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Genetic analysis of litter size, parturition length, and birth assistance requirements in primiparous sows using a joint linear-threshold animal model¹

B. Holm^*,,2, M. Bakken, O. Vangen and R. Rekaya

^* Norsvin, Hamar, Norway; and Department of Animal and Aquaculture Science, Agricultural University of Norway, Aas, Norway; and and Department of Animal and Dairy Science, University of Georgia, Athens 30606

	Abstract

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The aim of this study was to investigate whether selection for number of live born piglets has led to prolonged parturition and increased requirement for birth assistance, resulting in increased numbers of stillborn piglets. Data were collected from 6,718 primiparous Norwegian Landrace sows farrowing between 2001 and 2003. The need for birth assistance was recorded as a binary response. Physical intervention in the birth of piglets and/or hormonal treatment by the farmer was recorded as birth assistance. The duration of the parturition was analyzed as a binary trait (<4 h and >4 h). The statistical model used for analysis included contemporary groups of herd-year, litter breed, season of farrowing, parity in which the sow was born, a regression on the age of sow at farrowing, an additive genetic effect, and a service sire effect. A full Bayesian approach via Gibbs sampling was adopted to estimate the genetic relationships between these four traits. A total chain length of 100,000 iterations was run. The first 10,000 samples were discarded as burn-in, and the remaining 90,000 iterations were retained without thinning for post-Gibbs analysis. The highest direct heritability was estimated for the number of live-born piglets (h² = 0.07), followed by the duration of farrowing (h² = 0.05), the need for birth assistance (h² = 0.05), and the number of stillborn piglets (h² = 0.04). The genetic correlations revealed that the number of live and stillborn piglets was uncorrelated; however, the number of live piglets born had a moderate genetic correlation to the need for birth assistance (r_g = 0.24 ± 0.01) and duration of farrowing (r_g = –0.20 ± 0.01), whereas the number of stillborn piglets was highly correlated to the need for birth assistance (r_g = 0.74 ± 0.01) and the duration of parturition (r_g = 0.66 ± 0.01). The duration of farrowing and the need for birth assistance were genetically highly correlated (r_g = 0.89 ± 0.00). For all traits, the service sire variance was approximately one quarter in magnitude compared with its respective genetic variance. The results showed that selection for the number of live born piglets is not expected to influence the number of stillborn piglets. Increasing the number of live piglets born through selection should have a slight negative effect on farrowing duration and a minor increase in the need for birth assistance. Sows with a high genetic potential for birth assistance and prolonged parturition were more likely to give birth to greater numbers of stillborn piglets.

Key Words: Bayesian Analysis • Dystocia • Genetic Parameters • Litter Size • Parturition • Sow

	Introduction

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Selection for litter size has an unfavorable genetic effect on number of stillborn piglets (Johnson et al., 1999; Tribout et al., 2003). The exact causes of stillbirths are multiple and complex; however, increased duration of parturition (DP) and the need for birth assistance (BA) are often associated with a higher stillbirth rate. One of the major causes of stillbirths is asphyxiation due to hypoxia, and it is associated with later position in the birth order, broken umbilical cord, longer preceding birth intervals, and lower piglet hemoglobin (Zaleski and Hacker, 1993; Herpin et al., 1996). Additionally, piglets suffering from asphyxia during parturition are less viable, require more time to get to the udder, and have a lower rate of growth and neonatal survival the first 10 d of life (Herpin et al., 1996). Nonetheless, there are few reports on the genetic relationships between duration of the parturition, the need for BA, and stillborn piglets. Canario et al. (2003) found no significant genetic relationship between DP and other farrowing traits. However, all farrowings were stimulated by PG 1 d before expected farrowing and were closely observed. This may provide a different result than that found with data from conventional herds. In a survey based on 276 litters (Berge, 1949), the average DP in litters with 7 to 16 piglets was 3.6 h, and the number of stillbirths increased with increased duration. Genetic variation of BA was reported by Berg et al. (2001); however, the genetic relationship of BA with other farrowing traits was not examined. The aim of this study was to investigate whether selection for litter size has led to prolonged parturition, increased probability for the need for BA, and consequently, more stillborn piglets.

