Implementation of a sire-maternal grandsire model for evaluation of calving ease in the United States

The objective of this study was to add a maternal grandsire (MGS) effect to the existing sire model for national calving ease genetic evaluations. The Animal Improvement Programs Laboratory (AIPL) of USDA assumed responsibility for conducting the national genetic evaluation for calving ease and maintaining the associated database in 1999. Existing evaluations used a sire threshold model. Adding an MGS effect to the model was expected to improve accuracy by partially accounting for merit of mates and differences in maternal ability of the dams. Dystocia data were migrated to a relational database integrated with the AIPL production database. This database design allowed more rigorous data edits by comparison with the production data and improved MGS identification (ID) rate by utilizing pedigrees from the production records. Integration of dystocia data with production data increased MGS ID rate from 58 to 73%. In addition, nearly 200,000 duplicate records were identified using the new edit system. Sire and sire-MGS models were compared using over 10 million observations available for the August 2002 national genetic evaluation. The sire model included herd-year, season, sex of calf, parity of dam, birth year group of sire, and sire. For the sire-MGS model, MGS and birth year group of MGS were added, year-seasons rather than seasons were used, and sex of calf and parity of dam were combined into a single interaction effect. Herd-year, sire, and MGS were random effects. Variance components used for the sire model were those previously used in the national evaluation and for the sire-MGS model were estimated in a separate study. Correlations between predicted genetic merits for service sire calving ease from the two models was 85%, indicating general agreement, but with some significant differences in evaluations. A sire-MGS model was implemented in August 2002 for the national calving ease genetic evaluation system.     

Calving difficulty and stillbirths of pure Holsteins versus crossbreds of Holstein with Normande, Montbeliarde, and Scandinavian Red

Pure Holstein cows and Normande/Holstein, Montbeliarde/Holstein, and Scandinavian Red/Holstein crossbred cows were compared for calving difficulty and stillbirth rates. Scandinavian Red was a combination of Norwegian Red and Swedish Red. All cows calved from June 2001 to August 2004 at 7 commercial dairies. Statistical models for analysis included effects of herd-year-season of calving and sex of calf in addition to breed of sire and breed group of dam. Male calves had significantly more calving difficulty and stillbirths than heifer calves. First-calf Holsteins bred to Holstein, Brown Swiss, Montbeliarde, and Scandinavian Red bulls were used to determine effects of breed of sire. Calves sired by Scandinavian Red bulls (5.5%) and Brown Swiss bulls (12.5%) had significantly less calving difficulty than calves sired by Holstein bulls (16.4%) from Holstein first-calf heifers. Also, fewer stillbirths resulted from use of Scandinavian Red bulls (7.7%) compared with use of Holstein bulls (15.1%) for first-calf Holstein heifers. Scandinavian Red-sired calves (2.1%) had significantly less calving difficulty than Holstein-sired calves (8.4%) for multiparous Holstein dams. Non-Holstein breeds of sire had significantly fewer stillbirths than Holstein sires when mated to multiparous Holstein dams. To determine the effects of breed of dam, 676 pure Holsteins, 262 Normande/Holstein, 370 Montbeliarde/Holstein, and 264 Scandinavian Red/Holstein crossbred virgin heifers that had been bred to Brown Swiss, Montbeliarde, and Scandinavian Red bulls were utilized. All groups of crossbred cows had significantly less calving difficulty at first calving than pure Holsteins (3.7 to 11.6% vs. 17.7%). Furthermore, Montbeliarde/Holstein (6.2%) and Scandinavian Red/Holstein (5.1%) crossbreds had significantly lower stillbirth rates at first calving than pure Holsteins (14.0%).     

Genetic Analysis of Dystocia and Calf Mortality in Israeli-Holsteins by Threshold and Linear Models

Calvings of 106,751 Israeli Holstein heifers were analyzed for dystocia and calf mortality, scored dichotomously, and a composite trait, scored trichotomously. Dystocia was also studied with 146,973 second and third parity records. Models fitted included herd-year-season, sex of calf, calving age, calving month, sire of cow, sire of calf, and groups of sire of cow and of calf. Herd-year-season, sire of cow and calf, and residuals were random with diagonal variance-covariance matrices. Herd-year-season variance was assumed to be 10% of the residual component. Other variance components were estimated by REML for linear models and by the counterpart of REML for threshold models. Heritability estimates were two to five times greater in threshold than in linear models, but correlations between corresponding sire evaluations were all >.9. Linear model sire evaluations were skewed positively, whereas threshold model evaluations had symmetrical distributions. Heritability for dystocia was greater in first than in later parities. Correlations between first and later parity sire evaluations were <.5. Thus, the genetic control of dystocia seems to be different for heifers and cows. Correlations between sire of cow and calf evaluations were <.3. Correlations between dystocia and calf mortality evaluations were about .7.     

