Induction of ovulation in nonlactating dairy cows and heifers using different doses of a deslorelin implant

The objective of this study was to evaluate ovarian function after inducing ovulation with a deslorelin implant in nonlactating dairy cows and heifers. Cattle received GnRH on Day -9, and PGF_2α on Day -2. On Day 0, in Experiment 1, cows received either 100 μg GnRH (Control), a 750 μg (DESLORELIN 750) or 1000 μg (DESLORELIN 1000) deslorelin implant. On Day 0, in Experiment 2, cows received 100 μg of GnRH or a 450 μg (DESLORELIN 450) deslorelin implant. In Experiments 1 and 2, cows received PGF_2α on Day 16. Ultrasonography and blood sampling for plasma progesterone (P₄) were used to monitor ovarian activity. On Day 0, in Experiment 3, heifers received either 100 μg of GnRH or 750 μg (DESLORELIN 750) deslorelin implant. On Day 16, all heifers received PGF _2α. Blood samples were collected on Days 7, 13 and 16. In Experiments 1-3, deslorelin implants did not elevate plasma concentrations of P₄ in a systematic manner during the late luteal phase. In Experiments 1 and 2, deslorelin implants decreased the size of the largest follicle and the number of Class II and III follicles. In Experiments 1 and 2, deslorelin-treated cows failed to ovulate by Day 28. In conclusion, deslorelin implants induced ovulation, stimulated development of a normal CL, and delayed follicular growth during the subsequent diestrus period. For future applications, the dose of the deslorelin implant will have to be adjusted, and if used for timed-inseminations, nonpregnant cows will have to be resynchronized to minimize delayed returns to estrus and ovulation. © 2003 Elsevier Inc. All rights reserved.