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J. Anim. Sci. 2003. 81:624-629
© 2003 American Society of Animal Science

Results from nine generations of selection for increased litter size in swine

J. W. Holl and O. W. Robison

North Carolina State University, Raleigh, NC 27695

1 Correspondence:
phone: 919-515-4019; E-mail:
owrob{at}unity.ncsu.edu.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Direct selection for increased litter size was done for nine generations. The select line consisted of approximately 15 sires and 60 dams per generation, and selection was based on estimated breeding values for number of live pigs. A control line of approximately 10 sires and 30 dams was maintained with stabilizing selection. Heritabilities estimated in the select line using restricted maximal likelihood procedures, daughter-dam regression within sires, and half-sib analysis were 0.01, 0.04, and 0.00 for number of pigs born alive (NBA) and 0.02, 0.16, and 0.00 for total born per litter (TB). Corresponding estimates for the control line were 0.01, 0.06, and 0.23 and 0.02, 0.07, and 0.09 for NBA and TB, respectively. Realized heritabilities for NBA from multiple regression were 0.09 ± 0.08 in the select line and 0.11 ± 0.166 in the control line. Heritability estimated from regression of differences in response between lines on differences in cumulative selection differentials was 0.13 ± 0.07. At Generation 9, litter sizes, estimated breeding values, and cumulative selection differentials were 0.86 (P < 0.05), 0.63 (P < 0.01), and 9.05 (P < 0.01) pigs larger for the select line than for the control line. Phenotypic differences between lines for TB, adjusted backfat (BF), and days to 104 kg (DAYS) were not significant. Genetic trends in the select line were 0.053 ± 0.002 pigs/yr for NBA, 0.054 ± 0.013 mm/yr for BF, and 0.398 ± 0.110 d/yr for DAYS. Corresponding phenotypic trends were 0.145 ± 0.051 pigs/yr, -0.012 ± 0.089 mm per yr, and 0.307 ± 0.278 d/yr, respectively. Genetic trends in the control line were -0.026 ± 0.004 pigs/yr for NBA, 0.026 ± 0.022 mm/yr for BF, and -0.532 ± 0.182 d/yr for DAYS. Corresponding phenotypic trends were 0.001 ± 0.085 pigs/yr, -0.043 ± 0.147 mm/yr, and -0.519 ± 0.462 d/yr, respectively. Litter size can be increased by direct selection using breeding values estimated from an animal model, in conjunction with rearing selected gilts in litters of 10 pigs or less.

Key Words: Genetic Parameters • Genetic Trend • Litter Size • Pigs • Selection


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Increasing litter size is economically important (Tess et al., 1983). In mice, direct selection on litter size has been successful (Gion et al., 1990;Falconer, 1971), but was unsuccessful in swine (Ollivier, 1982). Due to evidence of negative maternal effects between the size of the litter in which a gilt was reared and size of the litter a gilt produces, Revelle and Robison (1973) suggested that successful selection for increased litter size would be accomplished if an optimal maternal environment was provided. In addition, Avalos and Smith (1987) and Bichard and David (1985) hypothesized that an increase in litter size would occur from using a family selection index.

Multiple-trait index selection has been shown to increase litter size and to affect other traits (Ferraz and Johnson, 1993;Neal et al., 1989; Johnson et al., 1999). Although some studies report genetic correlations of litter size with other traits (Young et al., 1978; HREF="#JOHANSSON-AND-KENNEDY-1983">Johansson and Kennedy, 1983;Hermesch et al., 2000), none has reported realized correlated responses in backfat or days to a slaughter weight.

Objectives of this study were to increase litter size using estimated breeding values from an animal model and to report direct and correlated responses to selection.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Implications
 Literature Cited
 
Founder animals included 15 boars and 32 females from the Nebraska control line, which was a Large White-Landrace composite (Neal et al., 1989). Animals were randomly mated, avoiding full- and half-sib matings, for two generations to expand the population. Pigs were then randomly assigned within litter either to a control line (Line C) or a select line (Line S) and selection was started.

Selections were based on breeding values for number of pigs born alive estimated from an animal model using multiple-trait REML (Boldman et al., 1993). The model included fixed effects of generation and random direct genetic effects. Selection in Line C attempted to maintain estimated breeding values at approximately zero. In Line S, 48 to 60 gilts and 15 boars per generation were selected. Selections were limited to no more than four gilts per litter. In Line C, approximately 30 females and 10 boars were selected. All matings were at random except to avoid full- and half-sibs matings.

