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* Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
Swedish Dairy Association, SE-631 84 Eskilstuna, Sweden
Corresponding author:
L. Steinbock; e-mail:
lena.steinbock{at}hgen.slu.se.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: stillbirth and calving difficulty • direct and maternal genetic effects • threshold model
Abbreviation key: SLB = Swedish Holstein, HF = Holstein Friesian, REML = restricted maximum likelihood
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Stillbirths and calving difficulties are influenced by both direct and maternal genetic effects, i.e. the genetic constitution of both the sire and maternal grandsire has an impact on the birth process and its outcome. In the Holstein population, large variation in stillbirth rates among bulls as sires and as maternal grandsires, ranging between 2 and 25%, has been observed in large progeny groups (Berglund and Philipsson, 1992; Philipsson et al., 1997). Stillbirth problems appear to be rather different today, compared with earlier experience, as stillbirths seem to be less closely related to high birth weight and to difficult calving than they used to be (Berglund and Philipsson, 1992; Steinbock et al., 1997). Nowadays, among SLB heifers, more than half of all stillborn calves are born at normal or easy calvings (Berglund, 1996; Steinbock et al., 1997).
A wide variation is obviously present in stillbirth rate at first calving (Berglund 1996; Philipsson et al.,1997). The questions are: to what extent is there genetic variation in stillbirth rate independent of calving difficulty, and is there any important genetic variation in second calvers? The objectives of this study were therefore to estimate genetic parameters for direct and maternal effects for both stillbirth rate and calving difficulty for first- and second-calvers separately, and to estimate the genetic correlation between the traits of the two parities. This information is needed both to define traits for the selection of bulls, and to optimize breeding programs designed to minimize calving difficulties and stillbirths.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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shows the structure of the edited data, together with incidences of stillbirth and calving difficulty.
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Statistical Methods
In first-calvers, variance and covariance components were estimated for the two traits using the single-trait sire-maternal grandsire model;
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where
| Yijklmnopq | = | denotes a 0 or 1 for live vs. stillborn and easy/normal vs. difficult calving,
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| µ | = | the overall mean,
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| ai | = | fixed effect of the ith combination of heifer’s calving age in months and sex of calf,
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| bj | = | fixed effect of jth year of calving,
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| ck | = | fixed effect of kth season of calving,
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| dl | = | fixed effect of lth genetic group of sire of calf,
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| fm | = | fixed effect of mth genetic group of calf’s maternal grandsire,
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| sln | = | random effect of calf’s sire,
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| mgsmo | = | random effect of calf’s maternal grandsire,
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| hyp | = | random effect of herd x year, and
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| eijklmnopq | = | random residual effect.
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In the analysis of second-calvers the combination of age of heifer and sex of calf was replaced by the effect of sex of calf only.
Heifers’ ages were divided into seven classes: 20 to 22, 23 to 24, 25 to 26, 27 to 28, 29 to 31, 32 to 34 and 35 to 38 mo at calving. Calving seasons were divided into five classes, due to the Swedish calving season, which begins in August, and due to the grazing periods. The classes were: late grazing period, August through September; fall, October; winter, November through February; spring, March through April; and early grazing period, May through July. The sires were distributed into 124 and 111 groups in first and second calving, respectively, and maternal grandsires were distributed into 130 and 112 groups in first and second calving, respectively. The groups were based on bull’s year of birth and on the proportion of HF-genes, expressed in quartiles, and was done in order to express the genetic difference between bulls due to the gradual Holsteinization present in the population. If there were fewer than 5 bulls in any group, the groups were assembled by year of birth only. Herd x year combinations with fewer than five calvings were put together in one group per herd over years.
Sire of sire and maternal grandsire as well as maternal grandsire of sire and maternal grandsire were included in the relationship matrix.
The analysis for stillbirth was also run with calving difficulty included as a fixed effect for both first- and second-calvers in order to explain the dependence of the genetic variation in stillbirth on calving difficulty. A bivariate analysis using the model described above was also made in order to obtain the genetic correlation between stillbirth and calving difficulty.
The calving performance traits were analysed with both linear mixed models and threshold models. Variance components from the linear mixed models were estimated using a restricted maximum likelihood (REML) procedure (Jensen and Madsen, 1994). In a Bayesian analysis, inference from a threshold model was based on a posterior distribution achieved with Gibbs sampling technique (Jensen, 1994). In the threshold model, the occurrence of a stillbirth or a difficult calving was assumed to be the result of a normally distributed variate—a liability value. The liability values for stillbirth were created by data augmentation, as described by Sørensen (1996), using random normal deviates from distributions conditional on the other effects in the model. The environmental variance was assumed to be 1 and the threshold was set to 0.
