The rate of genetic improvement is dependent upon use of tools
which allow producers to accurately measure performance and estimate
genetic merit. Traditionally, adjustment factors have been used
to standardize performance records and better allow for isolation
of the genetic component of observed variation. Current litter
adjustment factors in the swine industry are provided by the National
Swine Improvement Federation (NSIF, 1988) and are common to all
breeds. Litter adjustments for the American Landrace and Yorkshire
populations were updated by Brubaker et al. (1994) and were found
to differ between breeds and from the NSIF factors.
Reproductive performance of a sow can be affected by many factors
(Clark and Leman, 1986; Almond, 1992). Litter size can be affected
by contemporary group (environment), parity, age at farrowing,
previous lactation length, weaning to conception interval and
genetic merit. Litter weight is influenced by these factors as
well as the age at weaning and number weaned. Current adjustment
factors were derived by accounting for the factor of interest,
i.e. parity, and in some manner the environment. Other factors
such as genetic merit and age at farrowing have been ignored.
However, computing power and programs have advanced and now allow
for inclusion in the model of nearly all of the components of
variation simultaneously. The inclusion of these additional effects,
including genetic merit, should greatly increase the accuracy
of estimating adjustment factors.
Therefore, several objectives were involved in this study. First,
the derivation of traditional and possibly new litter adjustment
factors utilizing a more complete estimation model. Second, an
analysis of the need for breed-specific adjustments for the major
pure breeds of swine.
MATERIALS AND METHODS
Duroc, Hampshire, Landrace and Yorkshire litter records were obtained
from the National Swine Registry. Duroc, Hampshire, and Landrace
data included all records collected up to May 1996 and Yorkshire
data included all records collected up to November 1995. Data
sets included identification, herd, birth date, and pedigree of
the sow as well as contemporary group, farrowing date, number
born alive (NBA), litter weaning weight (LWT), number weaned (NW),
number after transfer (NAT), weaning date, and parity of the litter.
Age at breeding to produce the litter record was calculated as
farrowing date - birth date - 114 and weaning age of litter
as weaning date - farrowing date. Editing was conducted
to insure connectedness and complete records as well as to eliminate
biological extremes. The number of records, contemporary groups,
and dams as well as phenotypic means after editing are shown by
breed in Table 1.
Table 1. Number of litter records, contemporary groups, and dams as well as phenotypic means for number born alive (NBA), 21-day litter weight (LWT), and number weaned (NW) for Duroc, Hampshire, Landrace, and Yorkshire swine
No. of records
No. of cg
No. of dams
1 Unadjusted mean.
Estimation method for adjustment factors
A mixed model reduced animal model program (Feng, 1995) which
utilizes an iteration on data algorithm was used for simultaneous
estimation of all genetic and non-genetic effects for NBA, LWT,
and NW. Variance components used for the analysis were those
currently utilized by the national genetic evaluation of each
specific breed. The model for NBA included a direct genetic effect,
an uncorrelated random permanent environmental effect and fixed
effects of age and/or parity class and contemporary group. The
model for LWT and NW included the above effects as well as the
fixed effects of NAT and weaning age.
Age of dam at breeding was analyzed for possible influence on
reproductive performance within parity 1 and parity 2 litter records.
Initially, age classes within parities 1 and 2 were sub-divided
into 10 day intervals. Multiple subsequent analysis runs were
used to find the most appropriate class intervals, with additional
care taken to insure an adequate sample size within all age classes.
For analysis of LWT and NW, solutions for weaning age were obtained
for ages of 14 to 28 days. Multiplicative weaning age adjustments
for LWT were then obtained for adjustment of litter weight to
a 21-day base by dividing the solution for day of interest by
the solution for 21-day weight.
RESULTS AND DISCUSSION
Reproductive performance can be influenced by many factors, including
environment, parity, age at farrowing, lactation length, weaning
to mating interval and genetic merit (Clark and Leman, 1986).
However, due to computing limitations, genetic merit has not
been previously included in the estimation procedures. Therefore,
improvements in accuracy of the estimation procedures should be
achieved by inclusion of genetic merit in the estimation model.
