Synchronization of estrus with PGF 2α administered 18 days after a progesterone treatment in lactating dairy cows

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In a previous study we showed that estrus synchronization with 2 treatments of PGF 2α 13 d apart reduced conception rate at the synchronized estrus and that this reduction occurred mainly in cows in the early luteal phase at the second PGF 2α treatment. The objective of the present study was to determine the efficacy of a synchronization regimen in which PGF 2α was administered during the mid- to late-luteal phase to cows that had previously been synchronized with progesterone. Spring-calving cows from 6 dairy herds were used in this study. On Day −32 (Day 1 = the start of the breeding season), cows that had calved 2 or more weeks ago were randomly assigned to a synchronization (S, n = 732) or control (C, n = 731) group. Cows in Group S were treated with an intravaginal progesterone device (CIDR) for 12 d from Day −32 to Day −20, while those in Group C were left untreated. Similar percentages of cows in Group S (80.6%) and C (82.9%) had cycled by Day −7. The CIDR treatment synchronized the onset of estrus, resulting in 92.9% of cows in estrus being detected within 7 d after CIDR removal. Cows in Group S that had cycled by Day −7 were treated with PGF 2α (25 mg, im, Lutalyse) on Day −2. Cows in both groups that were anestrous on Day −7 were treated with a combination of progesterone and estradiol benzoate (EB) to induce estrus and ovulation (CIDR and a 10 mg EB capsule on Day −7, CIDR removal on Day −2, and injection of 1 mg EB 48 h after CIDR removal). The PGF 2α treatment synchronized the onset of estrus in 87.5% of the cows. Group S and C cows had similar conception rates to first (61.0 vs 58.3%) and second (58.4 vs 60.9%) AI; similar pregnancy rates over the AI period (82.8 vs 79.2%) and over the whole breeding season (91.9 vs 90.6%); and required a similar number of services per pregnancy to AI (1.7 vs 1.8). The interval from the start of the breeding season to conception for cows conceiving to AI or to combined AI and natural mating was shorter (P < 0.001) by 5.7 and 6.2 d, respectively, for the Group S cows. It is concluded that the treatment regimen tested in the present study achieved satisfactory estrus synchronization, had no detrimental effect on fertility at the synchronized estrus, and shortened the interval from start of the breeding season to conception.