Oestrous suppression and pregnancy prevention in cattle using GnRH agonists

"The aims of this thesis were: 1. to characterise ovarian function in heifers and cows treated long-term with the GnRH agonist deslorelin; and 2. to evaluate the potential of controlled-release deslorelin implants for the prevention of pregnancy in heifers and cows maintained in extensive environments." -- abstract. The aims in this thesis were: (1) to characterise ovarian function in heifers and cows treated long-term with the GnRH agonist deslorelin; and (2) to evaluate the potential of controlled-release deslorelin implants for the prevention of pregnancy in heifers and cows maintained in extensive environments. Study 1: The first study was conducted on three cattle stations. On Station 1, a total of 100 heifers and cows each received a GnRH agonist implant that contained 12 mg deslorelin. On Stations 2 and 3, a total of 168 heifers and cows each received a GnRH agonist implant that contained 8 mg deslorelin. The heifers and cows used at the three locations were confirmed non-pregnant at the start of the study. Animals were managed under standard conditions as single groups on each station. Bulls (4%) were kept with females for the duration of the study. Control, untreated heifers and cows were included at each site to confirm that bulls were not limiting to fertility in each herd. Pregnancy and time of conception were estimated based on foetal size as determined by rectal palpation. Pregnancy rates at Station 1 (12 mg implant) were: heifers (n=50), 0% at 6 months and 10% at 11 months; cows (n=50), 0% at 6 months and 6% at 11 months. Pregnancy rates at Station 2 (8 mg implant) were: heifers (n=41), 0% at 6 months and 10% at 12 months; cows (n=48), 4% at 6 months and 12% at 12 months. On Station 3 (8 mg implant) pregnancy rates were: heifers (n=40), 8% at 6 months and 28% at 11 months; cows (n=37), 8% at 6 months and 24% at 11 months. Pregnancy rates for control cattle at Station 1 were: heifers (n=10), 50% at 6 months and 800/0 (new group introduced at 6 months, n=10) at 11 months; cows (n=10), 100% at 6 months and 80% (new group introduced at 6 months, n=10) at 11 months. Pregnancy rates for control cattle at Station 2 were: heifers (n=8), 12% at 6 months and 62% at 12 months; cows (n=11), 64% at 6 months and 91% at 12 months. On Station 3 pregnancy rates for control cattle were: heifers (n=10), 78% at 6 months and 60% (new group introduced at 9 months, n=10) at 11 months; cows (n=10), 100% at 6 months, and no further control cows were introduced. At the completion of field observations at 12 months treated animals were slaughtered to determine pregnancy status and characteristics of ovaries and reproductive tracts. Final pregnancy data and estimated average number of days to first conception for deslorelin-treated cattle were: Station 1 (n=99), 9% pregnant and 336 ± 3 days; Station 2 (n=76), 26% pregnant and 231 ± 19 days; Station 3 (n=84), 10% pregnant and 244 ± 13 days. Treatment with a deslorelin implant in heifers and cows restricted ovarian follicular growth to early antral follicles (1-2 rom diameter), as observed by ultrasound scanning. Study 2 In Study 1, the return to normal fertility in heifers and cows treated with a deslorelin implant appeared to be related to increased pasture availability and accelerated rate of live weight gain, which may put the cattle in a better condition to achieve pregnancy. Study 2: was designed to specifically examine whether treatment with a deslorelin implant suppressed ovarian function for at least 12 months in heifers gaining live weight at a relatively fast rate. GnRH agonist implants that contained three doses of deslorelin were used in Study 2. A group of 198 heifers showing oestrous cycles at regular intervals were assigned to one of four groups: Control (n=50), not treated; GnRH agonist-low dose (n=50), 3 mg deslorelin; GnRH agonist-medium dose (n=50), 6 mg deslorelin; GnRH agonist-high dose (n=48), 12 mg deslorelin. All heifers and four bulls were maintained on natural pastures. Ultrasonography at monthly intervals was used to monitor ovarian follicular activity, corpus luteum development, conception and maintenance of pregnancy. During the 12-month study, heifers gained 153 ± 3 kg which was approximately 0.4 kg/day. The duration of suppression of ovarian activity was related to the dose of deslorelin in the implants. The 3 mg deslorelin implant suppressed ovarian activity for 3 months with cumulative pregnancies of 40/0 at 4 months, 57% at 8 months and 75% at 12 months. The 6 mg deslorelin implant suppressed nonnal ovarian function for 5 months with cumulative pregnancies of 2%, 36% and 54% at 4, 8 and 12 months, respectively. The 12 mg deslorelin implant suppressed ovarian cyclicity for almost 12 months with 6% cumulative pregnancies at 12 months. Pregnancies in control heifers, that were replaced at regular intervals, occurred throughout the study confirming that bulls were not limiting in fertility. In summary, deslorelin implants were found to suppress ovarian activity in heifers and cows. The duration of suppression of ovarian activity was related to dose of GnRH agonist. Treatment with a GnRH agonist implant that contained 12 mg deslorelin prevented pregnancies for up to 12 months in heifers undergoing continuing live weight gain. The thesis has shown that GnRH agonist implants have the potential for use as a non-surgical technique for the prevention of pregnancies in heifers and cows in extensive beef production systems typical of northern Australia.