrease fat in milk, as an example, would appear to be difficult. However, to simply increase the amount of milk solids by increasing total milk production is obviously possible, with an estimated realized 1% of the mean genetic trend for increased milk production in Holstein-Friesian cattle (Miller et al., 1969; Powell and Freeman, 1974) . Little economic heterosis exists in milk production, as indicated by Touchberry (19711. Heterosis does exist (6.4% for pounds of milk), but the milk production of the Holstein breed is too high to make the crossbreeding program economically feasible. Line crosses of poultry are used commercially to capitalize on the reproductive heterosis in egg number. Milk production is reduced by inbreeding (Young et al., 19691; however, the milk constituents as percentages are only slightly depressed. Nordskog et al. (1974) reports inbreeding depression in egg size also. A review of fat secretion in animal products suggests a moderate heritability, with some indication of hybrid vigor and inbreeding de- pression usually found with traits of moderate heritability. Even though the milk content is not involved in reproductive fitness as it once was, the high genetic correlations among the constituents of milk suggest that the fat secretion would be difficult to alter genetically without changing the other constituents of milk. The general conclusions to be drawn from the research evidence concerning the kind and relative amount of genetic variation available to change the fat content of animal products are as follows: · Fat deposition among animals treated alike is highly heritable. Major breed differences exist in rate of maturity and fat deposition. The small amounts of heterosis or inbreeding depression that exist suggest that there is little nonadditive genetic variance. Results of selec- tion for increase and decrease in external fat deposition (backfat) in swine suggest that heritability estimates are logical in magnitude.
Genetics of Fat Content in Animal Products 93 · Fat secretion is moderately heritable among animals treated alike. Major breed differences exist in the amount and percentage of fat. Heterosis and inbreeding depression results suggest a moderate amount of nonadditive genetic variance for milk and fat production; however, this has not been exploited economically because of the superiority of one breed for milk production and because of the management system that controls calf production where heterosis could be beneficial. Evi- dence suggests that a genetic trend of about 1 percent of the mean for milk production exists, indicating that sire selection in conjunction with artificial insemination is effective in increasing production both of fat and of solids-not-fat. Commercial poultry are usually the product of a line or strain cross made to capitalize on the hybrid vigor for egg number as a reproductive trait. High genetic correlations exist among the constituents of milk and probably egg composition. Total production can be increased. Improvement in protein content with a reduction in fat content appears to be highly unlikely. GENETIC PROBLEMS Lerner (1954) in his book on genetic homeostasis concludes from his review of the literature that there exists a definite antagonism after two or three standard deviations of selection change for a quantitative trait between artificial selection and natural selection. Practical evidence exists in domestic species as well that selection for an extreme results in picking up genetic trash that in the heterozygote was contributory to the extreme and was consequently selected, increasing the frequency of the unfavorable recessive gene. The advent of a high frequency of the dwarf gene in beef cattle breeds was the result of selection by the breeders of small, compact extremes with very mature form at an early age. Today the incidence of the "culard," or double muscling, condition in beef cattle is increasing as a probable result of selection for increased muscling and less fat. The condition is prevalent in some of the newly introduced breeds, because it is considered desirable by European butchers and exists in the British breeds. Severe reproductive conse- quences, especially in the female, exist when the culard condition is extreme. Current work suggests that the condition is a simple recessive with variable penetrance. See Keifer et al. (1972) for a description of the problem. The generally accepted example of a fluid population for changes in fat deposition is swine, where major type changes have occurred numer- ous times in the last century. See Craft (1958) for a description of these changes. Today selection in swine for reduced fat deposition is based either on the mechanical backfat probe (Hazel and Kline, 1952) or on
