Three experiments were conducted to determine if a larger dose of prostaglandin F2α
(PG) administered on day 6 of a 5-day Ovsynch timed artificial insemination (AI)
program would induce regression of the corpus luteum to facilitate AI and pregnancy
outcomes similar to a traditional 5-day program with two doses of PG. When applying
a 5-day program, cows that ovulate in response to the first GnRH injection have a
new corpus luteum (CL) that is 2 days younger when PG is administered in a 5- versus
7-day program. To regress successfully the younger CL, a second injection of PG must
be given 24 hours after the first PG injection to prevent reduced pregnancy rate after
the timed AI. These experiments demonstrated that administering 50 mg PG (10 mL
Lutalyse) on day 6 produced luteolysis as efficiently as 25 mg PG (5 mL Lutalyse) administered
on days 5 and 6 when the cut point for progesterone was 1 ng/mL 72 hours
after the first PG injection or 48 hours after the larger PG dose. In contrast, when the
cut point was 0.5 ng/mL, the larger dose of PG was less effective. Pregnancy outcomes
in cows did not differ between treatment doses except in one herd (Exp. 3). Although
pregnancy outcomes were reduced only in one herd with the larger PG dose, this difference
may be confounded with the earlier injection of the second GnRH injection 16
hours before timed AI, rather than failure of luteolysis in response to the larger dose of
PG. Delaying the timing of AI, injection of the second GnRH, or both may be warranted
to allow sufficient time for progesterone to decrease to basal concentrations in
response to a larger dose of PG on day 6 to prevent a reduction in fertility.
