Fewer days until the first service after calving were associated with lesser fertility in all herd size groups. This is in agreement with a previous study in Holstein cows that showed that early postpartum service was associated with low conception rates [26] and [27]. Under normal conditions, ongoing uterine involution appears to contribute to lesser fertility for approximately 40 days postpartum [28]. In addition, cows on the studied farms nursed their calves until 3 months of age; suckling and/or lactation has been shown to inhibit estrous activity and ovulation in cows [29]. These findings, and our results, indicate that AI too soon after calving does not reduce the number of days open because there is high possibility of failure to conceive. In addition, the associations of the length of the CFSI with fertility differed among the herd size groups for cows who were first serviced at 0 to 50 and 51 to 94 days after calving. These differences are likely related to differences in feeding management or in estrus detection.