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Effect of Exogenous Progesterone on Time of Occurrence of the LH Surge in Heifers^1,2,

Michigan State University⁴, East Lansing 48824

³ Agricultural Institute, Grange, Ireland. Address all correspondence to J. J. Ireland.

Abstract

Experiments were conducted to determine the effects of progesterone-releasing intravaginal devices (PRID) inserted into heifers at various stages of the estrous cycle on (1) serum concentrations of progesterone and (2) time of occurrence of the preovulatory luteinizing hormone (LH) surge. In Exp. 1, 40 heifers were each treated for 7 days with a PRID containing either 2 or 6.75% progesterone; the devices were inserted on days 8 to 10 (luteal groups) or days 17 and 18 (follicular groups) of the estrous cycle. One day before PRID removal, all heifers were given an IM injection of 30 mg prostaglandin F₂-tham salt (PGF₂). Both stage of cycle and percentage progesterone in the PRID affected (P<.05) serum concentrations of progesterone. After the removal of the PRID, the LH surge occurred earlier (P<.05) in heifers in the follicular group treated with 2% PRID (mean ± SEM, 24 ± 2.5 hr). In Exp. 2, 40 heifers at days 5 to 10 of the estrous cycle were each fitted with a PRID containing 4 or 20% progesterone for 12 or 14 days. At the time of PRID insertion, 32 heifers were given an IM injection of 30 mg PGF₂. The other eight heifers each received for 12 days a 4% PRID with an estradiol capsule (10 mg estradiol benzoate) adhered to it. The interval from removal of PRID to occurrence of the LH peak was shorter (P<.05) for heifers given 4% PRID for 14 days (27 ±3.0 hr) and for heifers given 4% PRID with estrogen capsule for 12 days (30 ± 3.8 hr) than for heifers in the other groups. Increasing the concentration of progesterone in the PRID from 4 to 20% did not result in increased serum concentrations (P>.05) of progesterone in heifers given PGF2a at the time of PRID insertion. The PRID were ineffective in maintaining luteal phase concentrations of progesterone in serum for periods longer than 7 days. If luteal phase concentrations of progesterone (4 to 7 ng/ml) are necessary throughout a synchronizing treatment for subsequent optimum fertility, then either a 7-day treatment period with PRID should be used or more effective methods should be developed to deliver higher concentrations of progesterone.

¹ Research supported by Abbott Labs, Chicago, IL.

² The authors gratefully acknowledge help from members of the Anim. Reprod. Lab. in all aspects of this research.

⁴ Dept. of Dairy Sci.