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Genetic parameters for within-litter variation in piglet birth weight and change in within-litter variation during suckling¹

L. H. Damgaard^*,2, L. Rydhmer, P. Løvendahl^* and K. Grandinson

^* Danish Institute of Agricultural Sciences, Department of Animal Breeding and Genetics. P.O. Box 50,DK-8830 Tjele, Denmark and and Swedish University of Agricultural Sciences, Department of Animal Breeding and Genetics, Funbo-Lövsta, S-75597 Uppsala, Sweden

² Correspondence: phone:
+45 8999 1339; fax: +45 8999 1300; E-mail:
Lars.damgaard{at}agrsci.dk.

	Abstract

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The objective of this study was to ascertain whether maternal additive genetic variance exists for within-litter variation in birth weight and for change in within-litter variation in piglet weight during suckling. A further objective was to estimate maternal genetic correlations of these two traits with mortality, birth weight, growth, and number of piglets born alive. Data were obtained from Lövsta research station, Swedish University of Agricultural Sciences, and included 22,521 piglets born in 2,003 litters by 1,074 Swedish Yorkshire sows. No cross fostering was used in the herd. The following seven traits were analysed in a multivariate animal (sow) model: number of piglets born alive, within-litter SD in birth weight, within-litter SD in piglet weight at 3 wk of age, mean weight at birth, mean weight at 3 wk of age, proportion of stillborn piglets, and proportion of dead piglets during suckling. Maternal genetic variance for the change in within-litter SD in piglet weight during suckling was assessed from the estimated additive genetic covariance components by conditioning on within-litter SD in birth weight. Similarly, mean growth of piglets during suckling was assessed from the additive genetic covariance components by conditioning on mean weight at birth. The heritability for within-litter SD in birth weight was 0.08 and 0.06 for within-litter SD in piglet weight at 3 wk. The genetic correlation between these two traits was 0.71. Little maternal genetic variance was found for the change in within-litter SD in piglet weight during suckling, and opportunity for genetic improvement of this trait by selective breeding seems limited. The genetic correlation of within-litter SD in birth weight with proportion of dead piglets during suckling was 0.25 and of within-litter SD in birth weight with mean growth of piglets was -0.31. The maternal genetic variance and heritability found for within-litter SD in birth weight indicates that genetic improvement of this trait by selective breeding is possible. In addition, selection for sows’ capacity to give birth to homogeneous litters may be advantageous for piglet survival, piglet growth, and litter homogeneity at weaning.

Key Words: Birth weight • Genetic variation • Heritability • Litter traits • Mortality • Pigs

	Introduction

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Selection for litter size at birth has been successful (Southwood and Kennedy, 1991; Bidanel et al., 1994; Estany and Sorensen, 1995). However, it has been shown that productivity of sows measured as number of produced pigs per year is also dependent on their capacity to give birth to piglets that survive and have high vitality at weaning. Litter size is unfavorably correlated to piglet survival and vitality, and selection for litter size has a negative impact on preweaning mortality and birth weight (Roehe, 1999; Knol, 2001; Lund et al., 2002). Sow productivity is influenced by a number of factors, two of which are within-litter variation in birth weight and the change in within-litter variation in piglet weight during suckling. Within-litter variation in birth weight has been shown to be positively related to preweaning mortality on the phenotypic scale (English and Smith, 1975; Roehe and Kalm, 2000). Only few recent studies have addressed the genetic aspects of within-litter variation in birth weight, and heritabilities in the range of 0.10 to 0.11 have been reported (Högberg and Rydhmer, 2000; Hermesch et al., 2001). Knol (2001) suggested that selection for piglet’s own ability to survive may simultaneously reduce within-litter variation in birth weight. The change in within-litter variation in piglet weight during suckling is a measure of the sows’ ability to nurture piglets equally during suckling, and it varies between sows on the phenotypic scale (Thompson and Fraser, 1986). Some sows are apparently better at ensuring homogeneous growth of their piglets than others. The genetic knowledge about this trait and its effect on piglet survival and vitality of piglets is very limited. The objective of this study was to ascertain whether maternal additive genetic variance exists for within-litter variation in birth weight and for change in within-litter variation in piglet weight during suckling. A further objective was to estimate maternal genetic correlations between these two traits and mortality, birth weight, growth, and number of piglets born alive.