Ordered categorical sire evaluation for dystocia in Holsteins

New statistical procedures for analysis of ordered categorical data were investigated. In using the standardized threshold model, scores are transformed to estimate unknown boundary points and fixed and random effects on an underlying scale of continuous response. The model for calving ease included sex of calf, parity of dam, and the unknown boundary points as fixed effects; herd-year-seasons and sires were random. Data were 5029 dystocia scores on a scale of 1 to 5 collected from June 1982 to January 1984 through the Mid-States Dairy Records Processing Center. The predicted probability of a difficult birth by an average sire indicated that twice as many males as females are born with difficulty. Difficult births declined between first and second parity by a factor of 5. Herd-year-seasons accounted for 13.3% of the variance and sires 3.2%. Heritability on the underlying scale was higher (.147) than on the observed scale (.061). Rank correlations among the threshold model sire solutions and best linear unbiased predictions were .94 when herd-year-seasons were fixed, .98 with random herd-year-seasons, and .99 with normalized scores. Predicted percentage of difficult births for all sires in the analysis ranged from 6 to 18%.     

Genetic evaluation of Holstein sires and maternal grandsires in the United States for perinatal survival

Stillbirth, defined as a calf that dies just prior to, during, or within 48 h of parturition, represents a reoccurring concern among breeders of dairy cattle in the United States. About 11% of parturitions of primiparous Holstein cows result in the death of a calf; 5.7% in multiparous cows. Genetic evaluations can be reported as perinatal survival to 48 h to emphasize the positive information about the trait. The purpose of this research was to: 1) estimate genetic parameters by restricted maximum likelihood for perinatal survival rates; 2) characterize the genetic evaluation of sires for the perinatal survival of their progeny and maternal grandsires for the perinatal survival of their daughters progeny; and 3) estimate genetic trends from 1984 to 1994. Data (n = 666,339) were from the National Association of Animal Breeders calving ease database. Over 600 new young sires were available each year. The binomial response variable, 1 = alive, 0 = stillborn within 48 h of parturition was analyzed by using a sire-maternal grandsire linear mixed model. The model included fixed effects for sex of calf, dystocia, and season of birth, and gestation length as a covariate; correlated random effects of sire and maternal grandsire; and uncorrelated random effects of herd-years. Parturitions of primiparous and multiparous cows were analyzed separately. In primiparous cows, heritability estimates were 1.1 and 2.2% for sire of the calf and maternal grandsire, respectively. The genetic correlation between sire and maternal grandsire predicted transmitting ability (PTA) for perinatal survival, was 0.31; simple product moment correlations among sire-MGS PTA were 0.43 and 0.46 for primiparous and multiparous cows, respectively. The PTA for sire of the calf ranged from -2.9 (lower survival) to 2.8% (higher survival). Mean PTA from 1984 to 1994 was quite variable from year to year. Evidence showed a slightly negative, but nonsignificant, genetic trend in perinatal survival (-0.04% per year for sires and -0.02% per year for maternal grandsires). Estimates of genetic parameters and genetic trends for data from multiparous cows are also reported. Correlations among PTA for perinatal survival, milk yield, and calving ease are given.     

Models for genetic analysis of dystocia and calf mortality

Calvings of 106,750 Israeli Holstein heifers were analyzed for dystocia and calf mortality, which were scored dichotomously. Models fitted included herd-year-season, sex of calf, calving age, calving month, sire of cow, sire of calf, and groups of sire of cow and calf. Herd-year-season, sire of cow and of calf, and residuals were random with diagonal variance-covariance matrices. Traits were analyzed with and without a herd-year-season effect, and dystocia was analyzed separately for male and female calvings and with both sexes combined. Variance components were estimated by REML for linear models and by the counterpart of REML for threshold models. Heritability estimates were two to four times larger in threshold than in linear models, but correlations between corresponding sire evaluations were all greater than .9. Correlations between evaluations computed with and without herd-year-season effects were .9 for sire of calf evaluations for dystocia and greater than .97 for all other evaluations. Correlations between sire evaluations computed separately for male and female calvings were between .3 and .7. Thus, calving difficulty is expressed differently in male and female calvings. Genetic trends were unfavorable for dystocia but favorable for calf mortality. Phenotypic trends were curvilinear with maxima in 1982 and 1983 for calf mortality and dystocia.     

