Dose and Frequency of PGF2 α Administration to Lactating Dairy Dose and Frequency of PGF2 Administration to Lactating Dairy Cows Exposed to Presynchronization and Either Five-or Seven-Cows Exposed to Presynchronization and Either Five-or Seven-day Ovsynch Protocols: Ovulation, Luteolysis, and Pregnancy day Ovsynch Protocols: Ovulation, Luteolysis, and Pregnancy Rates Rates

Summary Lactating Holstein cows in one herd were milked three times daily and enrolled in a 2 × 2 × 2 factorial design with eight treatments before first postpartum artificial insemination (AI). These treatments were employed to test ovulatory, progesterone, and luteolytic outcomes to three main effects: (1) two GnRH-PGF 2α presynchronization programs (PG-3-G vs. Double Ovsynch); (2) 5-vs. 7-day Ovsynch-duration programs; and (3) two doses (25 mg on consecutive days) or one dose (50 mg) of PGF 2α administered before timed AI. Results from this experiment demonstrate no differences in the presynchronization treatments of PG-3-G vs. Double Ovsynch; both are effective in initiating estrous cycles during warm-hot vs. cool-cold seasons. Although ovulatory responses were similar after the first GnRH administration, Double Ovsynch cows tended to have greater ovulation responses after the second GnRH administration. The single large, one-time administered 50-mg dose was effective in causing luteolysis in the 7-day program but slightly less effective in the 5-day program. Thus, when using the shorter 5-day program, the two 25-mg PGF 2α doses administered 24 hours apart are recommended. Insufficient numbers of cows were treated to make conclusions about pregnancy outcomes in this one-herd study. Pregnancy rates, however, were reduced in 5-day Ovsynch program when the single large 50-mg dose of PGF 2α was employed.


Introduction
Potential limitations to a successful timed artificial insemination (AI) program for cows submitted for first service include cows that are not cycling (anovulatory), synchronization of the ovulatory follicle that should ovulate just after the timed AI, and regression of the corpus luteum or corpora lutea (if more than one exists when PGF 2α is administered).
The advantages of these GnRH-PGF 2α presynchronization programs include inducing ovulation in anovulatory cows, decreasing the percentage of cows with lower circulating progesterone concentrations (< 0.50 ng/mL) at G-1, increasing the percentage of cows with medium progesterone concentrations (0.50 < progesterone ≤ 3.0 ng/mL) at G-1, and increasing the proportion of cows with a corpus luteum (CL) at G-1.In addition, these GnRH-PGF 2α presynchronization programs increase the percentage of cows with high progesterone (> 3.0 ng/mL) at the PGF 2α treatment that immediately precedes timed AI and tend to increase average circulating progesterone at PGF 2α .
Early studies in beef and dairy cows indicated that an improvement in pregnancy outcome seemed to occur when a shorter 5-day Ovsynch program was compared with a 7-day program.The problem with these published studies was the confounding of program duration (5 vs. 7 days) and use of one vs.two doses of PGF 2α (6 to 24 h apart) to induce luteolysis.In none of the experiments were the two doses of PGF 2α tested in both the 5-and 7-day programs.Therefore, a test of the two doses of PGF 2α must occur in both programs to interpret the data correctly.If the 5-day program is not superior in pregnancy outcome, then the difference may simply be the result of improved complete luteolysis in cows treated with two doses of PGF 2α , regardless of program duration.
Applying a second standard dose or a single larger dose of PGF 2α seems to maximize complete luteolysis before timed AI.Corpora lutea less than 10 days old are resistant to complete regression after a single standard dose of PGF 2α .Administering PGF 2α as a single large dose on day 7 or as two standard doses on days 5 and 6 after G-1 (5-day Ovsynch) usually results in 70 to 84% of cows with progesterone < 0.3 ng/mL on the day of the timed AI.Progesterone concentrations at or near baseline at the final GnRH treatment influence the characteristics of GnRH-induced LH release and subsequent pregnancy risk.Increased dose or frequency of PGF 2α at the end of Ovsynch program has enhanced luteolysis, reduced progesterone concentrations at timed AI, and in some cases slightly increased pregnancy risk compared with a standard single dose.Recent studies demonstrated increased complete luteolysis when 7-day Ovsynch programs included a second standard dose of PGF 2α compared with a single standard dose.
Therefore, we proposed to address three questions.(1) Does the additional preGnRH (PreG; Figure 1) injection of Double Ovsynch improve synchronization characteristics compared with the PG-3-G presynch program?(2) Will one large dose (50 mg) of PGF 2α produce complete luteolysis similar to two standard 25-mg doses given 24 h apart?(3) Is the 5-day Ovsynch program superior to the 7-day program when similar dose and frequency of PGF 2α injections are administered in both programs?
Blood samples were collected before G-1, before PGF 2α , and at 24, 48, and 72 h after PGF 2α (first or only PGF 2α injection) to determine concentration ranges of progesterone and assess complete luteolysis (progesterone < 0.5 ng/mL at 48 to 72 h after the first or only dose of PGF 2α ).Transrectal ovarian scans by ultrasonography were used to assess ovulation after G-1 and G-2.Although pregnancy risk was assessed, this experiment was not designed to test pregnancy risk but focused on: (1) ovulation responses to G-1 and G-2; and (2) proportion of cows with complete luteolysis, as illustrated in Figure 1.