	Material and Methods

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Data

Data were obtained from Lövsta research station, Swedish University of Agricultural Sciences. Records of individual piglet birth weights and individual piglet weights at 3 wk of age were collected from 28,883 piglets. The piglets originated from 2,520 litters born by 1,293 purebred Yorkshire sows during the period from 1983 to 2000. All analyzed traits were derived from individual observations of piglets within litters. Therefore, the general procedure for editing data was to exclude the whole litter if one or more piglets in the litter did not fulfill the editing criteria. Litters were discarded due to missing weight at birth, missing weight at 3 wk of age, missing weight of dead piglets that died before 3 wk of age, or unknown father of the litter. In addition, 101 litters with fewer than five piglets alive at 3 wk of age were excluded.

After editing, records were available on 22,521 piglets in 2,003 litters born by 1,074 sows. The pedigree for these sows included a total of 1,408 sires and dams. Most of these dams had own litter records. The paternal breeds of these litters were Yorkshire (1778), Landrace (59), Hampshire (70), Duroc (58), and a European wild boar x Yorkshire with 3 to 12% wild boar (38).

Observations on individual piglets within litters were used to derive the following seven traits, which were regarded as maternal: number of piglets born alive (LB), within-litter SD in birth weight based on liveborn piglets (SD0), within-litter SD in piglet weight at 3 wk of age (SD3), mean weight at birth based on liveborn piglets (MW0), mean weight at 3 wk of age (MW3), proportion of stillborn piglets (P_STB), and proportion of liveborn piglets that died before 3 wk of age (P_D). Descriptive statistics for these variables are presented in Table 1.

Model

Phenotypic and maternal genetic estimates of covariances of the seven traits LB, SD0, SD3, MW0, MW3, P_STB, and P_D were obtained by analyzing them in the following multivariate animal (sow) model:

where y was a vector of observations for the seven traits, b was a vector of fixed effects, s was a vector of random father of litter effects common to litters with the same father, p was a vector of random permanent environmental effects common to litters of the same sow, a was a vector of random additive genetic effects of the sow, and e was a vector of residual effects. Incidence matrices X and Z relate the appropriate independent fixed effects in b and random effects in s, p, and a to the vector of records.

Three fixed effects were the same for all traits: batch, breed, and parity effect. In addition, the traits SD3, MW3, and P_D included a fixed period effect. The batch effect groups sows, which have farrowed within the same period of time together, and can be considered as a year-season effect. One hundred and ten batches were defined, with an average of 18 sows in each batch (SD = 8 sows). Average batch length was d 29 (SD = d 16). The breed effect represents the five different paternal breeds of the litters. Parity was included in the model, with five levels corresponding to the first three parities, the fourth and the fifth parity grouped together, and parities higher than the fifth grouped together. Piglets were weighed between d 19 and d 23 after birth, and the period effect with five levels corresponds to these 5 d. Preliminary analyses in univariate repeatability models, with the procedure PROC MIXED in SAS (SAS Inst. Inc., Cary, NC), showed no significant interaction between the fixed effects.

The traits SD0, SD3, MW0, MW3, P_STB, and P_D were derived as a function of individual observations of piglets within litters. The variance of such a function depends on the number of records on which the calculation is based. Preliminary weighted analyses in univariate models for SD0 and SD3 showed that the heritabilities did not differ from estimates obtained from a nonweighted analysis. The heterogeneous residual variance was ignored in the final model, and the random effects were assumed to follow an independent multivariate normal distribution with mean zero and covariance structure:

where I and A were the identity matrix and the additive genetic relationship matrix.