Also, traditional adjustment factors have dealt primarily with
those differences associated with parity. However, age at farrowing
has been shown to account for significant variation in the litter
records of younger sows (Brooks and Smith, 1980; Hughes, 1982;
Culbertson and Mabry, 1995; Xue et al., 1996).
Production and economic efficiencies often lead producers to attempt
to mate females at younger ages. The current adjustments do not
differentiate between sows who conceive their initial litters
at younger ages. Therefore, sows which possess the economic benefits
of being able to reach puberty and to conceive at younger ages
may have their genetic evaluation for reproductive traits biased
downward. Consequently, the accurate identification of sires
and herds of increased genetic merit should be increased using
the new adjustments for NBA.
Adjustment factors, number of records within each class and current adjustments in the Duroc, Hampshire, Landrace, and Yorkshire populations for NBA are presented in Tables 2-5; respectively. Newly derived adjustment factors differ from those currently used as well as differ between breeds. In addition to the parity differences which follow the expected pattern, differences were found in parities 1 and 2 due to age of the sow at time of the litter record.
Table 2. Adjustment factors and numbers within class for number born alive (NBA) in Duroc swine
1 NSIF, 1988.
The pattern of these differences is in agreement with previous work (Sherritt, 1962; Strang, 1970; Clark and Leman, 1986) which shows an increase of .003 to .009 pigs per parity 1 litter per additional day of age of the sow at breeding. Duroc and Hampshire populations, compared to the current NSIF adjustments, were found to have, on average, smaller adjustments for the initial parities and larger adjustments for later parities. However, larger adjustments were also found for the younger females relative to the older females within parities 1 and 2. Landrace and Yorkshire adjustments for NBA were larger than the current adjustments derived by Brubaker et al. (1994). Again, age was found to be a highly important component of variation within parities 1 and 2 with adjustments for younger Yorkshire sows differing from older sows by almost 1 pig within parity 1 or 2.
Table 3. Adjustment factors and number within classes for number born alive in Hampshire swine
1 NSIF, 1988.
Table 4. Adjustment factors and numbers within class for number born alive in Landrace swine
1 Brubaker et al., 1994.
Table 5. Adjustment factors and numbers within classes for number born alive in Yorkshire swine
1 Brubaker et al., 1994.
Parity and/or age adjustments for 21-day litter weight in Duroc, Hampshire, Landrace and Yorkshire are presented in Tables 6-9; respectively. Duroc and Hampshire adjustments were found to be larger in magnitude than those currently recommended by NSIF. Landrace and Yorkshire adjustments, on average, were found to be similar to the current adjustments. However, age of the sow contributed considerable variation to parity 1 litter records in all breeds with larger adjustments estimated for the younger females. Age of the sow at conception of the littter recordwas found to have decreasing importance in parity 2.
Table 6. Parity and age adjustment factors and number within classes for 21-day litter weight in Duroc swine
1 NSIF, 1988.
Table 7. Parity and age adjustments and number within classes for 21-day litter weight in Hampshire swine
1 NSIF, 1988.
Table 8. Parity and age adjustments and number within classes for 21-day litter weight in Landrace swine
1 Brubaker et al., 1994.
Table 9. Parity and age adjustments and number within classes for 21-day litter weight in Yorkshire swine.
1 Brubaker et al., 1994.
Number after transfer and multiplicative weaning age adjustments
for 21-day litter weight are presented in Tables 10 and 11; respectively.
Adjustments for number after transfer and weaning age are in
general agreement with the previous studies (NSIF, 1992; Brubaker
et al., 1994) and relatively consistent across breed populations.
Accurate adjustments should standardize performance records across factors which are known to cause non-genetic variation. This study utilized a method of estimation which allowed for inclusion of genetic merit to better account for differences which affect reproductive performance in the sow herd. In addition, the effect of age of the sow at conception of parity 1 and parity 2 litter records was analyzed in the estimation procedure. Results from this study indicate that adjustment factors for number born alive and 21-day litter weight should include age of the sow to better standardize litter records and improve evaluation of the genetic merit, particularly of parity 1 and 2 litter records.
Table 10. Number after transfer (NAT) adjustments for 21-day litter weight in Duroc, Hampshire, Landrace and Yorkshire swine
Table 11. Weaning age multiplicative adjustment factor for 21-day litter weight in Duroc, Hampshire, Landrace and Yorkshire swine
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