Sire and maternal grandsire effects on calving difficulty and calf mortality in Israeli Holsteins

Associations among sire and maternal grandsire evaluations for calving difficulty and calf mortality were investigated in the Israeli Holstein population. Primiparous and multiparous calvings were analysed separately. Genetic correlations between calving difficulty and calf mortality were close to unity for primiparous calvings but lower for multiparous calvings. Genetic correlations between primiparous and multiparous sire evaluations were .94 and .72 for calving difficulty and calf mortality. Genetic correlations between sire and maternal grandsire evaluations were .6 for primiparous calvings and not-significant for multiparous calvings for both traits. These results correspond to findings that the direct genetic effect is larger than the maternal effect but that there is a slight negative correlation between them. We suggest that sire evaluations for calving difficulty of multiparous cows be used as a preliminary prediction of heifer calving difficulty and calf mortality and that the separate calving trait evaluations be combined into an overall calving selection index.     

Effects of body measurements and weight on calf size and calving difficulty of Holsteins

Body measurements (heart and paunch girths, wither height, chest depth, pelvic length and width, and body length), body weight, and calving evaluation data (calf birth weight, calf sex, calf presentation, and calving assistance needed) were collected from 1974 parities of 762 Holstein cows between 1968 and 1986. Degree of calving assistance was scored continuously from 1 (no assistance) to 10 (hard mechanical assistance). Phenotypic correlations of dam body traits with calf birth weight were all significantly positive when combined for all parities and ranged from .23 for paunch girth to .27 for body weight and heart girth. Correlations of dam body traits with calving assistance scores were all significantly negative across parities and ranged from -.24 to -.30. Correlations of calf birth weight with calving assistance were higher for first parity (.37) than for all parities (.20). Least squares analysis showed that cows with shorter wither height and shorter pelvises tended to require more calving assistance. Heavier calves, winter calvings, and earlier parity all were related to increased dystocia. Male calves were heavier than female calves and also were associated with greater calving difficulty.     

Undesired phenotypic and genetic trend for stillbirth in Danish Holsteins

The primary aim of this study was to evaluate the phenotypic and genetic trends for stillbirth in Danish Holsteins. Trends of calving difficulty and calf size were also evaluated. The second aim was to compare predicted transmitting abilities (PTA) of sires for stillbirth using a linear and a threshold model. Direct and maternal genetic effects were modeled by fitting correlated additive genetic effects of the sire and the maternal grandsire (MGS). For both the calf and the dam, covariates of breed proportions of Holstein-Friesian (HF) and the heterozygosity between HF and the original Danish Black and White (ODBW) were included. Records from 1.8 million first-calving Danish Holstein cows calving from 1985 to 2002 were used. In this period, the overall frequency of stillbirth increased from 0.071 to 0.090. An unfavorable genetic trend of stillbirth was found for both the direct and maternal effect. The background for the genetic trends was an intense use of HF sires as sires of sons, which increased the proportion of HF genes to 94% in the Danish Holstein calves born in 2002. The effect of the imported HF genes was higher direct effects of calf size, calving difficulty, and stillbirth compared with the ODBW genes. The maternal effect of stillbirth was poorer for HF than for ODBW even though HF had a better maternal calving performance than ODBW. The threshold and the linear models showed almost similar predictions of transmitting abilities of sires.     

Multiparity evaluation of calving ease and stillbirth with separate genetic effects by parity

