Livestock Production Science, 32 (1992) 283-293 Elsevier Science Publishers B.V., Amsterdam

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Estimates of genetic parameters for litter size in six strains of Iberian pigs

Miguel Perez-Enciso and Daniel Gianola Department o/Meat and Animal Science. University 0/ Wisconsin. Madison WI. USA

(Accepted 25 February 1992)

ABSTRAcr

Perez-Enciso, M. and Gianola, D., 1992. Estimates of genetic parameters for litter size in six strains of Iberian pigs. Livest. Prod. Sci., 32: 283-293.

Additive direct and maternal, permanent environmental and residual variances for total number of pigs born and number born alive in six closed strains of Iberian pigs were estimated by univariate restricted maximum likelihood using an animal model. Data were analyzed within strain, and also including all strains together. The largest data set analyzed included 8057 records and 3655 animals. Results were similar for both traits, irrespective of whether crossfostered dams were included in the analysis or not. Within-strain estimates of heritability of direct effects ranged from 0 to 13%, whereas heritability of maternal effects did not exceed 4%; permanent environmental effects accounted for between 0 to 18% of the phenotypic variance. Across strains, heritability of direct effects was 4% for total number born and 6% for number born alive, heritability of maternal effects was nil for both traits, and the variance of permanent environmental effects accounted for 7% of the phenotypic var­iance. The covariance between direct and maternal genetic effects, expressed as a percent of the total variation was negligible. The evidence suggests that maternal genetic effects and their covariance with direct genetic effects can be ignored in selection programs aimed to increase prolificacy in the Iberian pig.

Keywords: Iberian pig; litter size; maternal effects; restricted maximum likelihood.

INTRODUCTION

Improving prolificacy is an important objective in pig breeding. This is es­pecially true in the Iberian pig, due to its low prolificacy and because of a special market where other traditional objectives in pig breeding, such as in­creasing lean content, are desirable but of secondary importance (Dobao et al., 1988a). Although meat quality is important, it is difficult to measure in a reasonable time because most of the Iberian pig meat is directed towards products that are cured for a long period of time, up to two years, before

Co"espondence to: Miguel Perez-Enciso, Department of Meat and Animal Science, University of Wisconsin, Madison W153706-1284, USA.

0301-6226/92/$05.00 @ 1992 Elsevier Science Publishers B.V. All rights reserved.

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reaching the market. Further, litter size is relatively easy to measure, so in­cluding this trait in a selection program is often warranted.

From a reproductive point of view, Iberian pigs are often characterized as having low prolificacy, early maturity and good maternal abilities (Dobao et al., 1988a,b). Dobao et al. (l988b) discussed three methods for improving litter size under the current extensive management of Iberian pigs: 1; cross­breeding between Iberian strains, 2; crossbreeding with Chinese breeds, and 3; selection within purebreds utilizing information from relatives, i.e., an an­imal model. The first strategy can lead to an increase in mean prolificacy of about 6-7% (Dobao et aI., 1988b, Rodriguez et aI., 1990). Experimental re­sults involving crosses with the Jiaxing breed have not been satisfactory from an economic viewpoint (Dobao et aI., 1988b, 1990). On the other hand, the­oretical work investigating simple methods for incorporating family infor­mation in genetic evaluation for litter size gave encouraging results (Avalos and Smith, 1987). With today's developments in computing strategies, it would appear feasible to utilize all available information from relatives in an animal model. In either case, placing an optimum weight in each of the sources of family information requires knowledge of the variance components in the population. In practice, however, these are not known and they should be estimated as accurately as possible, trying to account for possible biases caused by selection.

Previous estimates of heritability oflitter size in pigs have been low, around 10%, on average (Haley et al., 1988). It has been suggested that the low her­itability of litter size may be due to the presence of maternal effects, resulting in negative covariances between the records of the dam and those of their daughters. However, evidence of maternal effects affecting litter size is far from being clear cut (Vangen, 1986; Haley et aI., 1988). Estimates of he rita­bility of litter size obtained in the Iberian pig have also been low (Dobao et aI., 1988b; Rodriguez et aI., 1990).

