The objective of this research was to determine the effect of birth weight on perinatal mortality (PM) (alive or dead at 48 h of age) and dystocia (unassisted or assisted). Data were 4528 records of births between 1968 and 1999 from the Iowa State University research dairy farm in Ankeny. The incidence of PM was 7.1%; dystocia was 23.7%. A logistic regression model was used to predict both PM and dystocia. The PM model included effects of year of birth, season (summer or winter), dystocia, parity (first or later), birth weight (kg), ratio of calfs birth weight to dam's weight (%), and gestation length (d). Odds of PM increased by 2.1%/yr. Calves born in the winter have a 36% higher risk of PM than calves born in the summer. Difficult births tend to result in PM 2.7 times more often than unassisted births. First-parity cows have a 2.4 times higher risk of PM than cows in later parities. Probabilities of PM for birth weights of 29, 35, 40, 46, and 52 kg were 2.1, 2.5, 3.4, 5.1, and 9.6%, respectively, when other factors were set at their average value. Similarly, ratios of calf to cow weight of 4.5, 5.7, 6.9, 8.1, and 9.3% yield probabilities of PM at 8.2, 4.2, 3.1, 3.5, and 5.7%, respectively. Finally, gestation lengths of 268, 273, 279, 284, and 290 d yield probabilities of PM of 5.5, 3.9, 3.1, 3.1, and 3.6%, respectively. The dystocia model included effects of year of birth, season, sex of calf, PM, parity, birth weight, and pelvic area (externally measured). Odds for dystocia decreased by 4.7%/yr. Calves born in the winter have a 15% higher risk of dystocia than calves born in the summer. Odds of male calves needing assistance were 25% greater than female calves. If a calf died in the first 48 h, then it is 2.7 times more likely that the calf needed assistance. First-parity cows have a 4.7 times higher risk of dystocia than cows in later parities. Odds of dystocia increase by 13%/kg increase in birth weight. An 11% decrease in odds for dystocia is associated with a one square decimeter (dm2) increase in pelvic area.

Calving data of North American Holsteins were analyzed using a linear sire model and a threshold sire model to estimate fixed effects and sire predicted transmitting abilities (PTA) for twinning rate. The data were 1,324,678 births of 37,174 sires of cows from the National Association of Animal Breeders (NAAB) calving ease database. All twins were assumed to be dizygotic. Triplets were not included in the analysis. The twinning rate was 5.02%. The sire model included relationships among sires and fixed effects of herd-years, season, parity, and sire groups. Different estimates of heritability due to models and restrictions on data have been demonstrated. Heritability estimates for the sire of the cow effect were 2.10% by the linear model analysis and 8.71% by the threshold model analysis. Sire PTA for twinning rate ranged from 1.6 to 8.0%. Mean incidence of twins increased from first to fifth and later parities: 1.63, 5.22, 6.66, 7.19, and 7.19%, respectively. Some changes with season were also evident. The incidence of twins was highest in April to June, at 5.88%, and lowest in October to December, at 4.23%. Sire group effects showed that sires born after 1990 had a higher incidence of twins than sires born before 1990. The mean twinning rate for sires born before 1980 was 4.55% compared with a mean of 5.58% for sires born after 1991. Characteristics of individual sire evaluations are discussed. Ample opportunity exists to change the incidence of twins in the Holstein population. Sire selection can be used to reduce the incidence of twins and also the increased cost of production associated with twins. From a national perspective, a reduction in the incidence of twin births can result in a saving of as much as $55 million per year.

The objective of this research was to estimate genetic parameters for a multiple-trait evaluation of dystocia (DYS), perinatal mortality (PM), birth weight (BWT), and gestation length (GL) in Holsteins. The data included 5,712 calving records collected between 1968 and 2005 from the Iowa State University dairy breeding herd in Ankeny. The incidence of PM was 8.8% and that of DYS 28.8%; mean BWT was 40.5 kg, and GL was 279 d. A threshold-linear animal model included the effects of year, season, sex of calf, parity, sire group, direct genetic, maternal genetic, and maternal permanent environment. Direct heritabilities for DYS, PM, BWT, and GL were 0.11 (0.04), 0.13 (0.05), 0.26 (0.04), and 0.51 (0.05), respectively. Maternal heritabilities were 0.14 (0.04), 0.15 (0.03), 0.08 (0.01), and 0.08 (0.02), for DYS, PM, BWT, and GL, respectively. The heritabilities are the posterior means of the Gibbs samples with their standard deviations in parentheses. The direct genetic correlation between PM and DYS was estimated at 0.67 (0.19), whereas the maternal genetic correlation was 0.45 (0.16). Direct and maternal PM and DYS are partially controlled by the same genes. Selection on only calving ease is not sufficient to control PM. With moderate genetic correlations between all 4 traits, BWT and GL should be included with DYS and PM in an evaluation of calving performance.

First and foremost I would like to thank God for forgiveness through His son Jesus Christ. I also want to thank my family. Your high expectations have always pushed me to succeed. I would also like to thank Dr. Jeff Berger for his guidance, time and effort he put in with me. Thank you to my committee members Dr. Jim Cornette and Dr. Jonathan Smith. Thank you to Dr. Luc Janss for explaining the intricate points of the threshold model. Thank you to Dr. Brian Kirkpatrick for his input and patience. Last but not least, I want to thank my future wife. Though I do not know you yet, I am praying that the Lord prepare us for one another. I look forward to the day we will be together.

James Johanson, former graduate research assistant; Jeff Berger, professor of animal science Summary and Implications A multiple trait animal model was implemented to study the direct genetic and maternal genetic relationship among four traits, dystocia (DYS), perinatal mortality (PM), birth weight (BW) and gestation length(GL), expressed at the birth of a calf. The sign and magnitude of genetic correlations among calving traits demonstrates the need to use multi-trait animal models for genetic evaluation of animals for calving performance. Adequate information exists to begin developing a calving performance index. This index will enable breeders of dairy cattle to optimize the health and well-being of replacement animals and reduce the incidence of dystocia and perinatal mortality.