Evaluations that analyze first and later parities as correlated traits were developed separately for calving ease (CE) from over 15 million calving records of Holsteins, Brown Swiss, and Holstein-Brown Swiss crossbreds and for stillbirth (SB) from 7.4 million of the Holstein CE records. Calving ease was measured on a scale of 1 (no difficulty) to 5 (difficult birth); SB status was designated as live or dead within 48 h. Scores for CE and SB were transformed separately for each trait by parity (first or later) and calf sex (male or female) and converted to a unit standard deviation scale. For variance component estimation, Holstein data were selected for the 2,968 bulls with the most records as sire or maternal grandsire (MGS). Six samples were selected by herd; samples ranged in size from 97,756 to 146,138 records. A multiparity sire-MGS model was used to calculate evaluations separately for CE and for SB with first and later parities as correlated traits. Fixed effects were year-season, calf sex, and sire and MGS birth years; random effects were herd-year interaction, sire, and MGS. For later parities, sex effects were separated by parity. The genetic correlation between first and later parities was 0.79 for sire and 0.81 for MGS for CE, and 0.83 for sire and 0.74 for MGS for SB. For national CE evaluations, which also include Brown Swiss, a fixed effect for breed was added to the model. Correlations between solutions on the underlying scale from the January 2008 USDA CE evaluation with those from the multiparity analysis for CE were 0.89 and 0.91 for first- and later-parity sire effects and 0.71 and 0.88 for first- and later-parity MGS effects; the larger value for later parity reflects that later parities comprised 64% of the data. Corresponding correlations for SB were 0.81 and 0.82 for first- and later-parity sire effects and 0.46 and 0.83 for first- and later-parity MGS effects, respectively. Correlations were higher when only bulls with a multiparity reliability of >65% were included. The multiparity analysis accounted for genetic differences in calving performance between first and later parities. Evaluations should become more stable as the portion of a bull's observations from different parities changes over his lifetime. Accuracy of the net merit index can be improved by adjusting weights to use evaluations for separate parities optimally.     

Relationship between external body measurements and calving difficulties in Canadian Holstein-Friesian cattle

Thirteen body measurements per cow were collected from 1980 to 1981 on 3,193 Canadian Holstein-Friesian cows by 576 sires in 100 herds. These data were analyzed to test effects of herd, age of cow, stage of lactation, and generation group; to predict sire proofs; to estimate heritabilities and genetic and phenotypic correlations among body measures; and to estimate correlations between sire proofs for body measures and sire (maternal grandsire of calf) proofs for calving ease. Herd and age of cow were important sources of variation for all external body measures. Stage of lactation was an important source of variation for heart girth, and for slopes from hip to pin and from thurl to pin. Effects of generation group were not significant for any body measures. Heritabilities were moderate to high (.21 to .45). Genetic correlations among all height characteristics were large and positive (.68 to .99). Daughters that were large with wide pins, long sloping rumps, and little slope from thurl to pin bone seemed to be favored for easy calving.     

Effects of inbreeding in the dam on dystocia and stillbirths in US Holsteins

Dystocia scores were recorded by producers on 120,434 Holsteins (218,213 records) from 1985 through 1996; dystocia scores 3 to 5 were coded as difficult births. Stillbirths were recorded for deaths within the first 48 h after birth. Data were restricted to registered cows for pedigree completeness, and inbreeding coefficients were calculated using 5-generation pedigrees. Computational restrictions required that subsets of the data be created by choosing herds at random but using all records from selected herds. Effects of inbreeding in the dam were estimated in a sire-maternal grandsire (of the calf) threshold model using Gibbs sampling. The model included fixed effects of calf sex and inbreeding of the dam and random effects of herd-year-season of birth, additive genetic, and residual effects. First, second, and third parities were analyzed separately. Solutions for sex of calf and inbreeding from different parities were converted to expected change in probability of dystocia or stillbirth per 1% increase in inbreeding. Inbreeding effects were largest for first-parity cows giving birth to male calves at a 0.42% increase in probability of dystocia/1% increase in inbreeding. Effects of inbreeding for first-parity dams giving birth to female calves were smaller, 0.30%/1% increase in inbreeding. Incidence of stillbirths increased 0.25 and 0.20% for male and female calves/1% increase in inbreeding for first parity births. Effects of inbreeding on dystocia and stillbirths declined with parity. Effects of inbreeding were small, especially in later parities, but were consistently unfavorable.     

Estimation of repeatability of calving ease in Canadian Holsteins

Restricted maximum likelihood was used to estimate repeatabilities and heritabilities for calving ease from a data set of 636,972 Holstein parturitions obtained from the Quebec Dairy Herd Analysis Service. Subsets of the data, chosen to reflect different stages of cow maturity, were analyzed separately and compared. Initial least squares analyses were used to define the fixed effects to include in the model. The effects of herd-year-season, sex of calf, parity, and sex by parity interaction were significant in all subsets. Age and parity by age interaction were significant in subsets with primarily younger animals. Variances for sire of calf, cow, and residual effects were then estimated by REML. Repeatabilities ranged from 5.98 to 7.87% and heritabilities for calving ease as a direct effect ranged from 1.97 to 4.70%.     

