SWINE GENETICS
NSIF
National Swine Improvement Federation
Purdue University Cooperative Extension Service
West Lafayette, Indiana
Fact Sheet Number 15 NSIF-FS15
Selection Programs for Seedstock Producers
Authors:
Erik R. Cleveland, University of Hawaii at Hilo;
Charles M. Stanislaw, North Carolina State University;
and Charles Smith, University of Guelph.
Reviewers:
Rodger K. Johnson, University of Nebraska;
William T. Ahlschwede, University of Nebraska;
Lynn A. Cole, Ohio seedstock producer;
Allan Schinckel, Purdue University.
Introduction
One goal for seedstock producers is to maximize genetic progress per
year within their herds. Genetic progress is maximized when the
following practices are used:
1. Maintaining contemporary groups of adequate size and minimum age
range,
2. Using equipment and techniques that allow collection of accurate
records on all animals in the herd,
3. Timely processing of records through a program that allows the
most accurate evaluation of the animals,
4. Using culling policies to shorten the generation interval to an
acceptable value and allow the replacement of breeding animals
with superior individuals,
5. Selecting animals based on an index of estimated breeding values
(EBVs) or expected progeny differences (EPDs), and
6. Using mating policies that will minimize rates of inbreeding.
Many of these concepts have been covered in other National Swine
Improvement Federation (NSIF) fact sheets. This fact sheet ties
together many of these concepts by providing recommendations for
seedstock producers interested in setting up effective selection
programs.
Topics covered in this fact sheet related to within herd
selection include setting up contemporary groups, culling breeding
animals, selecting replacements and developing policies to help
minimize inbreeding. Other topics include selecting outside breeding
stock, developing association breeding programs and planning matings
for purchased semen.
For discussion related to selecting and culling, it is assumed
that producers are using selection indexes based on EBVs or EPDs. When
using STAGES or the SWINE-EBV program (as discussed in NSIF fact sheet
#12), producers can compare index values for animals from different
contemporary groups. With the Nebraska SPF Performance Testing
program, the index values are not directly comparable for animals from
different groups. Thus, comparisons should only be made within a given
contemporary group. Because of the differences among indexes, culling
and selection strategies will be discussed separately for the Nebraska
program. Any mention of boar or gilt selection in this fact sheet will
refer to choosing animals with the highest index scores. However, a
certain percentage of higher indexing animals will be eliminated in
most groups due to disposition, underlines, feet & legs or other
physical problems. Using a large number of physical characteristics as
culling criteria, will limit the rate of improvement in performance
traits. Thus, producers should cull animals based on only a few traits
that have a definite, detrimental effect on animal function. The
calculations made in this fact sheet does not take this culling into
consideration so estimated and actual selection values can differ
somewhat. The actual number of animals to cull or select can vary
somewhat from estimates provided by fact sheet equations due to
environmental factors. For example, it may be necessary to select a
greater number of replacements in the summer due to the effects of
heat stress on reproduction.
Some seedstock operations consist of both nucleus and multiplier
herds. Nucleus herds send semen, off-test animals, and/or cull sires
and sows to their multiplier operations for breeding purposes. Several
breeding practices can be used to increase the rate of genetic
progress in these nucleus-multiplier systems. Seedstock producers are
encouraged to use artificial insemination (A.I.) since it allows a
greater use of elite sires in both nucleus and multiplier herds.
Furthermore, producers should include performance data from both
nucleus and multiplier herds in across-herd genetic evaluations to
obtain more accurate genetic merit estimates.
Contemporary Group
In setting up a within herd selection program, it is important
to decide which group of animals will make up the contemporary group.
A contemporary group should consist of litters that have farrowed
during a period of 3 weeks or less to reduce environmental differences
within the set of pigs. Ideally, each contemporary group should
consist of at least 15 to 20 litters to allow for more meaningful
comparisons. Smaller producers should aim for at least 10 litters from
at least 3 sires in each contemporary group.
Two examples will illustrate how to set up contemporary groups.