The number of fully formed pigs and live pigs and individual weights were recorded at birth. Pigs were cross-fostered to standardize size of litter reared by each dam. Litter size was reduced to 10 or less within 24 h of birth. At 28 d of age, pigs were weaned. At 140 d of age, backfat at the seventh rib and at a point midway between the last rib and last lumbar were recorded along with weight. Backfat was measured with a metal probe, averaged, and adjusted to 104 kg of BW.

Females were mated at 8 mo of age and allowed 5 wk to conceive. Females were used for one farrowing. The mean number of live pigs at birth was calculated for each line-generation subclass. Individual selection differential (ISD) was calculated as half of the difference between the size of the litter in which an individual was born and the average litter size of that generation and line. Cumulative selection differential (CSD) was calculated as the mean CSD of parents plus ISD.

Realized heritability was obtained from regression of line differences for the number born alive on line differences in CSD as well as the ratio of line differences in response to line differences in CSD in the last generation as described by Hill (1972), and by a multiple-regression procedure as described by Quijandria et al. (1983). The following model was used for the multiple regression procedure:


where Yijkl is the dependent variable of number of live pigs born to the animal, Ai is the fixed effect of the ith generation, Bj is the fixed effect of the jth line (C or S), Ck(j) is the effect of cumulative selection differential within the jth line, and Eijkl is random error. Heritability was also estimated from daughter–dam regression within sire of daughter, half-sib analysis of sires nested within generation, and from an animal model using multiple-trait REML (Boldman et al., 1993).

Genetic and phenotypic trends were estimated by regressing estimated breeding values or phenotypic values on generation number. In addition, line differences were regressed on generation number.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Implications
 Literature Cited
 
Estimates of heritability for each trait by line are summarized in Table 1Go. Restricted maximal likelihood estimates of heritabilities are from a multiple-trait model, but estimates were consistent regardless of the number of traits in the model. Heritability estimates from Line C were either higher than or equal to those from Line S for number born alive. Restricted maximal likelihood estimates of heritabililies were lower than those reported by Lamberson et al. (1991),See et al. (1993),Johnson et al. (1999), and Hermesch et al. (2000), but similar to the value reported by Haley and Lee (1992) using DFREML. Using daughter–dam regression, heritability estimates were lower than those reported (Haley et al., 1988;Lamberson et al., 1991).


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  		Table 1. Estimates of heritability by three methods for number born alive, total born per litter, adjusted backfat, and days to 104 kg in Line S and Line C
 
Adjusted backfat heritability estimates were higher in Line C than Line S for all methods, although both were lower than REML estimates reported by Ferraz and Johnson (1993). Heritability estimates for days to 104 kg were higher in Line S than in Line C for all methods.

Restricted maximal likelihood estimates of genetic correlations are given in Table 2Go. Genetic correlations between number born alive and total born were similar to literature estimates (Young et al., 1978; Bereskin, 1984a;Irvin and Swiger, 1984). Genetic correlation estimates between number born alive and adjusted backfat were higher than those reported by several authors (Young et al., 1978; HREF="#JOHANSSON-1981">Johansson, 1981;Johansson and Kennedy, 1983;Hermesch et al., 2000). Genetic correlation estimates of number born alive with days to 104 kg were similar to those reported by Young et al. (1978) and Bereskin (1984b), but were smaller than estimates reported by Johansson and Kennedy (1983). Between total born per litter and adjusted backfat, genetic correlation estimates were larger than other literature estimates (Young et al., 1978; HREF="#JOHANSSON-AND-KENNEDY-1983">Johansson and Kennedy, 1983;Bereskin, 1984b) in Line S and less than literature estimates in Line C.


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  		Table 2. Restricted maximal likelihood estimates of genetic correlations among number born alive, total born per litter, adjusted backfat, and days to 104 kg for Line S (below the diagonal) and Line C (above the diagonal)
 
Line by generation means are shown in Table 3Go. Means in Line C were relatively stable. Line S consistently had more born alive and more total pigs born per litter except for Generation 3. Mean adjusted backfat and days to 104 kg were similar between lines throughout the study.


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  		Table 3. Line S and Line C means over nine generations of selection for number born alive, total born per litter, adjusted backfat, and days to 104 kg
 
Cumulative selection differentials for number born alive by line within generations are shown in Figure 1Go. At Generation 9, CSD were 10.83 pigs in Line S and 1.78 pigs in Line C. Cumulative inbreeding in Line S was 0.09% and 0.07% in Line C.



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  		Figure 1.

