The objective of this study was to examine
changes in ovarian steroids in relation to the
onset of first standing estrus detected by
pressure-sensitive rump-mounted devices (ABS
HeatWatch® [HW]). Twenty Holstein heifers
were treated with PGF on days 5 to 8 (early) 2a
or 12 to 15 (late) of the estrous cycle to induce
estrus. The following traits were different
(P<.05) in early vs late heifers: serum progesterone
before PGF (2.3 vs 5 ± .3 ng/ml); 2a
interval from PGF to estrus (HW devices: 42 2a
vs 52.4 ± 2.8 hr or visual: 45 vs 57 ± 2.5 hr);
interval (40.6 vs 57.8 ± 2.2 hr) from PGF to 2a
estradiol-17b (E2) peak concentrations in
blood serum (17.7 vs 12.3 ± 1 pg/ml) at estrus;
duration of estrous cycle after treatment injection
of PGF (20 vs 21.4 ± .2 days); interval 2a
from onset of estrus to the peak in E2 (1.4 vs
–5.4 ± 1.8 hr); and E2 peak associated with the
first dominant ovarian follicle of the estrous
cycle following the treatment injection of
PGF (2.2 vs 5.2 ± .6 pg/ml), respectively. 2a
Durations of estrus and standing events were
similar regardless of when the corpus luteum
was regressed during either the early or late
luteal phase. When all sexual behavior was
evaluated after 41 injections of PGF , the 2a
number of standing events (28.1 vs 16.1 ± 3.1)
was greater (P<.05) in heifers after early-cycle
regression of the corpus luteum. Results indicated
that the number of standing events per
estrus and concentrations of E2 during estrus
were related to the duration of progesterone
exposure before a PGF -induced luteolysis. 2a
The first standing event of estrus detected by
the HW device corresponded closely to the
peak in E2 and LH at the onset of estrus.
