Short communication: Change in dose delivery of prostaglandin F2α in a 5-day timed artificial insemination program in lactating dairy cows.

We hypothesized that 50mg of prostaglandin F2α (PG) on d 6 would induce luteolysis in a traditional 5-d Ovsynch-72 program [GnRH 5 d before (d 0) and 72 h after (d 8) 25-mg PG doses (d 5 and 6 after GnRH); timed artificial insemination (AI) on d 8]. Experiment 1 monitored luteal regression of original and GnRH-induced luteal tissue (corpus luteum, CL) by transrectal ultrasonography and blood serum concentrations of progesterone after both 25-mg doses of PG (d 5 and 6; control; n=31) or a single 50-mg dose of PG on d 6 (n=30). Estrous cycles were presynchronized (GnRH 7d before 25 mg of PG); 11 d later, cows were enrolled in a 5-d Ovsynch-72 program (62 to 71 d in milk) and treatments were administered. Blood was sampled for progesterone analysis and luteal structures were measured on d 0 (original CL) and d 5 through 9 to monitor original and new GnRH-induced CL. Control PG reduced luteal tissue area of original CL on d 6 and 7 compared with PG administered only on d 6, but no difference between treatments was detected by d 9. In contrast, no differences were detected in luteal tissue area of the induced CL on d 5 through 9. Serum progesterone on d 5 through 9 differed only on d 6 for control and the 50-mg dose. Luteolysis occurred in all 31 controls, but luteolytic failure occurred in 2 of 30 cows receiving 50mg, in which no CL were present on d 0 but 1 or 3 new CL were present on d 5 in these 2 cows. Pregnancy outcome 32 d after AI was 14 of 30 (40%) compared with 15 of 30 (50%) for control versus 50-mg dose, respectively. Experiment 2 monitored luteolysis in nonpregnant repeat-service cows subsequently treated with the same 2 treatments as in experiment 1. Serum progesterone in 63 cows (serum progesterone ≥1 ng/mL on d 5) on d 5, 6, and 8 differed only on d 6 for control and the 50-mg dose. Luteolysis occurred in 32 of 34 controls and in 29 of 29 cows treated with 50mg. Pregnancy outcome 32 d after AI was 17 of 33 (52%) compared with 13 of 29 (45%) for control versus 50-mg dose, respectively. We concluded that the single 50-mg dose was equivalent to the control based on actual luteal tissue regression and decreased progesterone.