Mean cumulative selection differential (CSD) for control and select lines.

 
Regression of line differences in number born alive on line differences in CSD is shown in Figure 2Go. Realized heritability was 0.13 ± 0.07. Because selection is based on estimated breeding values, this estimate is inflated by an unknown factor. However, this estimate is similar to the estimate of realized heritability of 0.15 ± 0.05 reported by Lamberson et al. (1991). Realized heritability as defined by Hill (1972) was 0.10 ± 0.07. Realized heritabilities estimated using multiple regression were 0.09 ± 0.08 for Line S and 0.11 ± 0.16 for Line C and are similar to those reported by Haley et al. (1988). Realized heritability estimates were larger than estimates using REML, daughter–dam regression within sire, and half-sib analysis.



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  		Figure 2.

Mean difference in number born alive (NBA) between Line S and Line C regressed on the difference in cumulative selection differential (CSD) between Line S and Line C.

 
Mean estimated breeding values for number born alive, adjusted backfat, and days to 104 kg by generation are shown in Figures 3 to 5GoGoGo. Estimated breeding values for number born alive at generation nine differed by 0.63 pigs (P < 0.01). Differences between lines for adjusted backfat and days to 104 kg were not significant at generation nine.



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  		Figure 3.

Mean breeding value (BV) for number born alive in control and select lines.

 


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  		Figure 4.

Mean estimated breeding value for backfat for the select and control lines.

 


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  		Figure 5.

Mean estimated breeding value for days to 104 kg for the select and control lines.

 
Regressions of estimated breeding values on generation number are given in Table 4Go. Line S regressions were positive and highly significant for number born alive, adjusted backfat, and days to 104 kg. Line C regressions were negative for number born alive and days to 104 kg (P < 0.01), but positive (P > 0.05) for backfat. Correlated genetic responses for Line S are in agreement with estimated genetic correlations. However correlated genetic response for Line C disagrees with the estimated genetic correlation for days to 104 kg.


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  		Table 4. Coefficients (b) and standard errors (SE) for regression of estimated breeding values on generation by trait and line and the difference of the line means
 
Regressions of line differences in estimated breeding value on generation number are shown in Table 4Go, also. Difference between lines in number born alive increased by 0.079 pigs per year (P < 0.01). This estimate is slightly higher than the response of 0.053 pigs per generation to direct selection for litter size following selection for ovulation rate reported by Lamberson et al. (1991). Regression of line differences in estimated breeding values for adjusted backfat on year was not significant. Line differences in estimated breeding values for days to 104 kg increased by 0.930 d /yr (P < 0.01).

Phenotypic trends are shown in Table 5Go. Regression of number born alive was highly significant for Line S and similar to the response over five generations reported by Ollivier (1982). Adjusted backfat had a nonsignificant negative phenotypic trend, even though it had a highly significant positive genetic trend. Days to 104 kg had a positive phenotypic trend (P > 0.05), similar to the highly significant positive genetic trend. In Line C, phenotypic regressions were not significant even though genetic trends for number born alive and days to 104 kg were highly significant.


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  		Table 5. Coefficients (b) and standard errors (SE) for regression of phenotype on generation by trait and line and the difference of the line means
 
Regressions of line differences on generation number are given in Table 5Go. Mean difference between lines for number born alive increased by 0.144 pigs per year, but was not significant, despite having a highly significant positive genetic trend. Regression of line differences on generation for adjusted backfat was similar to the genetic trend (P > 0.05). Mean difference between lines for days to 104 kg increased nonsignificantly by 0.826 d/yr in agreement with estimated genetic trend.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Implications
 Literature Cited
 
Nine generations of selection for number born alive resulted in significant differences in litter size (0.86 pigs), estimated breeding value (0.63 pigs), and CSD (9.05 pigs).

Correlated response to selection for increased number born alive resulted in significant increases in estimated breeding values for adjusted backfat and days to 104 kg. However, phenotypic changes in adjusted backfat and days to 104 kg were not significant.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
Literature Cited
 
Litter size can be increased by direct selection using breeding values estimated by an animal model in conjunction with rearing selected gilts in litters of 10 pigs or less. In addition, expectations are that selection for litter size will increase estimated breeding values for adjusted backfat and days to 104 kg.

Received for publication May 1, 2002. Accepted for publication November 12, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 


Avalos, E., and C. Smith. 1987. Genetic improvement of litter size in pigs. Anim. Prod. 44:153–164.

Bereskin, B. 1984a. A genetic analysis of sow productivity traits. J. Anim. Sci. 59:1149–1163.

