Plasma Gonadotropin Concentrations in Intact Female and Intact and Castrated Male Prepubertal Ponies1

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Abstract
The patterns of FSH and LH secretion were determined for prepubertal male and female ponies from birth in May to 8 months of age. On the day of birth, LH concentrations were higher (P<0.05) in female than in male foals. The mean LH concentration was significantly higher for the female foal on the day of birth than on any other day. In 5 spring-born fillies FSH concentrations increased (P<0.05) from a low in the spring to a high through the summer and declined thereafter to low concentrations in the fall. Intact spring-born male foals did not show a similar pattern in FSH concentrations; mean concentrations remained approximately the same from spring through fall. There was a small but consistent rise (P<0.01) in LH concentrations during late June and early July (age, ∼2–2.5 months) in the spring-born males; thereafter, LH concentrations declined and remained low and constant through December. The small rise and decline in LH concentration was temporally associated with a rise and decline in testosterone. Following castration in August at 4 months of age, FSH and LH concentrations increased (P<0.05) in the spring-born male foals. Apparently the negative influence of the gonads on the hypothalamo-pituitary axis develops earlier in the male foal than in the female, since no change in either gonadotropin had been observed in 4-month-old fillies following ovariectomy in a previous report (Wesson and Ginther, 1979). Plasma samples from 2 groups of later-born male and female foals were also assayed and in general the patterns of FSH and LH secretion appeared similar to the patterns in spring-born foals.