	Materials and Methods

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Data
Data on purebred Norwegian Landrace sows were obtained from the Norwegian Pig Breeders’ Association’s (Norsvin) nucleus and multiplying herds. Both types of herds report on-farm events to Norsvin through the national recording scheme. Norsvin supervises the national recording scheme and is the only operational pig breeding company and semen provider in Norway. The use of AI is almost 100%. In 2001, Norsvin required all nucleus and multiplier herds to record the DP and the need for BA. The definition of a stillborn piglet was a piglet found dead behind the sow at, or immediately after, farrowing. The DP was categorized into 2-h intervals (<2 h, 2 to 4 h, 4 to 6 h, >6 h) and unknown. Birth assistance was recorded as a binary trait (yes or no). Given the way the trait is recorded by the farmers, physical intervention in the birth of piglets and/or hormonal stimulation of farrowing was scored as BA. The national recording scheme was altered to meet these new challenges. The edited data set contained records on 6,718 primiparous sows farrowing between June 2001 and July 2003, and included the following traits: number of live born (NBA) and stillborn (NSB) piglets; DP; and need for BA. A total of 78% of the litters was purebred Landrace litters, whereas the remaining were Landrace x Yorkshire litters. The DP was recorded as unknown in 23.9% of farrowings. To obtain sufficient observations in each class for DP, the number of classes was decreased to two new classes, <4 h and >4 h.

Pedigree File
The pedigree file on an individual basis was constructed using animals in the dataset as the starting point; their relatives were traced back five generations whenever possible. The final file included 18,099 animals.

Statistical Analysis and Computations
The following joint linear-threshold animal model was used in the analysis:

[1]

where y was a vector that represented the unobserved liabilities for BA, DP, and the actual observed phenotypic values for NBA and NSB. The vector b included the contemporary group of herd-year (212), season of farrowing (4; January to March, April to June, etc.), the parity in which the sow was born (1, 2, 3), the litter breed (purebred or crossbred litter), and the regression on the age (280 to 470 d) of the sow. The vector a included the additive genetic effects, s was a vector of random service sire effect, and e was the vector of residual effects. Only service sires with more than two records were included. Two hundred seventeen different service sires were represented in the data. Incidence matrices X, Z₁, and Z₂ were known with the appropriate dimensions relating the vector y to b, a, and s, respectively. To avoid extreme case problems associated with the threshold model, both possible responses of the binary traits must occur within each contemporary group. Six records within each contemporary group were set as a minimum.

A Bayesian implementation via Gibbs sampling was adopted. Conditionally on the position parameter vector, = (b',a',s')', and the residual (co)variance matrix, R, the observed responses and the liabilities were assumed to be normally distributed:

[2]

where R was a 4 x 4 residual (co)variances matrix. Given the problem of unidentifiability of the threshold model, two restrictions were required. Thus, the thresholds and the residual variances associated with both binary traits were set to zero and one, respectively.

[3]

where and were the residual variances for NSB and NBA, and was the residual covariance between the corresponding traits. To ensure proper posterior distribution, the following prior distributions were assumed for the parameters in the model:

A normal distribution with zero mean and a large variance (to convey little belief a priori) was assumed as prior for the vector b:

[4]

The classical multivariate normal distributions were assumed as prior for direct additive effects:

[5]

where G was a 4 x 4 (co)variance matrix of direct additive effects analog to R, and A was a matrix of additive genetic relationship.

Multivariate normal distributions were assumed for the service sire effects:

[6]

where P was a diagonal matrix with elements , , , and representing the service sire variances for DP, BA, NSB, and NBA, respectively. For all parameters included in the dispersion matrices, R, G, and P, uniform bounded priors were assumed.

The joint posterior density was obtained by the product of densities in [2] to [6].

[7]

defined only within the boundary of the bounded priors for the dispersion parameters.

Posterior Analysis
The augmented joint posterior distribution in [7] was in closed form, and the conditional posterior distribution of all the parameters of the models was derived as described by Wang et al. (1993) and Sorensen et al. (1995), being truncated normal for the liabilities, normal for the position parameters, and scaled-inverted Wishart distributions for the dispersion parameters.

Convergence diagnostics were assessed using the method of Raftery and Lewis (1992) as implemented in the CODA software (Best et al., 1995). The required length of the burn-in period was always less than 3,800 iterations in both analyses for all parameters. Thus, a total chain length of 100,000 iterations of the Gibbs sampler was run with a conservative 10,000 iterations as burn-in. The latter 90,000 iterations were retained without thinning for post-Gibbs analysis.

