Improved synchrony of estrus and ovulation with the addition of GnRH to a melengestrol acetate-prostaglandin F-2 alpha synchronization treatment in beef heifers

The objective of this experiment was to determine the effect of a GnRH inje ction within a melengestrol acetate (MGA)-PGF(2 alpha) (PGF) estrus synchro nization protocol on follicular dynamics and synchronization of estrus. Pub ertal crossbred beef heifers (n = 34) were randomly assigned to one of two treatments. Both treatment groups were fed MGA (0.5 mg.hd(-1).d(-1)) for 14 d and injected (i.m.) with PGF (25 mg of Lutalyse) 19 d after MGA withdraw al. Melengestrol acetate was delivered in a feed supplement of 1.8 kg.hd(-1 ).d(-1). Seventeen heifers received an injection of GnRH (100 mug Cystoreli n) 12 d after MGA withdrawal and 7 d before PGF. The control group (n = 17) received only MGA-PGF. Estrus was detected four times/d for 7 d beginning on the day PGF was injected. Transrectal ultrasonography was performed dail y on eight heifers from each treatment to monitor ovarian activity and char acterize changes in follicular dynamics after MGA withdrawal and until ovul ation after PGF. Each of the GnRH-treated heifers either ovulated or had a luteinized dominant follicle following GnRH and subsequently initiated a ne w follicular wave (8/8, 100%). All GnRH-treated heifers (17/17, 100%) and 9 4% of controls (16/17) exhibited estrus after PGF. Estrus was exhibited ove r a 132-h period (12 to 144 h) for control heifers compared with 60 h (48 t o 108 h) for GnRH-treated heifers. The peak synchronized period for both tr eatments was between 48 and 72 h after PGF, during which time 76% (13/17) o f the GnRH-treated heifers exhibited estrus compared with 63% (10/16) for c ontrols. Seventy-one percent (12/17) of the GnRH-treated heifers exhibited estrus from 48 to 60 h after PGF, compared with 38% (6/16) for controls (P < 0.05). In summary, injection of GnRH within a 14- to 19-d MGA-PGF protoco l increased the synchrony of estrus during the synchronized period and conc entrated the period of detected estrus. This protocol may offer potential f or the fixed-time insemination of replacement beef heifers.