SEASONAL-CHANGES IN THE CONCENTRATION OF ESTROGENS AND TESTOSTERONE IN THE PLASMA OF THE STALLION

A blood sample was taken from each of 15 stallions at monthly intervals for 14 consecutive months. Plasma concentrations of estrogens and testosterone were measured by radioimmunoassay methods. Estrogens in peripheral blood were present in much higher amounts than testosterone and were principally in a water-soluble, solvolyzable form (> 98%). The major component in the solvolyzed extracts behaved chromatographically as estrone. The mean plasma level (± S.E.) of estrogens averaged across months was 52.9 ± 4.5 ng ml-1. Individual stallions showed considerable month-to-month variation; for example, single monthly samples ranged from 29.5 to 160.6 ng ml-1 for the stallion with the highest single value. The highest mean monthly concentration was 69 ± ng ml-1 in May, and plasma levels were < 40 ng ml-1 during November and December. For the 11 Thoroughbred stallions in the study, the mean concentrations of estrogens were 73 ± 5.8 ng ml-1 for May to July and 45 ± 4.1 ng ml-1 for November to January (P > 0.001). The mean monthly concentrations (± S.E.) of testosterone ranged from 0.22 ± 0.05 to 0.90 ± 0.14 ng ml-1, and individual samples ranged from < 0.02 to 2.8 ng ml-1 of plasma. While the highest mean level of testosterone was seen in September, there was a significant difference (P < 0.01) between the values in the breeding season (May-July, 0.73 ± 0.07 ng ml-1) and the non-breeding season (November-January, 0.38 ± 0.08 ng ml-1). No marked seasonal changes were observed, however, in testosterone levels in several stallions. It was concluded that plasma estrogen levels may provide a more sensitive index of endocrine function of the testes in the stallion. © 1979.