These examples are based on a 90% farrowing rate.
Example #1. In this herd about 28 sows are bred every month. To
reduce the time period within a farrowing group, each month's breeding
period will be limited to 14 days. From each breeding, approximately
25 sows will farrow. Thus, each contemporary group will consist of
the 25 litters that farrowed during the 14 day period.
Example #2. In this operation the producer breeds 17 sows every
week with about 15 litters farrowing. To meet the stated guidelines,
the producer decides that a contemporary group will consist of 30
litters that are born during the two week period.
Culling Sows
STAGES and SWINE-EBV. For each contemporary group of sows, the
producer should keep a list of animal ear notches and index values.
Notations can be made on the list for those sows that will be culled
due to health, reproductive or feet & leg problems. After weaning,
these individuals will be culled while the remaining sows in the group
will be bred. During the assigned breeding period, gilts will be bred
as replacements for the cull sows. If additional gilts can be bred
during this period, they will serve as candidates to replace the lower
indexing sows. To serve as a replacement in the group, a gilt must
have a higher index score than the sow that will be culled. After
pregnancy testing the bred gilts, the low indexing sows can be removed
from the herd.
Nebraska SPF. With this program the index values for sows and
gilts are not directly comparable so some type of culling policy is
needed to help achieve an acceptable female generation interval. In
some herds a policy may not be needed if heavy culling of sows is
already occurring.
Let's calculate the female generation interval and parity
distribution of sows in a contemporary group with four culling
policies that might be used by herds enrolled in the Nebraska program.
The culling policies and calculations were based on the assumptions of
Johnson (1990):
1. 90% of selected gilts are cyclic and will express estrus,
2. gilts are bred during a 14 day period so 2/3 of the cycling
gilts will mate,
3. 90% of all bred females will farrow, and
4. 80% of the sows that farrow will be mated again.
____________________________________________________________________________
Table 1. Distribution of sows by parity within a contemporary
group and generation intervals for different culling policies.
____________________________________________________________________________
Sow Culling Policy % of Sows in each Parity Generation
____________________________________________________________________________
1 2 3 4 5 Interval
1. 20% parity 1
all parity 2 58 42 1.17
2. 20% parity 1 & 2
all parity 3 45 32 23 1.32
3. 20% parity 1, 2 & 3
all parity 4 38 28 20 14 1.44
4. 20% parity 1, 2, 3 & 4
all parity 5 35 25 18 13 9 1.56
____________________________________________________________________________
A brief description of the culling policies and the results of
the calculations are shown in Table 1. For each system 20% of the sows
in assigned parities are culled between farrowing and breeding. These
systems differs in which parity classes this 20% culling is used. For
example, in System #1 the 20% culling applies to only parity 1 sows.
However, in System #3 the 20% value applies to parities 1, 2, and 3
sows. Keep in mind that the 20% value is a suggested goal but the
actual culling percentage can vary somewhat. The four systems also
differ in regard to the final parity in which all remaining sows are
culled. For example, in System #1 all sows are culled after farrowing
their second litter. In System #3 all sows are culled after farrowing
their fourth litter.
Based on Table 1 it is possible to make some comparisons among
the four systems. System #1 results in the shortest female generation
interval and the lowest selection intensity due to the numbers of
gilts that are needed to replace the large number of cull sows. With
this system one can expect to wean less pigs per sow due to the high
percentage of first parity females. System #4 results in the longest
generation interval and the highest selection intensity. From a
practical standpoint, System #3 is the most attractive. System #3
produces a female generation interval of less than 1.5 years and a
relatively high selection intensity as shown by examples in the
section on gilt selection.
Selecting Gilts
STAGES and SWINE-EBV. Producers should select a certain
percentage (such as 20%) of the higher indexing gilts from each
contemporary group. These females will go into the gilt pool as
candidates for breeding. Within the pool, gilts showing estrus during
an assigned breeding period can be bred to serve as replacements for
cull sows in the group. For further details on this topic, see the
section on sow culling.