Restricted maximum likelihood (REML) is considered by animal breeders to be an appealing method for variance component estimation. This is due to its well-known asymptotic properties, plus the fact that REML takes into ac­count the uncertainty associated with estimating the fixed effects in the model, whereas maximum likelihood does not. REML is also attractive for predict­ing breeding values with unknown variance components (Gianola et aI., 1986 ).

The objective of this research was to obtain REML estimates of genetic parameters for total number of pigs born and number born alive in six strains of Iberian pigs. Maternal effects were assessed as well.

MATERIAL AND METHODS

The records came from an experimental herd in Oropesa (Toledo, Spain)

28S

founded in 1945. The data analyzed here were from six closed strains: Cal­deira (C), Ervideira (E), Campanario (P), Guadyerbas (G), Gamito (M) and Torbiscal (T). For'details about these strains and management condi­tions, see Dobao et al. (1983) and Toro et al. (1988). In short, C, E, P and G were founder strains but, at present, only G exists. Strain T resulted from a composite involving all four founder strains, while M was derived from G. The complete pedigrees were known, so all individuals could be traced up to founder animals. There was no selection for litter size, but body weight at 50 days and teat number were subject to a weak positive selection pressure (Toro et al., 1986; 1988).

This study was intended to estimate parameters with purebred animals so litter size records from crossbred females were removed. Crossfostering was a management practice in these herds. With crossfostered animals, maternal effects can be due to the genetic dam or to the nurse. This can cause ambigu­ities in the interpretation of results unless a complicated model is used for data analysis. In order to avoid these difficulties, records from dams raised by a nurse were also removed; with this edit, about 100/0 of the animals were removed from the analysis. A summary of the data is presented in Table 1. The number of records (animals) per strain ranged from 291 (237) for Cal­deira to 3517 (1456) for Torbiscal. The number of genetic dams with records varied from 129 (C) to 904 (T).

The two traits considered, total number of pigs born per litter (NOBT) and number born alive (NOBA), were analyzed separately using an animal model, and univariate REML for estimating dispersion parameters. Multivariate analyses were not deemed necessary because of the mild selection noted above. The mixed model utilized for both traits was:

y=XP+ Zollo + Zmlm + Zoc+e (1)

where y is a vector of observations; X, Zo and Zm are known incidence matri­ces; fJ is a vector of fixed effects; 80 is a vector of direct additive genetic ef­fects; 1m is a vector of maternal additive genetic effects; c is a vector of per­manent environmental effects; and e is a vector of residuals. All effects other than fJ were regarded as random. In addition to the appropriate variance components, a covariance between direct and maternal effects was included and estimated as well. The fixed effects included in fJ were farrowing season and parity. The number of farrowing seasons for each strain is given in Table I. In general, there were 2 farrowing seasons per year for C, E, and P; and 4 farrowing seasons per year for M, G, and T. There were four parity classes: frnt, second, third, plus a class including records from fourth and later parities.

Two different analyses were conducted. In the first one, strains were ana­lyzed separately; in the second, all strains were analyzed together, so that a fixed effect for strain was added to the model. Estimates of estimable func­tions of fixed effects were "estimated" generalized least-squares, with the un-

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TABLE 1

Description of the data used in the analysis

Strain No. of No. of No. of dams No. of animals records with records farrowina

seasons

Caldeira(C) 237 291 (304)1 129( 134)1 24 Ervideira(E) 281 348(406) 143(150) 33 Campanario(P) 299 372(386) 171 (179) 30 Gamito(M) 597 1093 ( 1213) 385(426) 72 Guadyerbas( G) 807 1442(1621 ) 496(549) 94 Torbiscal(T) 1456 3517(4164) 904 ( 1068) 60 All 3655 7063(8057) 2228(2506) 96

'Numbers within parentheses pertain to data sets with crossfostered females included.

