EFFECT OF TIMING OF ESTRADIOL BENZOATE INJECTION RELATIVE TO GONADOTROPIN TREATMENT ON SUPEROVULATORY RESPONSE, AND ON EMBRYO YIELD AND QUALITY IN BEEF HEIFERS

Variation in superovulatory responses in cattle may be related to the stage of follicular growth at the time of gonadotropin treatment. Wave s of follicle growth are regulated by both follicle-stimulating hormon e (FSH) and oestradiol, The objective of experiment 1 was to determine the dynamics of follicle wave emergence and the relationship with FSH and oestradiol concentrations, after treatment of heifers with oestra diol benzoate (ODB) in the presence of an intravaginal progesterone-re leasing device (CIDR-B). Experiment 2 examined the superovulatory resp onse, embryo yield and quality following treatment with porcine follic le-stimulating hormone (pFSH) at different times relative to ODB injec tion. In experiment 1, 28 beef heifers were treated with a CIDR for 9 days and allocated at random to one of four groups to receive either: (1) CIDR only, or 5 mg ODB given as a single intramuscular injection a t (II) day 0 (d0); (III) day 1.5 (d1.5); or (IV) day 3 (d3) post CIDR insertion. Ovaries were examined using daily ultrasound and blood samp les were collected twice daily for 11 days. In experiment 2, 96 heifer s were treated with a CIDR and 5 mg ODB as in experiment 1, and were a llocated using a 4 x 3 factorial design plan to a superovulation progr amme using three doses (400 IU; 600 IU; 800 IU) of pFSH. FSH was given for 4 days at 12-h intervals beginning 6.5 days after CIDR insertion. Heifers received prostaglandin analogue 12 h before CIDR removal and were inseminated (AI) at 48 and 60 h post CIDR withdrawal and embryos were recovered 7 days after AI. In experiment 1, the interval from CID R insertion to follicle wave emergence (FWE) was longer (P < 0.05) in heifers treated with ODB at d1.5 (5.4 +/- 0.4 days) and d3 (5.1 +/- 0. 6 days) compared to heifers treated with CIDR only (2.4 +/- 0.4 days). On the basis of time to proposed injection of pFSH heifers would have had follicle emergence 4.4, 2.3, 1.5 and 1.4 days prior to pFSH for g roups I, II, III and IV; respectively. In experiment 2, heifers treate d with ODB at d1.5 had a higher (P < 0.05) superovulatory response (18 .2 +/- 1.7) than heifers treated at d3 (12.8 +/- 1.7), but superovulat ory response in both groups did not differ (P > 0.05) from heifers tre ated at d0 (14.4 +/- 2.0) or with CIDR only (15.0 +/- 1.8). There were fewer (P < 0.05) freezable-grade embryos recovered from heifers treat ed with ODB at d0 (1.5 +/- 0.7) and d3 (2.1 +/- 0.5) compared to heife rs treated at d1.5 (3.0 +/- 0.6) or in heifers treated with CIDR only (3.4 +/- 0.7). Increasing the dose of pFSH caused a linear increase in the superovulatory response (11.7 +/- 1.0, 15.8 +/- 1.4 and 18.0 +/- 1.9) and in the number of embryos recovered (5.8 +/- 0.9, 7.0 +/- 0.8 and 9.1 +/- 1.0) for 409 IU, 600 IU and 800 IU, respectively. In concl usion, heifers treated with ODB had wide variation in time to follicle wave emergence and there was not a consistent beneficial effect of pr etreatment with ODB on embryo yield and quality following superovulati on. (C) 1998 Elsevier Science B.V. All rights reserved.