	Results

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Descriptive statistics for the four traits are presented in Table 1. The average litter size was large, considering that the dataset originated from primiparous sows, and almost 15% of the farrowings required birth assistance. A summary of the posterior distribution for the additive direct, service sire, and residual variances and heritabilities are presented in Table 2. Posterior means and standard deviations of genetic and residual correlations are presented in Table 3. The highest direct heritability was estimated for NBA, followed by BA, DP, and NSB. The variances for the service sire effects were significantly smaller than those for the corresponding additive genetic variances; however, their magnitude indicated that some variation between service sires existed. In general, the sire variance for all four traits explained 1 to 2% of the total variation. The genetic correlations revealed that NBA and NSB not were correlated; however, NBA had a moderate genetic correlation with BA and DP, whereas NSB was highly correlated with BA and DP. The increasing number of live born through selection is expected to have a small effect on decreasing the duration of the parturition. Increasing NBA is also expected to lead to a small increase in the probability that BA is needed. A decreased DP has a favorable relationship with NSB, whereas increased probability for the need for BA will have an unfavorable effect on NSB. Furthermore, prolonged DP was genetically highly correlated to the need for BA.

View this table: [in this window] [in a new window]   Table 2. Summary of the posterior distributions for additive genetic variance (), residual variance (), service sire variance (), additive heritability (), and service sire heritability () for the four first parity traits

	Discussion

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There is a paucity of reports of genetic correlations between these traits in the literature. Canario et al. (2003) used data from one nucleus herd under close surveillance by the herd manager where the farrowings were stimulated by PG 1 d before expected farrowing. This surveillance, as well as the hormonal stimulation, may have an effect on the frequency of stillbirths and the duration of the parturitions. The average number of stillborn piglets was lower in their study than in the current study (1.0 vs. 1.1). On the other hand, the use of field data, as in this study, may lead to an increased environmental variance, as there may be different definitions among herd managers. Regardless, including the entire effective breeding population was advantageous for the current study. First, should any attempt be made to include such traits in a breeding program, the traits must be recordable under commercial circumstances. Second, using the entire breeding population ensures that the estimated genetic parameters are valid for the population. In the current study, 23% of the records for DP were recorded as unknown. There were no differences in NSB or NBA if DP was recorded as known or unknown.

Infections may cause stillbirths and abortions and can thereby increase environmental variance when analyzing litter size (Christianson, 1992). It is likely that noninfectious causes accounted for the postparturition mortality in the current study because Aujeszky’s disease, porcine respiratory and reproduction syndrome, transmissible gastroenteritis, and swine influenza have never been reported in Norway (Jarp and Tharaldsen, 2002).

Stillbirths have an important economic role in pig farming because they account for more than one-third of the total loss of piglets until weaning (Grandinson et al., 2002). Additionally, piglets weakened from a difficult birth are more likely to die during the first critical days after farrowing (Herpin et al., 1996).

The current study indicated that the frequency of stillbirths would be unaffected when selecting for NBA piglets in primiparous sows. Moreover, sows with a high genetic potential for prolonged duration of farrowing were more likely to have a higher probability for BA and subsequently more stillborn piglets. Prolonged parturition was highly correlated with the need for BA. Noninfectious causes of fetal deaths are often multifactorial and difficult to diagnose, but most stillborn piglets die during the last part of parturition. The direct causes are often anoxia due to damaged or broken umbilical cord, partially loosened placenta, or decreased placental blood circulation (Herpin et al., 1996). No objective test of stillbirth was undertaken in the current study; therefore, some piglets recorded as stillborn may have died in the first hours after birth, but before the farmer inspected the litter. Producers can misclassify up to 40% of the dead piglets found around farrowing (Christianson, 1992).

The estimated heritability of NBA in the current study was relatively low (h² = 0.07). Literature based on 96 estimates for number of live born piglets range from 0.0 to 0.7, with an average of 0.09 over all parities and breeds (Rothschild and Bidanel, 1998). Peskovicova et al. (2002) summarize the heritability for NBA in the first parity to be 0.11. Breed and population differences may explain some of the differences in reported estimates. Our estimated heritability for NSB (h² = 0.04) concurred with Grandinson et al. (2002), but was lower than that estimated by Canario et al. (2003) (h² = 0.15). Our posterior mean for the heritability for the duration of the parturition (h² = 0.05 ± 0.00) was higher than obtained by Canario et al. (2003) (h² = 0.02 ± 0.02). That the current study used field data, and that each farrowing was not as closely monitored, may explain some of this deviation, as well as breed differences and differences in analyzing the trait.

