Incidence of premature estrus in lactating dairy cows and conception rates to standing estrus or fixed-time inseminations after synchronization using GnRH and PGF2α

Fixed-time AI (TAI) after GnRH-PGF2α-GnRH treatment is a method to achieve pregnancies in dairy herds without estrous detection. However, cows that fail to respond to the initial GnRH may have compromised TAI conception rates due to asynchronous ovarian response. This study documented the percentage of GnRH-treated Holstein cows (n=345) in two herds that displayed estrus at an inopportune time for optimum TAI conception rate (≤48 h post-PGF2α; premature estrus (PE)) and compared conception rates of two TAI protocols in cows that did not display PE. At biweekly herd health exams, cows diagnosed as not pregnant to a previous AI and cows >80 days postpartum with no AI were treated with 100 μg GnRH (day −7) and 25 mg PGF2α (day 0). Cows detected in PE by twice-daily visual observation from day −7 to day 2 were bred by AI 8–12 h later. Cows not detected in PE were randomly assigned by parity, body condition score, and postpartum interval to receive either: (1) 100 μg GnRH at 48 h after PGF2α and TAI 16 to 18 h later (Ovsynch); or (2) TAI at 72 h post-PGF2α and a concurrent 100 μg GnRH injection to those cows not detected in estrus between 48 and 72 h post-PGF2α (modified Ovsynch (MOV)). All hormone injections were im. Twenty percent (68/345) of the cows were detected in estrus before 48 after PGF2α, of which 5% (17/345) were detected in estrus before PGF2α (≤day 0). Herd influenced the percentage of cows in the PE group (herd A versus herd B; 25% versus 14%; P<0.05). Conception rates were not affected by treatment (PE versus Ovsynch versus MOV; 32% (21/65) versus 30% (37/125) versus 32% (47/145); P>0.10). However, within MOV-treated cows, conception rates were greater (P<0.05) in cows detected in estrus (46% (23/50)) compared with cows not detected in estrus (25% (24/95)). In conclusion, 20% of GnRH-treated cows displayed PE and necessitates estrous detection during this period if maximal pregnancy rates are to be achieved. Although additional estrous detection is required compared to Ovsynch, reduced cow handling and hormone usage, efficient use of expensive semen through greater conception rates in cows detected in estrus, and comparable TAI conception rates, suggests the MOV protocol may be a cost effective alternative to Ovsynch in many dairy herd reproductive management programs.