Nebraska SPF. The number of gilts that should be selected from
each contemporary group will depend upon the sow culling policy and
the length of the breeding interval. Based on the four sow culling
policies listed in table 1, the estimated number of gilts to select
from each contemporary group can be calculated by using the following
equation:
NG = (CP1 x NFB / MR) / (BID / 21)
where:
NG = no. of gilts selected from a contemporary group,
CP1 = coefficient for parity 1 sows (Table 1),
NFB = no. of females normally bred within a contemporary group,
MR = the coefficient for proportion of selected gilts that are
cyclic and will mate (examples will use .90), and
BID = the breeding interval in days for the contemporary group.
Two examples will illustrate the use of this equation.
Example #1. For this herd about 28 females are bred during a 14
day period for a contemporary group. About 25 litters farrow in each
group. Based on culling policy #3 the producer will sell all sows
after their fourth litter while 20% of the females are culled between
farrowing and breeding in each of the other parity classes (1 to 3).
With this system the coefficient for parity 1 sows is #3). The number
of gilts that should be selected from each contemporary group is: NG =
(.38 x 28 / .9) / (14 / 21) = 18 gilts.
Thus, the 18 top indexing gilts from each contemporary group
should be selected. If 18 top animals are selected from 100 gilts that
may be tested within this contemporary group, the selection rate will
be 18%. Obviously additional gilts will be needed if some animals are
culled due to health, reproductive or feet & leg problems. Producers
should aim for a gilt selection rate of less than 25% in each
contemporary group.
Example #2. The contemporary group consists of 34 females bred
during a 14 day period out of which 30 litters farrow. Since culling
policy #3 is used, the coefficient for parity 1 sows is .38 (Table 1).
The number of gilts that should be selected from each contemporary
group is: NG = (.38 x 34 / .9) / (14 / 21) = 22 gilts.
Thus, the two top indexing gilts are selected from each
contemporary group. If two top animals are selected from 120 gilts
that may be tested within this contemporary group, the selection rate
will be 18%.
Culling Sires
STAGES and SWINE-EBV. For each contemporary group, compare the
index values of young off-test boars to those of current sires. To
serve as a replacement, a boar must have a higher index score than the
sire that will be culled. Once the new boar is ready for breeding, the
lower indexing sire can be remove from the herd.
Nebraska SPF. Since index values for sires and young boars are
not directly comparable, a culling policy must be developed that is
easy to use, allows for equal use of boars and results in a rapid
turnover of sires. To meet these objectives, producers should cull
sires based on the number of females bred. Thus, each sire would be
culled after reaching a set breeding quota. If a producer selects a
quota of 20, then a sire would be culled after the 20th female is bred
or inseminated. A quota between 15 to 25 bred females per sire seems
reasonable for most herds. However, some herds may want to use even
lower quotas (such as 10 to 15 females per boar) to help increase the
number of sires used during the year. With larger numbers of sires,
one can expect lower rates of inbreeding in a closed system.
Number of Sires per Contemporary Group
Research has shown that at least three sires should be used in
each contemporary group. The number of sires needed per group is also
affected by the number of matings that must be performed, the type of
breeding system used (hand mating versus A.I.) and by the length of
the breeding period. The estimated number of sires needed in each
contemporary group can be calculated by using the following equation:
NBC = ((NSC x NMS) / BIW) / NMBW
where:
NBC = number of sires per contemporary group,
NSC = number of sows bred per contemporary group,
NMS = number of matings per sow,
BIW = breeding interval in weeks, and
NMBW = number of matings (or inseminations) allowed per boar per
week.
An example will illustrate the use of this equation. Assume the
herd under consideration has 28 sows bred in each contemporary group.
The breeding interval of each group is two weeks. Assume that each sow
is bred twice and that each boar is allowed four matings per week. The
number of sires needed in each contemporary group is:
NBC = ((28 x 2) / 2) / 4 = 7 boars.