known dispersion parameters replaced by the REML solutions. Also, esti­mates of dispersion parameters were obtained from a model that excluded maternal genetic effects. The objective ofthis was to assess these effects using likelihood ratio tests. Twice the difference in log-likelihoods between models with and without maternal effects was referred to a x: distribution with two degrees of freedom; this is an asymptotic, rather than an exact test and the degrees of freedom correspond to the two extra parameters in the model with maternal effects. If the test statistic exceeds the appropriate critical value of the r distribution, the null hypothesis of absence of maternal effects is re­jected. After assessing maternal effects, the whole data set was analyzed in­cluding also records from crossfostered dams, so as to obtain more precise parameter estimates. The number of records in this case is shown in Table 1.

Computations were carried out with the derivative free approach program DFREML (Meyer, 1988), with a modification that makes use of sparse ma­trix solvers (Boldman and Van Vleck, 1991). This modification resulted, in our case, in a reduction in CPU time of up to 400 times. The programs were run on the IBM 3090 of the Cornell National Supercomputer Facility. Itera­tion stopped when the variance of the likelihood function values was less than 10-9 or 10- 1°. Approximate standard errors of estimates of dispersion pa­rameters were obtained using a quadratic approximation to Fisher's infor­mation matrix (Smith and Graser, 1986). This approximation was only fea­sible in the model without maternal effects, because non-positive definite or singular covariance matrices were obtained with the full model.

RESULTS AND DISCUSSION

The mean and coefficient of variation for the traits considered are given in Table 2. Mean litter size was from 1 to 3 less piglets than values often found

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TABLE 2

Mean and coefficient of variation (CV) for number of pip born alive (NOSA) and total number born (NOBT). See Table I for strain abbreviations

C E P M G T AD

TABLE]

NOSA

Mean

6.90±O.1l 7.00±O.13 6.84±O.IO 7.43±O.05 7.06±O.OS 7.99±O.03 7.56±O.02

28 38 29 25 29 26 27

NOBT

Mean

7.28±O.!1 7.2S±O.13 7.37±O.IO 7.76±0.05 7.42 ± 0.06 8.37 ± 0.03 7.93 ± 0.02

27 35 27 23 30 25 26

Estimates of mean differences between strains for number of pigs born alive (NOBA) and total num­ber born (NOBT). Results from the model includina maternal effects. See Table I for strain abbreviations

Strain NOSA NOBT

E-C O.ll 0.01 p..c 0.09 0.28 M-C 0.8S 0.92 o-c 0.]9 0.44 T..c 1.26 1.]5

in other commercial breeds (e.g., Southwood and Kennedy, 1990). This con­firms the low prolificacy of the Iberian pig. The strain with the highest mean value for litter size was T. This is probably due to some retained heterosis, because Torbisca1 was developed as a composite line, and to a lower inbreed­ing depression, since it was the strain kept with the largest effective size. Ef­fects of heterosis and of inbreeding depression for reproductive traits are well documented in the Iberian pig (Dobao et al., 1988b; Toro et al., 1988; Ro­driguez et al., 1990).

Table 3 shows estimates of differences in mean litter size between strains. Dearly, T was the most prolific strain, followed by Gamito (M), Guadyerbas (G), Ervideira (E), Campanario (P) and Caldeira (C). Prolificacy in Tor­bisca1 was more than one piglet and a quarter higher than in Caldeira.

Estimates of contrasts between parities are in Table 4. Litter size increased with parity. Sorensen (1990) reported that litter size increased up to parity 6

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TABLE 4

Estimates of mean differences between parities for number of pigs born alive (NOBA) and total number born (NOBT). Results from the model including strain and maternal effects. Last level in­cludes all parities above third

Parity NOBA NOBT

2-1 3-1 4-1

TABLES

0.81 1.02 1.80

0.82 1.33 1.92

Parameter estimates for number of pigs born alive (NOBA) and number born total (NOBT) using all strains and a model with maternal effects; O'i: direct additive variance; O'~: maternal genetic var­iance; O'AM: genetic covariance between direct and maternal effects; 0':: permanent environmental variance; O'~: phenotypic variance; hi =O'VO'~; h~ =O'~/O'~; hAM=O'AM/O'~; c2=0'~/0'~

Trait

NOBA NOBT

0.178 0.26S

0.000 0.000

0.000 0.000

0.283 0.316

0.04 0.06

0.00 0.00

0.00 0.00

0.07 0.07

and then declined. In our data, however, NOBT and NOBA increased mono­tonically even after parity 8 (results not shown).

