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Effect of Termination of Pregnancy or Long-Term Progestogen Exposure on Subsequent Estrous Cycle Length and Concentration of Progesterone in Plasma of Heifers¹

University of Missouri, Columbia 65211

Abstract

An experiment was conducted to determine whether short estrous cycles following abortion of heifers between 70 and 75 d of gestation are due to factors associated with the previous presence of a conceptus or long-term exposure of the uterus and(or) ovaries to a progestogen. Fifty crossbred heifers were randomly allotted at estrus (d 0) to five groups: control (n = 10), pregnant (Preg.; n = 14), progestogen (norgestomet) implant (Norg.; n = 9), progesterone-releasing intravaginal device (PRID; n = 9), or hysterectomy (Hyst; n = 8). Control heifers were injected during the mid-luteal phase of an estrous cycle with 25 mg prostaglandin F₂ (PGF₂) and length of the subsequent estrous cycle was determined. Beginning 6 to 8 d after estrus, heifers in the Norg. or PRID groups were given norgestomet ear implants or intravaginal coils, respectively, every 10 d for 70 d. Heifers were hysterectomized 5 to 8 d after estrus. Seventy to 75 d after conception, progestogen treatment or hysterectomy, heifers were injected (i.m.) with 25 mg PGF₂ and the last norgestomet ear implants or PRIDs were removed. Interval from PGF₂ injection to first estrus ( ± SE) ranged from 2.5 ± .2 to 4.4 ± .7 d (P > .05). Length of the first estrous cycle ( ± SE) following PGF₂-induced luteolysis or progestogen withdrawal was shorter (P < .01) for the Preg. group (8.2 ± .4 d) than for the control, Norg. and PRID groups (21.5 ± .6 d; 19.3 ± 1.4 d; and 18.2 ± 1.3 d, respectively). In the Hyst. group, all corpora lutea were maintained beyond 25 d post-estrus. Mean length of the second estrous cycle in the Preg. group (18.0 ± .9 d) was shorter (P < .05) than length of the first estrous cycle in the control group but was within the normal range of estrous cycle lengths for heifers. In summary, PGF₂-induced abortion of heifers early in gestation resulted in short estrous cycles that were due to factors associated with the previous presence of a conceptus, rather than to long-term exposure to progestogen alone.

¹ Contribution from the Missouri Agric. Exp. Sta. Journal Series Number 10649.

² Dept. of Anim. Sci.

³ Dept. of Dairy Sci.

⁴ Dept of Vet Med. & Surgery.