Livestock Production Science 65 2000 57–70 www.elsevier.com locate livprodsci
Impact of biotechnology on crossbreeding programmes in
q
pigs
a , b
a b
Peter Visscher , Ricardo Pong-Wong , Colin Whittemore , Chris Haley
a
University of Edinburgh , West Mains Road, Edinburgh EH9 3JG, Scotland, UK
b
Roslin Institute , Roslin, Midlothian EH25 9 PS, Scotland, UK
Received 28 May 1999; received in revised form 8 November 1999; accepted 15 November 1999
Abstract
Crossbreeding programmes in pigs exploit between breed complementarity of additive genetic effects and heterosis generated by non-additive genetic effects. Within breed, improvement programmes may focus on additive effects and hence
the enhancement of complementarity, but non-additive variation is not generally used in within line selection or for mate selection at the multiplier or commercial level. In this paper, we discuss the impact of new biotechnological tools,
particularly molecular markers, multiple ovulation and embryo transfer MOET, and cloning, on structures and methods in crossbreeding. At the between line level, genetic marker information could allow better prediction of heterosis in novel
crosses from information on genetic distances. Within the crossbreeding structure, the same technique might be applied at the multiplier and commercial level to exploit specific combining abilities of particular animals. Combining simple MOET and
cloning protocols could radically alter the dissemination of crossbreeding benefits and their delivery to the farmer. The combination of MOET, cloning and genomic tools could result in speed genetics programmes, i.e. fast introgression and
recurrent selection methods. Thus, the ultimate impact of biotechnology will be increased rates of progress, efficient use of variation, reduced genetic lag, and the removal of one or two tiers in the breeding pyramid. The costs of new technologies
are discussed briefly.
2000 Elsevier Science B.V. All rights reserved.
1. Introduction breed or line complementarity. A typical practical
crossbreeding programme produces the slaughter Crossbreeding has been widely practised by pig
generation as a four-way cross. Such a crossing breeders since the 1960s. The benefits of crossbreed-
design utilises individual, maternal and paternal ing programmes in pigs were demonstrated in a
heterosis in the slaughter and parental generation, classical
paper by
Smith and
King 1964.
and has been shown to be optimum in terms of Crossbreeding is performed to utilise heterosis for
overall industry efficiency Smith, 1964; Sellier, reproduction fitness and growth traits, and for
1976; Clutter and Brascamp, 1998. In practice there any many other crossing designs employed, from
q
backcrosses and three-way crosses to crosses involv-
Based upon a paper presented at the 49th EAAP meeting in
ing more than four lines. In addition to the advantage
Warsaw, August 1998. Corresponding author.
of utilising heterosis, different breeds or lines may be
0301-6226 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 9 9 0 0 1 8 0 - 3
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. Visscher et al. Livestock Production Science 65 2000 57 –70
kept to breed for different objectives, for example MOET, in vitro production of embryos, molecular
specialised sire and dam lines selected for lean tissue genetics, and cloning on breeding programmes. The
growth rate and reproduction, respectively. With a likely impact of each of these techniques is discussed
suite of different lines the needs of a range of separately, as well as the effect of combining several
markets or environments can be addressed by select- techniques.
ing the appropriate combination of lines and crossing structure.
A practical consequence of the need to keep 2. Reproduction techniques
separate lines, together with the desire to keep the best genetics in-house and to disseminate genetic
2.1. Artificial insemination and MOET superiority to the slaughter pig, is the traditional
pyramid structure of the industry. In such a structure Although A.I. in pigs is fairly standardly used in
there are a few nucleus herds at the apex of the developed countries, we included a brief description
pyramid where genetic progress is achieved and for completion. A.I. and MOET increase the re-
many multiplier herds which multiply up either productive rate of the male and female, respectively.