Within-strain heritability estimates of direct genetic effects (h~) for NOBA ranged from 0.00 to 0.06. Heritability of maternal genetic effects was either nil or, at best, 0.04 in the M strain. The fraction of phenotypic variance ac­counted for by the covariance between direct and maternal effects was zero in all strains except in strain M (hAM = -0.03). This is not surprising since it is unreasonable to expect a high covariance when there is negligible variance for maternal effects. Repeatability of NOBA ranged from 0.04 to 0.15. Re­sults for NOBT were similar, with h~ ranging from 0.00 to 0.13, and hL- and hAM very close to zero or zero. Repeatability of NOBT ranged from 0.00 to 0.18. Due to low point estimates of many of the genetic variances and of the covariance, genetic correlations were often near 1, 0 or - 1.

Results of the across strain analysis are shown in Table 5. These estimates are more accurate than the within-strain ones because all information is used simultaneously. Heritability of direct effects was low, 0.04 and 0.06 for NOBA and NOBT, respectively, while hL- and hAM were zero for both traits. Seven percent of the variance of litter size was due to permanent environmental effects. Table 6 shows the results obtained with the model without maternal effects. Estimates of h ~ and c2 (the fraction of variance accounted for per­manent environmental effects) were very similar to those using a model in­cluding maternal effects (Table 5).

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TABLE 6

Parameter estimates ± approximate standard errors for number of pigs born alive (NOBA) and num­ber of pigs born (NOBT) using a model without maternal effects. Results from the across strain analysis. Symbols as in Table S

Trait

NOBA

NOBT

Crossfostercd females included

No Yes No Yes

0.186±O.l38 0.169±0.130 0.287±0.IS6 0.241 ±0.16S

0.276±0.137 0.306 ± 0.130 0.31O±0.lS4 0.3S9±0.lS4

0.04±0.03 0.04 ± 0.03 0.06±0.03 0.06±0.03

0.06±0.03 0.07±0.03 0.07±0.03 0.08 ± 0.03

The values of the likelihood ratio statistic obtained comparing the models with and without maternal effects in the across strain analysis were 0.018 and 0.061 for NOBA and NOBT, respectively. These values did not differ from zero (X~.O.05 =5.992). It is interesting to note, however, that the probabilities of a t random variable with 2 degrees of freedom being less than the likeli­hood ratio values observed are very small to be explained by chance alone (P=0.009 and 0.030 for NOBA and NOBT, respectively). This is probably related to a flat likelihood surface that does not change very much when ad­ditional parameters are added to the model. Nonetheless, programs were run starting from different points, but the converged solutions were very similar. In summary, the present evidence suggests that maternal genetic effects for litter size in Iberian pigs are probably small. In view of this result, a model without maternal effects would appear to be adequate for genetic evaluation of litter size in the Iberian pig, and there would be no reason for discarding records from crossfostered dams. Results from an analysis including these records (Table 6) gave similar results, suggesting that no large bias was intro­duced by discarding such records.

Although it has been shown that the size of the fraternity in which a female is reared can affect its prolificacy (Kirkpatrick and Rutledge, 1988), evi­dence of maternal effects on litter size in pigs is conflicting (Vangen, 1986; Haley et al., 1988). Available REML estimates of maternal variances give an ambiguous picture as well. For example, Southwood and Kennedy (1990) obtained significant estimates in some breeds, whereas Mercer and Crump ( 1990) did not. Maternal effects are often thought of as a microenvironment provided by the dam or nurse in which there is competition for limited re­sources (e.g., milk, maternal care). In some way, within-litter competition in a large litter would prevent female littermates from also attaining a high level of prolificacy. This is more likely to occur under intensive production systems or at a high level oflitter size (Vangen, 1986). However, management ofthe

290

Iberian pig is extensive, and mean litter size is low; this may explain the lack of evidence of maternal effects in our study.

