PHYSIOLOGICAL-BASIS OF INDUCED MOLTING AND TISSUE REGENERATION IN FOWLS

Some of the methods used to induce moulting in practice, the accompanying endocrinological and morphological changes and the coincidental feather rejuvenation, are discussed. The methods based on a quantitative feed restriction, followed by a variable degree of restricted energy intake after fasting, are reviewed together with methods based on a low calcium, and/or sodium or a high zinc diet. There is a more or less pronounced body weight loss, according to the method of egg laying stop used. The regression of the reproductive organs is pronounced but ovarian regression precedes the regression of oviduct and shell gland, the latter beginning under ovarian produced steroidal control. There was also a differential reduction of liver, ovary and oviduct depending on the mounting method used. The physiological-endocrinological background of moulting is not fully understood. The coincidental cessation of egg production and subsequent moulting when artificial methods of stopping egg laying are used requires further study. Fasting of laying hens causes lower progesterone and oestradiol plasma levels after 1 day. Luteinizing hormone (LH) levels were also lower, although this was not due to pituitary exhaustion, but rather to the lack of LH-releasing hormone stimulus, and/or decreased pituitary responsiveness. The changes in hormonal levels in hens moulted by the low Ca- or Na-method were generally slower and less pronounced than those in fasting hens. Possible changes in response of the ovary during moult or following different artificial methods of stopping egg production have still to be elucidated. Both thyroid hormones and prolactin have antigonadotropic effects and the levels of both increase following artificially induced cessation of egg production. The role of thyroid hormones, and especially of thyroxine, in the development and growth of feather papillae as well as the importance of a decrease in oestrogen activity for initiating moult are documented. An increase in the thyroid hormones/oestrogens ratio is probably of crucial importance for inducing new feather formation and could explain the occurrence of feather rejuvenation coincidental with cessation of egg production. This could be important in relation to energy balance after moulting.