ReBreed21-ET: Evaluation of a rapid resynchronization program that allows timed embryo transfer every 21 days.

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This study evaluated the efficiency of a rapid resynchronization of ovulation program to allow timed embryo transfer (TET) every 21d in heifer embryo recipients. Holstein heifers (n=510) had synchronized ovulation using a modified 5d CoSynch program for a TET (D7) after induced ovulation (D0). After TET, heifers were blocked by number of previous TETs and randomized into one of two resynchronization of ovulation programs: Resynch28 (n=279), a traditional resynch program for TET 35d after previous TET; or ReBreed21-ET, a program designed to allow TET 21d after previous TET. Once assigned into one of the two programs, heifers were kept in the same program during a 105-d TET period. In Resynch28, heifers received an intravaginal progesterone (P4) insert on D28, on D33 the P4 was removed, and pregnancy diagnosis was performed using rectal ultrasound to determine embryonic heartbeat. Nonpregnant heifers received a PGF2α treatment with a later GnRH treatment on D35 for a potential TET on D42 (35d after previous TET). Heifers in ReBreed21-TET received an intravaginal P4 insert on D14, on D19 the P4 was removed, and on D21 a GnRH treatment was given to synchronize a new ovulation. On D28, pregnancy diagnosis was performed using transrectal ultrasound to detect an embryonic heartbeat and nonpregnant heifers that had a corpus luteum (CL) ≥18mm in diameter received a TET (21d after the previous TET). Pregnancy per ET (P/ET) from the first TET was greater for heifers in ReBreed21-ET (52%) than Resynch28 (39.4%). In contrast, the subsequent TET (second and later) had similar P/ET for ReBreed21-ET (40.4%) and Resynch28 (40.8%). The overall pregnancy loss from D28 to D63 did not differ between programs (ReBreed21-ET [18.5%] and Resynch28 (16.3%]). Nevertheless, there were fewer pregnancy losses from D28 to D33 for Resynch28 (3.5%) than ReBreed21-ET (10.1%), while from D33 to D47, there was greater pregnancy loss for Resynch28 (10.1%) than ReBreed21-ET (4.9%). Time to pregnancy was approximately 8d earlier for ReBreed21-ET (45±3d) than Resynch28 (53±3d). Overall cumulative pregnancies at the end of a 105-d TET season were greater for ReBreed21-ET (75.2%) than Resynch28 (64%). Thus, the ReBreed21-ET program can improve the efficiency of TET programs.