Crossfostering was a practice in this herd, and it is known that it contrib­utes to the removal of postnatal maternal influences (Van der Steen, 1985). Data from crossfostered dams were first excluded from the analysis so as to meet the assumptions of model (1). This editing, however, may create bias because crossfostered females tend to be from either large or small litters. The results shown in Table 6, however, suggests that this bias can be ignored, at least in the Iberian pig. A further explanation for the lack of maternal effects is that in this herd females were not mated until they were considered to be "sufficiently developed" (L. Sili6, personal communication). This may re­duce the negative environmental effect that a large litter exerts on prolificacy of female littermates.

Across-strain heritabilities for NOBA and NOBT were 0.04 and 0.06, re­spectively, which are lower than previous estimates obtained in the Iberian pig (Dobao et al., 1988b; Rodriguez et al., 1990). Dobao et al. (1988b) re­ported heritabilities ranging from 0.03 to 0.19, with an average of 0.1 O. These estimates, however, were obtained with ANOVA methods, and they could be biased due to the inability of these methods to account correctly for relation­ships between animals. Rodriguez et a1. (1990) reported REML estimates of heritability of NOBA and NOBT in the strain T between 0.12 and 0.14, and the fractional contribution of permanent environmental effects to total vari­ance was near zero. The disagreement with the results obtained here may be due to the smaller samples of pigs used in their analyses. At any rate, since litter size is a trait affected by inbreeding depression (Toro et aI., 1988), it is unlikely that an additive model can provide an accurate representation of the biological picture. Unfortunately, it is difficult to set-up models including nonadditive genetic effects, and current data sets in Iberian pigs would pre­clude meaningful estimation of the unknown parameters.

Perez-Enciso et al. (1988) investigated by simulation the schemes pro­posed by Avalos and Smith ( 1987) to improve prolificacy in pigs, but consid­ering population sizes and selection intensities appropriate to the Spanish Ib­erian population. For the best scheme, Perez-Enciso et al. (1988) reported an expected annual increase of 0.17 piglets per litter, with an increase in inbreed­ing of 0.56% per year. However, they assumed h~ =0.10 and a repeatability of 0.22. Using the estimates obtained here, the annual increase in litter size would be about 0.09 piglets per litter per year. The rate of increase of inbreed­ing would be larger, however, because the optimum index puts more empha­sis on family records as h ~ decreases.

Perhaps additional genetic variation can be captured using more sophisti­cated and, maybe, more realistic models such as one where the residual dis­tribution is Poisson but inheritance is additive on a logarithmic scale (Foul­ley ct aI., 1987). Tempelman and Gianola (1991) obtained encouraging

291

simulation results with this model, even with an average Poisson parameter equal to 8, which would be close to the situation in Iberian pigs. Nevertheless, this model remains to be tested empirically.

CONCLUSION

In the light of the available data, maternal effects for litter size in the Iber­ian pig seem to be negligible. Across strains, heritability was very low, around 5%, and repeatability was between 0.11 and 0.14. A low heritability, and the risk of an increase in inbreeding makes it difficult to justify selection for pro­lificacy within pure strains of Iberian pigs. Dobao et al. (1988b) argued that the most cost-effective way to increase prolificacy in the Iberian pig would be by means of crosses between strains, and our results would tend to support this suggestion.

ACKNOWLEDGEMENTS

The tint author gratefully acknowledges a grant from INIA, Spain. We thank L. Sili6 and M.A. Toro for providing the data and useful comments. K. Meyer and K. Boldman are thanked for kindly providing their computer programs. The analyses were run on an IBM 3090 of the Cornell National Supercom­puter Facility, a center funded in part by the National Science Foundation, New York State, and the IBM corporation. This support is kindly acknowl­edged. Research was partially funded by the National Pork Producers Coun­cil of the United States of America.

