BREEDING & GENETICS                               PIH-27


             Guidelines for Choosing Replacement Females

William T. Ahlschwede, University of Nebraska
Daryl Kuhlers, Auburn University

Maynard Hogberg, Michigan State University
James F. Schneider, Holmen, Wisconsin

     The productivity of the sow herd is the foundation  of  com-
mercial  pork  production.  The sow herd also contributes half of
the genetic  makeup  of  growing-finishing  pigs.  These  factors
together indicate the importance of careful selection of replace-
ment gilts and wise decisions on their retention in the  breeding

     The choice of crossbreeding system plays an  important  role
in  the development of a gilt selection and sow-culling strategy.
With rotational crosses, all gilts are candidates for  selection.
It is reasonable to be quite selective. With the specialized ter-
minal crosses, the matings producing replacement gilts  are  less
productive than those producing market hogs. Hence, the number of
matings to produce replacement gilts  and  the  opportunities  to
select gilts are minimized.

     From among those gilts available for selection,  select  the
fastest  growing,  leanest  gilts  that  are sound and from large
litters; and among sows which have  farrowed  and  will  rebreed,
cull   only  those  with  physical  problems,  bad  dispositions,
extremely small litters (more than 3 pigs below herd average) and
poor mothering records.

     Pork producers with small sow herds will find  it  difficult
to  maintain  the  breeding  groups required to produce their own
gilts for terminal crosses. For these producers  to  profit  from
the   increased  productivity  of  terminal  crosses,  purchasing
replacement gilts may be desirable. It is recommended  that  pro-
ducers  purchasing replacement gilts establish a continuing rela-
tionship with a reputable supplier of healthy maternal  crossbred

The Sow's Job

     The success of a commercial pork operation depends upon  the
sow herd weaning large litters of pigs regularly. This means that
a large percentage of the sows and gilts show estrus  and  breed,
farrow  large litters of vigorous pigs, keep a high percentage of
the pigs alive and get their pigs off to a  good  start.  All  of
these  functions  are  affected  by  environmental situations and
management practices. All are affected by genetics. However,  the
large  genetic  influences  are  due  to  breed  differences  and
heterosis. Differences among gilts and sows in a breed group  are
much  less important. Direct selection from among gilts cannot be
made for these sow productivity characteristics.

     The size of litter in which a gilt is born and  the  weaning
weight of the litter genetically are traits of the gilt's mother.
Selecting gilts for these traits would be selection on the  dam's
record,  which dilutes the selection effort. Our understanding of
the genetic basis for these traits  indicates  that  economically
important  genetic  changes  can  be made by selection. The large
nongenetic variation in these traits, particularly  litter  size,
makes  it difficult to detect the change. The place for selection
on sow productivity is  at  the  seedstock  production  level  in
breeds  destined  for  use as sow lines.  Seedstock producers are
positioned to use Estimated Breeding Value  (EBV)  procedures  to
accelerate   genetic  improvement.   Purchase  boars  for  siring
replacement gilts from sources whose selection criteria  includes
sow  productivity.  Choose  boars with high EBV's for sow produc-
tivity traits.

     Keeping the baby pigs alive and getting them off to  a  good
start  generally  are  classified as maternal effects. Successful
management schemes assign the job of keeping pigs  alive  to  the
manager  as  well  as to the sow. If management intervenes on the
pigs' behalf by fostering pigs to equalize litter size,  by  hand
feeding  weak pigs, and by administering timely treatment of baby
pig health problems, the sow should not be held wholly  responsi-
ble  for  differences  in survival. In the matter of getting pigs
off to a good start, there are important differences  among  sows
which  show  up in weight gain of suckling pigs. For culling pur-
poses, evaluation of pig weights for indication  of  sow  milking
performance  should  be  made  before  the  pigs are 4 weeks old,
preferably at 3 weeks of age.

     Evaluating maternal performance early is  supported  by  two
types  of rationale. First, before 3 weeks of age, the pig relies
almost entirely on the sow as a source of nutrients. Under  usual
production  practices  there  is  no alternative. After the third
week the litter's need for nutrients often  surpasses  the  sow's
ability  to  produce milk. At that time, the pigs can turn to dry
feed to meet part or all of their needs.

