Heritability estimates of calving interval are obtained using fielddata,whichare subject to interference from the breeder. For example, measures of calving interval frequently omit dams culled for low production or reproductive problems, a strategy that can result in reduction of additive genetic variance. Additionally, this low heritability implies that variations in these traits are, to a large extent, influencedbyenvironmentalfactorssuch as herd management policies. New studies with censored data can overcome this problem