Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2017
Publication Date: 8/1/2017
Citation: Hutchison, J.L., Van Raden, P.M., Null, D.J., Cole, J.B., Bickhart, D.M. 2017. Genomic evaluation of age at first calving. Journal of Dairy Science. 100(8):6853-6861. https://doi.org/10.3168/jds.2016-12060.
Interpretive Summary: In order to minimize management costs and to produce animals that are profitable earlier in their life, selection for cows that have a younger age at first calving could improve dairy cattle efficiency. Since age at first calving is a phenotypic trait recorded on US dairy farms, we investigated the feasibility of using this data in genomic selection. Favorable correlations with fertility and lifetime production traits, in addition to high sire reliabilities suggest that age at first calving could be implemented as a new trait in genomic selection indices.
Technical Abstract: From their time of birth until their first lactation, dairy heifers incur management, health and feed expenses while not producing milk. Much effort has been made to estimate optimal ages of first calving (AFC) for cows to reduce these costs and ensure that animals are productive earlier in life. In order to estimate the optimal AFC for three dairy cattle breeds (Holstein, Jersey and Brown Swiss), we retrieved phenotypic records for over 14 million cows calving between 1997 and 2015 from the US national dairy database. The mean AFC for Holstein and Jersey has decreased by 2.4 and 2.7 months, respectively, since 2006. When comparing the effects of AFC on production and fertility traits, we found positive effects correlated with decreasing AFC for all but the earliest AFC group (18 to 20 months). We also identified an unfavorable correlation of lower AFC with increasing stillbirth rates in Holstein (0.047 least square mean (LSM) compared to a baseline of 24 months) and Brown Swiss (0.062 LSM). Finally, we identified favorable genetic correlations of lower AFC with NM$, HCR, CCR and DPR in Holstein and Jersey cattle, and favorable correlations for NM$ and HCR in Brown Swiss. In order to maximize lifetime production and reduce the effects of AFC on stillbirth, the optimal AFCs for Holstein and Brown Swiss are 21 to 22 months, and for Jersey it is 20 to 21 months. The majority of Brown Swiss and Holstein calvings are 5 and 2 months later than that current optimum, respectively. Calculated genomic PTA values for AFC showed an improvement of 20 percentage points in genomic young bulls compared to parent averages in Holstein, suggesting that genomic testing can improve selection for this trait.