     Second, there is evidence  which  suggests  that  heavy  pig
weights  up  through 4 weeks indicate high levels of milk produc-
tion, but heavy suckling pig weights after about 4 weeks  of  age
indicate lower levels of milk production. This is because pigs on
poor milking sows start creep feed earlier and eat more dry feed.
Hence,  evaluation  of  sow milking performance should be made at
about 3 weeks of age.

     Since fairly low rates of  sow  culling  are  suggested  and
equalizing  the  size  of  litters is expected, evaluation of sow
performance should identify those sows that obviously are milking
poorly.   Sows  that  are  slow  to come to their milk, that have
light pigs at 3 weeks or have pigs that die because of too little
milk should be marked for culling.

     In addition to farrowing and starting pigs, the sow supplies
half  the genetic composition of the offspring. Rate of gain, fat
thickness, and feed efficiency are commercially important  traits
which  respond  to  selection.   Increased  gain  and reduced fat
thickness can be selected  for  directly  in  replacements.  Feed
efficiency  is  favored indirectly by selecting the fast-growing,
low-backfat gilts.

     A balance between sow culling and gilt selection needs to be
established.   Replacement gilts are needed in sufficient numbers
to  replace  sows  as  they  are  culled.  Gilts  replacing  sows
represent  the  major opportunities for genetic change in the sow
herd. This change is primarily due to the genetic superiority  of
the  boar selected to sire replacement gilts. Replacing sows with
gilts also represents an opportunity to change the breed composi-
tion and heterosis level of the sow herd.

     Since sows generally produce larger litters of heavier pigs,
replacing sows with gilts may reduce production levels. This pro-
duction differential and the low relationship between the perfor-
mance  of successive litters argue for low rates of culling based
on sow performance in order to maintain high  levels  of  produc-
tion.  This must be balanced against the genetic change made pos-
sible by bringing gilts into production. A total gilt replacement
level of 15-20% is suggested for each farrowing.

     The gilt selection and sow culling scheme suggested  assumes
that  there  are no major genetic antagonisms between litter size
and maternal performance on one hand and rate  of  gain  and  low
backfat  thickness on the other hand. There is some evidence that
the so-called ``very meaty gilt'' does not make a good sow.  How-
ever,  there  is  no  documented  evidence  that  selecting fast-
growing, low-backfat gilts will adversely affect sow performance.

     Soundness. Soundness means being free from flaws or defects.
In selecting replacement females, being sound means being free of
flaws or defects which would interfere with  normal  reproductive
and maternal function. Three areas are of particular concern: (1)
reproductive; (2) mammary; and (3)  skeletal.  For  selection  as
replacement  stock,  sows and gilts should meet minimal levels in
each of these categories.

     Reproductive soundness.  Replacement  gilts  should  exhibit
normal    reproductive   development,   both   anatomically   and
behaviorally. The external genitalia should be normally developed
(Fig. 1).

     Most anatomical  defects  of  the  reproductive  system  are
internal  and not visible. Gilts with small vulvas (Fig. 1) indi-
cative of infantile  reproductive  tracts  should  not  be  kept.
Replacement  gilts should begin to show signs of puberty at least
a month prior to anticipated breeding. Sows which have difficulty
farrowing,  are  extremely slow farrowing, or have damaged repro-
ductive tracts (uterine prolapse  or  severe  uterine  infection)
should be culled.

     Mammary soundness. Replacement gilts should possess a suffi-
cient number of functional teats to nurse a large litter of pigs.
Current industry standards stipulate at least 6 well spaced func-
tional  teats  on each side.  Gilts with inverted or scarred nip-
ples should not be saved. New concrete, rough floors  and  corro-
sive  chemical  compounds  on  the floors of farrowing houses can
cause abrasions to gilts' underlines  which  result  in  nonfunc-
tional teats (Fig. 2). As the gilt approaches puberty, her under-
line should become more prominent, indicating normal development.

     Skeletal soundness. Gilts with feet and leg  problems  which
will interfere with normal breeding, farrowing, and nursing func-
tions should not be saved.  Sows that are unable to  get  up  and
down in farrowing crates should be culled.

Which Gilts and Sows to Select

     The fastest growing, leanest gilts which are sound and  from
large litters should be saved for replacement females. Sows which
fail to rebreed should be culled. Sows which had  small  litters,
failed to milk, or had problems farrowing should be culled. Small
litter means more than 3 pigs below the group average.

