Reproductive performance of grazing dairy cows following presynchronization and resynchronization protocols

Objectives were to compare the effect of presynchronization and resynchronization methods on fertility responses of grazing dairy cows at first and second artificial insemination (AI) and pregnancy rate during the entire breeding season. Lactating dairy cows (n=1,263) in 2 seasonal grazing farms were blocked, within farm, by parity, breed and days in milk. Within each block, cows were randomly assigned to 1 of 4 treatments arranged as a 2×2 factorial with 2 presynchronization and 2 resynchronization treatments. Cows had their estrous cycles presynchronized with either a PGF2α-based program (Presynch) consisting of 2 injections of PGF2α administered 14 d apart and starting the timed AI protocol 11 d later, or with a PGF2α-GnRH-based presynchronization program (G6G) consisting of an injection of PGF2α, followed 3 d later by an injection of GnRH and starting the timed AI protocol 6 d later. All cows received the first insemination on the same day, which was considered study d 0 and also d 0 of the breeding season. All cows received the 5-d timed AI protocol that consisted of GnRH on d −8, PGF2α on d −3 and −2, and GnRH+timed AI on d 0. Blood was sampled and analyzed for progesterone on d −8. On d 12, cows in each presynchronization treatment either remained as untreated controls (RCON) or received a controlled internal drug-release (CIDR) insert containing progesterone for 7 d (RCIDR). Estrus was observed daily starting on d 19 and cows in estrus were inseminated on the same day. On d 35, bulls were placed with the cows for an additional 65 d, completing a 100-d breeding season. Holstein cows were less likely to have progesterone ≥1ng/mL on d −8, and had less expression of estrus and pregnancy per AI (P/AI), which resulted in a slower rate of pregnancy and a smaller proportion of pregnancy at the end of the study than did Jersey or crossbred cows. In addition, body condition, days in milk, and plasma progesterone concentration at the first GnRH injection of the timed AI protocol had marked effects on the reproductive performance of lactating grazing dairy cows. A greater proportion of G6G cows had progesterone ≥1ng/mL at the first GnRH injection of the timed AI protocol compared with Presynch cows (82.0 vs. 74.3%). Presynchronization treatment did not influence P/AI, but cows in G6G had increased risk of pregnancy loss between d 30 and 65 after the first AI (12.9 vs. 8.1%). Nevertheless, an interaction between presynchronization and ovarian status was observe