Molt Induced by Gonadotropin-Releasing Hormone Agonist as a Model for Studying Endocrine Mechanisms of Molting in Laying Hens

Abstract

Molt induced by infusion of a gonadotropin-releasing hormone agonist (GnRH-A, ([D-Leu6,Pro9]-GnRH N-ethylamide]) has been used as a model for studying the endocrine mechanisms of molting. Molt induced by GnRH-A was also compared with that induced by feed withdrawal (FW). Thirty-three laying hens were divided into three groups: 1) sham infused (controls), 2) molted by GnRH-A infusion (GnRH-A), or 3) molted by FW. All groups had reduced day length. Birds were weighed and blood was sampled twice per week. Plasma was assayed for thyroxine (T4), triiodothyronine (T3), and progesterone (P4); egg production was recorded. A separate group of GnRH-A molted (n=8) and control (n=4) hens were killed during the molt and liver, ovary, and oviduct were weighed.Plasma T4 concentrations were elevated (P<.01), whereas plasma T3 did not change and plasma P4 decreased (P<.05) in the GnRH-A group compared with the control group. Plasma T4 was higher (P<.05) in the GnRH-A group than in the FW group, whereas plasma T3 was higher (P<.05) in the FW group than in the GnRH-A group. Plasma P4 concentrations declined to similar low levels in the GnRH-A and FW groups. The GnRH-A caused weight loss (P<.05) from the ovary, oviduct, and liver. The GnRH-A-molted hens lost less BW (P<.001) and were out of lay for 12 days less than FW-molted hens. Postmolt rates of lay were not significantly different for the two groups. It is concluded that: 1) GnRH-A-induced ovarian regression is associated with changes in plasma T4 and T3 concentrations that are different from those occurring during FW and 2) GnRH-A may be a useful agent for inducing molt and improved postmolt egg production.