Effects of GnRH Administered to Cows at the Onset of Estrus on Timing of Ovulation, Endocrine Responses, and Conception

Abstract

Two experiments examined effects of GnRH administered within 3 h after onset of estrus (OE) on ovulation and conception in dairy cows. In experiment 1, 46 cows received either saline, 250 microg of GnRH, or 10 microg of the GnRH analogue, Buserelin. Cows were observed for estrus, blood samples were collected, and ovulations were monitored by ultrasound. In controls, 76% of cows had intervals from estrus to ovulation of < or = 30 h and 24% had intervals > 30 h. Treatment with either GnRH or GnRH analogue (data combined) increased magnitude of LH surges and decreased intervals from estrus to LH surge or to ovulation. Treated cows all ovulated < or = 30 h after OE. Among control cows, plasma estradiol concentrations before estrus correlated positively with amplitudes of LH surges. Higher plasma progesterone was observed in the subsequent estrous cycle in GnRH-treated cows compared to control cows with delayed ovulations. Experiment 2 included 152 primiparous and 211 multiparous cows in summer and winter. Injection of GnRH analogue at OE increased conception rates (CR) from 41.3 to 55.5% across seasons. In summer, GnRH treatment increased CR from 35.1 to 51.6%. Across seasons, GnRH increased CR from 36.0 to 61.5% in cows with lower body condition at insemination and GnRH increased CR (63.2 vs. 42.2%) in primiparous cows compared to controls. Use of GnRH eliminated differences in CR for cows inseminated early or late relative to OE and increased CR in cows having postpartum reproductive disorders. In conclusion, GnRH at onset of estrus increased LH surges, prevented delayed ovulation, and may increase subsequent progesterone concentrations. Treatments with GnRH increased conception in primiparous cows, during summer, and in cows with lower body condition.

Effects of GnRH, PGF_2α and oxytocin treatments on conception rate at the time of artificial insemination in lactating dairy cows

“…In several previous studies, GnRH improved pregnancy rate by 7-21% in lactating dairy cows (Lee et al, 1983;Nakao et al, 1983;Stevenson et al, 1990;Kaim et al, 2003;Lopez-Gatius et al, 2006). On the other hand, in some studies the pregnancy rate was not affected by GnRH treatment follow- ing AI and was similar to our findings (Stevenson et al, 1984;Lee et al, 1985;Chenault 1990;Mee et al, 1990;Perry and Perry 2009).…”

“…However, the administration of GnRH at the time of AI, approximately 12 h after the initiation of standing oestrus, need not result in a greater surge of LH (Lee et al, 1985;Lucy and Stevenson 1986;Mee et al, 1990). In addition, the insufficient LH surge did not have any ovulatory effect (Lee et al, 1985;Lucy and Stevenson, 1986;Kaim et al, 2003) and did not improve pregnancy (Lee et al, 1985). Thus our result and previous studies indicated that the administration of GnRH at the time of AI did not affect pregnancy rate.…”

“…Earlier studies showed that the timing of GnRH injection according to the onset of oestrus affected gonadotropin release. Although exogenous GnRH at the onset of oestrus increased the preovulatory LH surge (Mee et al, 1990;Kaim et al, 2003;Perry and Perry, 2008), conception rate was increased in one study (Kaim et al, 2003) but not in others (Mee et al, 1990;Perry and Perry, 2009). However, the administration of GnRH at the time of AI, approximately 12 h after the initiation of standing oestrus, need not result in a greater surge of LH (Lee et al, 1985;Lucy and Stevenson 1986;Mee et al, 1990).…”

“…However the results are controversial after GnRH treatment of lactating cows. Some researchers found out that CR was improved (Kaim et al, 2003;Lee et al, 1983;Nakao et al, 1983;LopezGatius et al, 2006) while others reported that no effect on pregnancy rate was obtained (Stevenson et al, 1984;Lee et al, 1985;Chenault, 1990;Mee et al, 1990;Perry and Perry, 2009).…”

“…Gonadotropin-releasing hormone (GnRH) and its analogues administered at the time of AI are the most common treatments in management programmes for herds, recommended by veterinary practitioners (Stevenson et al, 1984;Chenault, 1990;Stevenson et al, 1990;Morgan and Lean, 1993). Improvement of the conception following GnRH treatment has been attributed to the prevention of an ovulation failure or a reduced variation in the interval between the onset of oestrus and ovulation by means of the induced preovulatory luteinizing hormone (LH) surge (Mee et al, 1993;Kaim et al, 2003). However the results are controversial after GnRH treatment of lactating cows.…”