71)Y S FLORIDA AGRICULTURAL EXPERIMENT STATION
Dairy Science Mimeo Report DY65-1
March 1, 1965
Trends in Inbreeding in the Dairy Breeds
C. J. Wilcox and D. K. Roy"l
Inbreeding is the practice of mating individuals more closely related to
each other by ancestry than they would be if selected at random from a
breed. Breeders and scientists have been interested in inbreeding for
centuries, primarily with the hope that they could use this system of
breeding to improve the genetic merit of their cattle. There is probably
even greater interest today, with increases in the knowledge of genetics
and in the use of AI.
Inbreeding depression is perhaps the most striking consequence of mating
related animals. Scientists suggest that it occurs because of the
increase in homozygosity in a population, with the accompanying decrease
in heterozygosity. It is manifested by a decline in fitness traits,
reproductive performance, and physiological efficiency. With dairy cows,
these include milk yield and body size.
Estimates of the decline in milk yield with inbreeding vary considerably,
but the decrease in Holsteins of 50 lb per lactation for each increase
of 1% inbreeding shown by Von Krosigk and Lush (23) is an excellent
example. The value 1% is called a coefficient of inbreeding and refers
to the estimated increase in homozygosity. Sutherland and Lush (22)
showed that for each increase of 1% inbreeding a decrease could be
expected for birth weight (0.2 lb), mature weight (5 to 7 lb), mature
wither height (.1 cm), over-all type score (circa 1 point), and so on for
On the other hand, inbreeding can be beneficial since heterosis frequently
results from crosses between inbred lines or between different races or
varieties. The production of such lines for subsequent crossing to obtain
hybrid vigor is one of the main purposes for which inbreeding may be
carried out. Dairymen may have this in mind when they cross families within
a breed. Another benefit of inbreeding is the production of genetically
uniform strains, particularly of laboratory animals, for use in bioassay
and in research in a variety of fields. At present, this does not have
practical value with dairy cattle, of course.
Bakewell (13) claimed in the 18th century that inbreeding produced pre-
potency and refinement. The animal breeding literature of the late 19th
century contained somewhat vague but persistent references to the mysterious
power of inbreeding to intensify and fix transmitting ability to a degree
far beyond what could be done merely by selecting and mating together
Assistant Geneticist and Research Assistant, respectively. Present
address of Mr. Roy: Department of Dairy Science, Louisiana State
University, Baton Rouge.
similar but unrelated individuals. It was observed long ago that to
unify a group of individuals into a breed, some inbreeding seemed necessary.
Inbreeding in order to test animals severely, prior to linebreeding to
them, had been practiced at least a little in founding most breeds,
although the principle appeared first to have been expressed clearly in
post-Mendelian times. Breeders hoped by this means to uncover undesirable
recessives in their breeding stock in order that they could take steps
to eliminate them. As experience accumulated on the ill effects of
inbreeding, breeders became more reluctant to inbreed.
Shorthorns. Many of the early British breeders practiced very close
inbreeding when the foundation was laid for new breeds and strains.
McPhee and Wright (11) found that the average inbreeding coefficient for
registered Shorthorns rose from near zero in 1790 to about 20% in 1825.
In the next 100 years this figure showed only a slight increase (to 26%
in 1920). In Thomas Bates' famous Duchess family the inbreeding coef-
ficient was kept steadily at about 40% during 40 years of breeding (8 cow
generations). This is probably a record in a practical breeding operation.
However, the Duchess family was known for low fertility and finally became
extinct. From 1900 to 1956, Dairy Shorthorns in England had become about
2.4% inbred, according to Clayton (5). This increase corresponded to
about 0.24% per generation.
Ayrshires. Trends in Ayrshires in Scotland were evaluated by Fowler (7)
for the period 1878 to 1927. During this period a steady rise in inbreed-
ing occurred from close to zero to about 5%. No appreciable difference
between the coefficients for males and females was noted. Breed structure
of New Zealand Ayrshires, 1910 to 1950, was evaluated by Stewart (21).
About 72% of the genes of heifers registered in 1950-51 were derived from
67 males and 43 females imported after 1910. In a sample of 100 heifers,
56.6% of the genes were derived from 6 herds.