REFERENCES

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

Boldman, K. and Van Vleck, LD., 1991. Derivative-free restricted maximum likelihood esti­mation in animal models with a sparse matrix solver. J. Dairy Sci., 74: 4337-4343.

Dobao, M.T., RodrigaDez, J. and Sili6, L., 1983. Seasonal influence on fecundity and litter per­formance characteristics in Iberian pigs. Livest. Prod. Sci., 10: 601-610.

Dobao, M.T., Rodrig8iiez, J., Sili6, L. and Toro, M.A., 1988a.lberian pig production in Spain. PiS News Info., 9: 277-282.

Dobao, M.T., Rodrig8iiez, J., Sili6, L., Toro, M.A. and De Pedro, E., 1988b. Genetica de la prolificidad en el cerdo lberico: revision de metodologfas y resultados. Investigacion Agraria (Producci6n y Sanidad Animates), 3: 109-134.

Dobao, M.T., Rodrig8iiez, J., Silio, L., Toro, M.A. and De Pedro, E., 1990. Utilization of Jiaxing crosses under the extensive management of Iberian pig. 2. Growth and carcass performance. Proc. Chinese Pia Symposium, p 260. INRA, Jouy-en-Josas, France.

FoulIey, J.L., Gianola, D. and 1m, S., 1987. Genetic evaluation oftraits distributed 8$ Poisaon­binomial with reference to reproductive traits. lbeor. Appl. Genet., 73: 870-877.

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Gianola. D., Foulley, J.L. and Fernando, R.L., 1986. Prediction of breeding values when vari­ances are not known. Genet. Sel. Evol., 18: 485-498.

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

Kirkpatrick, B.W. and Rutledge, J.J., 1988. Influence of prenatal and postnatal fraternity size on reproduction in swine. J. Anim. SeL, 66: 2530-2537.

Mercer, J.T. and Crump, R.E., 1990. Genetic parameter estimates for reproduction traits in purebred Landrace pigs. Proc. 4th World Congr. Appl. Genet. Livest. Prod., Edinburgh, vol. XIII: 489-492.

Meyer, K., 1988. DFREML-a set of programs to estimate variance components under an in­dividual animal model. J. Dairy SeL, 71 (suppI2): 33-34.

Perez-Enciso, M., Toro, M.A. and Sili6, L., 1988. Indices familiares y selecci6n para tamaiio de camada en el cerdo Iberico. Anaporc 74: 29-31.

Rodriguez, M.e., Bejar, F., Sili6, L. and Rodrigliiiez, J., 1990. Genetic parameters for prolifi­cacy in Iberian pigs. 41 st EAAP Meeting, Toulouse.

Smith, S.P. and Graser, H. U., 1986. Estimating variance components in a class of mixed models by restricted maximum likelihood. J. Dairy ScL, 69: 1156-1165.

Sorensen, D.A., 1990. An animal model for selection for litter size in the Danish pig breeding program. Proc. IV World Congress Genet. Appl. Livest. Prod., Edinburah, vol. XV: 435-438.

Southwood, 0.1. and Kennedy, B.W., 1990. Estimation of direct and maternal genetic variance for litter size in Canadian Yorkshire and Landrace swine using an animal model. J. Anim. Sci.,68: 1841-1847.

Tempelman, R. and Gianola. D., 1991. Evaluation of a Poisson animal model for the petic evaluation of embryo yields in a MOET population. J. Dairy ScL, 74 (suppl. I): 157.

Toro, M.A., Dobao, M.T., Rodrigliiiez, J., Sili6, L., 1986. Heritability ofa canalized trait: teat number in Iberian pigs. Genet. Sel. Evol., 18: 173-184.

Toro, M.A., Sili6, L., Rodrigliiiez, J., Dobao, M. T., 1988. Inbreeding and family index selection for prolificacy in pigs. Anim. Prod., 46: 79-85.

Van der Steen, H.A.M., 1985. Maternal influence mediated by litter size durina the sucklina period on reproduction traits in pigs. Livest. Prod. Sei., 13: 147-158.