94 R. L. WILLHAM subjective evaluation mainly directed toward increased muscling. Other objective means of evaluating fatness in the live animal, such as the use of high-frequency sound, are available. Surprisingly, after the breeders saw what a meaty pig should look like, they were able subjectively to select for increased meatiness with correspondingly less fat, especially in the shoulder, where much seam fat exists. The result of the selection was that meat-type pigs had much bigger hams. Christian ( 1968 ), considering the increased incidence of pale, soft, exudative pork muscle (PSE) and of the porcine stress syndrome (Pss), suggested that subjective selec- tion for augmented muscling could intensify these problems. Christian (1972) indicates that the heritability estimates of pork-quality measures are moderate but are antagonistically related to most measures of muscle quantity, although the correlations are low. The mode of in- heritance of PSS is not yet known, but Christian (1972) considers the possibility of simple inheritance. Ways are now being sought to detect susceptible swine, and the effort is much more promising than that to detect the carrier of the dwarf gene in cattle. Both PSE and PSS are critical in the swine industry today (Topel, 19681. Besides the inherited simple genetic problems involved or at least related to the reduction of fat deposition, the genetic correlations with other traits of economic importance need to be considered. The litera- ture for beef and swine suggest no major genetic antagonisms among the reproduction, production, or product traits at least from genetic correlations estimated using relative groups. Marbling in beef carcasses is the basis of the USDA quality grades. Marbling is positively correlated to fat deposition in the entire carcass; this is not unexpected, but it does not help produce cattle with minimum outside fat that grade USDA Choice. Koch (1974), using data from the U.S. Meat Animal Research Center, demonstrated this. Work on the selection study with swine suggests that there is no clear indication of a consistent decline in reproductive fitness due to selection for backfat thickness (Hetzer and Miller, 19701. However, the selection progress that has been made is slight in comparison with that made in studies involving laboratory species (Lerner, 1954~. Hetzer and Miller (1972) reported that selection for lower fat seems to increase the Duroc growth rate and that the opposite is indicated for the Yorkshire breed. Hetzer and Miller (1973), as expected, reported increased meatiness in the carcasses of the low-fat line. However, the Yorkshires seemed to show greater increases in meatiness and more decreases in fatness than the Durocs. Bereskin et al. (1974) present evidence suggesting that the low-fat lines are better mothers, indicative of more milk, when pig weight at weaning is considered. This appears
Genetics of Fat Content in Animal Products 95 reasonable; with a given amount of excess feed over maintenance, some priority in use must be made and this suggests physiological limits. Indications are that dairy cattle use this excess for milk production and even call upon body reserves, and that beef cows are able to nurse a calf and put on fat (Willham and Anderson, 19741. The ability of dairy breeds to rebreed with a calf at side under beef management is less than for beef breeds. Dairy cattle cumulate the insults of beef management until they fail to reproduce in the fixed breeding system. The high (0.8-0.9) positive genetic correlations between the amount traits of milk and between the percentage traits reported by Wilcox et al. (1971) have been mentioned. These correlations indicate that selection can be expected to change amount or percentage, but that all constituents will be changed in a like direction. Amount of butterfat in milk has usually been the trait selected, or at least a minimum per- centage has been set as desirable. Easy methods exist for separating milk to produce a commercial product with a low fat content. To con- sider the possible solids-not-fat reduction from selection makes selec- tion for reduced fat hardly seem worthwhile. Studies are under way in the north central part of the United States to monitor problems that may be encountered by selection for increased milk production alone. Numerous studies have investigated methods of reducing the cholesterol level in the egg yolk, but no genetic studies have been re- ported. The prospects of selecting for less fat secretion in animal products, produced to nurture the young, appear to be limited. Reduction in fat deposition by breeding could theoretically result in lean meat entirely devoid of fat content; however, before this stage is reached, production limits under current management systems, as well as natural selection, will end selection progress. Too little fat deposition could reduce the protective role of fat, especially in breeding stock. Severe reductions in adaptability would possibly result. Much before this, cries from consumers would be heard, especially in the United States consumers demanding tender, juicy, flavorful meat con- taining a relatively high proportion of fat. For beef from carcasses of British breeds to be graded USDA Choice, the carcasses must have a fat content averaging about 30%. BREEDING PROGRAMS Information concerning the genetic aspects of the fat content of animal products must be synthesized into a workable technology before it can be applied to a specific livestock enterprise. This technology can best be accomplished by developing a breeding program designed by considering
