Photoperiodicity and Wool Growth in Romney Rams and Wethers

Abstract

PHOTOPERIODIC control of the breeding season in several breeds of sheep has been demonstrated by Yeates¹ and others, and the effect of light as a stimulant to growth and shedding of hair in some mammals has also been investigated. Some wool-production responses of New Zealand Corriedale ewes to photoperiodic stimuli have been described by Hart^2,3 : ewes which had endured constant artificial light/dark rhythms produced for several months a greater weight of wool than they had done before the treatment and greater than control ewes subjected to the normal solar-light rhythm. It is thought that, succeeding the initial reception by the eyes of the light stimulus, impulses travel neurally to the hypothalamus and from there the activity of the adenohypophysis is controlled possibly by a neurohumoral link ; the activity of the adenohypophysis may then influence the rate of wool growth either by thyrotrophic or growth hormones. Hart³ supplied evidence supporting the view that “the photoperiodic response in wool growth may be due to an increased thyrotrophic activity of the pituitary” ; in one experiment he subjected a group of ewes to a fixed rhythm of two hours light to four hours darkness and after a latent period there was a marked response in wool growth by these animals. Hart cited this result in support of the ‘contrast sensitivity’ theory of reaction by the complex responsible for the expression of photoperiodic stimuli.