Short communication: Insertion of an intravaginal progesterone device at the time of gonadotropin-releasing hormone (GnRH) injection affects neither GnRH-induced release of luteinizing hormone nor development of dominant follicle in early diestrus of lact

Our objectives were to evaluate the acute effects of a controlled internal drug release (CIDR) insert containing 1.38 g of progesterone (P4) on the release of LH, follicular growth, and circulating concentrations of P4 in cows treated with GnRH at the time of CIDR insertion. Nonpregnant, lactating dairy cows (n = 27) were blocked by parity, predicted 305-d mature-equivalent milk production, and body condition score and randomly assigned to 1 of 3 treatments: (1) CIDR insertion concurrent with an injection of 200 µg of GnRH (n = 10; 2GP4); (2) CIDR insertion concurrent with an injection of 100 µg of GnRH (n = 10; 1GP4); and (3) injection of 100 µg of GnRH (n = 7; CON). Prior to onset of treatments, cows were submitted to a presynchronization protocol that consisted of a CIDR insert containing 1.38 g of P4 from d −7 to −2, 25 mg of PGF2α on d −2 and −1, and 100 µg of GnRH on d 0. Experimental treatments were applied on d 6, the early luteal phase of the estrous cycle. Concentrations of P4 in plasma were determined on d −2 and 0 and at 0, 15, 30, 60, 120, 240, 345, 600, and 1,200 min relative to treatment on d 6. Concentrations of LH were determined in plasma samples obtained at 0, 15, 30, 60, 120, 240, and 345 min relative to treatment on d 6. Ultrasonography examinations of ovarian structures were performed on d −2, 0, 2, and at 0, 600, and 1,200 min relative to treatment on d 6. Mean concentrations of P4 in the CON group (1.91 ± 0.28 ng/mL) were lower than in 2GP4 (3.40 ± 0.26 ng/mL) and 1GP4 (3.31 ± 0.24 ng/mL) groups, but concentrations in 2GP4 and 1GP4 were similar. Mean concentration of LH in response to the GnRH injection on d 6 was greatest in 2GP4 cows (3.08 ± 0.21 ng/mL) and did not differ between 1GP4 (2.23 ± 0.21 ng/mL) and CON (2.14 ± 0.25 ng/mL) cows. The diameter of the dominant follicle on d 6 was similar among treatments (2GP4 = 15.34 ± 0.50; 1GP4 = 15.31 ± 0.50; CON = 14.77 ± 0.62 mm). In conclusion, CIDR insertion concurrent with a 100- or 200-µg dose of GnRH neither altered GnRH-induced LH release nor had an acute effect on dominant follicle growth.