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Effect of administration of human chorionic gonadotropin after artificial insemination on concentrations of progesterone and conception rates in beef heifers^1,2

R. N. Funston^*,3, R. J. Lipsey, T. W. Geary and A. J. Roberts

^* University of Nebraska, West Central Research and Extension Center, North Platte 69101; and American Simmental Association, Bozeman, MT 59715; and and USDA, ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301

³ Correspondence: 461 West University Dr. (phone: 308-696-6703; e-mail: rfunston2{at}unl.edu).

The objective of this study was to determine whether administration of hCG approximately 5 d after AI would increase plasma progesterone concentrations and conception rates in beef heifers. Heifers from two locations (Location 1: n = 347, BW = 367 ± 1.72 kg; Location 2: n = 246, BW = 408 ± 2.35 kg) received melengestrol acetate (0.5 mg·heifer^–1·d^–1) for 14 d and an injection of PGF₂ (25 mg i.m.) 19 d later. Heifers were observed for estrus continuously during daylight from d 0 to 4.5 after PGF₂ and artificially inseminated approximately 12 h after the onset of estrus. Half of the heifers inseminated at Location 1 were assigned randomly to receive an injection of hCG (3,333 IU i.m.) 8 d after PGF₂, and a blood sample was collected from all heifers 14 d after PGF₂ for progesterone analysis. Half of the heifers inseminated at Location 2 were administered hCG on d 9 after PGF₂, and a blood sample was collected from all heifers 17 d after PGF₂. Heifers at Location 1 had a 94% synchronization rate, exhibited estrus 2.45 ± 0.03 d after PGF₂, and received hCG 5.55 ± 0.03 d after AI. Heifers at Location 2 had an 85% synchronization rate, exhibited estrus 2.69 ± 0.03 d after PGF₂, and received hCG 6.31 ± 0.03 d after AI. Progesterone concentrations were greater (P < 0.01) for hCG-treated heifers than for controls at both locations (8.6 vs. 4.6 ng/mL for treatment vs. control at Location 1, and 11.2 vs. 5.6 ng/mL for treatment vs. control at Location 2). Pregnancy status was determined by ultrasound approximately 50 d after AI. Conception rates (65 vs. 70% for treatment vs. control, respectively) did not differ at Location 1. Conception rates tended (P = 0.10) to be increased with hCG treatment at Location 2 (61 vs. 50% for treatment vs. control, respectively). A second experiment was conducted with 180 heifers at a third location to determine the effects of hCG administration 6 d after timed insemination at approximately 60 h after PGF₂ in heifers synchronized as in Exp. 1. Pregnancy rate to timed AI did not differ between hCG-treated (62%) and control heifers (59%). Final pregnancy rate after timed AI and bull exposure (92%) was not affected by treatment. In summary, administration of hCG 5 to 6 d after AI did not improve conception or pregnancy rates at two out of three locations evaluated, suggesting insufficient progesterone is not a major factor contributing to early pregnancy failure in beef heifers.

Key Words: Beef Heifers • Human Chorionic Gonadotropin • Progesterone

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