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Results (cont.). Results (cont.). Introduction. Table 3. Correlations of AFC and the traits included in the lifetime net merit index*.

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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