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Colorado State University, Fort Collins 80523
Abstract
Sixty-four mares were utilized in a 2 x 3 factorial experiment. The factors were: 1) treatment time, February 1 to 15 (Group A), April 1 to 15 (Group B), May 8, 23 or June 1 (Group C); and 2) .044 mg of altrenogest [17β-hydroxy-17-(2-propenyl)estra-4,9,11-trien-3-one]/kg body weight dissolved in neobee oil (4.4 mg/ml) and fed orally for 15 d vs 5 ml of neobee oil only. Mares in Group A and B were examined via rectal palpation twice weekly unless a follicle 
30 mm was detected, in which case palpations were performed daily until ovulation. During each treatment period, jugular blood was obtained from one-half of the control and treated mares every-other-day beginning 6 d before treatment and continuing throughout treatment and daily for 12 d after treatment. Concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and progesterone were determined by radioimmuno-assay. Treatment with altrenogest was ineffective in inducing estrus or ovulation in Group A mares. In addition, altrenogest did not affect (P>.10) concentrations of LH or progesterone during or after treatment, but did suppress (P<.10) follicle size in Group A mares during and shortly after treatment. In Group B mares, more (P<.10) treated mares displayed estrus and ovulated sooner (P<.05; 8.0 ± 10.6 and 25.1 ± 22.3 d, respectively) after drug withdrawal than control mares (25.9 ± 22.3 and 43.5 ± 18.6 d, respectively). Size of the largest follicle at the onset of treatment (Group B) did not alter (P>.10) the interval to estrus, but was negatively correlated (P<.05) with the interval to ovulation and the duration of the post-treatment estrous period. Number of follicles 20 mm, 30 mm and the size of the largest follicle were suppressed (P<.05) in Group B treated mares during the first one-half of the treatment period. Treatment with altrenogest had no effect (P>.05) on concentrations of LH and progesterone, but elevated (P<.05) concentrations of FSH during the middle of the treatment period compared with controls. Pregnancy rates after two cycles were 81 and 75% for treated and control Group B mares, and 79 and 85% for Group C mares, respectively.
1 Anim. Reprod. Lab. The authors wish to express appreciation to C. M. Clay, P. R. Loomis, D. C. Monroe, K. J. Imel, J. R. Zumbrunnen for their assistance. Supported in part by a grant from Roussel-UCLAF, 163 Ave. Gambetta, Paris, France 5020. Altrenogest was previously referred to as Allyl Trenbolone.
3 Present address: Lasma West, Solvang, CA 93463.
4 Present address: 5303 Wilmot Road, McHenry, IL 60050.
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