Prediction of transmitting ability of heifers from genetic evaluations of dams when dams and herdmates are required to have a first record

Estimated transmitting ability for milk production calculated from only first lactation records was regressed on milk proofs of the sire and maternal grandsire and either estimated transmitting ability of the dam calculated from only first lactations or estimated transmitting ability of the dam calculated from all lactations of a data set in which all cows were required to have milk records of first lactation. Records of 222,576 Holstein heifers in the northeastern United States were analyzed. Partial regression coefficients were similar to those from reports in which all records in the data file were used to estimate transmitting abilities. The partial regression coefficient for estimated transmitting ability of the dam from all lactation records (.17) was smaller than the approximate theoretical regression coefficient (.70). The partial regression coefficient for estimated transmitting ability of the dam calculated from first records (.72) was similar to the approximate theoretical regression coefficient (.80).     

Phenotypic trends in incidence of stillbirth for Holsteins in the United States

The objectives for this study were to determine 1) if there was a trend in stillbirths for the U.S. Holstein population, 2) if stillbirths are the same trait in primiparous and multiparous cows, and 3) what was the role of dystocia in stillbirths. A sample of 666,341 births from the MidStates Dairy Records Processing Center and the National Association of Animal Breeders was used to examine the influence of sire, herd, year, season, sex of calf, parity of dam, calving ease, and gestation length on the survival of the calf. Parity was scored as an ordered variable (1, 2, 3+). Calving ease was scored on a scale of 1 (no assistance) to 3+ (needed assistance). An increasing trend in stillbirths was found in primiparous and multiparous cows. The percentage of stillborn calves in primiparous cows increased from 9.5 in 1985 to 13.2 in 1996. Stillbirths in multiparous cows increased from 5.0 to 6.6% from 1985 to 1996. Variation about the trend was greater in primiparous cows than in multiparous cows. Dystocia was a major determinant of stillbirth incidence, but the association was stronger in primiparous cows. Sex of calf had different associations with stillbirth incidence in primiparous and multiparous cows. Gestation length and season of birth also had significant associations with stillbirth incidence. Logistic regression models with fixed and random effects were fit to the data to preserve the binary nature of the stillbirth response. The expected probability of stillbirths for an average herd and sire was 10% for primiparous cows and 5% for multiparous cows. Replacement of stillborn calves is a substantial cost to the dairy industry at more than $125.3 million per year. Because of the increasing incidence of stillbirths, these costs have increased by $75.9 million from 1985 to 1996.     

Implementation of a routine genetic evaluation for longevity based on survival analysis techniques in dairy cattle populations in Switzerland

Genetic evaluation of sires for functional longevity of their daughters based on survival analysis has been implemented in the populations of Braunvieh, Simmental, and Holstein cattle in Switzerland. A Weibull mixed sire-maternal grandsire survival model was used to estimate breeding values of sires with data on cows that calved since April 1, 1980. Data on Braunvieh and Simmental cows included about 1.1 million records, data on Holstein cows comprised about 220,000 records. Data contained approximately 20 to 24% right-censored records and 6 to 9% left-truncated records. Besides the random sire and maternal grandsire effects, the model included effects of herd-year-season, age at first calving, parity, stage of lactation, alpine pasturing (Braunvieh and Simmental), and relative milk yield and relative fat and protein percentage within herd to account for culling for production. Heritability of functional longevity, estimated on a subset of data including approximately 150,000 animals, were 0.181, 0.198, and 0.184 for Braunvieh, Simmental, and Holstein, respectively. Breeding values were estimated for all sires with at least six daughters or three granddaughters in the data. Breeding values of sires are expressed in months of functional productive life and published in sire catalogs along with breeding values for production traits.     

Short communication: Heritability of twinning rate in Holstein cattle

Multiple births or twinning in cattle is a naturally occurring reproductive phenomenon. For dairy cattle, twinning is considered a detrimental trait as it can be harmful to cow and calf as well as costly to the producer. The objective of this study was to examine recent US calving records for the Holstein breed to determine a current estimate of heritability for twinning rate along with effects of season and parity. Two models were used in this study: a linear sire model and a binary threshold-logit sire model. Both were mixed models considering fixed effects and random effects. Analyses were conducted using a restricted maximum likelihood method. Heritability estimates were 0.0192 ± 0.0009 and 0.1420 ± 0.0069 for the linear and threshold models, respectively. Repeatabilities from the linear and threshold-logit models were 0.0443 ± 0.0012 and 0.2310 ± 0.0072, respectively. The nonzero estimates of heritability indicate the potential to select against this trait for genetic improvement of Holstein cattle.     