     This selection and culling program  requires  identification
of  potential  replacement  gilts  at  birth. The gilts should be
evaluated for growth and leanness as they approach market weight.
The purpose of evaluating the growth and leanness is to eliminate
slow growing and fat gilts. This is  best  done  when  the  gilts
weigh between 180 and 200 lb. This is considered the final selec-
tion. At this time, the gilts are appraised  for  indications  of
normal  reproductive  development,  functional  appearance of the
underline and skeletal soundness.

     Ear-notching gilts  at  birth  with  litter  and  individual
notches,  along  with a written record of birth date, litter size
and breed composition will meet the needs of the  gilt  selection
program.  Less complex identification systems can be used if they
provide a method of identifying  gilts  from  large  litters  and
allow age determination at the time gilts are evaluated and added
to the breeding herd.  Some producers  have  notched  only  gilts
from large litters, using birth date as the number so the age can
be determined. Ear tags are helpful in identifying  sows  in  the
breeding herd (Fig. 3). Sufficient sow identification and farrow-
ing house records need to be kept in  order  to  cull  the  right

Purchasing Replacement Gilts

     The adoption  of  specialized  terminal  crosses  creates  a
dilemma  for  producers  with  sow  herds too small to maintain a
separate breeding group for the production of replacement  gilts.
Yet  the  added productivity of the maternal cross sows is needed
to provide competitive profit  levels.  For  producers  with  few
sows,  purchasing replacement gilts may be desirable. The produc-
tivity of specialized maternal crossbred sows usually  more  than
repays  the  purchase  price.  The health status of the purchased
gilts is a major consideration.

     When purchasing replacement gilts, the source of  the  gilts
is the primary concern. The source controls the health status and
the breed and cross of  the  gilts.  The  source  determines  the
genetic worth of the gilts by the selection practices employed in
the production of the gilts. The source is the agent  of  quality
control and provides service if needed. Hence, the choice of sup-
plier is the most critical decision when buying gilts.

     Health. By restricting gilt purchases to  a  single  source,
commercial  producers can establish the herd health level experi-
enced by the supplier. Hence, it is important to  choose  a  herd
with an excellent health status. Sources with defined herd health
programs and monitoring procedures are  preferred.  Consult  with
the  veterinarian  of the herd and with other customers to verify
the herd status. Inspection of the herd is in order.

     Breed and cross. The purpose of buying replacement gilts  is
to  get  breed  combinations  and crosses that cannot be produced
effectively at home. It is an opportunity to buy the  best  cross
for  your  farm.  Compromises in the breed and heterosis level of
the gilt must be accompanied by substantial savings  in  purchase
price.  One  buys  the lifetime performance of the sow when gilts
are purchased. To assure complete or 100% heterosis, the sire  of
the  gilt must be of a breed not represented in the sow producing
the gilt.

     Genetic worth. Within every breed or cross, a wide range  of
genetic  merit  or worth exists. Among groups within the breed or
cross, the major factor in genetic merit is the  selection  prac-
ticed among the parents of the group. While it is not possible to
determine exactly the genetic merit of the herd, it  is  possible
to  make  a  meaningful  assessment  of  potential  sources.  Two
indirect methods are suggested. The  first  is  to  evaluate  the
selection  practices  of the source herd. A replacement gilt sup-
plier who bases selection decisions on performance tests for rate
of  gain, leanness and sow productivity is preferred to one which
does not. The other source of information about the genetic merit
of a replacement gilt supplier is other customers. Often the sup-
plier will provide testimonials from satisfied customers.  It  is
preferred to check customer results independently.

     The availability of microcomputers and  the  development  of
new  software has made more accurate evaluation of breeding stock
feasible. If breeders test all litters for  several  generations,
they  can  estimate  the real genetic value of individuals. These
estimates are calculated as Estimated Breeding Values (EBV) or as
Expected  Progeny  Differences  (EPD). Since parents pass half of
their genetic compliment to their  offspring,  EPD  is  equal  to
one-half  of  the  EBV.   Computationally, performance records of
individuals are considered along with the records  of  all  rela-
tives,  weighted by the heritability and strength of family rela-
tionships, to arrive at the  best  estimate  of  an  individual's
genetic  worth.   Breeders  who use EBV's or EPD's to make selec-
tions in their own herds can make  more  rapid  genetic  progress
than those who do not.

