Abstract

In vitro exposure of oocytes to elevated temperatures hastened oocyte maturation; furthermore, performing IVF of heat-stressed oocytes 5 h earlier than the usual 24 h resulted in blastocyst development similar to that of non-heat-stressed controls (Edwards et al. 2005 J. Dairy Sci. 88, 4326–4333). If elevated ambient temperatures in vivo alter oocyte maturation in a similar fashion, then new strategies are needed to induce earlier release of the oocyte from the ovulatory follicle. Current objectives were to examine follicular growth after FSH administration and examine whether treatment with FSH and an exogenously induced LH surge would hasten ovulation. On Day 0 (8 to 9 days after estrus) of the experimental period, lactating Holstein cows (n = 31; 65–115 days in milk; 1–6 lactations) received an EAZI-BREED CIDR (Pfizer Animal Health, New York, NY, USA) plus 100 µg of gonadotropin-releasing hormone (GnRH, IM; Cystorelin, Merial Ltd, Iselin, NJ, USA). On Day 7, CIDRs were removed and cows were administered 500 µg cloprostenol (IM; Estrumate, Schering-Plough Animal Health, Union, NJ, USA). Concurrently, cows were randomly allocated to receive either 80 mg FSH (FSH; n = 15; Folltropin-V, Bioniche Animal Health, Belleville, ON, Canada) or 4 mL of sterile saline (SAL; n = 16). Forty-eight h later (Day 9), cows within the FSH and SAL groups were randomly subdivided to receive either a 100-µg dose of Cysterolin (GnRH) or 3000 IU of hCG (hCG, IM; Chorulon, Intervet Inc., Millsboro, DE, USA) generating 4 treatment combinations (FSH/GnRH, n = 3; FSH/hCG, n = 7; SAL/GnRH, n = 8; and SAL/hCG, n = 8). Ovarian activity was assessed by ultrasonography to evaluate growth of the ovulatory follicle. Following CIDR removal, frequent ultrasonography was utilized to confirm ovulation (disappearance of the dominant follicle). Data were analyzed using the MIXED procedure of SAS (SAS Institute, Inc., Cary, NC, USA). Five cows from the FSH group were removed from the combination treatment due to ovulation occurring before 48 h post-CIDR removal. Size of the ovulatory follicle at time of GnRH or hCG administration was not different between FSH or SAL groups (16.7 ± 0.7 v. 17.5 ± 0.6 mm, respectively). Total growth of the ovulatory follicle from CIDR removal to ovulation did not differ between FSH (3.04 ± 0.7 mm) and SAL (2.75 ± 0.7 mm)-treated cows. As calculated from time of CIDR removal, ovulation occurred earlier in FSH (63.6 ± 4.5 h) than in SAL (77.2 ± 4.4 h; P < 0.05)-treated cows. Combination of FSH/GnRH produced the earliest ovulation (74 ± 1.2 h) which was different only from FSH/hCG (78.6 ± 0.8 h; P < 0.05), but not from SAL/GnRH or SAL/hCG (77 ± 0.8 and 78 ± 0.8 h, respectively). Regardless of FSH or SAL treatment, cows treated with GnRH ovulated earlier than those treated with hCG (75.5 ± 0.7 v. 78.3 ± 0.6 h, respectively; P < 0.05). In conclusion, while FSH was unable to increase the size of the ovulatory follicle, earlier ovulation occurred when given alone or in combination with GnRH.