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Title:   Author  item
                                            Cole, Johnitem
                                            Hutchison, Janaitem
                                            Bickhart, Derekitem
                                            Null, Daniel
  Submitted to: Journal of Dairy Science
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 2/23/2013
                                    Publication  Date: 7/8/2013
                                    Citation: Cole, J.B., Hutchison, J.L., Bickhart, D.M., Null, D.J. 2013. Optimal age at first calving for U.S. dairy cattle. Journal of Dairy Science. 96(E-Suppl. 1):289–290 (abstr. W188).Interpretive Summary:
                                Technical Abstract: Heifer rearing is a major expense for the US dairy industry that accounts for 15 to 20% of the total cost of producing milk. Selecting for an optimal age at first calving (AFC) in US dairy cattle could reduce these costs while still providing animals with high lifetime yields. Records from 9,502,802 Holstein (HO), 611,939 Jersey (JE), and 46,580 Brown Swiss (BS) cows with first calvings from January 1, 1997 to present were selected for analysis. A linear model included the fixed effects of herd-year of calving, year-state-month of calving, and AFC, grouped into 50-d intervals. The traits analyzed included, milk yield, composition, and persistency, fertility, and longevity. The highest first-lactation milk yield occurred at an AFC between 651 to 700 d (21 to 23 mo) for HO, resulting in an increase in milk production of 58 kg compared to animals with an AFC between 701 to 750 d (23 to 25 mo). Similarly, fat, protein, and persistency of milk in HO showed an increase of 1.86 kg, 2.08 kg, and 0.05, respectively, for the same AFC interval. For JE, the highest yields occurred at 601 to 650 d (20 to 21 mo), with increases of 129 kg, 5.18 kg, 4.45 kg, and 0.12 for milk, fat, protein, and persistency of milk, respectively. The greatest yields for BS occurred between 701 to 750 d, although they did not differ from the yields of the 651 to 700 d or 751 to 800 d groups. Heifers from all breeds calving between 601 to 750 d showed an increase in heifer and cow conception rates compared to those calving later. Lifetime traits (up to 5 parities) showed the same optimal HO AFC, with differences between 651 to 700 d and 701 to 750 d of +589 kg, +21.14 kg, +18.23 kg, +11 d, and +0.90 d for milk, fat, protein, days in milk, and days open. For JE, differences between 601 to 650 d and 701 to 750 d were +1397 kg, +64.21 kg, +50.03 kg, +33 d, and +0.72 d, respectively. These results show that the optimal AFC is actually 75 to 130 d less than the breed averages published in 2010, suggesting that a reduction in AFC may improve profitability and fertility in HO and JE cattle, contrary to current age adjustment factors.


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