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ANIMAL GENETICS |
Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
Abstract
The aim of this study was to estimate direct and maternal genetic parameters for calving difficulty score, stillbirth, and birth weight at first and later parities for Charolais and Hereford cattle in Sweden. Calving traits have long been recorded for purebred beef cattle in Sweden, but only birth weight has been used in the selection in order to avoid calving difficulties. Linear animal model analyses included records on birth weight for 60,309 Charolais and 30,789 Hereford calves born from 1980 to 1999, and calving traits for 74,538 Charolais and 37,077 Hereford calves born from 1980 to 2001. The frequencies of difficult calvings and stillbirths were approximately 6% at first and 1 to 2% at later parities for both breeds. Fewer than half the stillborn calves were born from difficult calvings. Heritabilities estimated for birth weight in different univariate and bivariate analyses for Charolais and Hereford calves born at first and later parities ranged from 0.44 to 0.51 for direct effects and 0.06 to 0.15 for maternal effects. Heritabilities on the observable scale for calving difficulty score of Charolais and Hereford, scored in three classes, ranged from 0.11 to 0.16 for direct and 0.07 to 0.12 for maternal effects at first parity, and lower at later parities. All estimated heritabilities for stillbirth were very low (0.002 to 0.016 on the observable scale). Direct-maternal genetic correlations were negative, with few exceptions. Genetic correlations between the traits and between parities within traits were generally moderate to high and positive. Calving difficulty score should be included in the genetic evaluation of beef breeds in Sweden, whereas progeny groups in Swedish beef populations are too small for stillbirth to be considered directly.
Key Words: Beef Cattle • Birth Weight • Dystocia • Genetic Parameters • Stillbirths
Introduction
Calving performance and viability of the calf have been recorded in the Swedish beef recording scheme since the late 1960s. These traits are subjectively scored categorical traits, influenced by both direct (calf) and maternal effects and with generally low heritabilities reported in the literature (Koots et al., 1994a
). Swedish calving records for purebred beef calves have not previously been used in the genetic evaluation of beef breeds. Birth weight has been used as an indicator trait to avoid calving difficulties. High genetic correlations between calving ease and birth weight have been reported in reviews by Meijering (1984) and Koots et al. (1994b). This relationship is unfavorable because birth weight is positively correlated to growth rate after birth (Mohiuddin, 1993; Koots et al., 1994b).
The relationship between birth weight and calf mortality is less clear. There seems to be an optimal value for birth weight with regard to calf viability (Meijering, 1984). Koots et al. (1994b) reported positive genetic correlations between birth weight and perinatal mortality, suggesting that in most studies, birth weight was larger than optimum. Calving difficulty and stillbirth generally have higher incidences in the first parity vs. later parities and have been suggested in some studies to be genetically different, but correlated, traits in first- and second-parity cows (Weller et al., 1988; Luo et al., 2002; Steinbock et al., 2003).
The aim of this study was to estimate genetic parameters for direct and maternal effects on calving difficulty score, stillbirth, and birth weight at first and later parities of Charolais and Hereford cattle in Sweden. This information is needed to evaluate whether more direct measures of calving difficulty and stillbirth should be considered in the genetic evaluation based on field data.
Materials and Methods
Choice of Data
Field data on birth weight, calving performance and stillbirth for purebred Charolais and Hereford cattle were obtained from the Swedish Dairy Association, which is responsible for the Swedish beef recording scheme. In total, the available calving data included information on calving traits for 92,290 Charolais and 43,394 Hereford calves born from 1980 to 2001, and the weight data included information on birth weight for 69,351 Charolais, and 34,393 Hereford calves born from 1980 to 1999.
Records without information on age of dam, season of birth, type of birth (single or twin), sex of calf, or birth herd-year were not included in the analyses and neither were observations in any fixed contemporary group with fewer than three observations. Records where the calf was the result of embryo transfer or not single born were excluded from the analyses, as were observations of calves with dams older than 168 mo of age. Observations on birth weight outside the accepted range of 18 to 75 kg were discarded. The majority of discarded observations were excluded because of too-small herd-year contemporary groups. After these edits, the final data set included information on calving traits for 74,538 Charolais and 37,077 Hereford calves and on birth weight for 60,309 Charolais and 30,789 Hereford (Table 1).
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Trait Definitions
Birth Weight.