Number of Boars to Select per Contemporary Group
STAGES and SWINE-EBV. For herds using these programs, producers
do not need to select a set number of boars from each contemporary
group. From each group producers should select boars that excel in
index scores compared to current sires. In other words, a young boar
must have a higher index score than the sire that will be culled.
Nebraska SPF. The number of boars to select will depend upon
the number of sires used and the number of contemporary groups bred
during the year. First, the estimated number of sires used in a year
can be calculated based on the following equation:
NBY = (NCG x NSC) / BQ
where:
NBY = number of sires used in a year,
NCG = number of contemporary groups bred in a year,
NSC = number of sows bred in a contemporary group, and
BQ = breeding quota (number of bred females allowed per boar).
The following equation can then be used to determine the number
of boars to select from each contemporary group:
NBS = NBY / NCG
where:
NBS = number of boars to select from each contemporary group,
NBY = number of sires used in a year, and
NCG = number of contemporary groups bred in a year.
Two examples will illustrate the use of these equations. In both
examples, each boar is culled after breeding 20 females.
Example #1. For this herd 12 contemporary groups of 28 sows
each are bred during the year. The number of boars used during a year
is:
NBY = (12 x 28) / 20 = 17.
Now it is possible to calculate the number of boars to select
from each contemporary group as:
NBS = 17 / 12 = 1.4 or 1 to 2.
Thus, one or two top indexing boars should be selected from each
contemporary group. If one or two top animals are selected from 100
boars that may be tested within this contemporary group, the selection
rate will vary from 1 to 2%. A boar selection rate of less than 5%
within each contemporary group is recommended.
Example #2. In this herd 24 contemporary groups of 34 sows each
are bred during the year. The number of sires used in a year is
calculated as:
NBY = (24 x 34) / 20 = 41.
Next, the number of boars selected per contemporary group is
calculated as:
NBS = 41 / 24 = 2.
Thus, the two top indexing boars would be selected from each
contemporary group. If two top animals are selected from 120 boars
that may be tested within a contemporary group, the selection rate
will be 2%.
Minimizing Inbreeding in Closed Herds
All seedstock herds should use within herd selection of boars
and gilts to achieve genetic improvement. Within herd selection is the
key to effective seedstock improvement programs. However, some
inbreeding will eventually occur when only within herd selection is
used. High rates of inbreeding can easily offset the progress possible
from within herd selection. Thus, seedstock producers need to have an
understanding of what factors affect the rate of inbreeding so they
can take the appropriate action if they decide to close their herds.
In a closed herd the genetic relationship among sires is
important. If all selected replacement boars are sired by one
individual, the herd can expect a more rapid increase in inbreeding
buildup. Consequently, it is important to start a closed herd breeding
program with as many unrelated sires as possible. For example, it
would be desirable to use at least 15 unrelated sires during the year
prior to closing the herd.
Once the herd is closed to outside sires, the number of
replacement boars selected each year will become a very important
factor affecting the rate of inbreeding. The greater the number of
sires, the lower the rate of inbreeding. It would be desirable to use
at least 15 sires each year in a herd.
Another factor that affects the rate of inbreeding, but to a
lesser extent, is the size of the sow herd. The expected rate of
inbreeding is lower in operations with larger sow numbers.
The relationship among mates is another factor that affects the
rate of inbreeding. Mating closely related animals results in a
greater degree of inbreeding. To help minimize inbreeding in a closed
herd, it would be beneficial for a seedstock producer to use computer
software that calculates the relationship coefficients among all
selected animals. Based on these values, one could plan matings among
individuals that have smaller relationship coefficients.
Small and moderate size seedstock herds can expect faster rates
of inbreeding when they adopt a closed herd selection program as
compared to large operations. Thus, these herds may need to use some
outside breeding to help minimize inbreeding. One method of obtaining
outside breeding is to purchase semen or individual animals. Another
method is to participate in an association breeding program.
