SYNCHRONIZATION OF ESTRUS WITH PGF2-ALPHA ADMINISTERED 18 DAYS AFTER A PROGESTERONE TREATMENT IN LACTATING DAIRY-COWS

In a previous study we showed that estrus synchronization with 2 treat ments of PGF(2 alpha) 13 d apart reduced conception rate at the synchr onized estrus and that this reduction occurred mainly in cows in the e arly luteal phase at the second PGF(2 alpha) treatment. The objective of the present study was to determine the efficacy of a synchronizatio n regimen in which PGF(2 alpha) was administered during the mid- to la te-luteal phase to cows that had previously been synchronized with pro gesterone. Spring-calving cows from 6 dairy herds were used in this st udy. 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 synchroniza tion (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. Simila r percentages of cows in Group S (80.6%) and C (82.9%) had cycled by D ay -7. The CIDR treatment synchronized the onset of estrus, resulting in 92.9% of cows in estrus being detected within 7 d after CIDR remova l. Cows in Group S that had cycled by Day -7 were treated with PGF(2 a lpha) (25 mg, in, Lutalyse) on Day -2. Cows in both groups that were a nestrous 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 m g EB 48 h after CIDR removal). The PGF(2 alpha) treatment synchronized the onset of estrus in 87.5% of the cows. Group S and C cows had simi lar 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 fro m the start of the breeding season to conception for cows conceiving t o 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 satisfacto ry estrus synchronization, had no detrimental effect on fertility at t he synchronized estrus, and shortened the interval from start of the b reeding season to conception. (C) 1998 by Elsevier Science Inc.