Birth weight was recorded in kilograms up to 4 d after calving, either by the farmer or by a technician from the beef-recording scheme.
Calving Difficulty.
Calving performance was recorded in the Swedish beef-recording scheme as easy calving (unassisted), normal calving (assisted by one person), normal calving with malpresentation of the calf, difficult calving (assisted by more than one person), difficult calving with malpresentation of the calf, caesarean section, or calving induced by hormone injection. The frequency of caesarean sections and induced calvings was very low (<0.5% in all cases). Therefore, we grouped these together with difficult calvings and assigned the value 1 for unassisted calvings, 2 for normal calvings, and 3 for difficult calvings. Malpresentations were not treated separately (i.e., these were included with other normal or difficult calvings). Calves without a record of calving difficulty score (<0.01%) were assumed to have had an easy calving. No transformation of scores for calving difficulty was made since the use of equally spaced scores has been suggested to have negligible effects on heritability estimates (Naazie et al., 1991; Abdel-Azim and Berger, 1999).
Stillbirth.
The viability of the calf was recorded as one of the following classes: live-born, dead at birth, dead within 24 h, dead after 24 h, dead due to an accident, and malformed. In this study, we defined calves as stillborn if they were recorded dead at birth or dead within 24 h after birth, whereas the others were considered live-born. We assigned the value 0 for live-born and 1 for stillborn calves. Malformed calves (0.15% at the highest) were grouped with the stillborn because recording procedures did not allow calves to be recorded as both stillborn and malformed. Calves without records (<0.05%) were treated as live-born. Very few stillborn calves (<4%) had recorded birth weight.
Estimation of Genetic Parameters
The different breeds were analyzed separately. Both univariate and bivariate linear animal models were used. The basic bivariate model for estimating variance components for first parity traits was:
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where yi and yj are observed birth weight, calving difficulty score, or stillbirth. The vector 
contains for all traits the fixed effects of herd-year, season, and combination of sex of the calf and age group of the dam. For birth weight, the fixed effect of age at weighing in days was included in the model. The X and Z matrices are incidence matrices relating the observations to the fixed and random effects, respectively; m is a vector of maternal genetic effects; a is a vector of additive genetic effects of the animals; and e is a vector of random residuals. The models for later parity traits also included a random permanent environmental effect of the dam, pe. For random effects, the means were zero and the variances were as follows:
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where A is the relationship matrix. For bivariate analyses with first- and later-parity traits, the permanent environmental effect of dam was only included for later parities, and residual covariances were assumed to be zero since the traits were measured on different animals. Covariances between genetic and environmental effects were assumed to be zero, and no variances due to dominance or epistatic effects were assumed to exist.
(Co)variances were estimated using the average information algorithm (Jensen et al., 1997) for restricted maximum likelihood included in the DMU package (Jensen and Madsen, 1994). The convergence criterion was chosen so that the norm of update vector for the (co)variance components was less than 10-4. Asymptotic standard errors of (co)variance components were computed from the inverse average information matrix. Standard errors of genetic correlations were obtained by Taylor series expansions (Madsen and Jensen, 2000).
Heritabilities.
Direct and maternal heritabilities on the observable scale were calculated as 
and 
, respectively, where 
for all traits in the first parity and 
for later parities. Heritabilities on the underlying continuous scale were approximated from the heritabilities on the observable scale to enable comparisons with other studies, using a transformation described by Gianola (1982):
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where 
k is the score for response, 
k is the probability of response in the kth category (k = 1, 2, . . ., m), and zk is the ordinate of a standard normal density function corresponding to thresholds between categories k and k + 1. For two response categories as for stillbirth, it reduces to: 
, as shown in Dempster and Lerner (1949).
Results and Discussion
The total frequency of difficult calvings was 6.6% for Charolais and 6.2% for Hereford at first parity and 1.0% for Charolais and 1.2% for Hereford at later parities. The incidence of stillbirth was also higher in heifers (5.9% for Charolais and 5.6% for Hereford) compared with older cows (1.8% for both breeds). A higher incidence for first vs. later parities is commonly found in the literature (Philipsson, 1996). The frequencies of difficult calvings and stillbirth were 1.4 to 2.5 times higher in males than in females (Table 2), which is in agreement with earlier results (Philipsson, 1976b). There was a clear tendency for increased risk of calving difficulty for calves with high birth weights in our study, as shown in Figure 1. Of all first-parity calvings where the calf was stillborn, 42% in Charolais and 45% in Hereford were recorded as difficult (Table 3). The corresponding figures for calvings at later parities were 21 and 25%.