Association Breeding Programs
To minimize inbreeding and to maximize the rate of genetic
change in operations using within herd selection, seedstock producers
should consider forming or participating in an association breeding
program. In other countries these associations are called cooperative
or group breeding programs. Herds participating in an association
should be involved in intense within herd selection based on the same
selection index. Involvement in an across-herd genetic evaluation
program is highly desirable for participating herds to allow the
comparison of animals at different farms.
Breeding associations can consist of any number of herds. The
number of herds involved in the association is of less importance
compared to the total number of sows in the system. Associations with
more sows can support greater number of sires which will help minimize
inbreeding in the system. For this reason it is desirable that an
association breeding program contain at least 200 sows.
A variety of associations could be developed where semen, boars,
gilts and/or sows are exchanged among herds. From a health standpoint,
it would be desirable if only semen or lab pigs were transferred among
herds. Several methods of exchanging semen could be developed but only
two examples will be discussed in this fact sheet. In both examples
A.I. sires will be selected from herds involved in the association.
Association #1. Each herd has set up it's own selection program
using the procedures discussed in the previous sections. In this
association, hand mating is used for most sows. In a given herd, semen
can be collected when boars have rest periods between contemporary
groups or after reaching their breeding quotas. Collected semen can be
distributed to other herds in the system that have sows ready for
insemination. Each herd should use semen from at least one outside
boar in each contemporary group. This practice may bring about a small
change in the planned boar allotment, selection and culling policies.
However, this procedure will help provide links between operations for
across-herd evaluations.
Association #2. In this association the participating herds use
A.I. for most matings. Since A.I. boars are used across herds, a sire
list should be developed which might include information such as EBVs
(or EPDs), index values and pedigrees. It would be desirable for an
association to have at least 15 sires on the list at a given time as a
means to help minimize inbreeding in the system.
The association should develop a selection policy for boars
since these sires will be shared by all herds. For herds using STAGES
or SWINE-EBV, selection can be based on index values by comparing the
scores of young off-test boars and current sires. If the Nebraska SPF
program is used, producers might select the top 1 to 2% of the boars
from each contemporary group. If 100 boars are performance tested in a
contemporary group, then the one or two top indexing boars would be
selected from each set.
Next the association should develop a sire culling policy. For
herds using STAGES or the SWINE-EBV program, sires can be culled when
higher indexing boars are available for breeding. With the Nebraska
SPF program, the association should rank animals numerically on the
boar list based on their date of entry into the pool of A.I. sires.
Older sires would be at the top of the list and young boars would be
added to the bottom. The first individual on the list would be the
next sire culled once a new service age boar is ready to be added to
the listing. With this method the rate of culling is dependent upon
the boar selection policy.
It would be desirable that each herd inseminate sows with semen
from both home-raised and outside sires. Furthermore, semen from at
least 3 sires should be used in each contemporary group for accurate
evaluations.
Selecting Outside Breeding Stock
For contemporary groups it is desirable to have at least 15 to
20 litters born within a time interval of three weeks or less. Small
herds should aim for at least 10 litters in a group. With STAGES and
SWINE-EBV, low indexing sows can be culled when higher indexing gilts
are available. For the Nebraska SPF program, culling 20% of the sows
after weaning within parities 1, 2 and 3 plus total culling after the
fourth litter is a practical compromise between maximum productivity
and minimum generation interval. The culling rate will determine the
number of gilts to select from each contemporary group.
For STAGES and SWINE-EBV, sires can be culled when higher
indexing boars are available. With the Nebraska SPF program, a culling
policy should be developed to remove a boar from the herd after
approximately 15 to 25 females are bred or inseminated. The culling
rate will determine the number of boars to select from each
contemporary group.
The use of some outside breeding stock warrants consideration by
producers since it is a way to help prevent inbreeding and provide
links for across-herd genetic evaluations. In closed herds, rates of
inbreeding can be minimized by practices that increase the number of
sires used yearly.
Some of these practices are difficult for smaller seedstock
producers to follow. Therefore, they should consider forming a
breeding association and exchange semen among herds to help minimize
inbreeding and to increase the rate of genetic improvement.
