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* Animal Breeding and Genetics Group, Wageningen Institute of Animal Sciences, Wageningen University, Wageningen, The Netherlands;
and
IPG, Institute for Pig Genetics B.V., Beuningen, The Netherlands;
and
TOPIGS France, Le Haut de Serre, 55700 Brouennes, France
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Animal Breeding • Breeding Value • Pigs • Stillbirths
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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; PigCHAMP Breeding Herd Summary U.S.A., 2000). Postmortem examination of stillborn piglets allows categorization according to the estimated time of death relative to the moment of farrowing (Randall and Penny, 1967; Glastonbury, 1977; Carr and Walton, 1995). Such information gives insight into the underlying biological mechanisms of stillbirth. For example, death prior to farrowing may be associated with infection during pregnancy or placental insufficiency, whereas death during and immediately after farrowing is often the result of asphyxia experienced during delivery (reviewed by English and Morrison, 1984).
There is a small but significant genetic influence on stillbirth (Johnson et al., 1999; Hanenberg et al., 2001; Knol et al., 2002). Maternal genetic and direct genetic effects influence the occurrence of stillbirth (Knol et al., 2002), indicating that genes of both the sow and the piglets are involved. More insight into the biological background of genetic differences in stillbirth can be obtained by analysis of the estimated time of death of stillborn piglets originating from sows and litters with known genetic merit for farrowing survival (i.e., the complement of stillbirth).
The objectives of this study were to analyze the incidence of different categories of stillborn piglets in relation to genetic merit for farrowing survival of sows and litters and to analyze relationships of total number of piglets born, average BW of the litter, and within-litter variation in BW with genetic merit for farrowing survival of sows and litters.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Estimated Breeding Values for Farrowing Survival
From January 1998 until June 2002, TOPIGS breeding company used a recording protocol on two of their nucleus pig farms, including the farm reported in this study. For each litter, the individual weight of all piglets at farrowing (including stillborn piglets, but excluding mummified piglets) and the date of death in the case of failed survival were recorded. The data set that resulted from this protocol included information on 39,504 piglets born from litters of the dam lines, Lines D1 and D2. This data set was used to estimate breeding values for farrowing survival of sows (EBVfs_maternal) and litters (EBVfs_direct), thereby ignoring results obtained in this study. Estimated breeding values for farrowing survival of the sows are defined as the maternal genetic effect on farrowing survival, whereas EBVfs_direct is defined as the average of the direct genetic effects of the respective father and mother of the piglet. For estimation of breeding values, an animal model that included direct (piglet) and maternal (sow) effects was used. Heritabilities for the direct and maternal effects were 0.021 ± 0.007 and 0.025 ± 0.007, respectively, and the genetic correlation was -0.063 ± 0.284. In this model, farrowing survival was defined as a binary trait, with a zero score for stillborn piglets and a score of 100 if born alive. Sex, BW of the piglets in classes of 100 g, and litter size were taken as fixed effects, while litter effect of the mother of the piglets was taken as random effect. The model is described in more detail by Knol et al. (2002).
The range of EBVfs_maternal and EBVfs_direct in this study was continuous. The range of EBVfs_maternal was 13.6%, which indicates an expected phenotypic difference in stillbirth rate of 13.6% between the sow with the lowest (-8.4%) and highest (5.2%) EBVfs_maternal. Similarly, the range of EBVfs_direct was 9.1%, indicating an expected phenotypic difference in stillbirth rate of 9.1% between litters with lowest (-6.2%) and highest (+2.9%) EBVfs_direct.
Data Collection
Records of 336 purebred litters, produced by 307 first to eighth parity sows, were collected during 10 sampling periods. In total, 49% of all farrowings were induced. Induction of farrowing took place if sows showed no signs of approaching farrowing on or after the calculated farrowing date. Farrowings were not supervised. At first check-up after farrowing (on average, 12 h postpartum), the number of live-born and stillborn piglets was recorded for each litter. A piglet was classified as stillborn when it was found dead lying behind the sow, appeared wet, and was more or less covered with placental membranes. All piglets (alive and stillborn) were weighed, and subsequently, postmortem examination was performed on the stillborn piglets. Stillborn piglets were classified into four categories: 1) nonfresh stillbirths showing signs of degeneration, as evidenced by a (partial) brown skin color—these piglets probably died more than a week before onset of farrowing (Randall and Penny, 1967); 2) prepartum stillbirths showing no external signs of decay but with the same brick-red color of all their abdominal organs due to hemolysis and autolysis—these died in utero in the days closely preceding farrowing (Bille et al., 1974); 3) intrapartum stillbirths had a normal color of the abdominal organs but the presence of mucus and/or meconium in the trachea indicated that they died during farrowing; and 4) postpartum stillbirths had (partly) aerated lungs, but no colostrum in the stomach, and thus died shortly after birth.
