LAUSR

To determine the effects of two presynchronization strategies in conjunction with delayed fixed-time artificial insemination (TAI) on pregnancy rates to TAI (PR/AI), 1700 Angus beef heifers at three locations in South Dakota were enrolled in a completely randomized design with a 2 by 2 factorial arrangement of treatments. Within location, all heifers were randomly assigned to one of four treatments: 1) PG54 (n = 434), heifers were administered prostaglandin F2α (PGF; 25 mg im) 7 d prior [Day -14] to the initiation of the 7-d CO-Synch + controlled internal drug releasing (CIDR) protocol wherein they received gonadotropin-releasing hormone (GnRH; 100 μg im) and a CIDR insert on Day -7, PGF at CIDR removal on Day 0, and a second injection of GnRH concurrently with TAI 54 ± 2 h later; 2) PG72 (n = 426), heifers were exposed to the same treatment as PG54, however, TAI was performed 72 ± 2 h after CIDR removal; 3) PG-CIDR54 (n = 422), same as PG54 but heifers received a CIDR insert on Day -14 rather than Day -7, in addition to PGF administration; 4) PG-CIDR72 (n = 418), same as PG-CIDR54, however, TAI was performed 72 ± 2 h after CIDR removal. Estrus detection patches were applied to all heifers on Day 0 and were evaluated for activation at TAI. Pregnancy was diagnosed via transrectal ultrasonography between 30 and 47 d after TAI. The percentage of heifers exhibiting estrus between Day 0 and TAI was greater (P < 0.01) in the PG72, PG-CIDR54, and PG-CIDR72 treatments compared to the PG54 treatment (78.11, 86.59, and 91.09 vs. 31.05%, respectively). Furthermore, estrus response was greater (P < 0.01) in PG-CIDR72 heifers when compared to PG72. Pregnancy rates to TAI differed among treatments and were greater (P < 0.05) in the PG72 and PG-CIDR54 treatments when compared to PG-CIDR72 (48.8 and 50.4 vs. 38.4%, respectively), and were greater (P = 0.03) in PG-CIDR54 vs. PG54 (43.1%). Moreover, a tendency (P = 0.10) was determined on PR/AI between PG54 and PG72. In conclusion, presynchronization strategies and prolonged exposure to exogenous progesterone have the potential to alter estrus expression and improve fertility in replacement beef heifers.