Ovarian Cyclicity
On the basis of progesterone concentrations, the proportion of cows that had initiated estrous cycles at the start of the Ovsynch program did not differ between PG-3-G and Double Ovsynch (Table 1).More cows tended (P = 0.09) to return to having estrous cycles before the onset of ovulation synchronization programs during the cool-cold season compared with the warm-hot season (88.4 vs. 81.9%).

Ovulation Response to GnRH-1
Neither single nor multiple ovulation responses to G-1 differed between PG-3-G and Double Ovsynch treatments (Table 1).Season had differing effects on multiple ovulation in cows treated with PG-3-G and Double Ovsynch.During the warm-hot season, multiple ovulation occurred more frequently in PG-3-G than Double Ovsynch cows (17.4 vs. 6.4%),whereas during the cool-cold season, the reverse was detected (8.6 vs. 13.6%,respectively).

Luteolysis
Decreasing concentrations of progesterone (luteolysis or regression of the corpus luteum or corpora lutea) after the PGF 2α treatments just preceding timed AI are summarized in Table 2.At 24 hours after the first or only PGF 2α dose, the proportion of cows with progesterone concentration < 1 ng/mL was greater (P < 0.05) in Double Ovsynch than PG-3-G treatments; greater (P < 0.05) in 7-vs.5-day Ovsynch cows, and greater in cows treated with the 1 × 50 mg than 2 × 25 mg PGF 2α dose (no interactions).At 48 and 72 hours, more (P < 0.01) cows had progesterone < 1 ng/mL in the 7-vs.5-day Ovsynch cows.At 72 hours, more (P < 0.01) cows had progesterone < 0.5 ng/mL in the 7-vs.5-day Ovsynch cows and more (P < 0.01) cows receiving the 2 × 25 mg dose than the 1 × 50 mg dose.An interaction, however, was detected at 48 and 72 hours between the Ovsynch duration and the PGF 2α dose-frequency (Figure 2).Although mean progesterone was < 0.05 ng/mL at both time points, the single 1 × 50 mg was slightly less effective than the 2 × 25 mg dose in reducing concentrations of progesterone in the 5-vs.7-day Ovsynch-treated cows.Thus, the single large dose was equally effective as the 2 × 25-mg dose in the 7-day Ovsynch program, but less so when applied to cows in the shorter 5-day program.

Ovulation Response to GnRH-2
Single ovulation after G-2 differed slightly between presynch treatments (Table 3).Cows presynched with Double Ovsynch tended (P = 0.085) to have greater ovulatory responses than PG-3-G cows.Duration of Ovsynch (5 vs. 7 days) had no effect on ovulation response to G-2.Single ovulatory response to G-2 tended (P = 0.073) to be greater during the cooler-cold season than during the warmer-hot season (94.2 vs. 88.3%, respectively).
In contrast, multiple ovulation tended (P = 0.108) to be greater in PG-3-G than Double Ovsynch cows and greater (P = 0.004) in 5-vs.7-day cows.An interaction (P = 0.011), however, was detected between presynchronization treatments and Ovsynch duration (Table 3).Multiple ovulation was greater in PG-3-G cows in the 5-day compared with the 7-day program, whereas no differences in multiple ovulation occurred between program durations in Double Ovsynch cows.Season had no effect on the frequency of multiple ovulation.

Pregnancy Outcomes
No overall differences were detected in pregnancy rate between PG-3-G and Double Ovsynch treatments (Table 3) or between the 5-vs.7-day program, although numerically pregnancy rates were greater for PG-3-G and 7-day program cows.Because of the differences in the effectiveness of luteolysis (decreasing progesterone after PGF 2α during the timed AI week), an interaction between Ovsynch duration and PGF 2α dose-frequency was detected (Table 3).Cows receiving the 1 × 50 mg dose in the 7-day program had greater (P < 0.05) pregnancy rates than cows receiving the 1 × 50 mg dose in the 5-day program.
Results from this experiment demonstrate no significant differences in the presynchronization responses to PG-3-G vs. Double Ovsynch treatments.Both are effective in initiating estrous cycles during all seasons.Although ovulatory responses were similar after G-1, Double Ovsynch cows tended to have greater ovulation responses after G-2.The single large, one-time administered 50-mg dose was effective in causing luteolysis in the 7-day but less so in the 5-day cows.Thus, when using the shorter 5-day program, the two 25-mg doses administered 24 hours apart are recommended.Insufficient numbers of cows were treated to make conclusions about pregnancy outcomes in this one-herd study.Pregnancy rates, however, were reduced in 5-day compared with the 7-day Ovsynch program when the single large 50-mg dose of PGF 2α was employed.As expected, pregnancy rate was greater (P = 0.056) during the cooler-cold months compared with warmer-hot season (40.2 vs. 30.0%).Means differ (P < 0.05) within main effect category.c-d Means differ (P < 0.01) within main effect category.

Table 1 .
Ovarian cyclicity assessed at 7 days after two presynchronization treatments and ovulatory responses response to GnRH-1 (G-1)

Table 2 .
Percentage of cows with progesterone concentrations < 1 ng/mL at 24, 48, and 72 hours after the first or only dose of PGF 2α or < 0.5 ng/mL at 72 hours