PS2-2. Effect of Single or Double PGF2α Dose at Device Removal on Estrus Expression and Pregnancy Rate in Angus Cows.

Abstract The objective of this study was to compare the effects of a single versus double PGF2α dose administered at device removal on estrus expression rate (EER) and pregnancy rate per timed artificial insemination (P/TAI) in Angus cows. A total of 695 multiparous, suckled cows (45 – 90 d postpartum) from four Angus herds were synchronized with a 6-day GnRH/P4 protocol for TAI. On Day -9, body condition score (BCS; 1-5 scale) was recorded, and cows received 100 μg GnRH analogue and an intravaginal P4 device (DIB 0.5, Zoetis). On Day -3, the P4 device was removed and eCG (400 IU, Novormon, Zoetis) was administered. At that time, cows were randomly assigned to receive one PGF2α dose (Single, n = 351; 500 μg sodium cloprostenol, Ciclase, Zoetis) or two simultaneous PGF2α doses (Double, n = 344; 1000 μg sodium cloprostenol, Ciclase, Zoetis). Tail paint (Rust-Oleum, Rust-Oleum Corporation) was applied to detect estrus expression. On Day 0 (72-74 h post-device removal), cows received 100 μg GnRH i.m. concurrently with TAI. Cows were inseminated using frozen-thawed semen from commercially available Angus sires. A single technician performed all insemination within each herd, using two proven-fertility sires per herd. Tail paint was evaluated on Day 0 and estrus was defined as ≥ 30% paint removal. Pregnancy was diagnosed on Day 35 by transrectal ultrasonography (Easy-Scan Go, IMV imaging). Data were analyzed using SAS. The effects of treatment group, herd, and BCS on EER and P/TAI were analyzed using logistic regression models (Proc Logistic, SAS) with backward elimination (P ≥ 0.1). The P/TAI model included treatment, herd, BCS, sire, and estrus expression. Estrus expression rate was greater in cows receiving the double PGF2α dose than the single dose (76.3 vs. 66.7%; P < 0.01) and was also affected by herd and BCS (P ≤ 0.03). Pregnancy per TAI was affected by estrus expression and herd (P < 0.01) but did not differ between treatment groups (Single=53.6%; Double=58.1%; P = 0.66). In conclusion, administering a double dose of PGF2α at the end of a 6-day GnRH/P4 protocol increased EER compared with a single dose. However, P/TAI was not affected by the PGF2α dose in the present study.