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Title:   Author  item
                                            Cole, Johnitem
                                            Null, Daniel
  Submitted to: Journal of Dairy Science
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 3/9/2010
                                    Publication  Date: 6/24/2010
                                    Citation: Cole, J.B., Null, D.J. 2010. Age at First Calving in Holstein Cattle in the United States. Journal of Dairy Science. 93(E-Suppl. 1):594(abstr. W28).Interpretive Summary:
                                Technical Abstract: Heifer rearing costs account for 15 to 20% of the total expense of milk production, and the decline in fertility of U.S. Holsteins is well documented. Earlier age at first calving (AFC) may improve profitability and fertility. Records for 400,000 U.S Holstein cows born on or after January 1, 1997 were selected by random sampling of herd codes and used to estimate (co)variance components and breeding values. Phenotypic AFC averaged 788 ± 89 d, and ranged from 540 to 1095 d. The model included random animal and residual effects, and a fixed herd-year-season (HYS) of birth effect. Herd-year-season of birth was included to avoid confounding between sires and period-of-birth, and groups were required to include at least 10 observations. The four seasons were defined as: December to February, March to May, June to August, and September to November. Heritability averaged 0.027 ± 0.003 across the six datasets, which is lower than some literature estimates, and consistent with earlier, unpublished studies on U.S. Holsteins. Predicted transmitting abilities for AFC of active bulls ranged from -13 to +14 d, and averaged -1.9 ± 3.6. Correlations were calculated among sire PTA for bulls with reliabilities of lifetime net merit (NM$) of at least 90%. Age at first calving had favorable (negative) correlations with milk (-0.22), fat (-0.18), and protein yield (-0.23), SCS (-0.05), productive life (-0.01), NM$ (-0.18), heifer conception rate (HCR; -0.18), and persistency of milk (-0.12), fat (-0.13), and SCS (-0.02). Unfavorable (positive) correlations were found with cow conception rate (0.04) and protein persistency (0.04). Daughter pregnancy rate (DPR) was uncorrelated with AFC (0.001; P > 0.05). Excessive AFC has a negative effect on yield and lifetime profitability, which is reflected in these correlations. Genetic trend was estimated by regression of sire PTA for AFC on birth year, and was slightly negative, decreasing by -0.09 d per year (P < 0.01). Routine genetic evaluation of AFC is desirable because it provides dairy producers with an additional tool for managing reproduction in their herds.


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