Calving difficulty is a lowly heritable trait, and most differences between calvings are due to environmental causes rather than the service sire involved. Consequently, bull proofs for calving difficulty must be based on many calvings to be reliable. Table 1 contains sample NAAB calving ease summary information for some Select Sires bulls in July 1985. Three columns of information are presented. Percent probability of being easier than breed average combines information on expected percent of difficult first births and the amount of information available.
Dairy farmers make the judgment on what difficult is when they score each birth on their farm.
Sample NAAB calving ease proofs for some Select Sires bulls in July 1985. Table 2 shows the distribution of such proofs for active AI Holsteins in January 1985. Just over half the AI Holstein sires had no dystocia proof in January 1985. Genetically speaking, restricting sires for use on heifers to those with dystocia proofs has a cost. Distribution of percent difficult first births on active AI Holsteins in January 1985.