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and 5). The results were similar for the two breeds and for first and later parities. These estimates were within the range of heritabilities of birth weight reported in reviews by Mohiuddin (1993) and Koots et al. (1994a). They are also close to heritabilities previously estimated for Swedish Hereford and Charolais populations (Eriksson et al., 2002).
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Heritability estimates for calving difficulty score were similar in the different univariate (Table 4
) and bivariate (Tables 5 and 6) analyses. Direct heritabilities on the observable scale for calving difficulty score at first parity ranged from 0.11 to 0.16 and did not differ much between the two breeds. Corresponding maternal heritabilities were lower than the direct and ranged from 0.07 to 0.12. Lower estimates of maternal compared with direct heritabilities have also been found by Trus and Wilton (1988), Carnier et al. (2000) and Luo et al. (2002).
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), a difference in level between first and later parities remained. Direct heritabilities on the underlying scale for calving difficulty score, from univariate analyses and bivariate analyses with birth weight, were between 0.21 and 0.27 for first parity compared with 0.06 to 0.14 for later parities. The corresponding maternal heritabilities were in the ranges 0.12 to 0.18 for first parity and 0.01 to 0.07 for later parities. Lower heritabilities of calving traits for cows than for heifers are commonly found in the literature (Weller et al., 1988; Gregory et al., 1995a,b; Steinbock et al., 2003).
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and Koots et al. (1994a). Koots et al. (1994a) reported a tendency for higher heritabilities of reproductive traits, such as calving ease, in beef breeds vs. dairy breeds. Our estimates on the underlying scale were in the lower part of the range of heritability estimates presented for Charolais and Hereford by Trus and Wilton (1988) and Bennett and Gregory (2001) and for other beef and dual-purpose breeds by Hagger and Hofer (1990), Naazie et al. (1991), and Gregory et al. (1995a,b).
In comparison with estimates for beef-dairy crosses (McGuirk et al., 1998) and dairy breeds (Weller and Gianola, 1989; Luo et al. 2002; Steinbock et al. 2003), the heritabilities in our study were similar or somewhat higher.
Stillbirth.
Heritability estimates for stillbirth were generally low. All estimates on the observable scale were between close to zero and 0.02. On the underlying scale, there was a tendency for lower heritabilities for stillbirth at first parity than at later parities (Table 7) for both breeds. The heritabilities on the underlying scale estimated for stillbirth at first parity in our study were lower than most estimates reported in the literature, such as 0.10 to 0.15 for perinatal mortality presented by Koots et al. (1994a) and 0.04 to 0.14 by Meijering (1984). Our estimated direct heritabilities for stillbirth at later parities were, however, more comparable with these results and results from threshold model analyses for dairy cattle by Weller and Gianola (1989) for beef-dairy crossings by McGuirk et al. (1998) and for Simmental by Hagger and Hofer (1990). The comparatively low number of observations and data structure with only one offspring per heifer in the first-parity data made it difficult to separate genetic and environmental variances, and as a result, the heritabilities of stillbirth in first parity may be underestimated.
Correlations
Direct-Maternal Correlations.
Estimated genetic correlations between direct and maternal effects within traits were generally negative, with a few exceptions, and varied between high negative (-0.99) and low to moderate positive (0.32) (Tables 4, 5, and 6). Most correlations between direct and maternal genetic effects on different traits were negative or close to zero. Negative direct-maternal genetic correlations are commonly found for calving traits and birth weight in the literature (Meijering, 1984; Koots et al., 1994b; Bennett and Gregory, 2001), with few exceptions (e.g., Luo et al., 2002). The highest negative estimates (-0.99) for stillbirth in our study are, however, unrealistically high and were estimated with great uncertainty due to very low direct and maternal genetic variances.
Birth Weight and Calving Difficulty.
