Prevention of delayed ovulation
It is important to ensure a timely ovulation in relation to service.
By making sure that ovulation occurs within 7-18 hours after AI, a satisfactory conception rate can be acheived.
Administration of GnRH around time of service
Depending on the size and maturity of the dominant follicle, ovulation usually occurs within 24 hours after GnRH injection, which is similar to the time from onset of oestrus until ovulation. Click on the thumbnail sketch for full size image.
Mechanism of action of GnRH administration
It is postulated that administration of GnRH analogues at the time of insemination may modify the function of preovulatory ovarian follicles or characteristics and secretory capacity of the developing corpus luteum (Mee et al., 1993).
Experimental results reported suggest that GnRH:
- Might enhance or alter theca-lutein or granulosa-lutein differentiation in the pre- or postovulatory follicle or developing corpus luteum
- Might act on the developing corpus luteum to promote the conversion of small luteal cells to large luteal cells
Progesterone secretion is thus increased.
Timing of the GnRH treatment
It is best to use GnRH at the time of AI or up to 6 hours beforehand (Rosenberger et al., 1991.) The reasons are:
- chronological relationship between endogenous LH release
- duration of heat and ovulation
- survival time of sperm and oocyte
In practice however GnRH is usually administered at the same time as AI with very satisfactory results.
- Rosenberger et al. (1991) evaluated the effect of GnRH (Receptal, Intervet; 10mcg/Fertagyl, Intervet; 250mcg) administered to dairy cows during oestrus on plasma LH and conception in relation to the time of the treatment and insemination.
In groups with low conception rates at first post partum AI, treatment with GnRH improved insemination results. It was suggested that GnRH treatment could reduce variation in time of ovulation or prevent ovulation failure.
- Several earlier studies demonstrated that treatment with GnRH at the time of insemination in repeat-breeding dairy cows increased pregnancy rates (Stevenson et al., 1988, 1989; Lee et al., 1983; Phatak et al., 1986).
- The study of Morgan and Lean (1993) presented an extensive analysis of the possible effect of the treatment with GnRH at the time of insemination on conception rate in cattle.
The article compared results from numerous previous studies in which GnRH or GnRH analogues were used at AI and submitted them to meta-analysis.
There was a significant increase in the chance of pregnancy in cows treated with:
- GnRH analogue at the first postpartum insemination
- at second service after calving
- in repeat breeder cows treated with GnRH at the time of insemination.
Repeat breeder cows responded better to the treatment than the other groups, which supports a hypothesis that a proportion of repeat breeding cows had previously failed to conceive because of a failure in the timing or magnitude of GnRH, LH or FSH surge at oestrus.The effects on conception of treatment with GnRH at the time of insemination – meta-analysis by Morgan and Lean
Click on the icon on the right to open the slide show
- In a study involving 2437 animals, Heuwieser et al. (1994) analysed the relationship between administration of GnRH, body condition score and fertility in dairy cows. In this study conception rate improved when GnRH was administered at the first breeding post partum in cows with a body condition score below 3.0 regardless of their parity.
- Ullah et al. (1996) evaluated the effect of GnRH administration in lactating Holstein cows exposed to the heat stress and found that the treatment with GnRH at oestrus improved the fertility results in comparison to a non-treated group.
For more information see summary of : Effect of GnRH administration at oestrus in lactating Holstein cows during heat stress (Ullah et al., 1996)