96 R . L . WI L LHA M the direction sought, the description of the available genetic differences, and the decisions based on the descriptions to accomplish a move in the desired direction. The latter consideration is selection. Selection is the only force available to breeders to make directional genetic change in livestock populations. Such deliberation requires not only information on the genetic and economic values of fat content but also information on the other classes of traits that are of economic importance to a livestock enterprise. To some extent the information must relate to a given species. Why this is true can be seen by considering the current beef industry values found in Table 4. For simplicity, the traits of economic importance are ar- ranged in three classes: reproduction, production, and product. Traits in the reproduction class include calf crop percentage and calving difficulty. The production class involves both maternal traits, such as maternal performance and milk production, and market traits, such as average daily gain and feed efficiency. The product class involves both quantity and quality of the product or the carcass produced by the commercial animal. Because of the low reproductive rate in cattle, the reproductive class of traits is at least five times as important as improvement in the produc- tion traits. Currently, the production traits are twice as important eco- nomically as the product traits. This is the basic issue in considering fat deposition changes. Today, breeders in the beef industry simply do not have an economic incentive to devote great effort to the improvement of fat content. Where the females produce litters, reproduction is less important; female costs can be spread over numerous offspring. In swine, the product traits are more important economically than in cattle, and more has been done to improve them. In the "Heritability" and "Heterosis" columns of Table 4, the values are negatively correlated. That is, the reproductive class has the lowest TABLE 4 Current Beef Industry Values Breed Relative Differ Classof Economic Heritability Heterosis ences Traits Values ( % ) ( To ) ( % ) Reproduction 10 10 10 10 Production 2 40 5 50 Maternal - 20 7 40 Market 50 3 60 Product 1 50 0 5 .
Genetics of Fat Content in Animal Products 97 average heritability but the highest percentage of heterosis realized from crossbreeding, and the product traits have the highest heritability but almost no heterosis. As a general rule, this relationship exists over the red-meat species of domestic animals. These values determine the kinds of breeding programs that can be used within an industry. The produc- tion and product traits can usually be improved by selection, whereas the reproductive complex ("fitness") responds to crossbreeding. Little free additive genetic variance appears to remain after the eons of natural selection for fitness. In the last column of Table 4, we see that there are large breed differences in the production traits, both in those relating to maternal ability and in those relating to market traits. Differences of 50% of the mean are common in feedlot gain, for example. Breed differences also exist in the reproductive complex and in the product traits, even when breeds are taken to the same fatness rather than the same age or weight. Age and weight result in large breed differences because of the rate of maturity. A table similar to Table 4 could be constructed for each domestic species. Many common elements exist. The percentages of heritability and heterosis are relatively constant in meat, milk, and eggs. If we study this table, we see that improvement in the reproductive complex means economic improvement. This suggests a commercial crossbreeding pro- gram in which crossbred females are used to obtain heterosis in the reproductive complex. When breeds are selected for a cross, there is an opportunity to select those that complement each other. Selection on this basis can aid in developing a maternal line of cows and in producing a market steer with just enough fat deposition to meet current grading standards and demand top price. The heritability of the production and product traits is high enough that breeding stock herds producing germ plasm for the commercial producer can select for improvement in these two classes of traits and pass this improvement directly to the producer through superior breeding stock. Basic industry values set the possible breeding programs for a species; but in program design it is necessary to specify goals of the enterprise, the way in which differences among selection units (individuals or pos- sibly breeds) are to be measured, and the selection decisions based on the available measurements. Setting goals is a major problem among breeders. Changing the goal from one thing this year to another next has been the downfall of many operations. This and the need for fat in wartime have caused swine breeders to relax their reduction in fat deposition at least twice since 1900. Since 1950, however, breeders have been under steady pressure
