Evaluation of two different oestrus-synchronisation methods with timed artificial insemination and resynchronisation of returns to oestrus in lactating Holstein cows

To examine the outcomes of adding medroxyprogesterone acetate (MAP) to the ovsynch protocol with the traditional ovsynch protocol in both cycling and anoestrus cows, and to evaluate a resynchronisation protocol, 742 cows averaging more than 40 days postpartum were assigned to the following four treatments: (1) ovsynch (OVS): day 0: GnRH; day 7: PGF2α; day 9: a similar dose of GnRH; day 10: timed artificial insemination (TAI), approximately 16–20 h later; (2) ovsynch+MAP (MAP): the same ovsynch protocol plus an intravaginal insert made of polyurethane sponge impregnated with 300 mg of MAP immediately after the first GnRH treatment and on day 7, at the time of the PG treatment, the sponge was removed; (3) resynchronisation (MAP+ODB): 1 mg of oestradiol benzoate (ODB) on day 13 after TAI and a new sponge impregnated with MAP was inserted and; on day 20, 1 mg of ODB was given and the sponge removed; and (4) no resynchronisation (No MAP): only oestrus detection and AI at any repeat oestrus detected after TAI. Progesterone was measured in milk samples collected on days −17, −10, −3, 13 and 20 (TAI=day 0). Based on milk P4 at days −17 and −10, 27.4% of the cows were still anoestrus. At PG injection, 67.7% of the cycling and 21.3% of the anoestrus cows had elevated P4. Farm, days postpartum and parity variations were detected in both cases. On day 20 after TAI 42.6% of cycling and 8.3% of the anoestrous cows had elevated P4. Pregnancy rates were similar in both pre-breeding treatments (20%), but interactions (P<0.001) were detected between treatment and cycling activity (for anoestrous cows: MAP=34.9%; OVS=11.1%. Average interval from TAI to subsequent AI was 37±3 days. Resynchronisation resulted in more (P<0.001) cows in oestrus between days 18 and 25 after TAI. Conception rate in the MAP+ODB treatment was lower (P<0.05) than the No MAP group (22.8% versus 47.4%). It was concluded that the addition of a progestin to the ovsynch protocol resulted in increased pregnancy rates of cows treated during anoestrus. The benefit of including MAP with the ovsynch protocol for cycling cows is equivocal.