Restoration patterns for luteinising hormone and ovarian function following treatment with GnRH agonist implants (deslorelin) for 7, 14 or 21 days in cycling dairy cows.

Continuous GnRH agonist treatment of cows results in downregulation of GnRH responsiveness and a state of induced anoestrus. Inducing anoestrus in a precisely controlled manner could have several potential applications in dairy herd management. However, relatively little is known regarding the processes involved in restoring reproductive normality following an induced anoestrus. This study describes an experiment that was conducted to examine patterns of recovery of LH release and follicle growth in non-lactating Holstein cows immediately following cessation of treatment for 7, 14 or 21 days with a deslorelin implant. Oestrus cycles were synchronized at 7 days intervals and a deslorelin implant inserted in every cow 13 days after detected oestrus so that a group had implants for either 21 days (n = 9), 14 days (n = 10) or 7 days (n = 9). On the day of implant removal every ovarian follicle greater than 4 mm in diameter was ablated using ultrasound guided vacuum needle aspiration in an attempt to standardize follicle sizes. Daily ovarian ultrasound examinations were performed on each cow until 35 days after implant removal and again at 45, 59 and 74 days. A subgroup of four cows randomly selected from each treatment group had frequent serial blood samples collected over 8 h at 4 and 10 days after implant removal for LH profiling. There was no significant effect of treatment duration on any LH parameter and results were pooled. Mean LH pulse amplitude increased by 67% between 4 and 10 days after implant removal (0.34 ng/ml versus 0.57 ng/ml; 4 days versus 10 days post-implant, P < 0.001). Mean pulse frequency remained unchanged between the two samplings (5.9 pulses versus 6.9 pulses per 8 h; 4 days versus 10 days post-implant, P > 0.1). Smoothed mean LH concentrations were unaffected by treatment duration or time (0.36 ng/ml versus 0.41 ng/ml; 4 days versus 10 days post-implant, P > 0.1). The pattern of follicle growth and ovulation did not differ significantly between treatment durations and pooled means were used for comparative descriptions. The emergence of a new follicle wave could be detected beginning at 4 days after implant removal (mean 7.9 +/- 0.8 days). After emergence, a period of rapid follicle growth generally ensued with signs of oestrus occurring when the follicle reached 12.3 +/- 0.5 mm and ovulation when mean follicle diameter was 13.1 +/- 0.7 mm at 13.6 +/- 1.5 days after implant removal. Oestrus preceded ovulation in all cases where ovulation subsequently occurred. The mean interovulatory interval after implant removal was similar amongst groups (18.2 +/- 1.3 days). Follicle growth could be categorized into three groups based on the time to emergence and fate of the first wave DF. Spontaneous recovery was characterised by ovulation of the newly emerged DF. Failure to ovulate the first DF was associated with the formation of a persistent follicle by 35 days after implant removal with some 25% (7/28) of cows showing persistent follicles. Delayed emergence (>14 days after implant removal) was detected in 11% (3/28) of cows, but when a follicle did eventually emerge it was seen to ovulate normally. The correlation between number of days to first oestrus expression and LH pulse amplitude on 4 days post-implant removal was significant (R2 = 44%, P < 0.05). A model was then proposed for the restoration of reproductive function following GnRH agonist removal. In conclusion, duration of deslorelin treatment was associated with only small changes in LH and follicle parameters. The formation of persistent follicles delayed the recovery of more cows than delayed emergence of a new follicle wave after implant removal.