Use of dietary thyroxine as an alternate molting procedure in spent turkey breeder hens

Abstract

In the turkey industry, molting is traditionally achieved by reducing photoperiod and withdrawing feed and water for several days. Although it is the most effective method, this practice is discouraged in Canada and alternative strategies need to be established. Thyroid hormone levels naturally change during molt, and dietary thyroxine (T4) supplementation was previously shown to induce molt in chickens. This study aimed at evaluating the effectiveness of supplemental dietary T4 in inducing molt in spent turkey breeder hens. One hundred twenty 75-wk-old hens were randomly divided into 4 groups (5 floor pens/replicates, 5 hens each) with the control group kept under a 14-h photoperiod and fed a breeder's diet throughout, whereas hens from the 3 other groups were supplemented with 40 ppm (45.76 mg/kg) T4 for 10 d. One treatment group was maintained under 14 h of light and fed a breeder's diet, whereas the 2 others were subjected to a drop in photoperiod to 6 h during or after supplementation and then were fed a maintenance diet. Egg production, feed intake, BW, molt, and plasma levels of T4, prolactin, and luteinizing hormone were measured. All treated hens ceased laying by d 20; however, several individuals spontaneously returned to lay when left on 14 h of light, suggesting incomplete involution of the reproductive tract. Supplementation significantly reduced feed consumption and induced rapid BW loss. All hens returned to their initial weight by the end of the experiment. Most treated hens initiated molt by d 8 of supplementation and all completed molt by d 37. Plasma T4 in treated hens increased significantly by d 3 (P < 0.05) and remained significantly higher than in controls until d 9 (P < 0.01). Levels returned to initial values by d 35. Prolactin levels did not appear to be influenced by T4 but were mainly dependent on photoperiod and reproductive stage, whereas luteinizing hormone levels remained low throughout. In summary, dietary supplementation with 40 ppm (45.76 mg/kg) T4 was successful in inducing molt in turkey breeder hens. However, dropping the photoperiod was necessary to completely reset the reproductive system.