Brown Swiss. In 1937, Yoder and Lush (24) indicated that the inbreeding
of Brown Swiss was increasing at the rate of 0.5% per generation. This
was about the same as rates of the other dairy breeds, as later confirmed
by Shrode and Lush (19). The average generation interval was 5.4 years.
Holsteins. In 1936, Lush, et al. (10) reported on trends for Holsteins in
the United States. In 10 generations from 1881 to 1928 or 1931, inbreeding
had increased to a little over 4% and relationship to 3.4%. The average
relationship of individuals within a population is, of course, indicative
of the amount of inbreeding which can be expected. Obviously, the more
closely parents are related to each other, the higher will be the coef-
ficient of inbreeding. If the parents are not inbred, the inbreeding
coefficient of an individual will be half the coefficient of relationship
between the parents. Lush noted a faint tendency for the breed to form
into separate families, but this tendency was not extreme because these
families were mated with each other. This was somewhat higher than the
inbreeding for British-Friesians. Robertson and Asker (14) indicated in
1951 that the degree of inbreeding, 1910 considered as zero, had never
Ragab and Asker (12) analyzed the Friesian population in Egypt involving
heifers imported from Holland in 1954-55. The average inbreeding was
0.6%. Average relationship of 156 randomly selected pairs of pedigrees
was 1.9%. It was found that 12% of the genes of the herd were contributed
by the sires Adema 197, his son Adema 25437, and his grandsons Anna's
Adema 2650 and Jan 29845. Adema 197 had the highest direct relationship
to the herd (6.0%).
Abe and Ofuku (1) sampled Holstein pedigrees in Japan in 1958. Although
most registered stock traced back to a small number of breeders, the fact
that a large number of animals had been imported and little linebreeding
had been practiced had resulted in low inbreeding and relationship coef-
ficients. In Canada, Holstein AI sires were more highly inbred (0.9%)
than was the general population (0.4%) according to Hickman (8). Current
inbreeding had increased since earlier estimates were 0.56 to 0.58% and
0.17 to 0.21%, respectively.
Allaire and Henderson (2) have made one of the few extensive, recent U.S.
studies. For 17,490 Holstein cows by 200 AI sires, the average inbreeding
was estimated to be 0.4%. The mean coefficient of relationship of the
200 sires was 0.72%, although among sires of one stud it was 3.45%.
Jerseys. Barker (4) analyzed the breed structure of Jerseys in Australia.
Animals imported between 1900 and 1919 had made a genetic contribution
of 40% to the breed in 1950. At least two substructures existed within
the breed, based on the major herds in Queensland and those in the rest
of Australia. The total inbreeding was 4.19% from 1900 to 1950 and out
of this 1.31% was current inbreeding, 0.51% long term inbreeding and
2.37% strain inbreeding. During the period 1916-1925, the average in-
breeding of Jersey cattle in Britain was 3.9%, according to Smith (20).
A sample of high-producing cows showed their average inbreeding to be
Rottensten (18) studied Jerseys in Denmark, investigating four line
pedigrees of 142 breeding bulls and two line pedigrees of 382 elite cows
covering eight generations from 1914 to 1957. The level of inbreeding
during this period increased to 2.19% or 0.27% per generation, and it
was the same for males and females. The average relationship had increased
to 4.59% with no significant difference between males and females. The
coefficient of relationship for 20 prominent bulls was found to vary from
3.34 to 8.64%.
Other Breeds. The inbreeding of a sample of Red Dane sires, with 1911
considered as the base, was 11.2%, corresponding to an increase of about
1.6% per generation (15). Females, however, were less highly inbred,
about 4.7%. Rottensten also investigated inbreeding in the Red Danish
breed (16). Included were registered bulls born in 1927, 1937, 1947 and
1957, with pedigrees traced back to 1886. The average coefficients of
inbreeding per generation at these times was found to be 0.46, 0.60,
0.95, and 0.75%, respectively. He credited the increasing use of AI
during this period for the decrease in the coefficient between the groups
born in 1947 and 1957. AI in this case provided to dairymen a wider
genetic base than did natural service. He also found that the coefficients
of relationship for bulls born between 1894 and 1947 varied from 2.0 to
18% and the highest coefficient for any bull born since 1930 was 9.2%.
