Role of ovarian steroids in the control of moult induction in laying fowls.

1. Thirty-five Warren SSL hens were selected on the basis of variation in moulting response following a force-moult at 81 weeks of age. Fourteen hens were from a group which returned to layers mash ad libitum from day 9, while 21 came from a group with dietary restriction prolonged to day 28. Blood samples were taken on days 23 and 36, and hormone concentrations were measured. Moulting was recorded on days 0, 11, 23, 36, 52 and 68. 2. Progesterone (P4) but not oestradiol (E2) inhibited moulting during egg laying and the sharp fall in P4 concentration when laying ceased was a primary factor in the induction of moulting. 3. Thyroxine (T4) concentrations were closely correlated with all criteria of plumage renewal, but not with the onset of moulting. Although T4 was closely correlated with subsequent feather growth it is questionable whether T4 is a primer of moult-induction independent of P4. 4. Feathers which were pulled out required 12 +/- 2 d to be replaced beyond the feather-papillae stage. For remiges this timing was similar regardless of whether hens were moulting or still laying. The timing was similar for remiges and body-feathers in moulting hens, but the reappearance of body-feathers in laying hens was slightly delayed.