Statistical Analyses
Gestation length was calculated as the difference between the day of first mating and the day of farrowing. Mummified piglets were excluded from all analyses. The average BW of the litter included weights of all stillborn and live-born piglets. Within-litter variation in BW was calculated as the standard deviation of BW of total number of piglets born.
The LSMEANS of individual BW of the four categories of stillborn piglets and the live-born piglets were calculated by Model [1], using the Proc GLM of SAS (SAS Inst., Inc., Cary, NC):
 | [1] |
where Yijkl = individual BW; µ = overall mean; Sexi = sex of the piglet (i = male, female); Sowj = sow effect (j = 1 to 307); Statusk = status of the piglet (i.e., live-born or four categories of stillborn) (k = 1 to 5); and eijkl = residual error.
Relationships of stillbirth with EBVfs_maternal and EBVfs_direct (Model [2]) were analyzed by logistic regression, using Proc Genmod of SAS. Logistic regression was used because of the binomial distribution of stillbirth:
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where Yijkl = the total number of stillborn piglets per litter or the number of stillborn piglets per category as a proportion of the total number of piglets born; µ = overall mean; Linei = line of the sow (i = D1, D2); Parityj = parity of the sow (j = 1 to 8); SPk = sample period (k = 1 to 10); GL = gestation length; TNB = total number of piglets born; ABW = average BW of the litter; M = percentage males within the litter; EBVfs = estimated breeding value for farrowing survival of the sow or the litter; and eijkl = residual error. In preliminary analyses, relationships of stillbirth with EBVfs were adjusted for induction of farrowing. The effect of induction of farrowing was non-significant (P > 0.10) in all analyses and thus removed from Model [2]. Estimated breeding values for farrowing survival of sows and litters were also simultaneously included in Model [2] to determine if effects of EBVfs_maternal and EBVfs_direct were still present after adjusting for each other.
Model [3] was used to analyze the relationship of the total number of piglets born with EBVfs_maternal and EBVfs_direct, using Proc GLM of SAS:
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where Yijkl = total number of piglets born; µ = overall mean; Linei = line of the sow (i = D1, D2); Parityj = parity of the sow (j = 1 to 8); SPk = sampling period (k = 1 to 10); EBVfs = estimated breeding value for farrowing survival of the sow or the litter; and eijkl = residual error.
Model [4] was used to analyze relationships of average BW of the litter and within-litter variation in BW with EBVfs_maternal and EBVfs_direct, using Proc GLM of SAS. For analysis of within-litter variation in BW in relation to EBVfs, average BW of the litter was included in Model [4] as a covariate.
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where Yijkl = average BW of the litter or within-litter variation in BW; µ = overall mean; Linei = line of the sow (i = D1, D2); Parityj = parity of the sow (j = 1 to 8); SPk = sampling period (k = 1 to 10); GL = gestation length; TNB = total number of piglets born; M = percentage males within the litter; EBVfs = estimated breeding value for farrowing survival of the sow or the litter; and eijkl = residual error.
In all analyses, stepwise elimination of nonsignificant effects (P > 0.10) was applied.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Table 1 shows LSMEANS of BW for each category of stillbirth and for live-born piglets. There were no significant differences in BW between the different categories of stillborn piglets (P > 0.05). Live-born piglets were significantly heavier than all categories of stillborn piglets (P < 0.0001), except for prepartum stillbirths (P = 0.78).
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shows unadjusted litter averages for the different stillbirth categories, estimated logistic regression coefficients, and P-values for the relationships of stillbirth with EBVfs_maternal and EBVfs_direct. The total number of stillborn piglets per litter and the number of stillborn piglets in each category decreased with increasing EBVfs_maternal. An increase in EBVfs_direct was also associated with a decrease in total stillbirth but was not associated with a decrease in the number of intrapartum stillborn piglets. After adjusting for differences in EBVfs_direct, relationships of the incidence of stillbirth with EBVfs_maternal remained significant (P < 0.01) for all categories, except for nonfresh stillborn piglets (P = 0.30). On the other hand, relationships of the incidence of stillbirth with EBVfs_direct became nonsignificant (P > 0.05) for all categories after adjusting for differences in EBVfs_maternal. Figures 1a to 1e show the probabilities of stillbirth for all categories in relation to EBVfs_maternal and EBVfs_direct. For the range of observed EBVfs, probabilities of stillbirth in relation to EBVfs_maternal were higher than probabilities of stillbirth in relation to EBVfs_direct.
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shows that the probability of total stillbirth was lowest in third-parity sows and increased at higher parities.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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; Hanenberg et al., 2001; Knol et al., 2002). Knol et al. (2002) performed a genetic analysis of the number of stillborn piglets per litter using a model that included direct genetic and maternal genetic effects. They found a significant influence of both effects, indicating a role for both genes of the piglet and genes of the sow in the occurrence of stillbirth.
