The Relationship of Plasma Thyroid Hormone and Prolactin Concentrations to Egg Laying, Incubation Behavior, and Molting by Female Turkeys Exposed to a One-Year Natural Daylength Cycle

Abstract

To determine relationships to egg laying, incubation behavior and molting, plasma thyroxine (T4), triiodothyronine (T3), and prolactin levels of 20 female domestic turkeys were determined throughout a one-year cycle of natural daylengths. Endocrine changes were analyzed relative to periods of reproductive and molting cycles. Data were also analyzed relative to termination of lay after division into two groups based on the presence (I, n = 7) or absence (NI, n = 13) of incubation behavior. Plasma T4 levels of the flock increased to a peak and subsequently decreased during the onset of lay period from February to April. Plasma T4 generally increased throughout the decline of lay period from April to July and molting period from July to October, and then reached greatest levels at the end of the molting period. Plasma T3 reached a peak early in the onset of lay and decreased rapidly during the remainder of that period. Plasma T3 remained depressed during the decline of lay, increased throughout molting, and attained greatest levels as molt was completed. Prolactin was low throughout the prelay period and sharply increased midway through the onset of lay. Prolactin increased to a marked peak during the decline of lay, and then rapidly decreased as egg laying was terminated. Relative to NI birds, egg laying was terminated earlier and prolaclin levels were markedly greater during and following the termination of lay in I birds. Plasma T4 and T3 were generally lower in I birds during the molting period which followed the termination of lay and their molt was delayed. These results support the hypothesis that changes in thyroid hormone and prolactin levels are involved in physiological processes associated with the termination of egg laying and molting in female turkeys.