Project Number: 8042-31000-111-003-I
Project Type: Interagency Reimbursable Agreement
Start Date: May 1, 2023
End Date: Apr 30, 2026
Egg production in turkey hens exhibits a bell-shaped distribution, leading to groups of low and high egg producing hens performing below and above the average flock egg production, respectively. Egg production begins with follicle ovulation, which is initiated by a preovulatory hormonal surge by the hypothalamo-pituitary-gonadal axis but can be influenced by other neuroendocrine axes. The previous research has determined that low and high egg producing hens exhibit differential expression of two neuroendocrine axes, the hypothalamo-pituitary-gonadal axis and the hypothalamo-pituitary-thyroid axis prior to and during the preovulatory surge, with low egg producing hens showing upregulation of the thyroid axis. Culture of pituitary and follicle cells from low and high producing hens in vitro showed that high egg producing hens were more responsive to stimulation of the reproductive axis, with increased production of key reproductive hormones compared to low egg producing hens. Additionally, high egg producing hens displayed increased sensitivity to the effects of thyroid hormone in culture, with thyroid hormone treatment reducing the reproductive hormone producing capabilities of high egg producing hens to the decrease hormone levels produced by low egg producing hens under basal conditions. The thyroid axis has been characterized during the initiation and cessation of egg production, though its role during active egg production and its influence on ovulation is not well understood. To further understand the interplay between the hypothalamo-pituitary-thyroid axis and the hypothalamo-pituitary-gonadal axis and the implication of this relationship on ovulation and egg production rates, the following objectives are proposed: 1) Determine the influence of circulating thyroid hormone levels on the reproductive axis during peak egg production. ARS hypothesize thyroid hormone supplementation will downregulate the stimulatory pathways and upregulate the inhibitory pathways of the reproductive axis. 2) Define the hormonal profiles of the reproductive and thyroid axes in low, average, and high egg producing hens during the open period. ARS hypothesize that the egg production groups will exhibit differences in the reproductive and thyroid axis hormonal profile, with correlation amongst the hormones produced by the two neuroendocrine axes. 3) Characterize pituitary responsiveness to thyrotropin releasing hormone stimulation in low and high egg producing hens. ARS hypothesize that thyrotropin releasing hormone stimulation will reduce the upregulation of the reproductive axis seen in high egg producing hens in previous studies.
Objectives 1 and 2 will be completed with University of Maryland. Objective 3 will be completed by USDA. Commercial line breeding turkey hens will be used to complete these objectives. Objective 1: Egg production records will be used to determine average egg producing hens (roughly 90 hens). Three treatments will be applied in the drinking water for 4 weeks (beginning at initiation of egg production): no treatment, thyroid hormone (Synthroid), and methimazole (inhibits thyroid activity) (n=30 per treatment). Hens will be sampled inside and outside of the preovulatory surge. Egg production records, ovarian parameters, and blood will be collected at the end of treatment. Plasma samples will be analyzed for progesterone, estrogen, triiodothyronine, and thyroxine by radioimmunoassay. Additionally, the hypothalamus, pituitary, thyroid, and ovary will be collected from a subset of hens (n=5 per group). Levels of mRNA for key reproductive/thyroid genes will be assessed using real-time quantitative PCR (RT-qPCR). Results will be analyzed using a two-way ANOVA. Objective 2: Egg production records will be used to determine low, average, and high egg producing hens (roughly 60 hens, n=20 per group). The right jugular vein will be cannulated to allow for multiple blood samples to be collected without added handling stress to the birds. Blood samples will be taken every hour during the open period (8-10 hr window during which ovulation can occur physiologically). Plasma samples will be analyzed for progesterone, estrogen, triiodothyronine, thyroxine, and thyroid stimulating hormone using radioimmunoassays to create hormone profiles for key reproductive and thyroid hormones during the open period, and ultimately during the preovulatory surge. Results will be analyzed using a one-way MANOVA and regression analysis. Objective 3: Egg production records will be used to determine the top and bottom 15% of egg production, the high and low egg producing hens, respectively. Three treatments will be applied to the two egg production groups: saline injection (no treatment), thyrotropin releasing hormone (0.25 ug/kg) injection (stimulates the thyroid axis), and somatostatin (0.25 ug/kg) injection (suppresses thyrotropin releasing hormone action) (n=9 per treatment/egg production group). Hens will be injected daily for 4 weeks following the initiation of egg production. Hens will be sampled inside and outside of the preovulatory surge. Egg production records, ovarian parameters, blood, and tissues (hypothalamus, pituitary, thyroid, and ovary) will be collected at the end of the injection period. Blood samples will be assessed for steroid (progesterone and estrogen) and thyroid (triiodothyronine and thyroxine) hormone by radioimmunoassay. RNA will be extracted from collected tissue samples. Levels of mRNA for key reproductive/thyroid genes will be assessed using real-time quantitative PCR (RT-qPCR). Results will be analyzed using a two-way ANOVA.