Efficacy of a Second Injection of Prostaglandin F2α in Yearling Beef Heifers Following Previous Estrus Synchronization

Angus-based, yearling beef heifers (n = 1,858, 322 kg) managed at Rex Ranch, Ashby, NE, were utilized to determine the effects of administering a second prostaglandin F_2α (PGF; Lutalyse, Zoetis Animal Health, Parsippany, NJ) injection to heifers who did not previously respond to estrus synchronization. All heifers were exposed to a melengestrol-acetate (MGA) – PGF protocol. Heifers were offered 0.5 mg/d MGA for 14 d. On d 32, fertile bulls were placed with heifers for 24 h. On d 33, bulls were removed and all heifers received an injection of PGF and an estrus detection patch was applied. Following PGF injection, heifers were observed for estrus for 3 d and AI 12 h after detection of estrus (n =1,527). Heifers were considered in estrus when greater than 50% of the rub-off coating was removed from the patch. Heifers who did not show signs of estrus (n = 331) were placed in a separate pasture with fertile bulls at a 1:33 bull to heifer ratio. After 3 d with bulls, heifers with greater than 50% of the rub-off coating removed from the patch (n =151) were considered to have been bred and were removed and placed with the previously bred heifers. The remaining heifers, who did not show estrus, were randomly assigned to receive either a second PGF injection (n = 90; SPG) or no injection (n = 90; CON) and remained with bulls for 4 d. Following bull removal, SPG and CON heifers with greater than 50% rub-off coating removed were considered in estrus and returned to the herd. Pregnancy diagnosis was conducted via transrectal ultrasonography 47 d after SPG and CON were returned to the herd. Percentage of heifers expressing estrus was greater (P < 0.01) for SPG treatment (60% vs. 23% ± 13%, SPG [n=53] vs. CON [n=21]). However, pregnancy rate was similar (P = 0.38) between treatments (34% vs. 52% ± 11%, SPG [n=18] vs. CON [n=11]). In summary, administration of a second PGF injection to yearling beef heifers that didn’t respond to an MGA-PGF protocol did not improve pregnancy rates.