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Comparing split-time AI pregnancy rates among non-estrous heifers based on administration of GnRH at AI
This experiment was designed to evaluate split-time artificial insemination (STAI) in beef heifers following administration of the 14-day (d) controlled internal drug release (CIDR)-prostaglandin F2α (PG) protocol and to compare pregnancy rates among non-estrous heifers based on administration of GnRH at AI. Estrus was synchronized for 1,138 heifers across six locations. Heifers received a CIDR insert (1.38 g progesterone) on Day 0 with removal on Day 14. Estrus detection aids (Estrotect) were applied at PG (25 mg) 16 d after CIDR removal on Day 30. Treatments were balanced across locations for heifers using reproductive tract score and weight. Split-time AI was performed at 66 and 90 h after PG, and estrus was recorded at these times. Heifers in both treatments that exhibited estrus by 66 h were inseminated at that time and did not receive GnRH, whereas AI was delayed 24 h until 90 h after PG for heifers that failed to exhibit estrus by 66 h. For heifers in treatment 1 that were inseminated at 90 h, GnRH (100 μg) was administered concurrent with AI at 90 h. Heifers in treatment 2 that were inseminated at 90 h did not receive GnRH. Estrous response did not differ between treatments at 66 h after PG (1=70%; 2=71%; P = 0.58) or during the 24 h delay period (1=59%; 2=52%; P = 0.21). There was no effect of treatment on total AI pregnancy rate (1=54%; 2=56%; P = 0.60) or on AI pregnancy rate for heifers inseminated at 66 h (1=58%; 2=62%; P = 0.86) or 90 h (1=44%; 2=39%; P = 0.50) after PG. Ovulation was confirmed via ultrasonography for a subset of heifers that failed to exhibit estrus prior to 90 h after PG. For heifers that failed to exhibit estrus by 90 h, ovulation rate did not differ between treatments (1=52%; 2=50%; P = 0.64) nor did AI pregnancy rate (1=24%; 2=15%; P= 0.97). In summary, when split-time AI was used in conjunction with the 14-d CIDR-PG protocol in heifers, comparable pregnancy rates were achieved without administering GnRH.
Keywords: beef heifer, gonadotropin-releasing hormone, split-time artificial insemination