Pregnancy outcomes after change in dose delivery of prostaglandin F2α and time of gonadotropin-releasing hormone injection in a 5-day timed artificial insemination program in lactating dairy cows1

Abstract

We demonstrated that 50mg of PGF2α on d 6 successfully induced luteolysis in lactating dairy cows enrolled in a traditional 5-d Ovsynch-72 program [GnRH injection 5 d before (d 0; GnRH-1) and 56 (p.m. on d 7; GnRH-2) or 72h (d 8; GnRH-2) after a 25-mg injection of PGF2α (d 5 and 6 after GnRH injection); timed artificial insemination (AI) on d 8]. Our current objective was to determine pregnancy outcomes in lactating dairy cows after a 50-mg injection of PGF2α on d 6 or a 25-mg injection of PGF2α on d 5 and 6 in a 5-d Ovsynch program. Cows in herd 1 diagnosed not pregnant between 30 and 36 d since last AI were enrolled to receive either a 50-mg injection of PGF2α on d 6 (1×50; n=134) or a 25-mg injection of PGF2α on d 5 and 6 (2×25; n=139) after GnRH-1 (d 0), with GnRH-2 at 72h after PGF2α injection (d 5), concurrent with timed AI (d 8). Cows in herd 2 diagnosed not pregnant between 34 and 40 d were treated similarly: even-tagged cows received the 2×25 (n=422) treatment, and odd-tagged cows received the 1×50 (n=450) treatment, except that GnRH-2 was administered at 56h. Blood collected from cows in herd 1 at d 0, 5, 6, and 8 was assayed for progesterone. Luteolysis was defined to occur when progesterone concentration was ≤1ng/mL on d 5, and 72h later (d 8) was either <0.5ng/mL or <1ng/mL. Progesterone concentrations did not differ between treatments on pretreatment d 0 and 5, but were greater in 1×50 than 2×25 cows on d 6 (4.7±0.2 vs. 1.1±0.2ng/mL) and d 8 (0.43±0.04 vs. 0.19±0.04ng/mL), respectively. Luteolysis was greater in the 2×25 versus 1×50 treatment when the cut point was 0.5ng/mL, whereas no difference was detected when the cut point was <1ng/mL on d 8. Lack of complete luteolysis was greater in cows classified as early cycle on d 0 or having a new corpus luteum after d 0 because progesterone concentration was greater on d 5 and 6 than for cows classified as late cycle on d 0 or cows having low progesterone on d 0 and 5. Pregnancy per AI at 30 to 40 d did not differ between 2×25 and 1×50 cows having luteolysis by d 8 or in all cows (37.2 vs. 33.3%) in herd 1, respectively, but differed in herd 2 (24.7 vs. 19.5%; no treatment by herd interaction). We conclude that incomplete luteolysis by d 8 was greater in 1×50 cows using a cut point of <0.5ng/mL at AI. The difference in pregnancy outcome in herd 2 may have resulted from insufficient time for complete luteolysis before GnRH-2 at 56h compared with GnRH-2 at 72h (at AI) in herd 1.