Evaluation of two beef cow fixed-time AI protocols that utilize presynchronization

Presynchronization was evaluated as a method to improve estrus response before fixed-time AI (FTAI). The objective was to compare FTAI results in beef cows from two different presynchronization approaches. Blood samples were collected on Day-14 (Day 0 = CIDR removal) to determine progesterone concentration (>= 1 ng/ mL = high, <1 ng/mL = low). In a subset (n = 1289), an additional blood sample was collected between Day -21 and-29 to determine cyclicity (if both the Day-14 and Day-21 to-29 samples were classified as low progesterone cows were classified as noncycling). Cows (n = 1388) from 30 herds were grouped by days postpartum (DPP) and age, and randomly assigned to either of two protocols. Cows assigned to the PG 6-day CIDR & FTAI protocol (PG6d) received prostaglandin F2 alpha (PG) on Day-9, CIDR insertion and GnRH on Day -6, and CIDR removal and PG on Day 0. Cows assigned to the 7&7 Synch protocol (7&7) were administered PG and CIDR insertion on Day-14, GnRH on Day-7, and CIDR removal and PG on Day 0. For both protocols, FTAI occurred concurrently with GnRH 66 h after second PG. Pregnancy was determined by transrectal ultrasonog-raphy 30-40 d after FTAI. The GLIMMIX procedure of SAS was used to detect differences in estrus response and pregnancy success with herd as a random variable. Estrus response (0-66 h) was analyzed with two models, one included cyclicity and another replaced cyclicity with progesterone concentration at Day-14. In both models, cows assigned to the 7&7 protocol had greater (P < 0.01) estrus response than cows assigned to the PG6d protocol. The model including cyclicity, estrus response was impacted by the cyclicity by DPP interaction (P = 0.03), cyclicity by protocol interaction (P = 0.04), and the tendency of BCS by protocol interaction (P = 0.08). In the estrus response model that included progesterone concentration at Day-14, significant variables included the protocol by progesterone concentration at Day-14 (P = 0.01), and BCS (P < 0.01), while DPP (P = 0.08) and progesterone concentration at Day-14 (P = 0.07) were tendencies. Pregnancy success was influenced by estrual status (P < 0.01), body condition score (P = 0.04), and cycling status (P = 0.02), but was not influenced by protocol (P = 0.75; PG6d = 38 +/- 5% and 7&7 = 37 +/- 5%). In conclusion, effectiveness of presynchronization method depended on a cows' physiological status, and the 7&7 protocol increased estrus response compared with PG6d, but there was no difference in pregnancy success.