Synchronization of estrus, conception rate, and embryonic mortality in beef cattle following treatment with progesterone-releasing intravaginal devices or melengestrol acetate in conjunction with PGF₂α

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 F₂α (PGF) on the final day of treatment. Real time, B-mode, ultrasound 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 MGA-treated 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.