Superovulatory response following ablation-induced follicular wave emergence at random stages of the oestrous cycle in cattle.

Abstract

Based on the premise that superovulation in cattle is optimal when superstimulation is initiated at the time of follicular wave emergence, the present study was done in beef heifers to determine if the superovulatory response following a single bolus of gonadotrophin treatment after follicle ablation (induced wave) at random stages of the oestrous cycle is comparable to the same gonadotrophin treatment at mid-dioestrus (spontaneous wave). In Experiment 1, heifers were assigned to nonablation (n = 18) and ablation (n = 20) groups. In nonablated heifers, superstimulatory treatment was given as a single subcutaneous injection (Folltropin-V, 400 mg) at mid-di-oestrus to coincide with emergence of the spontaneous follicular wave 8 to 12 days after oestrus. In ablated heifers, the same superstimulatory treatment was given 1 day after ablation of all follicles > or = 5 mm at random stages of the oestrous cycle to coincide with emergence of the ablation-induced wave. In both the nonablation and ablation groups, PGF2 alpha (Estrumate, 500 micrograms) was given 48 h after the superstimulatory treatment and artificial insemination was done 60 and 72 h later. Reproductive tracts were collected at the time of slaughter 6 or 7 days after insemination. Observations made in Experiment 1, indicated that some ablated heifers had only partial luteal regression at the time of insemination, while some others exhibited behavioral oestrus as early as 24 h after PGF2 alpha treatment. The design was amended in Experiment 2 to address these problems. Heifers were assigned to nonablation (n = 17), ablation-alone (n = 20) or ablation plus progestogen (n = 20) groups. Follicle ablation, superstimulatory treatment, artificial insemination and collection of reproductive tracts were done as in Experiment 1. However, all heifers were given two doses of PGF2 alpha (500 micrograms/dose) 48 and 60 h after superstimulatory treatment to ensure complete luteal regression, and heifers in the ablation plus progestogen group received a norgestomet ear implant at the time of follicle ablation to prevent early ovulations. The implant was removed at the time of the second PGF2 alpha treatment. In Experiments 1 and 2, the means for the ovarian and superovulatory responses were not significantly different between groups. Averaged over the nonablation and all ablation groups for Experiments 1 and 2, the mean number. of corpora lutea, fertilized ova and transferable embryos were 22.9 vs 18.6, 7.3 vs 7.8 and 5.4 vs 5.6, respectively. In summary, follicle ablation at random stages of the oestrous cycle followed by a single bolus of gonadotrophin treatment 1 day later resulted in a superovulatory response that was comparable to the same superstimulatory treatment administered around the time of spontaneous wave emergence at mid-dioestrus. The ablation/superstimulation method described herein offers the advantage of initiating superstimulatory treatment forthwith and assuring that treatment is concomitant with wave emergence to achieve an optimal superovulatory response. Moreover, the full extent of the oestrous cycle is available for superstimulation and the need for detecting oestrus or ovulation and waiting 8 to 12 days to initiate treatment is eliminated.