Estimates

The covariance components were estimated using the average information restricted maximum likelihood (AI-REML) procedure (Jensen et al., 1997) in the statistical package DMU (Jensen and Madsen, 1994). The SE of heritabilities and correlations were obtained by Taylor series approximations. Estimates at least two times higher than their corresponding SE were assumed to be significantly different from zero. Heritability of each of the seven traits was estimated as:

Analysis of Covariance Components for Piglet Weight Changes During Suckling

Within-litter SD in piglet weight at 3 wk of age summarizes information from the within-litter SD in birth weight, and the variability developed during suckling. Maternal genetic variance for the variability developed during suckling was derived as the change in within-litter SD in piglet weight from birth to 3 wk of age. Maternal genetic variance for this trait was assessed from the estimated genetic covariance components. The conditional covariances of LB, SD3, MW0, MW3, P_STB, and P_D given SD0 were derived. The matrix of estimated genetic covariance components was partitioned as:

where ₀₁₁ was the estimated covariance components related to LB, SD3, MW0, MW3, P_STB, and P_D, ₀₂₂ was the estimated variance component related to SD0, and ₀₁₂ and ₀₂₁ were the corresponding estimated covariance components. Assuming that the estimates were multivariate normal distributed the conditional additive genetic covariances of LB, SD3, MW0, MW3, P_STB, and P_D given SD0 (_01|2) were derived according to Morrison (1976).

Similarly, mean weight at 3 wk of age summarizes information from the gestation and suckling periods. Conditioning on mean weight at birth assessed the maternal genetic variance for mean growth of piglets after birth only. Hence, the estimated additive genetic covariance components were used to derive the conditional covariances of LB, SD0, SD3, MW3, P_STB, and P_D, given MW0.

	Results

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Descriptive Statistics

Table 1 gives descriptive statistics for the litters included in the genetic analysis (thus, only litters with more than four piglets at 3 wk). Average weight of liveborn piglets was 0.28 kg higher than that of stillborn piglets. Stillbirths were observed in 34% of the litters, and postnatal mortality (i.e., during the first 3 wk after birth) was observed in 59% of the litters. Liveborn piglets, which died before 3 wk of age weighed less at birth (1.17 kg) than piglets surviving the first 3 wk (1.51 kg).

Fixed Effects

The parity effects for the seven traits were relative to the estimates of second parity effects, which were arbitrarily set at zero (Table 2). Only the contrasts to second parity were tested for significance. Within-litter SD in weights, SD0 and SD3, increased from the first to the third parity. The number of liveborn piglets increased from the first to the fifth parity. In later parities LB decreased to a level equal to the second parity. Mean weight at birth in the second and third parity was higher than in the first and later parities. After suckling, the situation changed, and mean weight of piglets at 3 wk of age was higher only in the second parity. The proportion of stillborn piglets, and that of dead piglets during suckling decreased from the first to second parity, subsequently increasing to a higher level in later parities.

The heritabilities of the analyzed traits ranged between 0.06 and 0.39 (Table 4). The highest heritability was found for mean weight at birth, whereas the proportion of dead piglets during suckling and within-litter SD in piglet weight at 3 wk had the lowest heritability. The estimated heritablities were significantly larger than zero for all traits except P_D. The estimated father of litter variance was much smaller than the three other variance components. However, it was significantly larger than zero for the traits LB, MW0, and MW3. The permanent environmental variance was higher than the corresponding maternal genetic variance for SD3 and P_STB.