A comparison of 4 predictive models of calving assistance and difficulty in dairy heifers and cows

The aim of this study was to build and compare predictive models of calving difficulty in dairy heifers and cows for the purpose of decision support and simulation modeling. Models to predict 3 levels of calving difficulty (unassisted, slight assistance, and considerable or veterinary assistance) were created using 4 machine learning techniques: multinomial regression, decision trees, random forests, and neural networks. The data used were sourced from 2,076 calving records in 10 Irish dairy herds. In total, 19.9 and 5.9% of calving events required slight assistance and considerable or veterinary assistance, respectively. Variables related to parity, genetics, BCS, breed, previous calving, and reproductive events and the calf were included in the analysis. Based on a stepwise regression modeling process, the variables included in the models were the dam's direct and maternal calving difficulty predicted transmitting abilities (PTA), BCS at calving, parity; calving assistance or difficulty at the previous calving; proportion of Holstein breed; sire breed; sire direct calving difficulty PTA; twinning; and 2-way interactions between calving BCS and previous calving difficulty and the direct calving difficulty PTA of dam and sire. The models were built using bootstrapping procedures on 70% of the data set. The held-back 30% of the data was used to evaluate the predictive performance of the models in terms of discrimination and calibration. The decision tree and random forest models omitted the effect of twinning and included only subsets of sire breeds. Only multinomial regression and neural networks explicitly included the modeled interactions. Calving BCS, calving difficulty PTA, and previous calving assistance ranked as highly important variables for all 4 models. The area under the receiver operating characteristic curve (ranging from 0.64 to 0.79) indicates that all of the models had good overall discriminatory power. The neural network and multinomial regression models performed best, correctly classifying 75% of calving cases and showing superior calibration, with an average error in predicted probability of 3.7 and 4.5%, respectively. The neural network and multinomial regression models developed are both suitable for use in decision-support and simulation modeling.     

Genetic evaluation of stillbirth in United States Holsteins using a sire-maternal grandsire threshold model

A sire-maternal grandsire threshold model was used for genetic evaluation of stillbirth in US Holsteins. Calving ease and stillbirth records for herds reporting at least 10 dead calves were extracted from the Animal Improvement Programs Laboratory database. About half of the 14 million calving ease records in the database had a known livability score, mostly from herds processed by Dairy Records Management Systems (Raleigh, NC). Calf livability scores of 2 and 3, representing calves born dead and calves that died within 48 h of parturition, respectively, were combined into a single category. The model included effects of herd-year, year-season, parity-sex, sire, birth year group of sire, maternal grandsire (MGS), and birth year group of MGS. Herd-year, sire, and MGS were random effects. Mean predicted transmitting abilities, expressed as the expected percentage of stillbirths, were 7.9 and 8.6 for direct and maternal stillbirths, respectively. Mean reliabilities for both the direct and maternal effects were 45%. Correlations among domestic and Interbull stillbirth solutions on the underlying scale for bulls with at least 90% reliability ranged from 0.63 to 0.90 across countries for direct stillbirths and from 0.69 to 0.96 for maternal stillbirths, indicating that results were generally consistent with those from other countries. There was no evidence of a genetic trend for either trait. More complete recording of stillbirth scores would improve reliabilities and could allow for evaluations of other breeds.     

Using an incomplete gamma function to quantify the effect of dystocia on the lactation performance of Holstein dairy cows in Iran

The aim of the present study was to estimate the effect of dystocia on lactation performance, using an incomplete gamma function. Data from March 2000 to April 2009 comprising 100,628 lactations of 65,421 cows in 204 dairy herds collected by the Animal Breeding Center of Iran were used. Of 100,628 births, 91.8% required no assistance, whereas 8.2% required assistance of some sort. Factors associated with the presence of dystocia were calving season, calving year, herd, calf sex, parity, and age of dam. Peak yield for primiparous cows with dystocia at calving occurred on d 87.2 [standard error (SE) 0.47], and for primiparous cows with easy calving, the peak of lactation was on d 83.3 (0.25). Peak yield was lowered by 0.39 (SE 0.07), 2.20 (SE 0.15), 2.22 (SE 0.21), and 2.54 (SE 0.32) kg for cows with incidence of dystocia compared with normal cows in parity 1 to 4, respectively. Dystocia was associated with decreased 305-d lactation performance in all parities, mostly in early lactation. Although more difficult births occurred in heifer calvings, loss in lactation performance was greater in second or later lactations following a difficult birth.