Uniform, improper prior distributions were used for fixed effects. Vague priors for variance components were calculated for calving difficulty from Ducrocq (2000). They were also used as prior information in the analysis on stillbirth as (co)variances on stillbirth were not found in the literature. To test the sensitivity to prior information the data was also analysed using information calculated from the linear analysis. There were however no important differences in the results between the two sets of prior information. A chain of 11,000 samples for first-calvers was used and 14,000 samples for second-calvers. The first 300 samples were regarded as burn-in period and discarded. The effective number of independent observations and lag correlations were calculated as described by Sorensen et al. (1995).
The expectations of the estimated variances, covariances are given by Kriese et al. (1991) and were calculated together with the heritabilities for the direct and maternal effects as:
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The genetic correlation between direct and maternal effects was calculated as:
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where the subscripts dir and mat represent direct and maternal additive effects, respectively. Variances and covariances with appropriate subscripts of maternal grandsires and sires were taken from the output of the REML procedure and Gibbs sampling, respectively. Monte Carlo standard errors of the posterior means and effective number of independent observations were calculated as described by Sorensen et al. (1995). For first-calvers, the Gibbs chain reached equilibrium distribution almost immediately. For the second-calvers, the chain was less stable, but also seemed to have reached equilibrium distribution relatively early.
For purposes of comparison, heritabilities from the linear analyses were transformed to the underlying scale according to Dempster and Lerner (1950).
EBV from linear analyses were used to calculate correlations between results for bulls represented with at least 100 offspring as sires and maternal grandsires, respectively, in first and second calving. The method proposed by Calo et al. (1973) was used to adjust the calculated correlations to genetic correlations.
The ETA for bulls as sires and maternal grandsires were presented as percentage of stillbirths and difficult calvings respectively and the range in values among bulls was calculated by adding the population means from Table 1
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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presents the incidence levels for stillbirth and calving difficulty for first- and second-calvers as an average over the period studied. The incidences were much higher in first calving for both traits. The incidence of neither stillbirth nor calving difficulty in second calving changed particularly over the 10 yr studied, whereas in first calving there was a large increase in stillbirth rate, from below 6% in 1985 to almost 9% at the end of the period.
Age of the heifer at calving in connection with sex of calf had a considerable impact on calving performance (Figure 1). For bull calves there was a considerable difference in stillbirth rate between young and old dams, whereas it was 10 to 14% up to the age of 26 mo, it decreased to 8% at the higher ages. Although stillbirth rates for heifer calves were not as high, the trend was similar to but less pronounced than for bull calves. Regarding calving difficulty, there was an appreciable difference between bull and heifer calves, but there did not seem to be the same distinct connection with age. However, the older heifers, 30 mo of age and above, did not have as much calving difficulty as the younger ones.
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). In the summer months (May to September), when the heifers are out at pasture, the incidence of difficult calving was lowest. The stillbirth rate closely followed the same trend as the incidence of calving difficulty except that it was only low in late summer and early autumn.
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the sires and maternal grandsires of the calves from heifers are divided into five genetic groups in order to show how the proportion of HF-genes affects the rate of stillbirth and calving difficulty, respectively. For stillbirth, sires with 50% or more HF-genes gave a higher stillbirth rate than the average 7.1% in all data. For maternal grandsires, only the bulls without HF-genes gave a stillbirth rate above average of 10.1%. For calving difficulty, the sires without HF-genes gave almost no difficulty at all. For the pure HF-sires too, the rate was below the average of 8.4%. Most calving difficulty was caused by sires with 50% HF-genes (14.3%). Also for maternal grandsires the highest rate of calving difficulty was reached by those with 50% HF-genes; the others had rates on or slightly below average.
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and 3. On the visible scale for first calving, heritabilities for stillbirth were 3.8% and 2.8% for direct and maternal effects, respectively. For calving difficulty the corresponding values were 6.2% and 4.8%. In second calving the heritabilities were very low—between 5% and 20% of the estimates from first calving.
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When adjusting for calving difficulties in the model, heritabilities for stillbirth decreased to about half the values, without adjustment (Table 3).
Genetic correlations between stillbirth rate and calving difficulty were 0.80 for the direct effects and 0.74 for the maternal effects in first-calvers (not shown in a table). The other estimates of direct and maternal heritabilities, and direct-maternal genetic correlations, derived from the bivariate analyses were very close to the estimates derived from the single-trait linear analyses and are therefore not presented in detail.
The correlations between EBV for bulls in first and second calving, and the calculated corresponding genetic correlations, are presented in Table 4. For stillbirth, the genetic correlations for both the direct and maternal effects were 0.4 to 0.5; for calving difficulty they were slightly higher: 0.6 to 0.7.
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shows the range in ETA for bulls. The difference between bulls both as sires and as maternal grandsires of the calves is considerable for both calving traits in heifers. In second calving, where the incidences were considerably lower, the range between bulls is much smaller.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Heritabilities
The estimates of heritabilities for stillbirth rate at first calving obtained from linear analyses have almost doubled, compared with previous Swedish results (Philipsson, 1976b). This is partly an effect of the increased incidence level. For calving difficulty, however, the heritabilities were lower than in the earlier study, which was probably based on more accurate recording, being an experimental field study. The heritabilities estimated for second calving were less than a third of the values for first-calvers when the threshold model was used, and, as could be expected, were even lower when estimated on the visible scale.
