Increased plasma progesterone levels can be achieved by inducing the formation of accessory corpora lutea, which can be obtained by treatment with hCG at on average 4-6 days after insemination (Binelli et al., 2001). Apart from the initiation of additional corpora lutea this treatment is believed to provide further LH support to the corpus luteum verum resulting from the ovulation of the dominant follicle.
Santos et al. (2001) administered hCG on day 5 after AI in high producing dairy cows and noted that the treatment induced formation of accessory CL, enhanced plasma progesterone concentrations and improved conception rates when evaluated on days 28, 45 and 90, especially in cows losing body condition between AI and 28d after AI. Similarly Breuel et al. (1989); Sianangama et al. (1992); Rajamahedran and Sianangama (1992) reported a significant increase in pregnancy rate with hCG administered at 7 days post AI. Keneda et al. (1981) and Kerbler et al. (1997) achieved an improvement in pregnancy rates with administration of hCG at a dose of 1500 i.u. after AI.
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Results of administration of hCG post insemination reported in the literature
Human chorionic gonadotrophin is usually administered at a dose of 1500 i.u. on the day of embryo transfer.
It was shown that administration of hCG at this moment directly supports the development and function of the corpus luteum resulting from ovulation, but also induces ovulation/luteinisation of receptive follicles of the first wave of subsequent follicular development. This results in formation of induced corpora lutea, increase of progesterone levels and reduction in oestradiol concentrations.
Small et al. (2002) evaluated the influence of hCG (Chorulon, Intervet; 2500 i.u./cow) administration at day 7 in recipients of transferred embryos and inseminated cows. They found that treatment with hCG at the time of ET, 7 days after AI, improved timed-AI pregnancy rates in twin-bearing cows and first calf heifers. The authors postulated that treatment with hCG at 7 days post AI may be used to improve pregnancy rates in metabolically stressed cows and first calf heifers.
Nishigai et al. (2002) administered hCG at 6 days after oestrus in embryo recipients. The results of their trial showed that administration of hCG (1500 i.u./cow) 6 days after oestrus improved the pregnancy rate for non-surgical frozen embryo transfer 7 days after oestrus by enhancing luteal function and depressing oestradiol secretion.
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Results of administration of hCG post insemination reported in the literature
A number of recent studies have been devoted to the analysis of the effect of GnRH treatment in mid-cycle (usually 11-14 days post insemination) on embryo survival and the resulting pregnancy rate.
Mann et al. (1995) concluded that GnRH attenuated the luteolytic signal, allowing embryos more time to develop their anti-luteolytic ability. Depending on the stage of follicular development, treatment with GnRH analogues during the luteal phase causes advanced atresia, luteinisation or ovulation followed by luteinisation of the responding follicle.
Administration of GnRH between 11 and 13 days after service produced a marked increase in pregnancy rates (MacMillan et al., 1986; Mee et al., 1990; Peters et al., 1992; Stevenson et al., 1990; Ryan et al., 1994). Peters (2000) summarised results of various studies that analysed the effects of GnRH injections between days 11 and 13 of oestrus cycle on pregnancy rates in cows and noted wide variability both in respect to the experimental design and the degree of improvement in pregnancy rates obtained. This analysis suggested that in certain circumstances GnRH treatment after insemination may produce significant benefits.
A very recent study of Lopez-Gatius et al. (in press) demonstrated that GnRH treatment at the time of insemination and 12 days later increases the conception rate in high producing dairy cows during the warm season. Although lower than double treatment, strong benefits were also registered following a single GnRH
treatment at insemination.
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Overview of the positive results of GnRH treatment at mid-cycle reported in literature