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OCR for page 85
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
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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
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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
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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.
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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 ) .
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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).
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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
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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.
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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
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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
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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
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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
.
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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
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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,
OCR for page 99
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.
OCR for page 100
100
R. L. WILLHAM
Hetzer, H. O., and L. R. Miller. 1973. Selection for high and low fatness in
swine: correlated responses of various carcass traits. J. Anim. Sci. 37:1289.
Kieffer, N. M., T. C. Cartwright, and J. E. Sheek. 1972. Characterization of the
double muscled syndrome. I. Genetics. Tex. Agric. Exp. Stn. Consolidated
PR-311-3131.
Koch, R. M. 1974. Potential for producing "thin-rinded" choice through breeding.
Presented as part of a symposium at the Midwestern-section meetings of the
American Society of Animal Science in Chicago, Ill.
Lasley, J. F. 1972. Genetics of Livestock Improvement, 2d ed. Prentice-Hall, Inc.,
Englewood Cliffs, N.J.
Lerner, I. M. 1951. Natural selection and egg size in poultry. Am. Nat. 85:365.
Lerner, I. M. 1954. Genetic Homeostasis. John Wiley & Sons, Inc., New York.
Miller, P. D., W. E. Lentz, and C. R. Henderson. 1969. Comparison of contem-
porary daughters of young and progeny tested dairy sires. Unpublished mimeo-
graphed paper presented at the American Dairy Science Association meeting,
Minneapolis, Minn. Cornell University, Ithaca, N.Y.
Nordskog, A. W., H. S. Tolmau, D. W. Casey, and C. Y. Lin. 1974. Selection in
small populations of chickens. Poult. Sci. 53:1188.
Omtvedt, I. T. 1968. Some heritability characteristics and their importance in a
selection program. Page 128 in D. G. Topel, ed. The Pork Industry: Problems
and Progress. Iowa State University Press, Ames.
Powell, R. L., and A. E. Freeman. 1974. Genetic trend estimators. J. Dairy Sci.
57:1067.
Terrill, C. E. 1951. Selection for economically important traits of sheep. J. Anim.
Sci. 10:17.
Terrill, C. E. 1958. Fifty years of progress in sheep breeding. J. Anim. Sci. 17:944.
Topel, D. G. 1968. The Pork Industry: Problems and Progress. Iowa State Uni-
versity Press, Ames.
Touchberry, R. W. 1971. A comparison of the general merits of purebred and
crossbred dairy cattle resulting from 20 years (4 generations) of crossbreeding.
Proceedings, Nineteenth Annual National Breeders' Roundtable, sponsored by
Poultry Breeders of America, 521 E. 63rd, Kansas City, Mo. 64110.
Warwick, E. J. 1958. Fifty years of progress in breeding beef cattle. J. Anim. Sci.
17:922.
Wilcox, C. J., S. N. Gaunt, and B. R. Farthing. 1971. Genetic interrelationships of
milk composition and yield. South. Coop. Ser. Bull. No. 155.
Willham, R. L., and J. H. Anderson. 1974. Iowa station annual report to NC-1.
Mimeograph of annual progress. Iowa Agricultural and Home Economics Ex-
periment Station. Ames, Iowa.
Young, C. W., W. J. Tyler, A. E. Freeman, H. H. Voelker, L. D. McGilliard, and
T. M. Ludwick. 1969. Inbreeding investigations with dairy cattle in the North
Central Region of the United States. N.C. Reg. Res. Publ. 191. Agric. Exp. Stn.
Univ. Minn. Tech. Bull. 266.
Representative terms from entire chapter:
fat content