The estimated genetic correlations between direct effects on calving difficulty score and birth weight were 0.62 for Charolais and 0.72 for Hereford (Table 5). The corresponding maternal genetic correlations were lower: 0.46 for Charolais and even slightly negative for Hereford (-0.28). Except for this unexpected negative correlation, the results were in agreement with estimates from previous studies. Bennett and Gregory (2001) reported direct and maternal genetic correlations of 0.81 and 0.34, respectively, between birth weight and calving difficulty for beef cattle. In a review, Koots et al. (1994b) presented high direct genetic correlations between birth weight and calving ease of -0.73 and -0.67 for cows and heifers, respectively. Note that the signs of the correlations are dependent on trait definitions. Moderate to strong (0.44 to 0.93) genetic correlations between birth weight and calving difficulty have also been estimated for beef or dual-purpose breeds (Hagger and Hofer, 1990; Naazie et al., 1991; Gregory et al., 1995a,b).
Birth Weight and Stillbirth.
The estimated genetic correlations between stillbirth and birth weight were somewhat stronger and with the same sign as those between birth weight and calving difficulty score. As previously mentioned, the correlations with stillbirth were estimated with uncertainty. In the review by Koots et al. (1994b), the average genetic correlation between direct effects on birth weight and perinatal mortality for cows was positive and high (0.65), whereas it was moderate (0.41) for heifers. A high genetic correlation of 0.72 was estimated for Swiss Simmental by Hagger and Hofer (1990) when cow and heifer calvings were regarded as the same trait. McGuirk et al. (1998) estimated correlations of 0.42 and 0.80 between calf size and calf mortality when Charolais and Hereford sires were used for crossbreeding with dairy cows.
Correlations Between Calving Traits.
Both direct and maternal genetic correlations estimated between calving difficulty score and stillbirth were very high—in no case lower than 0.95. The correlations were estimated with uncertainty and they were higher than most estimates found in the literature, which range from 0.5 to 0.8 (Philipsson, 1996). Steinbock et al. (2003) reported correlations of 0.80 between direct effects and 0.74 between maternal effects on dystocia and stillbirth in first-calving Holsteins, and somewhat lower correlations were found by Luo et al. (1999). High genetic correlations between calving performance traits and calf mortality have been estimated for Simmental by Hagger and Hofer (1990) and for Charolais and Hereford sires used in dairy herds by McGuirk et al. (1998).
First and Later Parities.
Genetic correlations were generally high, but not in unity, between calving traits at first and later parities (Table 6). Except for the unexpected zero estimate between direct effects on stillbirth in Herefords, the lowest genetic correlations between calving traits in first and later parities was 0.66 and was estimated for direct effects on calving difficulty score in Charolais. Genetic correlations between parities for stillbirth were uncertain, especially for the numerically smaller Hereford breed.
Koots et al. (1994b) reported high positive average genetic correlations between calving ease in heifers and cows, both for direct (0.81) and maternal (0.75) effects, and a lower correlation of 0.32 between direct effects on perinatal mortality in heifers and cows. Most other studies of correlations between calving traits in different parities have been conducted for dairy breeds. Genetic correlations of 0.4 to 0.7 between stillbirth at first and second parity of Holsteins were found by Harbers et al. (2000) and Steinbock et al. (2003). For calving difficulty or calving ease, correlations between parities of 0.6 to 0.7 were reported by Luo et al. (2002) and Steinbock et al. (2003), whereas less than 0.5 was reported by Weller et al. (1988).
Permanent Environmental Effects and Residual Correlations.
The variance of permanent environmental effects of dam amounted to more than 5% of the total variance of birth weight for both breeds, and close to 5 and 3% of the total variance for calving difficulty score of Charolais and Hereford, respectively (Table 4). It seemed somewhat less important for stillbirth (about 2%) in both breeds.
Residual correlations between birth weight, calving difficulty score and stillbirth at first parity on the observable scale were low, but in all cases positive, ranging from 0.02 to 0.32 (Table 5). Environmental correlations between calving difficulty score and stillbirth and birth weight can be expected to be underestimated due to the involvement of categorical traits and nonlinearity (Meijering, 1984). In addition, few stillborn calves had information on birth weight; consequently, a residual correlation between these traits was difficult to estimate.
General Discussion
The results in this study were based on linear analyses. Calving ease and stillbirth are both categorical traits, and the use of threshold methodology for these traits has been recommended (Janss and Foulley, 1993). In our study, however, initial attempts to analyze the calving data with threshold models failed due to extreme category problems and gave unrealistic heritabilities even with highly simplified statistical models. These problems were probably due to the data structure with small contemporary groups, a low use of AI, and thus few offspring per sire and possible confounding between sire and herd-year effects.