A similar study was completed by Rottensten (17) with Black Pieds. They
also exhibited a decrease in inbreeding between 1947 and 1957. This
decrease may have been due to the fact that pedigrees of bulls born in
1957 were not traced as far back as others, however. The fraction of the
total gene pool which could be attributed to 46 outstanding bulls born in
1928, 1938, 1948, and 1958 was found to range from 1.55% to 17.01% in
1948, and 0.07% to 12.21% in 1958.
Langlet (9) found in Black Pied Lowland bulls that inbreeding and relation-
ship coefficients were 2.46 and 3.39% respectively for the AI groups and
0.78 and 3.35% for naturally mated groups; he considered these figures
sufficiently low to be disregarded, as there was no evidence of an undes-
irable narrowing of blood lines.
From study of published research, it is obvious that not enough is known
of the present situation in the U.S. dairy breeds, especially since the
advent of widespread AI. Members of the purebred breed associations are
concerned about possible increases in the average relationship of animals
within their breeds. For example, at the 89th Annual Meeting of the
Ayrshire Breeder's Association (3), it was pointed out that the genetic
base of the Ayrshire breed was becoming progressively narrower. It was
found that 6 Ayrshire bulls in AI units sired 3,003 recorded animals or
almost 23% of all Ayrshires recorded during the year. Similar examples
of concern can be found among breeders of each dairy breed.
In bisexual organisms such as the dairy cow, the rate of increase in
inbreeding per generation is slightly more than where N equals the
actual number of males in the population used for breeding. All of the
five major dairy breeds in the U.S. are large enough to avoid extreme
inbreeding due to population size, unless AI reduces the number of sires
used to an unexpectedly low number. Inbreeding among registered AI
Ayrshires could be expected to increase at a rate of 0.16% per generation,
or about 0.03% per year, based on 80 males in use (6) and about 6500
registered AI offspring. This is doubtless lower than the actual rate,
because of a number of factors.
Mating and selection policies followed by purebred breeders and AI sire
committees will certainly affect the inbreeding of all dairy cattle.
Registered sires are used in AI without notable exception, and grade and
registered cattle in the U.S. are genetically quite similar. The per-
centage of sires selected for use in AI which are sons of AI sires is
probably increasing. Although the latter practice is doubtless advisable
to achieve maximum genetic change in milk yield, some loss due to
increased inbreeding can be expected. Yet, the net effect of these two
opposing forces should be a steady increase in yield for the foreseeable
future, unless a drastic increase in inbreeding occurs.
Measurements of inbreeding and relationship, and their effects upon
various quantitative traits, have been made. However, the changes which
seem inevitable because of increased use of AI, popularity of certain
outstanding sires and their relatives, and the consequences of finite
population size in the pure breeds, are worthy of considerable additional
research. This research should be partly retrospective, to obtain clear
and reliable information on what has happened and is happening now.
There should also be research designed to obtain information on controlled
populations in both the presence and absence of selection for the quanti-
tative trait of interest. A number of projects already under way at
various experiment stations fulfill these objectives to some degree. A
great deal more research is needed, however.
For the dairy cattle breeder, linebreeding and inbreeding, when practiced
with discretion, continues to have a place for the production of breeding
stock. At present, the traditional recommendation that the commercial
dairyman avoid close inbreeding seems still to be warranted.
Inbreeding depression occurs in many traits, but inbreeding can at times
be utilized in the production of heterosis and is necessary in the
development of genetically uniform strains. The dairy breeds do not
appear to be markedly inbred as a whole, although many highly inbred
individuals exist. The average degree of inbreeding is probably increasing
because of a decrease in the effective population size due to AI and to
selection programs of registered cattle breeders. Present research should
be directed toward the evaluation of the effects of inbreeding and other
systems of mating, with at least a portion designed, controlled research
with and without the presence of selection.
The authors wish to thank Dr. Marvin Koger, Department of Animal Science,
for suggestions and critical comments.
(1) Abe, T., and Ofuku, S. Genetic Analysis of the Holstein Breed in
Japan. I. Genetic Contribution of Important Breeders and Animals
and Groups of Animals Imported in Different Times. Japan J. Zootech.
Sci., 33:187. 1962. Animal Breeding Abs., 31:46. 1963.
