In alpacas, giving the drug Cetrorelix (a GnRH blocker) makes the main ovarian follicle shrink and a new group of follicles appear about a week later, which helps farmers plan timed insemination. This is a veterinary breeding study and doesn’t give useful guidance for human health or performance.

The objectives of the study were to determine the effects of dose (double vs single) of a GnRH antagonist (Cetrorelix) and follicular status (known status vs random status) on ovarian follicular wave synchrony. In Experiment 1, we tested the hypotheses that treatment of alpacas with two doses of Cetrorelix, will cause 1) regression of the dominant ovarian follicle during both its mid-growing and mid-static phases, and 2) emergence of a new follicular wave at a consistent time after treatment. Ovarian ultrasonography was performed daily and alpacas (n = 7 per group; Day 0 = wave emergence) were given 1.5 mg Cetrorelix acetate im twice at an interval of 24 h starting on Day 5 (mid-growing dominant follicle, D5), Day 10 (mid-static phase, D10) or remained untreated (Control). The mean day of onset of dominant follicle regression was earlier in the D5 group than in the D10 and Control groups (Day 8.7 ± 0.89, 12.9 ± 0.34, 13.6 ± 0.90, respectively; P < 0.001). The largest diameter of dominant follicle was smallest in the D5 group (P = 0.003). The inter-wave interval was shortest in the D5 group (12.7 ± 0.36 days), intermediate in Control (14.6 ± 0.81) and longest in the D10 group (17.7 ± 0.52; P < 0.001). The interval from treatment to wave emergence was similar between D5 and D10 groups (7.7 ± 0.36 days vs 7.7 ± 0.52 days; P > 0.99). In Experiment 2, we tested the hypothesis that a single dose of 1.5 mg Cetrorelix will induce emergence of a new follicular wave synchronously among a group of alpacas on random days of the follicular wave. A new wave emerged 5.3 ± 0.70 days later (n = 15) and was more synchronous (lower variance) than in the pre-treatment control period (Barlett's test p = 0.049, Levene's test p = 0.025). In conclusion, treatment with a single or double dose of 1.5 mg Cetrorelix caused the demise of dominant follicle leading to synchronous emergence of the next follicular wave 5 days (single dose) or 8 days (two doses) later. Results document the potential utility of Cetrorelix in breeding management protocols that enable pre-scheduled, synchronous insemination.