In the present study, direct and maternal genetic effects on stillbirth were estimated by the calculation of EBVfs_direct and EBVfs_maternal, respectively. According to expectation, the total number of stillborn piglets per litter decreased with increasing EBVfs_maternal and EBVfs_direct, thereby confirming the genetic influence of both the sow and the piglets on the occurrence of stillbirth. Subsequently, we analyzed whether this decrease in stillbirth with increasing EBVfs was due to decreases in mortality before, during, or immediately after farrowing. For the sows, the decrease in stillbirth with increasing EBVfs_maternal was due to a decrease in mortality in all categories (i.e., before, during, and immediately after farrowing). For the litters, the decrease in stillbirth with increasing EBVfs_direct was due to decreases in the number of piglets dying before and immediately after farrowing, but not to a decrease in the number of piglets dying during farrowing. Apparently, the sow has a genetic influence on the probability of mortality during farrowing, whereas this influence is not present for the piglets. In agreement with other studies (Randall and Penny, 1967; Glastonbury, 1977), the probability of mortality during farrowing was much higher than before or immediately after farrowing. This underlines the major role of asphyxia in the etiology of stillbirth in the pig (Randall and Penny, 1967; Randall, 1971).
Because the genetic correlation between the direct and maternal genetic effects did not differ significantly from zero (see Materials and Methods), relationships of stillbirth with EBVfs_maternal and EBVfs_direct could be analyzed simultaneously. After adjusting for differences in EBVfs_direct, the relationship between stillbirth and EBVfs_maternal remained signficant for all stillbirth categories, except for nonfresh stillbirths. On the other hand, adjusting for EBVfs_maternal resulted in nonsignificant relationships of all categories of stillbirth with EBVfs_direct. Combined with our findings that the probability of intrapartum stillbirth was not related to EBVfs_direct and that the probability of stillbirth in relation to EBVfs was higher for the sow than for the litter (see Figures 1a
to 1e), this suggests that the sow exerts a stronger genetic influence on the probability of stillbirth than the piglets.
The total number of piglets born decreased with increasing EBVfs_direct but was not related to EBVfs_maternal. This suggests that selection against the direct genetic component of stillbirth will reduce litter size, whereas selection against the maternal genetic component will not affect litter size. Johnson et al. (1999) concluded that selection for sows with larger litters will increase the number of stillborn piglets per litter. They could not conclude whether this correlated selection for increased stillbirth was due to a decrease in direct genetic merit of the piglets, to a decrease in maternal genetic merit, or both. Results of the current study suggest that a decrease in direct genetic merit for farrowing survival may have been responsible for the observed increase in stillbirth in the study of Johnson et al. (1999).
Average BW of the litter decreased with increasing EBVfs_maternal but was not related to EBVfs_direct. Thus, decreased stillbirth in sows with high EBVfs_maternal is concomitant with a decrease in average BW of their piglets. This seems to contradict the often reported negative phenotypic relationship between stillbirth and average BW of the litter (England, 1974; Zaleski and Hacker, 1993; Leenhouwers et al., 1999), but it is in line with the positive genetic correlation of 0.37 between number of stillborn piglets and average BW of the litter that was reported by Grandinson et al. (2002). Further, this result is comparable to results of Leenhouwers et al. (2002), who reported a trend for reduced piglet weight with increasing genetic merit for total piglet survival (i.e., farrowing and preweaning survival). Biological explanations for the decreased stillbirth in litters from sows with high EBVfs_maternal are currently unknown, but a maternally determined greater degree of physiological maturity of the piglets may play a role. A negative relationship between average BW of the litter and physiological maturity was previously reported by several authors (Mersmann et al., 1984; Herpin et al., 1993; Leenhouwers et al., 2002).
In conclusion, decreased stillbirth with increasing genetic merit for farrowing survival of sows is associated with a reduction in stillbirth in all categories, whereas decreased stillbirth with increasing genetic merit for farrowing survival of litters is associated with decreases in the number of piglets dying before and immediately after farrowing. The genetic background of the sow seems to have a larger influence on stillbirth than the genetic background of the litter. Sows with higher genetic merit for farrowing survival give birth to litters with a lower average BW. Litters with a higher genetic merit for farrowing survival have fewer total number of piglets born.
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Implications |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Footnotes |
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2 Correspondence: P.O. Box 338, 6700 AH (phone: +31-317-482335; fax: +31-317-483929; E-mail: jascha.leenhouwers{at}wur.nl).
Received for publication September 24, 2002. Accepted for publication June 10, 2003.
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Literature Cited |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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