Selecting Outside Breeding Stock
Using outside breeding has both advantages and disadvantages.
Using semen from A.I. centers provides genetic links among operations
which is useful in across-herd genetic evaluations. For operations
that plan to close their herds to outside introductions within one or
two years, the purchase of boars or semen is a way to establish more
pedigree lines. With more pedigree lines, one can expect lower future
rates of inbreeding. One disadvantage of using outside breeding stock
is the risk of introducing diseases into an operation. This
disadvantage can be minimized by purchasing semen instead of buying
boars. Another disadvantage is the lower selection accuracy compared
to within herd selection. If a producer is selecting outside animals,
across-herd genetic evaluations should be to improve the selection
accuracy. Based on these evaluations, select high ranking sires or
their sons. In operations using intense within herd selection,
home-raised boars can be expected to be superior to many outside boars
that are available for sale.
The number of sires needed during a year in the cooperative can
estimated from:
NBYBC = (TNS x NIS) / ENI
where:
NBYBC = number of A.I. sires used during the year in the
breeding cooperative
TNS = total number of sows bred during the year in the
cooperative
NIS = number of inseminations per sow
ENI = expected number of inseminations per boar across all
herds.
After calculating NBYBC, it is possible to calculate the number
of boars that should be selected from each contemporary group as:
NBS = NBYBC / NCG
where:
NBS = number of boars selected from each contemporary group
NBYBC = number of A.I. sires used during the year in the
breeding cooperative
NCG = the number of contemporary groups bred in a year for the
cooperative.
Let's use the following example to illustrate the use of the two
equations. For the example the cooperative has a total of 550 sows
bred during the year. Each sow receives two inseminations while each
boar provides semen for about 40 inseminations during his allotted
stay as an A.I. sire. Assume that the cooperative has 20 contemporary
groups of sows bred during the year. The estimated number of A.I.
sires needed is:
NBYBC = (550 x 2) / 40 = 28.
Based this calculation, the number of boars to select from each
contemporary group is:
NBS = 28 / 20 = 1.4 or 1 to 2 boars.
Thus, producers should select 1 to two top indexing boars from
each contemporary group.
Development of an A.I. boar culling policy requires a different
plan than used in a closed herd breeding program. Cooperative herds
will differ in contemporary group size and breeding interval. Thus, it
is probably not practical to cull animals based on number of
inseminations since a given boar might removed before he has a chance
to be used in all member herds. Instead, it is more practical for
producers to develop a culling policy based on the time period the
boar is in the pool of A.I. sires. The cooperative might choose a
maximum time period of 1 to 4 months. For this system to work,
producers could develop a list of A.I. boars with pertinent data
including index values and date of entry into the pool of sires. Each
sire would be culled after reach the specified time limit. When a new
boar is ready for service, it would be entered on the list and would
replace the individual that had been used for the longest period as a
sire.
Summary
In setting up a breeding program, producers should decide what
will make up a contemporary group. A contemporary group should consist
of at least 20 littters that are born during a time interval of three
weeks or less. The producer must develop a sow culling policy. We
suggest culling 20% of the sows in each parity class (1 to 3) between
farrowing and breeding. With this policy, all sows would be culled
after farrowing their fourth litter. The number of gilts to be
selected from each contemporary group can be determined based on an
equation provided in the factsheet. For within herd selection, the
boar culling policy should be based on a quota of no more that 30 to
50 matings per sire. The number of sires and selected boars per
contemporary group can be determined based on provided equations. For
a within herd breeding program, at least 15 sires should be used per
year to help minimize rates of inbreeding. Small seedstock producers
should consider forming a breeding cooperative with the exchange of
semen among herds to help minimize inbreeding and to increase the rate
of genetic improvement.
References
Johnson, R.K. 1990. Closed herd breeding programs. Record of
Proceedings North American Swine Improvement Conference. 15:55.
NEW 12/92 (3M)
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