purebred or crossbred pigs to satisfy the need for a A.I. in pigs has had a much slower start than in
large number of pigs in the commercial tier at the cattle, but is increasing rapidly. In some countries
base of the pyramid. From the breeder’s breeding more than 50 of breeding females are inseminated
company or national scheme perspective, the current artificially. By using A.I., genetically superior nu-
structure of the pig industry is not only necessary but cleus boars can be used extensively, particularly at
also potentially lucrative, because commercial pig the nucleus and multiplier level. At the nucleus level,
farmers need a constant supply of replacement A.I. has made it possible to link several farms to
crossbred gilts and boars which cannot be home create a large ‘super nucleus’, thereby increasing
bred. genetic gain at nucleus level and a decrease in
There are drawbacks, however, with the current genetic lag between the nucleus herds and the
crossbreeding structures: commercial population. In addition, the introduction
of A.I. is changing the marketing strategy of breed- 1. The cost of maintaining several lines and the cost
ing companies, from selling live boars to selling of making crosses between lines to produce the
semen. However, compared to dairy cattle the impact crossbred animals
of superior boars on the population will be more 2. The genetic lag that is potentially introduced by
limited, because of the relatively lower reproductive the need to produce crossbred product for sale to
rate of A.I. boars compared to dairy bulls. commercial producers
Compared to cattle, multiple ovulation and em- 3. The difficult in actually selecting for crossbred
bryo transfer MOET and in vitro fertilisation performance when the nucleus populations tested
techniques in pigs have encountered many practical are purebred.
obstacles, including the difficulty with cryopreserva- tion of semen and embryos, polyspermy, and non-
In future, new biotechnology may help overcome surgical methods of embryo transfer see e.g., Prather
these drawbacks by facilitating a reduction in the and Day, 1998. For these reasons such techniques
number of lines that need to be kept, by the removal are not yet routinely used in practical breeding
of one or two tiers from the traditional breeding programmes.
pyramid through changing the way in which genetic superiority is disseminated to the commercial level,
2.2. In vitro production of embryos and by the effective use of crossbreeding information
to select within pure lines. In this paper, we will Further developments of reproductive technology
review the impact of biotechnology on crossbreed- for pigs are likely to occur. Among these we can
ing programmes in pigs. In particular, we will foresee the development of in vitro embryo pro-
discuss the impact of artificial insemination A.I., duction IVEP, e.g. from follicles collected from the
P . Visscher et al. Livestock Production Science 65 2000 57 –70
59
slaughter house or from live females, non-surgical control over the multiplication process for the breed-
embryo transfer techniques, embryo storage and ing companies. Effectively, such a scheme might
freezing techniques and cloning. The high female remove the need for a purebred multiplication tier,
reproductive rate of the pig raises the question how and reduce the crossbred tier in the industry, thereby
and why MOET or IVEP can be beneficial in reducing genetic lag. The use of Chinese pigs is not
breeding programmes. MOET by itself is not likely a necessary part of this programme, but used for
to be cost-effective if the aim is to increase female illustration because of their superior reproductive
selection intensity, and is not a standard practice in performance. It could be argued that using the
breeding programmes except for the hygienic trans- current Chinese genotypes e.g., the Meishan is not
fer of stock. However, as will be argued below, in desirable because of the small size of the neonates. It
combination with other techniques such as marker is possible that a desirable incubator female can be
assisted selection and marker assisted introgression bred either from current white dam lines by
programmes, these techniques may have a role to increasing litter size and keeping piglet weight
play. The production of embryos in vitro is still in its constant or from Meishan genotypes increasing
infancy stage in pigs, although progress has been litter weight and maintaining litter size.
made recently Prather and Day, 1998. If large scale IVEP becomes a reality, what will be the conse-
2.3. Cloning quences for breeding programmes? At the nucleus
level, superior females could have as much influence The prospect of cloning through nuclear transfer
on genetic progress as males through an increase in Wilmut et al., 1997 offers the possibility to reduce
selection intensity. For example, if the female selec- the genetic lag between the nucleus, multiplier, and
tion intensity could be increased from 1.27 to 1.76, commercial tiers, since genetically superior per-
corresponding to a proportion selected of 25 and formance tested animals can be cloned and supplied
10, respectively, and the male selection intensity is to the commercial farmer. The difference with large
2.06 selected fraction of 5, the annual genetic scale IVEP is that the clones are genetically identi-
gain for growth traits would be increased by, approx- cal, whereas the embryos from a single donor and
imately, 2.06 1 1.76 2.06 1 1.27 5 15. For re- single sire vary in genetic merit. A large scale
production traits, where the amount of information nuclear transfer operation requires a constant source
on males and females is not equal, the proportional of ‘good’ oocytes, and protocols for in vitro develop-
increase would be larger. ment of oocytes, using for example the abundant
Apart from the effect of an increased selection primordial follicles, need to be developed in parallel
intensity at the nucleus level, IVEP together with to the refinement of techniques for nuclear transfer.