The genetic correlations of SD0 with SD3, LB, MW0, and P_D were moderately to highly positive. The genetic correlations of SD3 with LB and MW0 were also positive (Table 5). The genetic correlations of LB with MW0 and P_STB were moderately negative, whereas the genetic correlation of LB with P_D was moderately positive. Genetically, MW0 was positively correlated to MW3 and P_STB but negatively correlated to P_D. Note that the genetic correlations between proportion of stillborn and number of liveborn piglets and mean weight at birth have opposite signs, compared to the corresponding correlations between postnatal mortality and the same traits. The SE of genetic correlations ranged between 0.09 and 0.39, so the genetic correlations were estimated with a high degree of uncertainty, probably due to the relatively small number of litters. The residual correlations were estimated with smaller SE, which ranged between 0.02 and 0.03.

Conditioning on within-litter SD in birth weight approximately halved the maternal genetic variance for within-litter SD in piglet weight at 3 wk from 0.0069 to 0.0034 kg. The genetic correlation between SD3 and P_D decreased from approximately zero to a negative value of -0.27. Furthermore, the genetic correlation between and SD3 and MW0 decreased from a positive value of 0.34 to a slightly negative value of -0.16. Similarly, conditioning on mean weight at birth reduced the maternal genetic variance for mean weight at 3 wk from 0.19 to 0.12 kg. The genetic correlation between MW3 and SD0 decreased from a positive value of 0.16 to a negative value of -0.31, and the genetic correlation between MW3 and P_D increased from close to zero to a positive value of 0.24. No other genetic correlations of SD3 and MW3 with the other traits changed considerably by conditioning on SD0 or MW0, respectively.

	Discussion

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Maternal genetic variance was detected for within-litter SD in birth weight. The heritability was low (0.08) but significantly larger than zero, suggesting that genetic improvement of within-litter SD in birth weight by selective breeding can be successful. This confirms previous findings on a subset of this data (1,697 litters born 1983 to 1996), for which Högberg and Rydhmer (2000) reported a heritability of 0.10 for within-litter SD in birth weight. In comparison, Hermesch et al. (2001) found a heritability of 0.11 for within-litter CV in birth weight. The heritability for within-litter SD in piglet weight at 3 wk was slightly lower than the corresponding heritability at birth. The genetic correlation between within-litter SD in birth weight, and within-litter SD in piglet weight at 3 wk was high (0.71), indicating that a genetic change in one of these traits will simultaneously induce an almost parallel change in the other. Maternal genetic variance for the change in within-litter SD in piglet weight during suckling was assessed from the estimated additive genetic covariance components. Conditioning on the within-litter SD in birth weight showed that this trait explained 51% of the maternal genetic variance for the within-litter SD in piglet weight at 3 wk. Hence, only minor maternal genetic variance exists for the change in within-litter SD in piglet weight during suckling, and the potential for genetic improvement of this trait by selective breeding therefore seems limited. Thompson and Fraser (1986, 1988) found substantial phenotypic variation between litters, in terms of the within-litter variation in piglet weight gain. According to this study, the reasons for the variation between sows appears to be mainly environmental.

In this study, no genetic relation was found between within-litter SD in birth weight and the proportion of stillborn piglets. Similar results have been reported on the phenotypic scale, where Leenhouwers et al. (1999) found no relation between number of stillbirths and within-litter variation in birth weight. On the other hand, there was a moderate positive genetic correlation between within-litter SD in birth weight and the proportion of dead piglets during suckling. A cross-fostering experiment by Milligan et al. (2001) provided little support for the hypothesis that high birth weight variation results in decreased survival. Several studies have, however, reported that preweaning mortality increases with increasing within-litter variation in birth weight on the phenotypic scale (English and Smith, 1975; Fahmy et al., 1978; van der Lende and de Jager, 1991; Roehe and Kalm, 2000). Our results indicate that the reason for such a relationship is partly genetic. This is further supported by Knol (2001), who reported that selection for piglet’s own ability to survive simultaneously reduced the within-litter variation in birth weight.

A moderately positive and therefore unfavorable genetic correlation was found between within-litter SD in birth weight and number of piglets born alive. The nutritional resources per piglet in uterus decrease with increasing litter size (Père et al., 1996), and maybe an increased competion in larger litters results in a larger weight variation at birth. The genetic correlation indicates that selection for sows’ ability to give birth to an increased number of piglets born alive may, at the same time, impair their ability to give birth to homogeneous litters, which is important for postnatal survival of piglets.

