Incidence of premature estrus in lactating dairy cows and conception ratesto standing estrus or fixed-time inseminations after synchronization usingGnRH and PGF(2 alpha)

Fixed-time AI (TAI) after GnRH-PGF(2 alpha)-GnRH treatment is a method to a chieve pregnancies in dairy herds without estrous detection. However, cows that fail to respond to the initial GnRH may have compromised TAI conceptio n rates due to asynchronous ovarian response. This study documented the per centage of GnRH-treated Holstein cows (n = 345) in two herds that displayed estrus at an inopportune time for optimum TAI conception rate (less than o r equal to 48 h post-PGF(2 alpha); premature estrus (PE)) and compared conc eption rates of two TAI protocols in cows that did not display PE. At biwee kly herd health exams, cows diagnosed as not pregnant to a previous AI acid cows > 80 days postpartum with no AI were treated with 100 mug GnRH (day - 7) and 25 mg PGF(2 alpha) (day 0). Cows detected in PE by twice-daily visua l observation from day -7 to day 2 were bred by AI 8-12 h later. Cows not d etected in PE were randomly assigned by parity, body condition score, and p ostpartum interval to receive either: (1) 100 mug GnRH at 48 h after PGF(2 alpha) and TAI 16 to 18 h later (Ovsynch); or (2) TAI at 72 h post-PGF(2 al pha) and a concurrent 100 mug GnRH injection to those cows not detected in estrus between 48 and 72 h post-PGF(2 alpha) (modified Ovsynch (MOV)). All hormone injections were im. Twenty percent (68/345) of the cows were detect ed in estrus before 48 after PGF(2 alpha). of which 5% (17/345) were detect ed in estrus before PGF(2 alpha) (less than or equal to day 0). Herd influe nced the percentage of cows in the PE group (herd A versus herd B; 25% vers us 14%: P < 0.05). Conception rates were not affected by treatment (PE vers us 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 d isplayed PE and necessitates estrous detection during this period if maxima l pregnancy rates are to be achieved. Although additional estrous detection is required compared to Ovsynch, reduced cow handling and hormone usage, e fficient use of expensive semen through greater conception rates in cows de tected in estrus, and comparable TAI conception rates, suggests the MOV pro tocol may be a cost effective alternative to Ovsynch in many dairy herd rep roductive management programs. (C) 2001 Elsevier Science B.V. All rights re served.