Introduction

1. Results (cont.) Results (cont.) Introduction Table 3. Correlations of AFC and the traits included in the lifetime net merit index* Heifer rearing costs account for 15 to 20% of the total expense of milk production, and the decline in cow fertility over the last 40 years is well-documented. Genetic selection for younger age at first calving (AFC) may improve profitability and fertility. Table 2. Summary statistics of PTA AFC for active bulls Data • First calvings from January 1, 1997 to present for Brown Swiss (BS), Holstein (HO), and Jersey (JE) • Calving ages between 18 months and 36 months • Sires had to be ≥ 60 months (5 years) old • The final dataset included 40,932 calvings for BS, 8,234,537 HO calvings, and 521,806 JE calvings • Average phenotypic AFC ranged from 735 d for JE to 826 d for BS (Table 1) • Heritability was 0.03 for HO and JE and 0.06 for BS • Average PTA AFC for active bulls ranged from -1.39 for JE to -0.29 for BS (Table 2) • Genetic trend was estimated by regression of sire PTA AFC on birth year, and was slightly negative for all breeds (Figure 1) • Correlations of AFC with net merit were -0.33 for HO, -0.40 for JE, and + 0.09 for BS (Table 3) Model Animal model used to predict variances and PTA: y = hys + a + e y = age at first calving hys = fixed effect of herd-year-season of birth a = random additive genetic effect e = random residual error *Correlations among PTA for bulls with reliabilities of AFC ≥ 0.90 (BS n=15, HO n=1,557 and JE n=126). Figure 1. Sire genetic trend for AFC Conclusions • Genetic differences exist among bulls for AFC • The heritability of AFC is similar to that of conception rate and daughter pregnancy rate • Increasing AFC results in lower yield, lower longevity, and lower lifetime profit • Routine evaluation of AFC is desirable because it provides dairy producers with an additional tool for managing reproduction in their herds Results Table 1. Summary of phenotypic AFC for cows