The increased genetic variation in stillbirth is probably an effect of the ongoing "holsteinization" during the period under study. An association between a higher proportion of HF-genes and increased stillbirth rate was found in our earlier study (Berglund and Philipsson, 1992). When bulls were sires of calves there was an obvious connection between the proportion of HF-genes and the higher incidence of calving difficulty and stillbirth, whereas the maternal effects show no obvious pattern. However, the results are difficult to interpret due to the time-lag between the generations of bulls with similar HF levels as sires and maternal grandsires, and their expression of direct and maternal genetic effects including the covariance between these.
The adjustment of stillbirth for calving difficulty in the model caused at both calvings a reduction of the heritability to about half compared with no adjustment. Thus, the genetic variation in stillbirth rate is partly independent of the incidence of calving difficulty. This is supported by the genetic correlations between stillbirth and calving difficulty as direct (0.80) and maternal effects (0.74) in heifers.
Correlations Between Parities
Genetic correlations between results for bulls with records from first and second calvings were of moderate size for both traits (Table 4). Between 20 and 50% of the genetic variation is common to both parities. For Holsteins in The Netherlands, the genetic correlations for direct and maternal effects between first and second calving were 0.52 and 0.78, respectively, for stillbirth (Harbers et al., 2000). In the U. S. Holstein population it was found that stillbirths at first and second calving should be regarded as different traits and analysed separately (Meyer et al., 2000). Despite the medium-high correlations, the low incidence and heritability values at second calving mean that these records would not contribute much to the accuracy in estimation of breeding value of bulls for stillbirth and calving difficulty. The genetic contribution to a visible variation in stillbirth and calving difficulty in second-calvers is rather small, as is obvious from the variation shown in ETA among bulls in Table 5.
Ducrocq (2000) estimated correlations between EBV of bulls for calving difficulties between heifers and older cows and obtained clearly higher estimates (0.8 and 0.7 for sires and maternal grandsires, respectively) than ours for both the direct and the maternal effects. Ducrocq (2002) got a higher estimate for the direct effect, the complete opposite of the result in our study. The heritabilities estimated on the underlying scale in that study were much lower, though, than those obtained in our study. The differences in heritabilities between first- and second-calvers was not as marked as in our study, although the incidences of difficult calvings were 9.6% and 3.3% in first- and second-calvers, respectively.
Correlations Between Traits
The small but consistent antagonistic genetic correlations that were found to exist between the direct and maternal effects in first-calvers, for stillbirth and for calving difficulty, indicate the need to evaluate bulls both as sires and as maternal grandsires of calves. These results correspond quite well with the literature. Harbers et al., (2000) also found a slight negative genetic correlation (-0.07) between direct and maternal effects for stillbirth in first-parity Holsteins. Manfredi et al., (1991) estimated the genetic correlation between direct and maternal effects for calving difficulty to -0.1, while Ducrocq (2000) obtained a genetic correlation of 0 between direct and maternal effects for calving difficulty in Holsteins in all parities. The change in sign and in the correlation between direct and maternal effects for calving difficulty and stillbirth when adjusting for calving difficulty at second calving are most probably due to the uncertainness of these estimates (the heritabilities and incidences are very low).
The genetic correlations between stillbirth and calving difficulty in first-calvers were rather high (0.78 and 0.84), and in the range of what was found in the review by Meijering (1984). In a more recent study in Britain, a lower correlation, 0.40, was found between the two traits when all parities were included (McGuirk et al., 1999). The results emphasize the importance of including both traits in the genetic evaluation of bulls.
Season and Age
The variation in results of stillbirth and calving difficulty over seasons was less than quite small in our study, differing 4%-units between lowest and highest rate of calving difficulty and only 2%-units for stillbirth rate (Figure 2). By comparison, the fluctuation over the year, e.g. in McGuirk et al., (1999), was 7.5%-units in calving difficulty. In earlier reviews by Meijering (1984) and McGuirk et al., (1999), the highest rates evidently occur in late autumn and winter, whereas we found the highest rate of calving difficulty in spring and almost no difference in stillbirth rate from October to July. The low rates of calving difficulty in summer and early autumn in our study might be attributable to less intense supervision of the calving while out at pasture, and that during that period heifers are in good physical condition.
The age of the heifer at calving is an important factor to take into account, from a management point of view. Our results suggest that it is essential not to let the heifers conceive too early. Calving difficulty rates were rather high up to about 30 mo of age, when giving birth to a bull calf. The difference between bull and heifer calves in our study was probably an effect of bull calves being heavier than heifer calves.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication March 27, 2002. Accepted for publication November 27, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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