Genetic Parameter Estimates and Breed Effects for Calving Difficulty and Birth Weight in a Multi-Breed Population

There are multiple breeds of beef cattle available to utilize in breeding systems to maximize production and economics. Calving difficulty (dystocia) is a significant cost to beef production and is more prevalent in first-calf heifers. The objectives of this study were to estimate genetic parameters and breed differences for calving difficulty and birth weight as a first step towards the development of across-breed adjustment factors for calving difficulty. Two models were employed to analyze birth weight (BWT) and calving difficulty (CD) recorded on 4,579 first parity females from the Germplasm Evaluation program at the United States Meat Animal Research Center (USMARC). Both bivariate animal models fit CD and BWT either using CD scores based on USMARC scoring system (Model 1) or CD assigned as Z scores based on the midpoint incidence rate (Model 2). Heritability estimates (SE) for BWT direct, CD direct, BWT maternal and CD maternal for model 1 were 0.35 (0.10), 0.29 (0.10), 0.15 (0.08), and 0.14 (0.08), respectively. Heritability estimates for BWT direct, CD direct, and BWT maternal for model 2 were similar to model 1. The estimate of CD maternal from model 2 was 0.13 (0.08). Genetic correlation estimates were positive between BWT direct and CD direct (0.63 ±0.17; model 1) and (0.64±0.17; model 2). All other genetic correlations were not significant. Bos Indicus influenced breeds tended to have the largest estimates of BWT direct. Calving difficulty direct breed effects ranged from -1.01 to 7.50 for model 1 and from -1.06 to 7.36 for model 2. Calving difficulty maternal breed effects ranged from -1.55 to 5.27 for model 1 and from -1.55 to 5.25 for model 2. Diverse biological types of cattle have different effects on both birth weight and calving difficulty. These differences can be used to match breeds to complement needs of production systems. Across-breed adjustment factors are needed for producers to accurately make selection decisions between sires of different breeds to improve calving difficulty. Advisor: Matthew L. Spangler