(2) Allaire, F. R., and Henderson, C. R. Inbreeding Within an AI Dairy
Cattle Population. J. Animal Sci., 23:847. 1964.
(3) Anonymous. Ayrshire Program Works. Hoard's Dairyman. p 616.
May 25, 1964.
(4) Barker, J. S. F. The Breed Structure and Genetic Analysis of the
Pedigree Cattle Breeds in Australia. I. The Jersey. Aust. J. Agric.
Res., 8:561. 1957. (A Correction 10:769. 1959).
(5) Clayton, G. A. Aspects of Breed Structure in Pedigree British
Shorthorn Cattle. Proc. British Society of Animal Prod. p 107. 1956.
(6) Dairy-Herd-Improvement Letter. U.S. Artificial Insemination (AI)
Participation Report for 1963. USDA, ARS-44-144. May 1964.
(7) Fowler, A. B. The Ayrshire Breed of Cattle: A Genetic Study.
J. Dairy Res., 4:11. 1933.
(8) Hickman, C. G. Genetic Analysis of the Canadian Holstein Breed.
Contribution No. 66. Animal Res. Inst., Canada Dept. of Agric.,
(9) Langlet, J., and Gravert, H. 3. Inbreeding and Relationship Coef-
ficients in Black Pied Lowland Bulls Used for Artificial Insemination
and Natural Mating in Schleswig-Holstein. Zuch tungs-kunde, 33:373.
1961. Animal Breeding Abs., 31:48. 1963.
(10) Lush, J. L., Holbert, J. C., and Willham, 0. S. The Genetic History
of the Holstein-Friesian Cattle in the United State:;. J. Heredity,
(11) McPhee, H. C., and Wright, S. Mendelian Analysis o: the Pure Breeds
of Livestock. III. The Shorthorns. J. Heredity, 1i:205. 1925.
(12) Ragab, M. T., and Asker, A. A. The Genetic Analysis of Friesian
Herd in Egypt. Indian J. Dairy Sci., 10:131. 1957.
(13) Rice, V. A., and Andrews, F. N. Breeding and Improvement of Farm
Animals. McGraw-Hill Book Co., Inc. New York. 1942.
(14) Robertson, A., and Asker, A. A. The Genetic History and Breed
Structure of British-Friesian Cattle. Emp. J. Exp. Agric., 19:113.
(15) Robertson, A., and Mason, I. L. A Genetic Analysis of the Red Danish
Breed of Cattle. Acta Agric. Scand., 4:257. 1954.
(16) Rottensten, K. Inbreeding and Consanguinity in the Red Danish Breed.
Aarsberetn. Inst. Sterilitetsforskn, K. Vet.-og Landbohojsk, 5:1961.
Animal Breeding Abs., 31:341. 1963.
(17) Rottensten, K. Inbreeding and Consanguinity in the Black Pied Danish
Breed. Aarberetn. Inst. Sterilitetsforskn. K. Vet.-og Landbohojsk
(kbh), 113:1962. Animal Breeding Abs., 31:341. 1963.
(18) Rottensten, K. Inbreeding and Consanguinity in the Jersey Breed in
Denmark. Aarberetn. Inst. Sterilitetsforskn. K. Vet.-og Landbohojsk,
19:1961. Animal Breeding Abs., 31:191. 1963.
(19) Shrode, R. R., and Lush, J. L. The Genetics of Cattle. Adv. in
Genetics, 1:209. 1947.
(20) Smith, A. D. B. Inbreeding in Cattle and Horses. Eugenics Review,
(21) Stewart, A. Expansion and Structure of the New Zealand Pedigree
Ayrshire Breed, 1910 to 1950. N.Z. J. Sci. Tech. Agric., 36:493. 1955.
(22) Sutherland, T. M., and Lush, J. L. Effects of Inbreeding on Size
and Type in Holstein-Friesian Cattle. J. Dairy Sci., 45:390. 1962.
(23) Von Krosigk, C. M., and Lush, J. L. Effects of Inbreeding on
Production in Holsteins. J. Dairy Sci., 41:105. 1958.
(24) Yoder, D. M., and Lush, J. L. A Genetic History of the Brown Swiss
Cattle in the U.S. J. Heredity, 28:154. 1937.