Vangen, 0., 1986. Genetic control of reproduction in pigs: from parturition to puberty. Proe. m World Congress Genet. Appl. Livest. Prod., Lincoln (Nebraska), vol XI: 168-179.

RESUME

Perez-Enciso, M. et Gianola, D., 1992. Estimation des parametres genetiques de la taille de Ia portee dans six soucbes de pores Iberiques. Livest. Prod. Sci., 32: 283-293 (en anglais)

La variance des efTets additifs directs et maternals, la variance d'environnement permanent et la variance residuelle du nombre total de porcelets et du nombre de porcelets vivants a Ia naissance ont ete estimees dans six souches fermees de porcs Iberiques par la methode du max­imum de vraisemenblance restreint univariate, en utilisant un modele animal. Les donnees etaient anaIysees intra-souche et aussi en incluant toutes les souches en m@me temps. Le fichier de donnees Ie plus important comportrait 8057 enregistrements et 3655 animaux. Les resultats sont similaires pour les deux caracteres, que I'on indue ou non dans I'analyse les truies pour lesquelles des adoptions rkiproques etaient pratiquees. Les estimations intra-soucbe de l'beri­tabilite des efTets directs vont de 0 a 13%, tandis que I'heritabilite des efTets maternels ne de­passe pas 4~; les efTets d'environnement permanent expliquent 0 a 18% de la variance pheno­typique. Entre soucbes, l'beritabilite des efTets directs est de 4% pour Ie nombre total de porceletl

293

• Ia naissance, et de 6~ pour Ie nombre de porcelets vivants, l'heritabilite des effets matemels est nulle pour les deux caracteres, et la variance des effets d'environment permanent represente "" de la variance phenotypique. La covariance entre les effets genetiques directs et matemels, exprimee en pour cent de la variation totale, est negligeable. Ces resultats suggerent que les effets genetiques matemels et leur covariance avec les effets genetiques directs peuvent etre iporCs dans les programmes de selection destines a ameliorer la prolificite ehez Ie pore Iberique.

KURZFASSUNG

Perez-Enciso, M. und Gianola, D., 1992. Die Schltzung genetischer Parameter fUr die Wurf­groBe in sechs Linien des lberischen Landschweines. Livest. Prod. Sci .• 32: 283-293 (auf english)

Mit einem Tiermodell in der univariaten REML-Method wurden Varianzkomponenten fUr additive direkte und maternale. permanent umweltbedingte und Resteffekte fUr die Merkmale insgesamt und lebend geborene Ferkel pro Wurf in sechs geschlossenen Linien des Iberischen Landschweines geschitzt. Die Daten wurden sowohl innerhalb als aueh tiber Linien hinweg analysiert. Der groOte Datensatz umfaBte 8057 Leistungen und 3655 Tiere. Die Ergebnisse waren fUr beide Merlcmale ihnlich. unabhingig davon, ob umgesetzte Ferkel einbezogen wurden oder nieht. Innerhalb Linien geschltzte Heritabilititen fur direkte Effekte lagen zwischen 0-13%, wihrend die Heritabilitit der matemalen Effekte 4% nieht tiberstieg. Permanente Umwelteef­fekte machten 0-18% der phinotypischen Varianz aus. Ober Linien hinweg geschitzt war die Heritabilitit der direkteo Effekte 4% fUr insgesamt uod 6~ fUr legend geborene Ferkelje Wurf, die matemale Heritabilitit war fUr beide Merkmale null, und die permanenten Umwelteffekte betrugen 7% der phinotypisehen Varianz. Die Kovarianz zwischen direkten und matemalen genetischen EtTekten, ausgedriiekt als Prozent der totalen Varianz, war zu vemaehlissigen. Diese Befunde bedeuten, daB matemale genetische Effekte und deren Kovarianz mit direkten genetischen Effekten in Selektionsprogrammen zur Verbes­seung der Fruehtbarkeit bie lberischen Landschweinen vemaehlissigt werden ki>nnen.