non-surgical embryo transfer and embryo storage In terms of the amount of variation at the commer-
technology could alter the dissemination structure of cial level the difference in variability of performance
the industry. One could envisage a few large-scale between animals derived from cloning or IVEP may
‘embryo farms’ in which embryos from superior be relatively small. For a trait with a heritability of
nucleus sows are produced and artificially insemi- 0.40, the cloned animals will still vary by 60 of the
nated with semen from nucleus boars from a variation of a population of unrelated individuals,
different selection line. The embryos would be whereas the animals produced by IVEP from a single
implanted into recipients which sexually mature sire–dam pair will vary by 80 of the total pheno-
early and which have a large reproductive capacity, typic variance. Hence, the difference is only a
for example purebred Meishan genotypes or existing proportional reduction of 0.25 for the cloned ani-
dam lines highly selected for large reproductive mals. However, these calculations assume that the
capacity. The piglets born from the recipients would genetic variation is additive in nature. It is possible
be transported to commercial farms. The advantage that the broad heritability including all sources of
of such a scheme is that fewer animals are trans- non-additive variation is substantially larger than 0.4
ported from nucleus to the multiplier tier, fewer sows for some traits and so the benefits in terms of
are needed are the multiplier level, and a greater reduced variability may be greater.
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. Visscher et al. Livestock Production Science 65 2000 57 –70
Using a cloning technique on its own is likely to For the traditional breeding scheme, we suppose
result in a one-off lifting of the genetic merit at the an integrated breeding scheme with a typical
commercial level. Substantial increases in sustain- pyramid structure of parents, grandparents GP,
able rates of improvement in the nucleus herds will great grandparents GGP and nucleus animals
depend upon the application and factors such as the GGGP. Grandparents produce on average 18
heritability, accuracy of selection, and rate of in- female parents in their lifetime, assuming a prod-
breeding. Reasonable improvements in gains are uctive life of 2 years and a reproductive rate of 18
possible where improvement is currently difficult. It live piglets per year. We further assume that GGP
is generally believed that for pig breeding, there is produce on average 15 female GP in their lifetime
no or little effect of the application of cloning on the and GGGP produce on average 10 female GGP in
rate of progress in the nucleus population. This may their lifetime. There are approximately 190 million
be true, but ignores the difference between rate of pigs slaughtered in the EU annually, and for the
progress at the nucleus and rate of progress at the purpose of these examples we express the numbers
commercial level. At present, the rate of progress at of animals per 10 million slaughter pigs. These
the commercial level for slaughter-traits is the aver- simple assumptions suggest that, per 10 million
age of the rates of progress for these traits in the sire slaughter pigs, there are approximately 500 000
and dam lines. If cloning and embryo transfer could parents females in the population, whose genes could
be applied cheaply, the slaughter generation con- be ultimately supplied by a small nucleus of 200
tain no genes from the dam line so the rate of sows. See Fig. 1 for the numbers of females in all the
progress at the commercial level would be the same tiers of the breeding pyramid. Hence, with the
as the rate of progress in the sire line. This would be ‘traditional’ breeding pyramid to breed F females,
1
of great economic advantage, because selection only 200 nucleus sows could be required to supply a
pressure on reproductive genes in the dam lines are irrelevant at the commercial level, and therefore an
inefficiency of the present breeding schemes. For example, if the rate of progress in the dam lines for
slaughter-traits is 90 of the corresponding rate in the sire lines, the rate of progress at the commercial
level would be 95 of what could be achieved with cloning. Hence, by completely separating the genes
for production and reproduction, a larger rate of progress could be achieved at the commercial level.
As with the implementation of large scale IVEP, cloning could change the structure in the industry.
We will argue below that the most powerful way to utilise cloning in animal breeding is in combination
with molecular technologies.
2.4. Reducing genetic lag To look at the potential impact of reproduction
techniques on the structure of the breeding pyramid, we examine three simple breeding schemes: a tradi-
tional breeding scheme, a modern breeding scheme, and a future scheme. These are only example of
many possible breeding schemes, and for both the traditional and the modern scheme, many variations
Fig. 1. Traditional breeding pyramid based upon 10 million
will exist in practice.
slaughter pig per year.
