198 Superovulatory response in cows undergoing aromatase inhibitor treatment

The aromatase inhibitor letrozole has been used in the treatment of infertility in women by inducing mild ovarian superstimulation or augmenting the ovarian response to FSH treatment. The effect has been attributed to an apparent up-regulation of FSH receptors on granulosa cells as a result of increased androgens (Weil et al. 1999 J. Clin. Endocrinol. Metab. 84, 2951-2956). The objective of this study was to determine whether letrozole will augment the superstimulatory response in FSH-treated cattle. Mature, non-lactating Holstein cows (n=30) were given 2 luteolytic doses of PGF2α 12h apart and scanned daily by transrectal ultrasonography to detect ovulation. Transvaginal ultrasound-guided ablation of follicles ≥5mm was performed in all cows as a group, at 5 to 8 days after ovulation. On the day of ablation, cows were assigned randomly to 2 groups (n=15 per group) and given either an intravaginal letrozole-releasing device or a blank device for 5 days. Coinciding with expected wave emergence (Day 0), cows in both groups were given 8 doses of 50mg of pFSH (Folltropin; Vetoquinol, Lure, France) IM at 12-h intervals, and PGF2α on Days 3 and 3.5. At the time of the second PGF2α, vaginal devices were removed and paint was applied to the tailhead to facilitate detection of oestrus. On Day 5, cows were given gonadotropin-releasing hormone (100mg gonadorelin) IM and artificially inseminated 12 and 24h later. Ova/embryos were collected by transcervical uterine flush on Day 12. The ovaries were examined by ultrasonography on Day 0, 3.5, 5, 6.5, and 12 to record the follicular and luteal response. Nominal data were compared between groups by t-test and by ANOVA for repeated-measures and are expressed as mean±s.e.m. Binomial data were compared by chi-squared test. The number of follicles at wave emergence did not differ between letrozole and control groups (24.6±3.1 and 26.5±3.3 respectively; P=0.68). There was no treatment effect or treatment×day interaction in the number of follicles >8mm on Day 3.5, 5, and 6.5. Letrozole-treated cows exhibited oestrus later than controls (50.3±1.1h v. 40.7±2.0h after first PGF2α; P<0.001) and had less variance in interval to oestrus (residuals, 3.1±0.48h v. 6.7±0.87 h; P<0.01). The number of CL on Day 6.5 was lower in the letrozole group than in the control group (9.1±1.1v. 12.3±1.1; P=0.05). The proportion of ovulations (number of CL on Day 12 over number of follicles ≥3mm on Day 0) was lower in the letrozole group than in the control group (0.65±0.05v. 0.82±0.08; P=0.02). The total number of ova/embryos collected per cow did not differ between letrozole and control groups (5.0±1.9v. 5.4±1.8; P=0.75), nor did the number of transferable embryos differ (3.0±1.2v. 4.3±1.5; P=0.56). In conclusion, treatment with a letrozole-releasing device during ovarian superstimulation did not improve the superovulatory response or embryo collection rate in mature Holstein cows, but letrozole treatment resulted in more synchronous oestrus, which may be used in the design of a fixed-time AI protocol following superstimulatory treatment in cattle. This research was supported by the Alberta Livestock and Meat Agency.