Abstract
Our objectives were to determine fertility of heifers after estrus synchronization using PGF2αpreceded by either progesterone, GnRH, or both. Beef (n = 193) and dairy (n = 246) heifers were assigned randomly to three treatments: 1) 50 μg of GnRH and a used intravaginal progesterone-releasing insert were administered on day −7, followed by 25 mg of PGF2αon day −1, and CIDR removal on day 0 (CIDR + GnRH + PGF); 2) the same as 1) but without the GnRH (CIDR + PGF); and 3) the same as 1) but without the CIDR (GnRH + PGF; modified Select Synch). Rates of estrus detection were lower in dairy than in beef heifers, and greater in heifers treated with the CIDR. In dairy heifers, conception and pregnancy rates were greatest in the CIDR + PGF treatment, followed by the CIDR + GnRH + PGF and GnRH + PGF treatments. The opposite trend was observed among treatments in beef heifers. All estrus-synchronization treatments produced acceptable estrus detection and pregnancy rates.
Highlights
The importance of dairy and beef heifers as future replacements cannot be overstated
Our objectives were to determine fertility of heifers after estrus synchronization using PGF2α preceded by either progesterone, GnRH, or both
Beef (n = 193) and dairy (n = 246) heifers were assigned randomly to three treatments: 1) 50 μg of GnRH and a used intravaginal progesterone-releasing insert were administered on day −7, followed by 25 mg of PGF2α on day −1, and CIDR removal on day 0 (CIDR + GnRH + PGF); 2) the same as 1) but without the GnRH (CIDR + PGF); and 3) the same as 1) but without the CIDR (GnRH + PGF; modified Select Synch)
Summary
The importance of dairy and beef heifers as future replacements cannot be overstated. Estrus can be synchronized either by shortening the luteal phase with PGF2α or by artificially extending the luteal phase with progestins. The “gold standard” for synchronizing estrus in beef heifers is the MGA + PGF protocol (feed 0.5 mg of melengesterol acetate [MGA] per day for 14 days and inject PGF2α 17-19 days later). The major disadvantage of that protocol is its long duration (31-33 days) before insemination begins. Similar to using MGA, behavioral estrus and ovulation are suppressed during treatment with the CIDR. Unlike MGA, fertility is normal at the first estrus after CIDR treatment. Short-term treatment with the CIDR produced tight synchrony of estrus, but conception rates were variable and related to treatment duration
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