The objective of these experiments was to determine the
effects of and/or interactions among estrous synchronization
treatments, reproductive status, and stage of the estrous
cycle on estrous response (ER), first service conception
rates (CR), pregnancy rates (PR), and embryonic mortality
(EM) after 25 d of gestation. Angus or Angus crossbred
cattle (n=391) at two locations were assigned to receive
either melengestrol acetate for 7 d (MGA-PGF; .5 mg/hd/d,
n=136) or progesterone releasing intravaginal device for 7 d
(PRID-PGF; n=139) or to serve as untreated controls (n=116).
All animals in MGA and PRID treated groups coincidentally
received 25 mg prostaglandin F2a (PGF) on the final day of
treatment. Real time, B-mode, ul trasound with a 7.5 mHz
linear-array transducer was used to conduct three ovarian
scans at 7-d intervals beginning 7 d prior to initiation of
treatment. Jugular blood samples were collected at each
scanning period. Serum was harvested and stored at 4°C
until radioimmunoassayed for progesterone (P4). Serum P4
levels in conjunction with ovarian scans were used to
determine cycling status and stage of the estrous cycle at
initiation of treatment. Cattle treated with PRID-PGF
exhibited a greater synchronized ER (P < .06) than MGAtreated
cattle. Cycling animals had a greater ER than noncycling
animals, regardless of treatment (P < .01).
Anestrous postpartum cows and prepubertal heifers treated
with PRID-PGF exhibited a greater ER (P < .05) within 7 d
than either MGA-treated or untreated control animals.
Conception rates of cattle treated with PRID-PGF beginning
late (> Day 16) in the estrous cycle were improved over
those of MGA-treated cattle (P < .13) at the same stage.
Pregnancy rate at 21 d was higher in PRID-treated cattle
than untreated controls (P < .01). Ultrasound scans
for embryonic viability were conducted at 25, 45, and 65 d
of gestation. Calving data was collected to characterize EM
between 65 d and term. The majority of embryonic loss
occurring after 25 d of pregnancy occurred before 45 d.
Synchronization treatment had no effect on the extent of EM
occurring after 25 d of gestation. Embryonic mortality
occurring between d 45 and 65 (2%) and between d 65 and term
(3%) when combined were similar in magnitude to EM occurring
between 25 and 45 d of gestation (4.8%). In conclusion,
PRID for 7 d combined with PGF was a superior
synchronization treatment for the mixed group of cyclic and
anestrous cattle.
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