The service sire effect was relatively small for all traits, which agrees with other reports regarding litter size (Serenius et al., 2003) and BA (Berg et al., 2001). The effect of the service sire on litter size can originate through differences in sperm quality and prepartum survival, for example, whereas the slight effect on BA and DP can, to some extent, be a direct genetic effect arbitrated through the piglet’s birth weight (Roehe, 1999).

Johnson et al. (1999) concluded that selection criterion in pigs should be NBA piglets, rather than the total number of piglets born. Lund et al. (2002) reached a similar conclusion in a genetic study on Finnish Landrace and Yorkshire. This study supported these findings because NBA was found to be genetically uncorrelated to NSB. The NSB piglets increased significantly with selection for increased total number of born piglets (Johnson et al., 1999). Consequently, live piglets constituted only 50% of the total increase. In addition, the increased NSB piglets was partly related to lower birth weight. Nevertheless, they could not conclude whether this correlated effect was attributable to the sow, the piglet, or both. Leenhouwers et al. (2003) estimated breeding values of both sows and litters for farrowing survival and concluded that the decrease in direct genetic merit of the litter for piglet survival before, during, or immediately after farrowing may be responsible for the increase in NSB reported by Johnson et al. (1999). In their study, almost 50% of the farrowings were induced if the sow showed no signs of approaching farrowing on the calculated date. Nevertheless, a significant influence of both direct and maternal effect on number of stillborn piglets indicates that the genetics of the sow and the piglet affect the incidence of still births (Knoll et al., 2002; Leenhouwers et al., 2003). Furthermore, the sow exerts a stronger genetic influence on the probability of stillbirth than the piglet (Leenhouwers et al., 2003). The current study indicated that NSB, as a trait of the sow, had a high genetic correlation to the duration of the parturition and to the need for birth assistance.

The estimated heritability for BA is in agreement with results obtained by Berg et al. (2001). In their study, mastitis-metritis-agalactia treatments were less frequent in sows without the need for BA, and the frequency of diarrhea was lower in litters when the sows were not affected by mastitis-metritis-agalactia, emphasizing the importance of having a sow with no need for birth assistance. It is likely that prolonged parturition and frequency of stillbirths have the same underlying reason when viewed as a sow trait. Dead piglets have a higher probability of retention than live piglets, thereby blocking the passage for the other piglets, resulting in a prolonged parturition (Berge, 1949). The current study suggested that DP and BA had the same underlying causes (r_g = 0.89 ± 0.00). The exact cause of the increased farrowing duration is often not known, but likely relates to the endocrine control of parturition. Increased farrowing duration increases stillbirth rates, and the time interval between two successive live born piglets is significantly lower than that between a live born and stillborn piglet (Christianson, 1992). The longer intervals may be due to a lack of fetal efforts and mechanical stimulation of the birth canal during farrowing. Canario et al. (2003) also observed that an increase in litter size is associated with a faster piglet delivery. Results of the current study imply that selection for more live born piglets will decrease the DP, and as the number of piglets increases, the DP per piglet will be even further decreased. Prolonged farrowing and/or sporadic asphyxia during farrowing does not necessarily lead to intrapartum death, but is likely to result in weakened piglets that are less viable at birth and that have a lower chance of surviving the critical first week of postnatal life (Zaleski and Hacker, 1993; Herpin et al., 1996). Less vigor at birth may lead to less forceful suckling, thereby decreased colostrum intake. Birth asphyxia clearly correlates with piglet behavior and postnatal development of thermoregulation, and asphyxia during parturition is directly responsible for 24% of early postnatal mortality (Herpin et al., 1996).

	Implications

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Variance components for the number of live born and stillborn piglets, the need for birth assistance, and duration of parturition have been successfully estimated with an animal model and a joint linear-threshold analysis. The results imply that selection for number of live born piglets in primiparous sows does not affect number of stillborn piglets. The breeding objective should therefore include the number of live born piglets as a measure of litter size. Prolonged parturition is highly connected to the need for birth assistance. Sows with a high genetic potential for prolonged duration of farrowing are more likely to have a higher probability for birth assistance and more stillborn piglets.

	Footnotes

 
¹ This study was financed by the Norwegian Research Council and Norsvin.

² Correspondence: Norsvin, P.O. Box 504, No-2 (phone: 476-494-8042; fax: 476-494-7960; e-mail: bjarne.holm{at}iha.nlh.no).

Received for publication January 29, 2004. Accepted for publication May 16, 2004.

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