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* Animal Improvement Programs Laboratory,
Agricultural Research Service, USDA, Beltsville, MD 20705-2350 Dairy Records
Management Systems, Raleigh, NC 27603
1 Corresponding author: millerrh{at}juno.com
Voluntary waiting period and adoption of synchronized
breeding (ovulation synchronization followed by timed artificial
insemination) were characterized from 33 million services of
Holsteins and Jerseys in Dairy Herd Improvement herds. Calving month,
calving year, and parity had large effects on days to first
service for both breeds. Holstein cows that calved during March
and April were bred later than those that calved during other
months (February and March for Jerseys), whereas cows that calved
during September and October were bred earlier. First-parity cows
had longer days to first service than did second-parity cows.
Herd-year voluntary waiting period was measured as the days
postpartum by which 10% of cows had received a first insemination.
Median days to reach 10% of cows bred were 55.5 d. Over 65% of
herds had 10% of cows inseminated by 60 d postpartum, the
voluntary waiting period assumed for national evaluations for
daughter pregnancy rate. Herd-years with synchronized breeding at
first insemination were identified through 
2 analysis based on
deviation of observed frequency of first inseminations by day of the
week from an expected equal frequency and by the maximum
percentage of cows inseminated on a particular day of the week. Herds
that were identified as having synchronized breeding had fewer
days to first service (17.0), more services (0.16/cow), and
fewer days open (9.1) than did herds that were classified as
having traditional estrus detection. Synchronized herds also had
a standard deviation for days to first service that was only 38%
as large as that for herds that bred on observed estrus. Adoption
of synchronized breeding for first services steadily increased
from 1.9% of herd-years (2% of cows) for 1996 to 19.9% of herd-years
(34.9% of cows) for 2005. Procedures for genetic evaluation of
daughter pregnancy rate should be examined to determine if herd
regimen for reproductive management affects results.
Key Words: voluntary waiting period • days to first service • synchronization • timed artificial insemination
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