98 R. L. WILLHAM to produce meat-type hogs. As a result, definite industry change has been made. Because of lack of economic incentive, no such change has been made in the beef cattle industry. The grading system for cattle supports a degree of fat deposition that is too high to be attained by the half- continental breeds recently imported. The high cost of concentrates, used extensively to fatten beef in the past, may in the future prohibit their use in feeding livestock. Thus, the search will be for genetic combinations that will fatten on high forage rations at a young age. Assuming that there is economic incentive, breeding values need to be considered. Because of the high heritability for most carcass traits, selection based on the animal's own performance results in maximum genetic change per unit of time. Thus, considerable effort has been de- voted to developing methods of measuring the carcass composition of live animals. The development of the mechanical backfat probe by Hazel and Kline (1952) was a significant breakthrough in the swine industry. Since this development, use of high-frequency sound and other objective means of evaluating fat deposition and even lean mass have been used on live animals. Also, breeders have been able to subjectively evaluate muscling and fatness, and much change has occurred in both the swine and beef industries. Evaluation of carcass composition in a live animal eliminates the expense of sib or progeny testing of males for use in a selection program. Assuming that fatness of live animals is to be measured, numerous opportunities are open to make selection decisions concerning the rate of maturity or amount of fat required in animals being fattened for market. The breeder is obliged to weigh the importance of quality of product along with quantity and efficiency of production. Mass selec- tion or the selection of parents on the basis of their own composition is the method of choice with high heritability. When carcass information is required, the progeny test of sires and the use of these data as sib tests of the sons is possible. Improvement in the amount of milk and in number of eggs, because these characters have moderate-to-low heritability and are expressed in only one sex, will probably have to be the objective of selection. The dairy industry is making genetic improvement for milk production through sire selection and the use of artificial insemination. The poultry industry consists of a few companies supplying the highly selected line crosses for egg production and broiler production. SUMMARY This chapter described the kind and relative amount of genetic variation available that can be used to change the fat content of animal products,
Genetics of Fat Content in A nimal Products 99 considered the genetic problems related to making genetic change in fat content, and defined breeding programs by which such change can be made. The heritability of fat deposition in red-meat domestic animals is high and is related to rate of maturity in the young animal destined for slaughter. Large breed differences exist, but small hybrid vigor or in- breeding depression effects suggest primarily additive genetic variance for the ability to deposit fat. The heritability of fat secretion in milk and eggs is moderate to low, with large positive genetic correlations existing among the constituents of the product, suggesting that total amount or percentage can be improved but that the relative amount of the constituents cannot be altered readily. The evidence is clear that subjective selection for increased muscling in both swine and cattle can lead to an increase in frequency of undesirable recessive genes that affect the biological process in detrimental ways. The culard condition in cattle and the PSE and PsS syndromes in meat-type swine are ex- amples. Simple breeding programs are available by which the fat content of animal products can be altered. Economic incentives and a simple way to measure fat content in live animals are necessary before these programs can be implemented. REFERENCES Agricultural Research Service. 1974. Germ Plasm Evaluation Program. Progress Report. Publ. No. ARS-NC-13. U.S. Meat Animal Research Center, Clay Center, Nebr. Bereskin, B., H. O. Hetzer, W. H. Peters, and H. W. Norton. 1974. Genetic and maternal effects on pre-weaning traits in crosses of high and low-fat lines of swine. J. Anim. Sci. 39:1. Christian, L. L. 1968. Limits for rapidity of genetic improvement for fat, muscle, and quantitative traits. Page 154 in D. G. Topel, ed. The Pork Industry: Prob- lems and Progress. Iowa State University Press, Ames. Christian, L. L. 1972. A review of the role of genetics in animal stress susceptibility and meat quality. In F. Grisler and Q. Kolb, eds. The proceedings of the Pork Quality Symposium. University of Wisconsin-Extension 72-0. Craft, W. A. 1958. Fifty years of progress in swine breeding. J. Anim. Sci. 17:960. Cundiff, L. V. 1970. Crossbreeding cattle for beef production. J. Anim. Sci. 30: 694. Cundiff, L. V., and K. E. Gregory. 1968. Improvement of Beef Cattle through Breeding Methods. N.C. Reg. Publ. 120. Res. Bull. 196. Hazel, L. N., and E. A. Kline. 1952. Mechanical measurement of fatness and car- cass value on live hogs. J. Anim. Sci. 11 :313. Hetzer, H. O., and W. R. Harvey. 1965. Selection for high and low fatness in swine. J. Anim. Sci. 26: 1244. Hetzer, H. O., and R. H. Miller. 1970. Influence of selection for high and low fat- ness on reproductive performance of swine. J. Anim. Sci. 30:481. Hetzer, H. O., and R. H. Miller. 1972. Rate of growth as influenced by selection for high and low fatness in swine. J. Anim. Sci. 35:730.
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