REPRODUCTIVE HORMONE LEVELS OF EARLY POSTPUBERTAL RAM LAMBS IN RELATION TO BREED, ADULT TESTIS SIZE AND SEMEN QUALITY

To evaluate the usefulness of early postpubertal endocrine characteris tics for predicting reproductive potential, blood serum levels of test osterone, luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin were determined in rams at 6, 8 and 12 months of age. T wo years later, scrotal size and semen quality were measured. The rams , housed indoors year-round were of a meat-type sire (Canadian Arcott) , two fecund-type dams (Outaouais and Rideau Arcotts) and Finnish Land race breeds. Daylength was maintained at 16-h light and 8-h darkness ( long days) from weaning to 6 months. At that time, daylength was reduc ed to 9-h light per day (short days) until the rams reached 3 years of age. The lighting regimen was then changed again to long days for 3 m onths, then back to short days (8 h of light). Indices of semen qualit y and testis size in adult rams were calculated from 10 records, colle cted biweekly, beginning at the end of a period of long days. Hormone levels, adult testis size and semen quality varied with breed. In gene ral, young Canadian rams had significantly lower testosterone and FSH levels than Finnish Landrace rams but as adults had a larger scrotal c ircumference and produced more sperm per ejaculate (42.2 cm and 7.0 x 10(9) compared to 33.0 cm and 3.3 x 10(9)). Outaouais and Rideau rams had hormone levels similar to those of Finnish Landrace rams but testi cular size and sperm production were intermediate between those of Can adian and Finnish Landrace rams. There were no breed differences among ram lambs in LH or prolactin levels. FSH levels, at all ages, were co rrelated negatively with scrotal size(P < 0.01). These findings indica te that FSH levels in ram lambs may be useful for predicting adult tes tis function. (C) 1998 Published by Elsevier Science B.V. All rights r eserved.