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Testosterone Effects on Mares during Synchronization with Altrenogest: FSH, LH, Estrous Duration and Pregnancy Rate¹

Louisiana State University^,2, Baton Rouge 70803 and and Colorado State University^,3, Fort Collins 80523

Abstract

Twelve mares fed altrenogest for 14 d were used to study the effects of a single injection of testosterone propionate on concentrations of follicle-stimulating hormone (FSH) during diestrus, and to relate the normal and perturbed patterns of FSH secretion to subsequent estrous characteristics and fertility. Seven of 12 mares received testosterone propionate at 200 µg/kg of body weight on d 5 of progestogen feeding. Mares were teased and blood samples were drawn daily, all mares were artificially inseminated at the first estrus after progestogen treatment. Testosterone propionate treatment caused a 50% reduction in concentration of FSH in plasma within 24 h, and this effect persisted through 48 h after injection. Within 4 d after the suppression of FSH secretion, concentrations of FSH rebounded and were significantly elevated compared with control values during the last 4 d of progestogen feeding. Testosterone propionate at this dosage also elicited estrous behavior in five of seven treated mares within 24 h after injection. After withdrawal of progestogen feeding, the interval to onset of estrus, duration of estrus, and magnitude of the luteinizing hormone peak were similar between groups (P>.05). Six of seven treated mares and three of five control mares became pregnant to breeding on the first estrus after progestogen treatment. Because manipulation of the normal pattern of FSH secretion during diestrus did not affect estrous characteristics or fertility of the subsequent estrus, such treatment may have potential as a means of synchronizing FSH surges during diestrus in the mare.

¹ Supported by the Louisiana Agr. Exp. Sta., Baton Rouge and published with the approval of the Director. We thank F. Randy Wright for his technical assistance; Drs. Harold Papkoff and Leo E. Reichert, Jr. for their gifts of purified gonadotropins; the National Pituitary Agency for the anti-hFSH serum and TSH radioimmunoassay reagents and Roussel-UCLAF, Paris, France, for their gift of Altrenogest, formally called allyl trenbolone.

² Dept. of Anim. Sci.

³ Anim. Reprod. Lab.

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