1401
Luteolysis and pregnancy outcome in 5-day Resynch dairy cows after 1 or 2 injections of prostaglandin F2α
Our objective was to determine pregnancy outcome after 50 mg PG administered on d 6 or 25 mg PG delivered on d 5 and 6, respectively, in a 5-d Ovsynch-Resynch (GnRH 5 d before [d 0] and 56 [p.m. on d 7] or 72 h [d 8] after 25-mg doses of PG [d 5 and 6 after GnRH]; timed artificial insemination [AI] on d 8). Lactating Holsteins in herd 1 diagnosed not pregnant between 30 and 36 d since last AI were enrolled randomly to receive either 50 mg PG on d 6 (1 x 50; n = 134) or 25 mg PG on d 5 and 6 (2 x 25; n = 139) after GnRH-1 (d 0), with GnRH-2 at 56 h after PG (d 5) and timed AI 16 h after GnRH-2. In herd 2, even-tagged cows received the 2 x 25 (n = 422) treatment and odd-tagged cows received the 1 x 50 (n = 450) treatment after a not pregnant diagnosis between 34 and 40 d since last AI. Blood collected from all cows in herd 1 at d 0, 5, 6, and 8 was assayed for progesterone. Defined luteolysis occurred when progesterone was ≥ 1 ng/mL on d 5 and 72 h later was < 0.5 ng/mL or < 1 ng/mL on d 8. Progesterone was similar between treatments on pretreatment d 0 and 5, but was greater (P < 0.01) in 1 x 50 than 2 x 25 cows on d 6 (4.7 ± 0.2 vs. 1.1 ± 0.2 ng/mL) and d 8 (0.43 ± 0.04 vs. 0.19 ± 0.04 ng/mL), respectively. Luteolysis was greater (P < 0.01; 93.3 vs. 78.5%) in the 2 x 25 vs. 1 x 50 treatment when the cutpoint was 0.5 ng/mL on d 8, whereas no difference was detected when the cutpoint was < 1 ng/mL (100 vs. 96.3%), respectively. Luteolytic failure was greater in cows classified as early cycle on d 0 or having a new corpus luteum after d 0 than for cows classified as late cycle on d 0 or having low progesterone on d 0 and 5. Luteolytic failure also was greater (P< 0.01) in 1 x 50 than 2 x 25 cows with a cutpoint of 0.5 ng/mL at AI and pregnancy per AI in combined herds was slightly reduced (30.4 vs. 25.1%), respectively.
Keywords: luteolysis, pregnancy per AI, progesterone