Holstein cows milked twice daily were
assigned to be inseminated at their first
detected estrus (control) after 42 days in
milk or received PGFza (PG) after 42
days, if they had a high milk progesterone
(P4) test on any of 3 consecutive Mondays
until first inseminated. Milk P4 tests and
injections of PG were given on Mondays,
and most of the breeding occurred on
Thursdays and Fridays. The proportion of
cows inseminated within 21 days of the
beginning of the breeding period was greater
in the milk P4 + PG group (52.8%)
than in the control (38.9%). Compared to
controls, use of PG reduced days to first
service by 12.2 ± 3.1 d, calving intervals
by 23.3 ± 8.9 d, rate of reproductive
culling, and cost per pregnancy. We concluded
that using PG as a management tool
in an AI program is warranted and cost
effective. However, the milk P4 test would
not be justifiable unless its cost were significantly
lower than the cost of a weekly
injection of PG.
