Effect of intrauterine administration of gonadotropin releasing hormone with glycerol on serum LH concentrations in lactating dairy cows

The objectives of the study were to assess: (1) preovulatory serum LH concentrations and (2) synchrony of ovulation after im or iu administration of GnRH with or without the addition of glycerol. Cows were presynchronized with 2 injections of PGF2α given 14 d apart (starting at 26 ± 3 DIM) followed by Ovsynch (OV; GnRH-7 d-PGF2α-48 h-GnRH) 12 d later. At the time of the second GnRH of OV (GnRH2), cows were blocked by parity and randomly allocated to 1 of 4 treatments: (1) control (CON; n = 8) received 2 mL of sterile water im; (2) im (IM; n = 8) received 100 μg of GnRH im; (3) cows were infused with 200 μg GnRH into the uterus (IU; n = 9); and (4) iu administration of 200 μg GnRH plus glycerol 7% v/v (IUG; n = 8). Serum circulating progesterone concentrations at hour 0 did not differ (P > 0.05) among groups. Concentrations of LH were greater (P < 0.05) in IM than IU, IUG, and CON cows at hours 1, 1.5, 2, and 3. All cows ovulated within 48 h in the IM (8/8) group followed by IU (6/9) and IUG (4/8) groups, and only two out of eight cows ovulated in the CON group. Although iu administration of GnRH in the IU and IUG groups resulted in lower serum concentrations of LH than IM cows, IU or IUG cows were able to ovulate within 48 h after GnRH2 administration.