Within-litter SD in birth weight was genetically positively correlated to mean weight at birth. Selection for higher mean weight at birth may simultaneously impair sows’ ability to give birth to homogeneous litters. However, part of the genetic correlation may originate from a simple scale effect, as SD increases with increasing piglet weights.

The genetic correlation between mean weight at birth and mean weight at 3 wk was 0.59, which is in agreement with findings of Kerr and Cameron (1995), indicating that breeding for higher mean weight at birth is very likely to increase mean weight at weaning as well.

A moderate negative, and therefore favorable, genetic correlation was found between mean weight at birth and proportion of dead piglets during suckling. In contrast, there was a moderate positive genetic correlation between mean weight at birth and proportion of stillborn piglets. Thus, there is a risk that selection for higher mean weight at birth would decrease postnatal mortality, but simultaneously increase proportion of stillborn piglets. The net result of an attempt to improve piglet mortality by selection for higher birth weight might prove to be very limited. Similar genetic correlations have recently been reported by Grandinson et al. (2003), from an analysis of first parity data from the same experimental herd. The breed comparison of Herpin et al. (1993) indicates that high birth weight is not equal to high piglet maturation at birth. Likewise, Knol (2001) concludes that selection for increased birth weight will not improve piglet survival.

There was a moderate negative genetic correlation between the change in within-litter SD in piglet weight during suckling and proportion of dead piglets during suckling. Homogeneous growth of piglets within litters can be established in two very different ways. The piglets can either be nurtured equally or the piglets with the most deviant weight, which are most often the smallest piglets, may die. These two factors have not been differentiated in this study, and the genetic correlation should therefore be interpreted accordingly. However, the results emphasize the importance of multitrait selection in genetic improvement of sow productivity in order to avoid unfavorable genetic responses.

The genetic effect for the change in within-litter SD in piglet weight during suckling was moderately positively correlated to the genetic effect for the number of liveborn piglets. Thus, selection for increased number of piglets born alive may, at the same time, impair sows’ genetic ability to nurture piglets equally. The small maternal genetic variance found for the change in within-litter SD in piglet weight during suckling suggests that the correlated response from selection on related traits may be limited. However, ignoring the trait is unwarranted before the response to selection has been quantified and investigated further.

Maternal genetic variance for mean piglet growth during suckling was assessed from the estimated additive genetic covariance components. Conditioning on mean weight at birth reduced the maternal genetic variance in mean weight at 3 wk of age by 36%. Hence, maternal genetic variance exists for mean piglet growth during suckling. There was a positive genetic correlation between mean piglet growth and proportion of dead piglets during suckling, indicating that high mean piglet growth may occur at the expense of higher postnatal mortality. This result emphasises once again the importance of multitrait selection in genetic improvement of sow productivity. On the other hand, a moderate negative genetic correlation was found between mean piglet growth during suckling and within-litter SD in birth weight. Selection for homogeneous litters at birth seems to be beneficial to sows’ genetic ability to mediate growth of their piglets during suckling.

	Implications

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The heritability found for within-litter variation in birth weight is of approximately the same size as the heritability of litter size. Thus, genetic improvement of within-litter variation in birth weight by selective breeding seems possible. In addition, selection for sows’ capacity to give birth to homogeneous litters may be advantageous for a number of other traits, such as piglet survival, growth of piglets, and the homogeneity of litters at weaning. Sow productivity is a complex trait influenced by a number of factors that are highly related, either positively or negatively. This study emphasized the importance of multitrait selection programs in genetic improvements of sow productivity.

	Footnotes

 
¹ We thank the staff at Lövsta research facility for recording more than 50,000 piglet weights and for taking good care of all animals.

Received for publication November 29, 2001. Accepted for publication August 20, 2002.

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