Age at First Calving in U.S. Dairy CattleJ. B. Cole and D. J. Null*

Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350Abstr. W28 2010

-3-2-1012345

1980 1985 1990 1995 2000 2005Birth Year

PTA

AFC

BS

HO

JE

Introduction

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

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

Model

Breed n Mean SD Min Max

BS 40,932 826 92 549 1095

HO 8,234,537 777 85 540 1095

JE 521,806 735 86 540 1095

Breed n Mean SD Min Max

BS 47 -0.29 4.53 -10.49 9.28

HO 737 -1.07 3.02 -9.64 16.22JE 135 -1.39 2.60 -9.17 5.85

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 (cont.)

•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)

Trait BS HO JEMilk 0.22 -0.39 -0.40Fat 0.49 -0.32 -0.40Protein 0.12 -0.43 -0.53Somatic Cell Score 0.05 -0.10 -0.21Daughter Pregnancy Rate -0.16 -0.13 -0.18Productive Life 0.00 -0.06 -0.14Cow Conception Rate -0.31 -0.06 0.13Heifer Conception Rate - -0.33 -0.20Maternal Calving Ease 0.05 0.22 -Sire Calving Ease -0.18 0.08 -Maternal Stillbirth - 0.15 -Sire Stillbirth - 0.04 -Feet and Leg Composite 0.21 -0.16 -0.09Size Composite -0.29 -0.19 -0.29Udder Composite -0.43 -0.16 -0.14Net Merit 0.09 -0.33 -0.40

Table 3. Correlations of AFC and the traits included in the lifetime net merit index*

Table 1. Summary of phenotypic AFC for cows

Figure 1. Sire genetic trend for AFC

Table 2. Summary statistics of PTA AFC for active bulls

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.

*Correlations among PTA for bulls with reliabilities of AFC ≥ 0.90 (BS n=15, HO n=1,557 and JE n=126).

Results (cont.)

Results