ContentsA series of studies was conducted to examine the effect of various GnRH -PG -GnRH regimes on the timing of ovulation using ultrasound in cows. In Study 1 cows received GnRH on day 5 of the oestrous cycle followed 7 days later by PG. Six cows received a second GnRH 48 hours after PG. These cows ovulated between 72 and 96 hours after PG compared to 96 and 120 hours for those not receiving a second GNRH. In Study 2, 32 cows received a first GNRH injection at a random stage of the cycle. When PG was injected 7 days later 29/32 were in the luteal phase. Eleven cows received a second GnRH at 56-60 hours after PG and 10 cows at 72 hours, the other 11 receiving no hrther treatment. Ovulation was most tightly synchronised in the group receiving GnRH at 56-60 hours, 10/11 ovulating between 72 and 96 hours after PG. In Study 3 six cows received PG on day 12 of the cycle followed by GnRH 60 hours later. Five of the six cows ovulated between 84 and 96 hours after PG. In Study 4, 6 cows received GnRH on day 17 of the cycle followed by PG 7 days later and GnRH 60 hours after PG. The first GnRH successhlly prolonged the luteal phase in 516 cows. Although the remaining cow ovulated 48 hours after PG, the others ovulated between 84 and 96 hours after PG. It is concluded from these studies that GnRH given 48-60 hours after PG is most critical in determining the time of ovulation, GnRH 72 hours after PG is clearly too late. The first GnRH treatment does not appear to influence the timing of ovulation but could be important in determining oocyte maturation through reprogramming follicular development several days before luteolysis is induced. The first GnRH also has the effect of delaying luteolysis if given late in the luteal phase.
Insemination at an inappropriate time is one of many constraints to good fertility in dairy cows. As a result, many studies have attempted to improve the synchrony of oestrus in controlled breeding programmes. However, the success of insemination depends not merely on the detection of oestrus, but also on the timing of ovulation relative to insemination. Thus a better understanding of the factors associated with the precise timing of behavioural oestrus and ovulation is required. In this study the time of ovulation has been determined, by ultrasound scanning, in relation to a variety of follicular phase events in dairy cows following both natural luteolysis and luteolysis induced by treatment with a prostaglandin F2a analogue. The objectives were firstly to determine whether differences existed in the timing of follicular phase events following natural and induced luteal regression and secondly to determine the degree of variation that exists between the timing of ovulation and the timing of other follicular phase events.
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