P . Visscher et al. Livestock Production Science 65 2000 57 –70
61
market of 10 million slaughter pigs with improved genes. In reality the number of nucleus sows sup-
plying such a market is much larger, because of inefficiencies in non-integrated breeding schemes,
competition between breeding companies, the need to maintain several lines, and the need to have large
enough lines to avoid high levels of inbreeding depression. The time between selection of the nu-
cleus sows and the slaughter of commercial animals is approximately 7 years, assuming that nucleus sows
have their first litter 6 months after being selected, females in all tiers have four litters with a farrowing
interval of 6 months, and an age of first farrowing of 12 months. Under these assumptions, the time when
the ‘average’ GGP, GP, P, and slaughter pigs are born is, approximately, 15, 36, 57, and 78 months since
the selection of the nucleus sows. If males are passed down the breeding pyramid as live animals, the lag
from the male side would be similar to that of the females. However, the use of A.I. to link the
purebred tiers see Fig. 1 reduces the lag on the male side, so that the overall lag can be reduced
Fig. 2. ‘Modern’ breeding pyramid based upon 10 million slaugh-
from 7 years to approximately 5–6 years.
ter pig per year.
To reduce lag, the above scheme can be amended, by combining the GGGP and GGP into a super
nucleus scheme by using A.I. Fig. 2. Thus the modern breeding scheme removes one tier of the
is more difficult than selection for growth traits, we breeding pyramid. The lag from the females in the
assume a larger nucleus for the recipient line breed- breeding pyramid is reduced by approximately 21
ing scheme. Finally, we assume that future recipients months, to approximately 5 years.
females produce about 40 piglets per year. These A further tier can be removed if we consider a
simple assumptions suggest a large reduction in the future breeding scheme, in which embryos of the
number of sows present at any point in time, and a slaughter generation genotype are implanted into
large reduction in genetic lag Fig. 3. Hence, in recipient females which are bred for lifetime re-
comparison with the traditional breeding scheme the productive rate as many live piglets born as pos-
sow population has halved, the lag between nucleus sible. We further assume that there is an unlimited
and commercial level is virtually zero or negative, if supply of embryos clones from donors, that embryos
only the top nucleus animals are used to produce are produced at the nucleus level using selected
clones, and only 1000 parents of replacement recipi- nucleus genotypes, and that commercial pig produc-
ents are needed. The number of tiers has now been ers buy embryos clones which are non-surgically
reduced to only three. implanted into the recipients. There can be a separate
In summary, novel reproductive techniques such breeding programme for recipient females. For ex-
as cloning, or technical improvements of current ample, the best recipient may be from an F cross
techniques such as A.I., MOET, and IVEP, are likely
1
from two Asian breeds or a cross between an Asian to have a direct impact on genetic improvement at
and white line or a present-day dam line, and the nucleus level and the speed of dissemination of
commercial pig producers ‘breed’ their own replace- superior genetics to the commercial tier. However,
ments by buying recipient F embryos. Since each of these techniques on their own are unlikely to
1
selection for reproduction traits for the recipient line have an impact on crossbreeding strategies as such.
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. Visscher et al. Livestock Production Science 65 2000 57 –70
through generations and relationship between ani- mals can be estimated using identity-by-decent
IBD measures. The study of associations between these IBD measurements with phenotypic perform-
ance has allowed the detection of linkage between marker loci and a QTL, and thereby the identification
of genome regions with major effects in quantitative traits. In the pig genome, several regions affecting
quantitative traits have already been found, mainly in studies using crosses between divergent breeds see,
e.g. Rothschild, 1998; Walling et al., 1998. Given the trend in the discovery of molecular markers and
advances in the statistical techniques to map QTL, the list of known single genes and genome regions
affecting quantitative traits is expected to sharply increase.
What is the impact of this powerful technology on pig breeding programmes and, in particular, on the
use of crossbreeding? Previously we discussed ways in which genetic markers might be used in pig
breeding programmes Visscher and Haley, 1995, 1998. Markers may be used to combine alleles
Fig. 3. Possible future breeding pyramid based upon 10 million slaughter pig per year.
variants of a gene or QTL from different breeds or lines using gene introgression programmes or by
3. Molecular techniques creating synthetic lines or to improve the response