Bereskin, B. 1984b. Genetic correlations of pig performance and sow productivity traits. J. Anim. Sci. 59:1477–1487.

Bichard, M., and P. J. David. 1985. Effectiveness of genetic selection for prolificacy in pigs. J. Reprod. Fert. 33:127–138.

Boldman, K., L. A. Kriese, L. D. Van Vleck, and S. D. Kachman. 1993. A Manual for Use of MTDFREML—A Set of Programs to Obtain Estimates of Variances and Covariances. ARS, USDA, Washington, DC.

Falconer, D. S. 1971. Improvement of litter size in a strain of mice at a selection limit. Genet. Res. 17:215–235.[Medline]

Ferraz, J. B. S., and R. K. Johnson. 1993. Animal model estimation of genetic parameters and response to selection for litter size and weight, growth, and backfat in closed seedstock populations of large white and landrace swine. J. Anim. Sci. 71:850–858.[Abstract]

Gion, J. M., A. C. Clutter, and M. K. Nielsen. 1990. Alternative methods of selection for litter size in mice: II. Response to thirteen generations of selection. J. Anim. Sci. 68:3542–3556.

Haley, C. S., E. Avalos, and C. Smith. 1988. Selection for litter size in the pig. Anim. Breed. Abstr. 56:317–332.

Haley, C. S., and G. J. Lee. 1992. Genetic factors contributing to variation in litter size in British Large White gilts. Livest. Prod. Sci. 30:99–113.

Hermesch, S., B. G. Luxford, and H. U. Graser. 2000. Genetic parameters for lean meat yield, meat quality, reproduction and feed efficiency traits for Australian pigs. 3. Genetic parameters for reproduction traits and genetic correlations with production, carcase and meat quality traits. Livest. Prod. Sci. 65:261–270.[Medline]

Hill, W. G. 1972. Estimation of realised heritabilities from selection experiments. II. Selection in one direction. Biometrics 28:767–780.[Medline]

Irvin, K. M., and L. A. Swiger. 1984. Genetic and phenotypic parameters for sow productivity. J. Anim. Sci. 58:1144–1150.

Johansson, K. 1981. Some notes concerning the genetic possibilities of improving sow fertility. Livest. Prod. Sci. 8:431–447.

Johansson, K., and B. W. Kennedy. 1983. Genetic and phenotypic relationships of performance test measurements with fertility in Swedish Landrace and Yorkshire sows. Acta Agric. Scand. 33:195–199.

Johnson, R. K., M. K. Nielsen, and D. S. Casey. 1999. Responses in ovulation rate, embryonal survival, and litter traits in swine to 14 generations of selection to increase litter size. J. Anim. Sci. 77:541–557.[Abstract/Free Full Text]

Lamberson, W. R., R. K. Johnson, D. R. Zimmerman, and T. E. Long. 1991. Direct responses to selection for increased litter size, decreased age at puberty, or random selection following selection for ovulation rate in swine. J. Anim. Sci. 69:3129–3143.[Abstract]

Neal, S. M., R. K. Johnson, and R. J. Kittok. 1989. Index selection for components of litter size in swine: Response to five generations of selection. J. Anim. Sci. 67:1933–1945.[Abstract/Free Full Text]

Ollivier, L. 1982. Selection for prolificacy in the pig. Pig News Info. 3:383–388.

Quijandria, B., M. Zaldivar, and O. W. Robison. 1983. Selection in guinea pigs: II. Direct response for litter size and body weight. J. Anim. Sci. 56:820–828.

Revelle, T. J., and O. W. Robison. 1973. An explanation for the low heritability of litter size in swine. J. Anim. Sci. 37:668–675.[Abstract/Free Full Text]

See, M. T., J. W. Mabry, and J. K. Bertrand. 1993. Restricted maximum likelihood estimation of variance components from field data for number of pigs born alive. J. Anim. Sci. 71:2905–2909.[Abstract]

Tess, M. W., G. L. Bennett, and G. E. Dickerson. 1983. Simulation of genetic changes in life cycle efficiency of pork production. II. Effects of components on effeciency. J. Anim. Sci. 56:354–368.[Abstract/Free Full Text]

Young, L. D., R. A. Pumfrey, P. J. Cunningham, and D. R. Zimmerman. 1978. Heritabilities and genetic and phenotypic correlations for prebreeding traits, reproductive traits, and principal components. J. Anim. Sci. 46:937–949.[Abstract/Free Full Text]


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