The use of three classes of calving difficulty score made the assumption of linearity less severe for this trait compared with stillbirth, where only two classes could be used. The genetic parameters estimated for calving difficulty score in this study were within the normal range of estimates in the literature, whereas analyses of stillbirth often gave erratic results.
Even though linear methodology for categorical traits is not as theoretically adequate, it has been used for calving traits in many studies and this has been shown to work for practical purposes (Meijering, 1985; Weller et al., 1988; Hagger and Hofer, 1990). In studies of calving difficulty and stillbirth by Hagger and Hofer (1990) and Steinbock et al. (2003), heritabilities estimated in linear analyses and transformed to the underlying scale were close to those estimated in threshold analyses.
Ramirez-Valverde et al. (2001) compared models for calving difficulty in field beef data, and found small differences in predictive ability of breeding values between linear and threshold models. They concluded that especially for low-accuracy animals (with few offspring), the animal model including maternal effects was superior to the sire-maternal grandsire model. They also found that greater improvement in predictive ability was achieved by changing from univariate to bivariate models (including birth weight) compared with changing from linear to threshold models.
A biological explanation for genetic correlations between parities of less than unity, as found in the present study, might be that partly different causes are important for calving difficulty at first and later calvings. A large calf in relation to the inlet pelvic dimensions of the cow is the most common cause of dystocia (Naazie et al., 1989) and is more important in heifers than in cows (Philipsson, 1976a; Meijering, 1984). Posterior or abnormal presentations of the calf, weak labor, uterine torsion, and insufficient dilatation of the cervix are of importance especially in older cows (Meijering, 1984).
Although the average birth weight of calves of heifers was 7 to 10% lower than for calves of later parity cows in our study, heifers are also smaller than cows and the frequency of difficult calvings was considerably higher at first parity compared with later parities.
The phenotypic relationship between birth weight and calving difficulty is shown in Figure 1. The frequency of difficult calvings started to increase markedly above a certain level of birth weight. The location of this apparent threshold was dependent on the parity and breed. The nonlinear phenotypic relationship between calving difficulty or stillbirth and birth weight has previously been reported by Philipsson (1977) and Berger et al. (1992). Very small calves may have reduced viability and larger-than-average calves are more likely to experience difficult calvings.
In this as well as several previous studies (Philipsson, 1976a; Berger et al., 1992; Steinbock et al., 2003), more than half of the stillborn calves did not have difficult calvings. The causes of stillbirth for calves with easy or normal calvings are largely unknown (Meijering, 1984). It is, however, also not known in our field data to what extent unassisted calvings were supervised, and if some calves that died shortly after birth could have been saved if all calvings had been supervised. This concern was also raised in a study on American Angus field data by Berger et al. (1992), who reported higher calf mortality rates among unassisted vs. assisted births in cows.
The phenotypic association between calving difficulty and stillbirth was more pronounced for bull calves than for heifer calves. Philipsson (1976b) reported large gender differences in calving difficulty and stillbirth that could partly be explained by higher birth weight of bull calves. Bull calves were, on average, 6 to 7% heavier than heifer calves at birth for both breeds in our study.
Implications
Direct and maternal effects on calving difficulty score should be included in the genetic evaluation of pure beef breeds in Sweden, using multiple-trait evaluation with calving difficulty score at first and later parities as correlated traits. Stillbirth will need to be considered through the correlated traits birth weight and calving difficulty score because Swedish beef progeny groups are too small for stillbirth to be considered directly. To better understand the consequences of selecting against calving difficulty, we need to study the genetic correlations with economically important growth and carcass traits.
Footnotes
1 Grants from the Swedish Farmers’ Foundation for Agricultural Research and Agria Research Foundation are gratefully acknowledged. The Swedish Dairy Association generously provided data. Appreciation is expressed to W. F. Fikse for valuable scientific discussions. 
2 Correspondence: P.O. Box 7023 (phone: +46-18-672007; fax: +46-18-672648; e-mail: Susanne.Eriksson{at}hgen.slu.se).
Received for publication May 9, 2003. Accepted for publication September 29, 2003.
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= first-parity Hereford (Heref.), 
= later-parity Hereford, 
= first-parity Charolais (Char.), and 
= later-parity Charolais.
), direct (
) and maternal (
) heritabilities on the observable scale for birth weight (BiW), calving difficulty score (CD), and stillbirth (SB) estimated in single-trait analyses