     Quality control and services. One would prefer a  source  of
replacement  gilts  which shipped to customers only sound healthy
gilts of the age and size agreed upon. This is part of the  repu-
tation of the source. Even with the best of sources, the buyer is
encouraged to inspect the candidate gilts and reject those  which
are unsound. Willingness to stand behind the gilts and help solve
any problems which may develop is the service included  with  the
gilts.   While  the amount of service supplied will vary with the
source, it is important that the nature  and  amount  of  service
provided be established prior to purchase.

     These factors suggest the importance of establishing a  con-
tinuing  relationship  with a supplier of replacement gilts. This
helps maintain a herd health program and in the timing  of  ship-
ments  of  gilts.  As the relationship continues, the supplier is
better able to provide service. Both parties know what to expect.

     Timing Purchases. It is recommended that  replacement  gilts
be  purchased at least 30 days before their anticipated breeding.
During this period they should be isolated from the main breeding
herd, but should be exposed to the organisms of the breeding herd
via cull sows and fecal material.

Management for Development

     While little direct selection can be  practiced  for  litter
and  maternal  performance,  several  management practices can be
used to help insure optimal performance from gilts. The  way  the
gilt is managed at birth and as she approaches puberty can affect
her  subsequent  reproductive  performance.  Moving  pigs   among
litters  at  birth  to equalize litter size is a common practice.
Equalizing litter size after the gilt  pigs  from  large  litters
have  been  identified  may  also  give  gilts the best chance to
develop normally. Research suggests that gilts  reared  in  large
litters  are  less productive than gilts reared in small litters.
Transfer male pigs from litters containing potential  replacement
gilts  to  other  sows,  so  that replacement gilts are reared in
average sized litters.

     Following weaning, gilts should be fed and managed in a  way
that will accelerate their growth and development until they near

     Replacement gilt candidates should be self-fed  a  balanced,
well-fortified  diet  during the growing period. When gilts reach
180-200 lb they should be evaluated and selected and placed on  a
restricted  diet  fortified  for limit feeding. Added accuracy in
evaluating growth and leanness by extending  the  feeding  period
does  not  justify  the  cost of the added feed. The added weight
gained on full feed is mostly fat which is  not  needed  and  may
interfere with subsequent reproduction. In addition, the stimula-
tion from sorting and moving the gilts and reducing  their  level
of  feed  at  180-200  lb may trigger puberty (first heat) in the
gilts. These changes, coupled with fenceline boar contact, should
help  induce  earlier  puberty and insure a higher pregnancy rate
and larger litters from the gilts.


                     Gilt Selection Calendar


When                                     What
Birth             o    Identify gilts born in large litters.  Hernias,
                       cryptorchids and other abnormalities should
                       disqualify all gilts in a litter for replacements.

                  o    Record birth dates, litter size, breed composition
                       and identification.

                  o    Equalize litter size by moving male pigs from
                       large litters to sows with small litters. Pigs
                       should nurse before moving.

                  o    Keep notes on sow behavior at time of farrowing
                       and check on: (a) disposition, (b) length of
                       farrow, (c) any drugs such as oxytocin
                       administered, (d) condition of udder, and (e)
                       extended fever.

3-5 weeks         o    Wean litters.  Feed balanced, well-fortified diets
                       for maximum growth and development.

                  o    Screen gilts identified at birth by examining
                       underlines, and reject those with fewer than 12
                       well-spaced teats. If possible, at this time
                       select and identify as replacement gilt candidates
                       about 2-3 times the number needed for replacement.

180-200 lb        o    Evaluate gilts for growth, leanness, and

                  o    Select for replacements the fastest growing,
                       leanest gilts that are sound and from large
                       litters.  Save 25-30% more than needed for

                  o    Remove selected gilts from market hogs.  Place on
                       restricted feed.  Increase mineral fortification

                  o    Give fenceline contact with boar.

                  o    Observe gilts for sexual maturity.  If puberty
                       records are kept, give advantage to those gilts
                       that have cycled most frequently when final
                       culling is made.

Breeding          o    Make final cull when the breeding season begins
time                   and keep sufficient extra gilts to offset the
                       percentage of nonconception.

                  o    Make sure all sows and gilts are ear-tagged or


Related Publications

     Additional information can be found  in  the  following  PIH
fact sheets:

PIH-23    Swine Diets
PIH-39    Crossbreeding Programs
          for Commercial Pork Production
PIH-52    Minerals for Swine
PIH-59    Infectious Swine Reproductive Diseases
PIH-101   Selection for Feet and Leg Soundness

REV 6/92 (7M)

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