Accounting for voluntary waiting period changes in US dairy herds: Adjusting daughter pregnancy rate and introducing first-service to conception.
The genetic evaluation of female fertility traits in dairy cattle in the United States has progressed over the past 2 decades, with 4 additional traits integrated into the national evaluation system since the introduction of daughter pregnancy rate (DPR) in 2004. However, concerns have arisen in the dairy sector, with reports of producers extending the voluntary waiting period (VWP), which is the time of initial breeding after calving, due to more persistent lactation yields. Since its inception, DPR calculations in the US evaluation have assumed a fixed 50-d VWP, which may not reflect modern reproductive strategies. Furthermore, producers may determine that some of their cows should have their VWP extended while others may follow the standard time. A re-evaluation of the current female fertility evaluation is necessary to ensure traits align with current management practices. Therefore, this study explores the addition of a potential new trait, First Service to Conception (FSC), along with a revised DPR formula that permits more flexibility with VWP. About 32 million records from the National Cooperator Database, covering 5 major dairy breeds (2003-2023), were used for this study. Data for cows calving before 2003 were unavailable because insemination records were not recorded prior. The findings suggest FSC enhances female fertility evaluations, providing a more comprehensive reproductive assessment independent of VWP. Another approach was an adjustment to the DPR calculation (DPRadj) to account for herd VWP on a herd-year and lactation group basis. The mean DPR value was 45.06%, while DPRadj increased the mean value to 52.58%. The mean FSC was 62.64 d. In an analysis using first lactation cows for cow traits and heifers for heifer traits, genetic correlations of FSC with other fertility traits were 0.97 with Cow Conception Rate (CCR) and DPR, 0.98 with DPRadj, 0.40 with Heifer Conception Rate (HCR), and 0.32 with Early First Calving (EFC). Predicted transmitting abilities (PTA) were calculated to assess the implications of adding FSC to, or replacing DPR with DPRadj within the CDCB multi-trait evaluation index, Net Merit $ (NM$). The highest correlation among the top 10% of bulls occurred when replacing DPR with DPRadj (0.99). Adding FSC to the original model reduced the latter correlation to 0.88, which indicates slight re-ranking of top bulls. Among animals with PTA reliability above 50%, the lowest correlation was between the original index and the index replacing DPR with FSC (0.95). This work advances genetic evaluation in dairy cattle, improving reproductive efficiency and productivity.