Semester

Summer

Date of Graduation

2002

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Animal and Nutritional Sciences

Committee Chair

Robert A. Dailey.

Abstract

A longtitudinal study was performed in prepubertal heifer calves to determine when the individual components of the hypothalamic-pituitary-ovarian axis were capable of functioning. In study 1, the hypothalamus and anterior pituitary of prepubertal dairy heifers were challenged with exogenous hormones (estradiol and GnRH, respectively) to determine at what age gonadotropin surges could be induced. Two-, four-, six-, and eight-month-old heifer calves were challenged with GnRH or estradiol. Blood was sampled from the jugular vein every 15 minutes for 6 hours or hourly for 30 hours, for the two treatments, respectively. Heifers in each age group released LH in response to each treatment. The LH response to GnRH increased with age (p < 0.05), whereas time to onset of the estrogen-induced surge was delayed in younger heifers (p < 0.05). A synchronous release of FSH was not associated with the release of LH with either treatment. In study 2, the ability of the hypothalamus to respond was examined more closely in prepubertal beef heifer calves. In this study, whether short-term separation of the calf and dam differentially affected the response to an exogenous estradiol challenge also was examined. Two-, three-, four-, and five-month-old heifer calves were challenged with estradiol and blood was sampled every 2 hours for 30 hours. Estradiol induced gonadotropin surges at all ages regardless of the separation of the calf from the dam or continuous association. The mean amounts of LH and FSH released in response to estrogen decreased with age (p < 0.05). Short-term weaning decreased mean LH, but did not affect FSH release. Time to onset of the LH surge was delayed in 3- and 4-month-old calves compared to 5-month-old calves (p < 0.05), but 2-month-old calves did not differ from any other age group. The hypothalamic and pituitary components of the reproductive axis can be induced to elicit an LH surge very early in development. However, the immature female does not begin spontaneous estrous cycles until much later in development. One difference that has been shown in these studies is the lack of responsiveness of FSH to estradiol stimulation in dairy heifer calves.

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