Effects of month and age on prolactin concentrations in stallion serum

Prolactin concentrations in stallion serum were measured by a newly developed radioimmunoassay based on anti-dog prolactin serum and radiolabelled horse prolactin. Samples of serum from a total of 444 stallions were obtained at a commercial abattoir monthly from April to the following March. Ages of stallions were estimated from eruption and wear patterns of incisors. In the analysis of variance, both month (P less than 0.01) and age (P less than 0.05) were significant sources of variation whereas there was no interaction between these factors. Monthly means for prolactin concentrations were greatest between May and August with peak concentrations occurring in July (7.1 +/- 0.5 ng/ml). Lowest mean concentrations of prolactin (1.2 ng/ml) were observed during the winter months (December and January). Concentrations of prolactin were lowest in stallions less than or equal to 5 years old and were highest in stallions greater than 10 years old. It appears that prolactin secretion in the stallion is strongly influenced by season, as it is in the mare and in other seasonally breeding animals. Moreover, prolactin secretion in the stallion tends to increase as the stallion ages.